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Synchrotron X-ray powder diffraction methods have been used to obtain accurate structures of the lanthanoid tantalates, LnTaO4, at room temperature. Three different structures are observed, depending on the size of the Ln cation: P21/c (Ln = La, Pr), I2/a (Ln = Nd-Ho), and P2/c (Ln = Tb-Lu). BVS analysis indicated that TaV is six-coordinate in these structures, with four short bonds and two longer bonds. Synchrotron X-ray powder diffraction methods were also used to observe the impact of Ta doping on the orthoniobates, Ln(Nb1-xTax)O4 (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Yb, and Lu). Where both the niobate and tantalate oxide were isostructural (fergusonite structure, space group I2/a), complete solid solutions were prepared. In these solid solutions, the unit cell volume decreases as the Ta content increases. The subtle interaction evident between the LnO8 and BO6 sublattices in the fergusonite-type oxides was not observed in the related pyrochlore oxides. A combined synchrotron X-ray and neutron powder diffraction study of the series Ho(Nb1-xTax)O4 was used to determine accurate atomic positions of the anions, and hence, bond lengths. This revealed a change in the (Nb/Ta)-O bond lengths, reflective of the difference in the valence orbitals of Nb(4d) and Ta(5d). Examination of the partial density of states demonstrates differences in the electronics between Nb and Ta, leading to a difference in the bandgap. This study highlights the importance of the long B-O contacts in the fergusonite structures, and its potential impact on the I2/a to I41/a phase transition.

Materials with negative thermal expansion are beneficial as they can be appropriately tuned to be compatible for device engineering. Here, we report our first-principles investigation on Hg2X2 and HgX2 (X= Cl, I), which exhibits negative thermal expansion behaviour along c-direction. The first-principles studies reveal an anisotropic thermal expansion behaviour in the investigated compounds over a broad range of temperature. The computed coefficients along the c-axis are found to be negative for Hg2Cl2 and HgCl2. Hg2I2 exhibits negative thermal expansion behaviour up to 100 K, while HgI2 exhibits positive thermal expansion, whose values are qualitatively in good accord with the experimentally reported ones. The Grüneisen parameters along with elastic compliance matrix are used to calculate the thermal expansion coefficients of these materials. A detailed study of vibrational spectra was also done to get a better understanding of thermal expansion. © 2022 Elsevier Ltd

Materials with negative thermal expansion are beneficial as they can be appropriately tuned to be compatible for device engineering. Here, we report our first-principles investigation on Hg2X2 and HgX2 (X= Cl, I), which exhibits negative thermal expansion behaviour along c-direction. The first-principles studies reveal an anisotropic thermal expansion behaviour in the investigated compounds over a broad range of temperature. The computed coefficients along the c-axis are found to be negative for Hg2Cl2 and HgCl2. Hg2I2 exhibits negative thermal expansion behaviour up to 100 K, while HgI2 exhibits positive thermal expansion, whose values are qualitatively in good accord with the experimentally reported ones. The Grüneisen parameters along with elastic compliance matrix are used to calculate the thermal expansion coefficients of these materials. A detailed study of vibrational spectra was also done to get a better understanding of thermal expansion. © 2022 Elsevier Ltd

The stereochemical activity of lone pair electrons plays a central role in determining the structural and electronic properties of both chemically simple materials such as H2O, as well as more complex condensed phases such as photocatalysts or thermoelectrics. TlReO4 is a rare example of a non-magnetic material exhibiting a re-entrant phase transition and emphanitic behavior in the long-range structure. Here, we describe the role of the Tl+ 6s2 lone pair electrons in these unusual phase transitions and illustrate its tunability by chemical doping, which has broad implications for functional materials containing lone pair bearing cations. First-principles density functional calculations clearly show the contribution of the Tl+ 6s2 in the valence band region. Local structure analysis, via neutron total scattering, revealed that changes in the long-range structure of TlReO4 occur due to changes in the correlation length of the Tl+ lone pairs. This has a significant effect on the anion interactions, with long-range ordered lone pairs creating a more densely packed structure. This resulted in a trade-off between anionic repulsions and lone pair correlations that lead to symmetry lowering upon heating in the long-range structure, whereby lattice expansion was necessary for the Tl+ lone pairs to become highly correlated. Similarly, introducing lattice expansion through chemical pressure allowed long-range lone pair correlations to occur over a wider temperature range, demonstrating a method for tuning the energy landscape of lone pair containing functional materials.

Corneal ectasia and Stevens–Johnson syndrome (SJS) are uncommon ocular conditions,1 both of which can be managed with scleral contact lenses to improve visual acuity, provide constant corneal lubrication, and decrease photophobia. Very few reports of co-existing keratoconus and SJS have been published. Rathi et al.2 described three cases (14–25-year-old males) with drug-induced SJS who developed keratoconus between two and seven years after the onset of SJS. They demonstrated a significant improvement in visual acuity and decrease in photophobia and ocular symptoms with full-time scleral lens wear. Saaed et al.3 described nine cases of keratoconus that developed on average 10 years after the onset of SJS and were successfully managed with scleral lenses. This clinical communication highlights the use of scleral lenses in the management of co-existing keratoconus and SJS.

This case report describes a unique presentation of bilateral sequential acute corneal hydrops that manifested several years after laser in situ keratomileusis. Initial management included anterior chamber perfluoropropane gas injection and corneal suturing. Longer-term visual rehabilitation involved the use of scleral lenses which significantly reduced lower- and higher-order ocular aberrations.

The present work reports the complete study of structural, vibrational, mechanical, and electronic properties of BeAl2O4 (known as Chrysoberyl) using first-principles computing methods. The calculated ground-state properties agree quite well with previous experiments. The computed phonon dispersion curves do not show imaginary frequencies confirming the dynamical stability. In addition, the calculated elastic constants also ensure the mechanical stability through fulfillment of mechanical stability criteria. Apart from that, the theoretically determined phonon frequencies agree quite well with previous Raman and infrared experiments at ambient conditions. Various thermodynamic properties are also being calculated as a function of temperature. The thermal expansion computed within the quasi-harmonic approximation is found to be positive in agreement with the experiments. From the electronic structure we can see that Chrysoberyl possess a direct band gap of 8.3 eV with the oxygen- 2p states dominating close to fermi level in the valence band. The influence of pressure on different properties is discussed. The highlight of the present work is the presence of optical isotropy in Chrysoberyl although the crystal structure is highly anisotropic.

The present work reports the complete study of structural, vibrational, mechanical, and electronic properties of BeAl2O4 (known as Chrysoberyl) using first-principles computing methods. The calculated ground-state properties agree quite well with previous experiments. The computed phonon dispersion curves do not show imaginary frequencies confirming the dynamical stability. In addition, the calculated elastic constants also ensure the mechanical stability through fulfillment of mechanical stability criteria. Apart from that, the theoretically determined phonon frequencies agree quite well with previous Raman and infrared experiments at ambient conditions. Various thermodynamic properties are also being calculated as a function of temperature. The thermal expansion computed within the quasi-harmonic approximation is found to be positive in agreement with the experiments. From the electronic structure we can see that Chrysoberyl possess a direct band gap of 8.3 eV with the oxygen- 2p states dominating close to fermi level in the valence band. The influence of pressure on different properties is discussed. The highlight of the present work is the presence of optical isotropy in Chrysoberyl although the crystal structure is highly anisotropic.

The present work reports the complete study of structural, vibrational, mechanical, and electronic properties of BeAl2O4 (known as Chrysoberyl) using first-principles computing methods. The calculated ground-state properties agree quite well with previous experiments. The computed phonon dispersion curves do not show imaginary frequencies confirming the dynamical stability. In addition, the calculated elastic constants also ensure the mechanical stability through fulfillment of mechanical stability criteria. Apart from that, the theoretically determined phonon frequencies agree quite well with previous Raman and infrared experiments at ambient conditions. Various thermodynamic properties are also being calculated as a function of temperature. The thermal expansion computed within the quasi-harmonic approximation is found to be positive in agreement with the experiments. From the electronic structure we can see that Chrysoberyl possess a direct band gap of 8.3 eV with the oxygen- 2p states dominating close to fermi level in the valence band. The influence of pressure on different properties is discussed. The highlight of the present work is the presence of optical isotropy in Chrysoberyl although the crystal structure is highly anisotropic.

Purpose: Visual rehabilitation following penetrating keratoplasty is the primary indication for approximately 15% of all scleral lens fittings. Since corneal biomechanics are altered following penetrating keratoplasty, the aim of this study was to quantify changes in anterior corneal optics following short-term scleral lens wear in eyes with corneal grafts. Methods: Scheimpflug images were obtained before and after a period of scleral lens wear (mean 6.3 ± 1.4 h), from eyes that had previously undergone penetrating keratoplasty (10 eyes of nine participants, mean age 31 ± 9 years). Corneal power and thickness data were examined over the central 6 mm, including regional analyses of the central (0–3 mm) and the mid-peripheral cornea (3–6 mm annulus) using customised software to deterime corneal power vectors M (best fit sphere), J0 (90/180 astigmatism) and J45 (45/135 astigmatism). Anterior corneal aberrations were extracted using corneal elevation data. Results: Corneal power vector J45 increased following lens wear (by 0.22 ± 0.05 D, p = 0.003) across the central 6 mm, while M displayed regional variations following lens wear indicating larger changes further from the corneal centre (p = 0.004). The change in corneal power vector M was also correlated with the magnitude of central corneal swelling (r = 0.65, p = 0.04). The anterior corneal aberration terms of oblique astigmatism, hoirzontal coma, and spherical aberration also varied following lens wear (all p ≤ 0.01). The mean change in the corneal spherocylinder derived from the elevation data following lens wear was +0.14/−0.54 × 44 for a 6 mm corneal diameter. Conclusions: Clinically significant alterations in anterior corneal topography and higher order aberrations were observed following short-term scleral lens wear in eyes that had undergone penetrating keratoplasty. Spherocylindrical changes were approximately

We present a highly versatile system ReS2, which transforms from a semiconductor to a two dimensional metal under uni-axial compressive strain along ‘a’ direction in both bulk and monolayer. The 2D nature is realised from highly flat Fermi surfaces and anisotropic transport properties. Moreover the layer independent electronic structure properties are revisited and thermoelectric properties of ReS2 in bulk, monolayer and bilayer forms reveal the competing thermoelectric (TE) coefficients in each form. The in-plane power-factor shows an enhancement over ‘c’-axis value as a function of strain, which is almost two orders of magnitude. In addition, strain induced tunable in-plane anisotropy of almost one order has been observed in both bulk and monolayer ReS2 (around 20%), which further open up the possibility of TE application as nanowires. Our analysis unveils a wide range of application for ReS2 in the field of thermoelectrics as bulk and thin films for a large temperature range. The magnitude of TE coefficients are comparable with other well established transition metal dichalocogenides.

In this work, we report the structural, magnetic, electrical and thermal transport properties of the Heusler-type alloy Ru2NbAl. From the detailed analysis of magnetization data, we infer the presence of superparamagnetically interacting clusters with a Pauli paramagnetic background, while short-range ferromagnetic interaction is developed among the clusters below 5 K. The presence of this ferromagnetic interaction is confirmed through heat capacity measurements. The relatively small value of electronic contribution to specific heat gamma (similar to 2.7 mJ mo1(-1) K-2 ) as well as the linear nature of temperature dependence of the Seebeck coefficient indicate a semimetallic ground state with a pseudogap, which is also supported by our electronic structure calculations. The activated nature of resistivity is reflected in the observed negative temperature coefficient and has its origin in the charge carrier localization due to antisite defects, inferred from magnetic measurements as well as structural analysis. Although the absolute value of thermoelectric figure of merit is rather low (ZT = 5.2 x 10(-3) ) in Ru2NbAl, it is the largest among all the reported nondoped full Heusler alloys.

In this work, we report the structural, magnetic, electrical and thermal transport properties of the Heusler-type alloy Ru2NbAl. From the detailed analysis of magnetization data, we infer the presence of superparamagnetically interacting clusters with a Pauli paramagnetic background, while short-range ferromagnetic interaction is developed among the clusters below 5 K. The presence of this ferromagnetic interaction is confirmed through heat capacity measurements. The relatively small value of electronic contribution to specific heat gamma (similar to 2.7 mJ mo1(-1) K-2 ) as well as the linear nature of temperature dependence of the Seebeck coefficient indicate a semimetallic ground state with a pseudogap, which is also supported by our electronic structure calculations. The activated nature of resistivity is reflected in the observed negative temperature coefficient and has its origin in the charge carrier localization due to antisite defects, inferred from magnetic measurements as well as structural analysis. Although the absolute value of thermoelectric figure of merit is rather low (ZT = 5.2 x 10(-3) ) in Ru2NbAl, it is the largest among all the reported nondoped full Heusler alloys.

In this work, we report the structural, magnetic, electrical and thermal transport properties of the Heusler-type alloy Ru2NbAl. From the detailed analysis of magnetization data, we infer the presence of superparamagnetically interacting clusters with a Pauli paramagnetic background, while short-range ferromagnetic interaction is developed among the clusters below 5 K. The presence of this ferromagnetic interaction is confirmed through heat capacity measurements. The relatively small value of electronic contribution to specific heat gamma (similar to 2.7 mJ mo1(-1) K-2 ) as well as the linear nature of temperature dependence of the Seebeck coefficient indicate a semimetallic ground state with a pseudogap, which is also supported by our electronic structure calculations. The activated nature of resistivity is reflected in the observed negative temperature coefficient and has its origin in the charge carrier localization due to antisite defects, inferred from magnetic measurements as well as structural analysis. Although the absolute value of thermoelectric figure of merit is rather low (ZT = 5.2 x 10(-3) ) in Ru2NbAl, it is the largest among all the reported nondoped full Heusler alloys.

We present a highly versatile system ReS2, which transforms from a semiconductor to a two dimensional metal under uni-axial compressive strain along ‘a’ direction in both bulk and monolayer. The 2D nature is realised from highly flat Fermi surfaces and anisotropic transport properties. Moreover the layer independent electronic structure properties are revisited and thermoelectric properties of ReS2 in bulk, monolayer and bilayer forms reveal the competing thermoelectric (TE) coefficients in each form. The in-plane power-factor shows an enhancement over ‘c’-axis value as a function of strain, which is almost two orders of magnitude. In addition, strain induced tunable in-plane anisotropy of almost one order has been observed in both bulk and monolayer ReS2 (around 20%), which further open up the possibility of TE application as nanowires. Our analysis unveils a wide range of application for ReS2 in the field of thermoelectrics as bulk and thin films for a large temperature range. The magnitude of TE coefficients are comparable with other well established transition metal dichalocogenides.

In this work, we report the structural, magnetic, electrical and thermal transport properties of the Heusler-type alloy Ru2NbAl. From the detailed analysis of magnetization data, we infer the presence of superparamagnetically interacting clusters with a Pauli paramagnetic background, while short-range ferromagnetic interaction is developed among the clusters below 5 K. The presence of this ferromagnetic interaction is confirmed through heat capacity measurements. The relatively small value of electronic contribution to specific heat gamma (similar to 2.7 mJ mo1(-1) K-2 ) as well as the linear nature of temperature dependence of the Seebeck coefficient indicate a semimetallic ground state with a pseudogap, which is also supported by our electronic structure calculations. The activated nature of resistivity is reflected in the observed negative temperature coefficient and has its origin in the charge carrier localization due to antisite defects, inferred from magnetic measurements as well as structural analysis. Although the absolute value of thermoelectric figure of merit is rather low (ZT = 5.2 x 10(-3) ) in Ru2NbAl, it is the largest among all the reported nondoped full Heusler alloys.

First principles calculations are performed using density functional theory and density functional perturbation theory for SnAs. Total energy calculations show the first order phase transition from an NaCl structure to a CsCl one at around 37 GPa, which is also confirmed from enthalpy calculations and agrees well with experimental work. Calculations of the phonon structure and hence the electron-phonon coupling, λepλep, and superconducting transition temperature, Tc, across the phase diagram are performed. These calculations give an ambient pressure Tc, in the NaCl structure, of 3.08 K, in good agreement with experiment whilst at the transition pressure, in the CsCl structure, a drastically increased value of View the MathML sourceTc=12.2K is found. Calculations also show a dramatic increase in the electronic density of states at this pressure. The lowest energy acoustic phonon branch in each structure also demonstrates some softening effects. Electronic structure calculations of the Fermi surface in both phases are presented for the first time as well as further calculations of the generalised susceptibility with the inclusion of matrix elements. These calculations indicate that the softening is not derived from Fermi surface nesting and it is concluded to be due to a wavevector-dependent enhancement of the electron-phonon coupling.

A detailed study on the ternary Zr-based intermetallic compound Zr2TiAl has been carried out using first-principles electronic structure calculations. From the total energy calculations, we find an antiferromagnetic L1(1)-like (AFM) phase with alternating ( 1 1 1) spin-up and spin-down layers to be a stable phase among some others with magnetic moment on Ti being 1.22 mu B. The calculated magnetic exchange interaction parameters of the Heisenberg Hamiltonian and subsequent Heisenberg Monte Carlo simulations confirm that this phase is the magnetic ground structure with Neel temperature between 30 and 100 K. The phonon dispersion relations further confirm the stability of the magnetic phase while the non-magnetic phase is found to have imaginary phonon modes and the same is also found from the calculated elastic constants. The magnetic moment of Ti is found to decrease under pressure eventually driving the system to the non-magnetic phase at around 46 GPa, where the phonon modes are found to be positive indicating stability of the non-magnetic phase. A continuous change in the band structure under compression leads to the corresponding change of the Fermi surface topology and electronic topological transitions (ETT) in both majority and minority spin cases, which are also evident from the calculated elastic constants and density of state calculations for the material under compression.

In the present work, we report a combined experiment and theoretical study on high pressure structural stability of Al2Cu type- Mo2B up to ∼40 GPa. Experiments using rotating anode x-ray source indicate that the ground state tetragonal structure of Mo2B (space group I4/mcm) to be stable up to the highest pressure studied. In addition, the experimental results were complemented by first principles density functional calculations within the projector augmented wave method. The calculated structural parameters are in excellent agreement with present experiments as well as previous reports. The estimated bulk modulus using Birch-Murnaghan equation of state from both experiment (302 GPa) and theory (310 GPa) reveals the ultra-incompressible nature of Mo2B. The much higher bulk modulus of this semi boride as compared to elemental Mo is discussed in terms of Mo-Mo bond distance in their crystal structures. The calculated electronic structure of Mo2B shows a strong Fermi surface nesting along P-P direction, which might also trigger lattice instability and is addressed in detail.

In the present work, we report a combined experiment and theoretical study on high pressure structural stability of Al2Cu type- Mo2B up to ∼40 GPa. Experiments using rotating anode x-ray source indicate that the ground state tetragonal structure of Mo2B (space group I4/mcm) to be stable up to the highest pressure studied. In addition, the experimental results were complemented by first principles density functional calculations within the projector augmented wave method. The calculated structural parameters are in excellent agreement with present experiments as well as previous reports. The estimated bulk modulus using Birch-Murnaghan equation of state from both experiment (302 GPa) and theory (310 GPa) reveals the ultra-incompressible nature of Mo2B. The much higher bulk modulus of this semi boride as compared to elemental Mo is discussed in terms of Mo-Mo bond distance in their crystal structures. The calculated electronic structure of Mo2B shows a strong Fermi surface nesting along P-P direction, which might also trigger lattice instability and is addressed in detail.

In the present work, we report a combined experiment and theoretical study on high pressure structural stability of Al2Cu type- Mo2B up to ∼40 GPa. Experiments using rotating anode x-ray source indicate that the ground state tetragonal structure of Mo2B (space group I4/mcm) to be stable up to the highest pressure studied. In addition, the experimental results were complemented by first principles density functional calculations within the projector augmented wave method. The calculated structural parameters are in excellent agreement with present experiments as well as previous reports. The estimated bulk modulus using Birch-Murnaghan equation of state from both experiment (302 GPa) and theory (310 GPa) reveals the ultra-incompressible nature of Mo2B. The much higher bulk modulus of this semi boride as compared to elemental Mo is discussed in terms of Mo-Mo bond distance in their crystal structures. The calculated electronic structure of Mo2B shows a strong Fermi surface nesting along P-P direction, which might also trigger lattice instability and is addressed in detail.

In the present work, we report a combined experiment and theoretical study on high pressure structural stability of Al2Cu type- Mo2B up to ∼40 GPa. Experiments using rotating anode x-ray source indicate that the ground state tetragonal structure of Mo2B (space group I4/mcm) to be stable up to the highest pressure studied. In addition, the experimental results were complemented by first principles density functional calculations within the projector augmented wave method. The calculated structural parameters are in excellent agreement with present experiments as well as previous reports. The estimated bulk modulus using Birch-Murnaghan equation of state from both experiment (302 GPa) and theory (310 GPa) reveals the ultra-incompressible nature of Mo2B. The much higher bulk modulus of this semi boride as compared to elemental Mo is discussed in terms of Mo-Mo bond distance in their crystal structures. The calculated electronic structure of Mo2B shows a strong Fermi surface nesting along P-P direction, which might also trigger lattice instability and is addressed in detail.

A detailed study on the electronic structure and Fermi surface (FS) of superconducting Heusler compounds ScAu2(Al, In) and ScPd2(Sn, Pb) has been carried out using first principles electronic structure calculations. The spin orbit coupling is found to play a major role in understanding the band structure and FS. Analysis of the data shows the importance of spin orbit coupling effect in the above compounds. The bands which cross Fermi level (EF) are found to be dominated by the Sc d t2g -states. The calculated total density of states are in good agreement with the experimentally reported value for ScPd2Sn. Under compression we find a change in the Fermi surface topology of ScPd2Sn at V/V0 = 0.95 (pressure of≈15 GPa), which is explained using the band structure calculations.

The crystal structure and the Yb valence of the YbFe2Ge2 heavy fermion compound was measured at room temperature and under high pressures using high-pressure powder X-ray diffraction and X-ray absorption spectroscopy via both partial fluorescence yield and resonant inelastic X-ray emission techniques. The measurements are complemented by first-principles density functional theoretical calculations using the self-interaction corrected local spin density approximation investigating in particular the magnetic structure and the Yb valence. While the ThCr2Si2-type tetragonal (I4/mmm) structure is stable up to 53 GPa, the X-ray emission results show an increase of the Yb valence from v = 2.72(2) at ambient pressure to v = 2.93(3) at ∼9 GPa, where at low temperature a pressure-induced quantum critical state was reported.

A detailed study on the electronic structure and Fermi surface (FS) of superconducting Heusler compounds ScAu2(Al, In) and ScPd2(Sn, Pb) has been carried out using first principles electronic structure calculations. The spin orbit coupling is found to play a major role in understanding the band structure and FS. Analysis of the data shows the importance of spin orbit coupling effect in the above compounds. The bands which cross Fermi level (EF) are found to be dominated by the Sc d t2g -states. The calculated total density of states are in good agreement with the experimentally reported value for ScPd2Sn. Under compression we find a change in the Fermi surface topology of ScPd2Sn at V/V0 = 0.95 (pressure of≈15 GPa), which is explained using the band structure calculations.

The crystal structure and the Yb valence of the YbFe2Ge2 heavy fermion compound was measured at room temperature and under high pressures using high-pressure powder X-ray diffraction and X-ray absorption spectroscopy via both partial fluorescence yield and resonant inelastic X-ray emission techniques. The measurements are complemented by first-principles density functional theoretical calculations using the self-interaction corrected local spin density approximation investigating in particular the magnetic structure and the Yb valence. While the ThCr2Si2-type tetragonal (I4/mmm) structure is stable up to 53 GPa, the X-ray emission results show an increase of the Yb valence from v = 2.72(2) at ambient pressure to v = 2.93(3) at ∼9 GPa, where at low temperature a pressure-induced quantum critical state was reported.

Electronic and thermoelectric properties of ternary chalcopyrite-type CuAlCh 2 (S, Se and Te) were studied using the first principles density functional calculations implemented in the full potential linear augmented plane wave (FP-LAPW) method. The thermoelectric properties are calculated by solving the Boltzmann transport equation within the constant relaxation time approximation. The calculated band gap using the Tran-Blaha modified Becke-Johnson potential (TB-mBJ), of all the compounds are in good agreement with the available experiment and other theoretical reports. Thermoelectric properties like thermopower, electrical conductivity scaled by relaxation time are calculated as a function of carrier concentrations at different temperatures. The calculated thermoelectric properties are compared with the available experiment and other theoretical calculations of similar materials.

High pressure structural stability studies were carried out on YGa2 (AlB2 type structure at NTP, space group P6/mmm) up to a pressure of ~35 GPa using both laboratory based rotating anode and synchrotron X-ray sources. An isostructural transition with reduced c/a ratio, was observed at ~6 GPa and above ~17.5 GPa, the compound transformed to orthorhombic structure. Bulk modulus B0 for the parent and high pressure phases were estimated using Birch–Murnaghan and modified Birch–Murnaghan equation of state. Electronic structure calculations based on projector augmented wave method confirms the experimentally observed two high pressure structural transitions. The calculations also reveal that the ‘Ga’ networks remains as two dimensional in the high pressure isostructural phase, whereas the orthorhombic phase involves three dimensional networks of ‘Ga’ atoms interconnected by strong covalent bonds.

The crystal structure and the Yb valence of the YbFe2Ge2 heavy fermion compound was measured at room temperature and under high pressures using high-pressure powder X-ray diffraction and X-ray absorption spectroscopy via both partial fluorescence yield and resonant inelastic X-ray emission techniques. The measurements are complemented by first-principles density functional theoretical calculations using the self-interaction corrected local spin density approximation investigating in particular the magnetic structure and the Yb valence. While the ThCr2Si2-type tetragonal (I4/mmm) structure is stable up to 53 GPa, the X-ray emission results show an increase of the Yb valence from v = 2.72(2) at ambient pressure to v = 2.93(3) at ∼9 GPa, where at low temperature a pressure-induced quantum critical state was reported.

The crystal structure and the Yb valence of the YbFe2Ge2 heavy fermion compound was measured at room temperature and under high pressures using high-pressure powder X-ray diffraction and X-ray absorption spectroscopy via both partial fluorescence yield and resonant inelastic X-ray emission techniques. The measurements are complemented by first-principles density functional theoretical calculations using the self-interaction corrected local spin density approximation investigating in particular the magnetic structure and the Yb valence. While the ThCr2Si2-type tetragonal (I4/mmm) structure is stable up to 53 GPa, the X-ray emission results show an increase of the Yb valence from v = 2.72(2) at ambient pressure to v = 2.93(3) at ∼9 GPa, where at low temperature a pressure-induced quantum critical state was reported.

High pressure structural stability studies were carried out on YGa2 (AlB2 type structure at NTP, space group P6/mmm) up to a pressure of ~35 GPa using both laboratory based rotating anode and synchrotron X-ray sources. An isostructural transition with reduced c/a ratio, was observed at ~6 GPa and above ~17.5 GPa, the compound transformed to orthorhombic structure. Bulk modulus B0 for the parent and high pressure phases were estimated using Birch–Murnaghan and modified Birch–Murnaghan equation of state. Electronic structure calculations based on projector augmented wave method confirms the experimentally observed two high pressure structural transitions. The calculations also reveal that the ‘Ga’ networks remains as two dimensional in the high pressure isostructural phase, whereas the orthorhombic phase involves three dimensional networks of ‘Ga’ atoms interconnected by strong covalent bonds.

Electronic and thermoelectric properties of ternary chalcopyrite-type CuAlCh 2 (S, Se and Te) were studied using the first principles density functional calculations implemented in the full potential linear augmented plane wave (FP-LAPW) method. The thermoelectric properties are calculated by solving the Boltzmann transport equation within the constant relaxation time approximation. The calculated band gap using the Tran-Blaha modified Becke-Johnson potential (TB-mBJ), of all the compounds are in good agreement with the available experiment and other theoretical reports. Thermoelectric properties like thermopower, electrical conductivity scaled by relaxation time are calculated as a function of carrier concentrations at different temperatures. The calculated thermoelectric properties are compared with the available experiment and other theoretical calculations of similar materials.

A detailed study on the electronic structure and Fermi surface (FS) of superconducting Heusler compounds ScAu2(Al, In) and ScPd2(Sn, Pb) has been carried out using first principles electronic structure calculations. The spin orbit coupling is found to play a major role in understanding the band structure and FS. Analysis of the data shows the importance of spin orbit coupling effect in the above compounds. The bands which cross Fermi level (EF) are found to be dominated by the Sc d t2g -states. The calculated total density of states are in good agreement with the experimentally reported value for ScPd2Sn. Under compression we find a change in the Fermi surface topology of ScPd2Sn at V/V0 = 0.95 (pressure of≈15 GPa), which is explained using the band structure calculations.

Electronic and thermoelectric properties of ternary chalcopyrite-type CuAlCh 2 (S, Se and Te) were studied using the first principles density functional calculations implemented in the full potential linear augmented plane wave (FP-LAPW) method. The thermoelectric properties are calculated by solving the Boltzmann transport equation within the constant relaxation time approximation. The calculated band gap using the Tran-Blaha modified Becke-Johnson potential (TB-mBJ), of all the compounds are in good agreement with the available experiment and other theoretical reports. Thermoelectric properties like thermopower, electrical conductivity scaled by relaxation time are calculated as a function of carrier concentrations at different temperatures. The calculated thermoelectric properties are compared with the available experiment and other theoretical calculations of similar materials.

The crystal structure and the Yb valence of the YbFe2Ge2 heavy fermion compound was measured at room temperature and under high pressures using high-pressure powder X-ray diffraction and X-ray absorption spectroscopy via both partial fluorescence yield and resonant inelastic X-ray emission techniques. The measurements are complemented by first-principles density functional theoretical calculations using the self-interaction corrected local spin density approximation investigating in particular the magnetic structure and the Yb valence. While the ThCr2Si2-type tetragonal (I4/mmm) structure is stable up to 53 GPa, the X-ray emission results show an increase of the Yb valence from v = 2.72(2) at ambient pressure to v = 2.93(3) at ∼9 GPa, where at low temperature a pressure-induced quantum critical state was reported.

High pressure structural stability studies were carried out on YGa2 (AlB2 type structure at NTP, space group P6/mmm) up to a pressure of ~35 GPa using both laboratory based rotating anode and synchrotron X-ray sources. An isostructural transition with reduced c/a ratio, was observed at ~6 GPa and above ~17.5 GPa, the compound transformed to orthorhombic structure. Bulk modulus B0 for the parent and high pressure phases were estimated using Birch–Murnaghan and modified Birch–Murnaghan equation of state. Electronic structure calculations based on projector augmented wave method confirms the experimentally observed two high pressure structural transitions. The calculations also reveal that the ‘Ga’ networks remains as two dimensional in the high pressure isostructural phase, whereas the orthorhombic phase involves three dimensional networks of ‘Ga’ atoms interconnected by strong covalent bonds.

The high-pressure structural behaviour of a series of binary thorium compounds ThX (X = C, N, P, As, Sb, Bi, S, Se, Te) is studied using the all-electron full potential linear muffin-tin orbital (FP-LMTO) method within the generalized gradient approximation (GGA) for the exchange and correlation potential. The calculated equlibrium lattice parameters and bulk moduli, as well as the equations of state agree well with experimental results. New experiments are reported for ThBi and ThN. Calculations are performed for the ThX compounds in the NaCl- and CsCl-type crystal structures, and structural phase transitions from NaCl to CsCl are found in ThP, ThAs, ThSb and ThSe at pressures of 26.1, 22.1, 8.1 and 23.2GPa, respectively, in excellent agreement with experimental results. ThC, ThN and ThS are found to be stable in the NaCl structure, and ThBi and ThTe in the CsCl structure, for pressures below 50GPa. The electronic structures of the ThX compounds are studied using the quasiparticle self-consistent GW method (G: Green function, W: dynamically screened interaction)

Ab initio calculations were performed on the scheelite-type MGeO 4 (M = Hf and Zr) compounds, which find a wide range of applications such as in X-ray imaging. We have studied the high-pressure structural stability, elastic constants, electronic structure, and optical properties of these compounds through density functional theory calculations. Two different density functional approaches, plane wave pseudopotential method (PW-PP) and full potential linearized augmented plane wave method (FP-LAPW), were used for the present study. The ground-state structural and vibrational properties are calculated and found to be in good agreement with experimental data. The compressibility of Zr and Hf germanates is found to be anisotropic as the a-axis is less compressible over the c-axis due to the presence of Ge-O bonds along the a-axis, which is further confirmed from the ordering of the elastic constants that follows C11 > C33. The electronic structure of the compounds has been calculated through recently developed Tran Blaha-modified Becke Johnson potential. The calculated electronic structure shows that the compounds are insulators with a gap of 5.39 eV for ZrGeO 4 and 6.25 eV for HfGeO4, respectively. Optical anisotropy of these compounds is revealed from the computed optical properties such as complex dielectric function, refractive index, and absorption coefficient. In addition, it is observed that Ti-doped ZrGeO4 and HfGeO4 turn out to be good phosphors as the pristine compounds have an energy gap greater than the visible range; upon Ti doping, the band gap reduces, and, as a result, emission spectra occur in the visible region and are well explained in the present study

sing first-principles calculations we have studied the valence and structural transitions of the rare earth monotellurides RTe (R = Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) under pressure. The self-interaction corrected local spin-density approximation is used to establish the ground state valence configuration as a function of volume for the RTe in both the NaCl (B1) and CsCl (B2) structures. We find that in ambient conditions all the RTe are stabilized in the B1 structure. A trivalent (R3+) rare earth ground state is predicted for the majority of the RTe, with the exception of SmTe, EuTe, DyTe, TmTe and YbTe, where the fully localized divalent (R2+) rare earth configuration is found to be energetically most favourable. Under pressure, the trivalent RTe undergo structural transitions to the B2 structure without associated valence transition. The divalent RTe on the other hand are characterized by a competition between the structural and electronic degrees of freedom, and it is the degree of f-electron delocalization that determines the sequence of phase transitions. In EuTe and YbTe, where respectively the half-filled and filled shells result in a very stable divalent configuration, we find that it is the structural B1 → B2 transition that occurs first, followed by the R2+ → R3+ valence transition at even higher pressures. In SmTe, DyTe and TmTe, the electronic transition occurs prior to the structural transition. With the exception of YbTe, the calculated transition pressures are found to be in good agreement with experiment.

The effect of pressure on the valence state of Eu in EuFe 2As2 has been investigated at high pressures up to 43 GPa at 10 K using resonant inelastic x-ray scattering and x-ray absorption spectroscopy using partial fluorescence yield. Two distinct density functional approaches have been used to complement the experiments. Our experimental results show that the Eu valence increases from a divalent state to a nearly trivalent state under application of pressure in consistence with theoretical simulations. Furthermore, our calculations show that the Eu magnetic moments prevail at high pressure up to 45 GPa

The effect of pressure on the valence state of Eu in EuFe 2As2 has been investigated at high pressures up to 43 GPa at 10 K using resonant inelastic x-ray scattering and x-ray absorption spectroscopy using partial fluorescence yield. Two distinct density functional approaches have been used to complement the experiments. Our experimental results show that the Eu valence increases from a divalent state to a nearly trivalent state under application of pressure in consistence with theoretical simulations. Furthermore, our calculations show that the Eu magnetic moments prevail at high pressure up to 45 GPa

The high-pressure structural behaviour of a series of binary thorium compounds ThX (X = C, N, P, As, Sb, Bi, S, Se, Te) is studied using the all-electron full potential linear muffin-tin orbital (FP-LMTO) method within the generalized gradient approximation (GGA) for the exchange and correlation potential. The calculated equlibrium lattice parameters and bulk moduli, as well as the equations of state agree well with experimental results. New experiments are reported for ThBi and ThN. Calculations are performed for the ThX compounds in the NaCl- and CsCl-type crystal structures, and structural phase transitions from NaCl to CsCl are found in ThP, ThAs, ThSb and ThSe at pressures of 26.1, 22.1, 8.1 and 23.2GPa, respectively, in excellent agreement with experimental results. ThC, ThN and ThS are found to be stable in the NaCl structure, and ThBi and ThTe in the CsCl structure, for pressures below 50GPa. The electronic structures of the ThX compounds are studied using the quasiparticle self-consistent GW method (G: Green function, W: dynamically screened interaction)

Ab initio calculations were performed on the scheelite-type MGeO 4 (M = Hf and Zr) compounds, which find a wide range of applications such as in X-ray imaging. We have studied the high-pressure structural stability, elastic constants, electronic structure, and optical properties of these compounds through density functional theory calculations. Two different density functional approaches, plane wave pseudopotential method (PW-PP) and full potential linearized augmented plane wave method (FP-LAPW), were used for the present study. The ground-state structural and vibrational properties are calculated and found to be in good agreement with experimental data. The compressibility of Zr and Hf germanates is found to be anisotropic as the a-axis is less compressible over the c-axis due to the presence of Ge-O bonds along the a-axis, which is further confirmed from the ordering of the elastic constants that follows C11 > C33. The electronic structure of the compounds has been calculated through recently developed Tran Blaha-modified Becke Johnson potential. The calculated electronic structure shows that the compounds are insulators with a gap of 5.39 eV for ZrGeO 4 and 6.25 eV for HfGeO4, respectively. Optical anisotropy of these compounds is revealed from the computed optical properties such as complex dielectric function, refractive index, and absorption coefficient. In addition, it is observed that Ti-doped ZrGeO4 and HfGeO4 turn out to be good phosphors as the pristine compounds have an energy gap greater than the visible range; upon Ti doping, the band gap reduces, and, as a result, emission spectra occur in the visible region and are well explained in the present study

sing first-principles calculations we have studied the valence and structural transitions of the rare earth monotellurides RTe (R = Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) under pressure. The self-interaction corrected local spin-density approximation is used to establish the ground state valence configuration as a function of volume for the RTe in both the NaCl (B1) and CsCl (B2) structures. We find that in ambient conditions all the RTe are stabilized in the B1 structure. A trivalent (R3+) rare earth ground state is predicted for the majority of the RTe, with the exception of SmTe, EuTe, DyTe, TmTe and YbTe, where the fully localized divalent (R2+) rare earth configuration is found to be energetically most favourable. Under pressure, the trivalent RTe undergo structural transitions to the B2 structure without associated valence transition. The divalent RTe on the other hand are characterized by a competition between the structural and electronic degrees of freedom, and it is the degree of f-electron delocalization that determines the sequence of phase transitions. In EuTe and YbTe, where respectively the half-filled and filled shells result in a very stable divalent configuration, we find that it is the structural B1 → B2 transition that occurs first, followed by the R2+ → R3+ valence transition at even higher pressures. In SmTe, DyTe and TmTe, the electronic transition occurs prior to the structural transition. With the exception of YbTe, the calculated transition pressures are found to be in good agreement with experiment.

The high-pressure structural behaviour of a series of binary thorium compounds ThX (X = C, N, P, As, Sb, Bi, S, Se, Te) is studied using the all-electron full potential linear muffin-tin orbital (FP-LMTO) method within the generalized gradient approximation (GGA) for the exchange and correlation potential. The calculated equlibrium lattice parameters and bulk moduli, as well as the equations of state agree well with experimental results. New experiments are reported for ThBi and ThN. Calculations are performed for the ThX compounds in the NaCl- and CsCl-type crystal structures, and structural phase transitions from NaCl to CsCl are found in ThP, ThAs, ThSb and ThSe at pressures of 26.1, 22.1, 8.1 and 23.2GPa, respectively, in excellent agreement with experimental results. ThC, ThN and ThS are found to be stable in the NaCl structure, and ThBi and ThTe in the CsCl structure, for pressures below 50GPa. The electronic structures of the ThX compounds are studied using the quasiparticle self-consistent GW method (G: Green function, W: dynamically screened interaction)

Ab initio calculations were performed on the scheelite-type MGeO 4 (M = Hf and Zr) compounds, which find a wide range of applications such as in X-ray imaging. We have studied the high-pressure structural stability, elastic constants, electronic structure, and optical properties of these compounds through density functional theory calculations. Two different density functional approaches, plane wave pseudopotential method (PW-PP) and full potential linearized augmented plane wave method (FP-LAPW), were used for the present study. The ground-state structural and vibrational properties are calculated and found to be in good agreement with experimental data. The compressibility of Zr and Hf germanates is found to be anisotropic as the a-axis is less compressible over the c-axis due to the presence of Ge-O bonds along the a-axis, which is further confirmed from the ordering of the elastic constants that follows C11 > C33. The electronic structure of the compounds has been calculated through recently developed Tran Blaha-modified Becke Johnson potential. The calculated electronic structure shows that the compounds are insulators with a gap of 5.39 eV for ZrGeO 4 and 6.25 eV for HfGeO4, respectively. Optical anisotropy of these compounds is revealed from the computed optical properties such as complex dielectric function, refractive index, and absorption coefficient. In addition, it is observed that Ti-doped ZrGeO4 and HfGeO4 turn out to be good phosphors as the pristine compounds have an energy gap greater than the visible range; upon Ti doping, the band gap reduces, and, as a result, emission spectra occur in the visible region and are well explained in the present study

The effect of pressure on the valence state of Eu in EuFe 2As2 has been investigated at high pressures up to 43 GPa at 10 K using resonant inelastic x-ray scattering and x-ray absorption spectroscopy using partial fluorescence yield. Two distinct density functional approaches have been used to complement the experiments. Our experimental results show that the Eu valence increases from a divalent state to a nearly trivalent state under application of pressure in consistence with theoretical simulations. Furthermore, our calculations show that the Eu magnetic moments prevail at high pressure up to 45 GPa

sing first-principles calculations we have studied the valence and structural transitions of the rare earth monotellurides RTe (R = Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) under pressure. The self-interaction corrected local spin-density approximation is used to establish the ground state valence configuration as a function of volume for the RTe in both the NaCl (B1) and CsCl (B2) structures. We find that in ambient conditions all the RTe are stabilized in the B1 structure. A trivalent (R3+) rare earth ground state is predicted for the majority of the RTe, with the exception of SmTe, EuTe, DyTe, TmTe and YbTe, where the fully localized divalent (R2+) rare earth configuration is found to be energetically most favourable. Under pressure, the trivalent RTe undergo structural transitions to the B2 structure without associated valence transition. The divalent RTe on the other hand are characterized by a competition between the structural and electronic degrees of freedom, and it is the degree of f-electron delocalization that determines the sequence of phase transitions. In EuTe and YbTe, where respectively the half-filled and filled shells result in a very stable divalent configuration, we find that it is the structural B1 → B2 transition that occurs first, followed by the R2+ → R3+ valence transition at even higher pressures. In SmTe, DyTe and TmTe, the electronic transition occurs prior to the structural transition. With the exception of YbTe, the calculated transition pressures are found to be in good agreement with experiment.