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Structural phase transitions drive several unconventional phenomena including some illustrious ferroic attributes which are relevant for technological advancements. On this note, we have investigated the ferroelastic structural transition of perovskite-type trigonal Ba2ZnTeO6 across Tc ∼ 150 K. With the help of Raman spectroscopy and density-functional theory (DFT)-based calculations, we report new intriguing observations associated with the phase transition in Ba2ZnTeO6. We observed the presence of a central peak (quasi-elastic Rayleigh profile), huge softening in the soft mode, hysteretic phonon behavior, and signatures of coexistent phases. The existence of a central peak in Ba2ZnTeO6 is manifested by a sharp rise in the intensity of the Rayleigh profile concomitant with the huge damping (or softening) of the soft mode (at ∼31 cm−1) near Tc, shedding light on the lattice dynamics during the phase transition. This is further corroborated by our phonon calculations that show that the soft mode (Eg) in the high-symmetry structure involving TeO6 octahedral rotation (with Ba and Zn translation) condenses into Ag and Bg modes in the C2/m low-symmetry phase. While most of the phonon bands split below Tc confirming the phase transition, we observed thermal hysteretic behavior of phonon modes, which signifies the first-order nature of the transition and the presence of coexisting phases as corroborated by our temperature-dependent X-ray diffraction and specific heat measurements. [ABSTRACT FROM AUTHOR]

The impact of Ta doping on two orthoniobates SmNbO4 and HoNbO4 has been studied using a combination of high-resolution powder diffraction and Density-Functional Theory calculations. In both ANb1−xTaxO4 (A = Sm, Ho) series the unit cell volume decreases as the Ta content increased demonstrating that the effective ionic radii of Ta is smaller than that of Nb in this structure. The average Sm–O distance and volume of the SmO8 polyhedra were invariant of the Ta content across the SmNb1−xTaxO4 solid solution whereas the average M–O (M = Nb or Ta) distance and MO6 polyhedral volume decrease with Ta doping. The analogous Ho oxides HoNb1−xTaxO4 do not form a complete solid solution when the samples were prepared at 1400 °C, rather there is a miscibility gap around x = 0.95, with HoTaO4 exhibiting the M′-type P2/c structure rather than the M-type I2/a structure of HoNbO4. Increasing the synthesis temperature to 1450 °C eliminates the miscibility gap. The energy difference between the P2/c and I2/a structures of HoTaO4 is found to be nearly 30 meV per f.u. with the total energy of the P2/c phase of HoTaO4 being more negative. First-principles calculations, carried out using Density-Functional Theory, reveal significant covalent character in the Nb–O bonds, which is reduced in the corresponding tantalates. Anisotropy in the Born Effective Charge tensors demonstrates the impact of the long M–O bond identified in the structural studies showing that the Nb and Ta cations are effectively six-coordinate. The similarity in the frequency of the intense Raman peak near 800 cm−1 due to the symmetric stretching of the Ta–O bonds is consistent with the description of that both polymorphs of HoTaO4 contain TaO6 octahedra. [ABSTRACT FROM AUTHOR]

The potential crystal structures and properties of urea as a function of pressure were studied using ab initio based electronic structure calculations. The enthalpy-pressure behavior shows that urea undergoes a pressure induced structural phase transition from P4̅21m (phase I) →P212121 (phase III) at 0.66 GPa with a volume collapse of 4.83%, driven by softening of the acoustic mode along the Γ–X direction. Another phase transition from the P212121→P21212 structure was identified at 3.09 GPa. The violation of Born stability criteria in the P212121 structure along with softening of the acoustic mode in the U–R direction was responsible for the pressure induced phase transition. Furthermore, the application of pressure led to the breaking and formation of N–H…O bonds in the crystal structure of urea during the phase transition, i.e., the H-acceptor capacitance of the oxygen atoms was varied between phases I/IV and -III. Band structure calculations were performed using a hybrid functional (Heyd, Scuseria and Ernzerhof, HSE) which includes a part of exact Fock-exchange. The computed electronic band structure showed that the urea polymorphs are insulators with a direct band gap of 6.21, 6.85 and 6.99 eV for phase-I, -III and -IV, respectively, at selected pressures. We have also presented the dielectric functions (real (ε1(̉)) and imaginary (ε2(̉)) parts), refractive index and absorption coefficients to explore the optical characteristics of the urea phases. The geometric interpretation of intermolecular interactions were quantitatively visualized using Hirshfeld surface analysis. Our results provide a complete picture of various properties of urea polymorphs that lay the foundation for further understanding of structures and their applications. [ABSTRACT FROM AUTHOR]

Developing novel energetic materials of high detonation performance and low sensitivity is one of the primary objectives related to explosive research. By employing ab initio calculations, a series of energetic ionic salts based on 5,5′-bitetrazole-1,1′-diolate (BTO) were thoroughly investigated to understand the structure–property–performance interrelationship. The physicochemical and detonation characteristics of these energetic ionic salts including structural, electronic, vibrational and performance parameters (heats of formation, detonation pressures, and detonation velocities) were discussed in detail. The strong intermolecular hydrogen bonding environment between the BTO2− anion and various cations is mainly responsible for prominent detonation performance and enhanced molecular stability. Such strong intermolecular hydrogen bonds are observed in hydrazine and hydroxylammonium cations compared to other cations. To predict an accurate band gap, electronic band structures of the studied energetic ionic salts (EIS) were calculated using the HSE06 hybrid functional and they are found to be wide band gap insulators with a bandwidth ranging from 4.33–5.05 eV. Careful inspection of various EIS revealed that the hydroxylammonium and hydrazine cations produce the highest density relative to other cations when combined with the BTO anion. The detonation characteristics of BTO2− are computed using EXPLO5 code. In particular, HA-BTO and TKX-50 exhibit high detonation pressure (38.85 and 40.23 GPa) and detonation velocities (9.94 and 9.91 km s−1), superior to those of traditional nitrogen-rich energetic materials with moderate sensitivities. These results highlight the importance of hydrogen bonding interactions in designing energetic salts for next-generation explosives, propellants, and pyrotechnics. [ABSTRACT FROM AUTHOR]

An ab initio study on the impact of hydrostatic pressure and strain on the electronic properties of an unexplored class of ternary Zintl phases KZnX (X = P, As, Sb) is reported. Density functional theory (DFT) based studies revealed that all the three materials are direct band gap semiconductors under ambient conditions. We have theoretically demonstrated that KZnX can be driven into different metallic phases under pressure. In contrast, by applying strain the compounds can be realized as topological insulators. This is confirmed by the observed non-trivial topological character in the electronic band structure of the present ternary systems. For the precise determination of low energy band topology, the Tran Blaha modified Becke–Johnson (TBmBJ) exchange potential was used by incorporating spin–orbit coupling. The concomitant change of electronic band shapes as a function of pressure indicates a semi-metallic nature in KZnX (X = P, As, Sb) at 30 GPa, 21 GPa and 11 GPa respectively. Based on an analysis of the parity eigenvalues, we anticipate that a band inversion occurs between the Zn-s and X-p states, thus demonstrating a weak topological behaviour in semi-metallic states. Also, a weak non-trivial topologically insulating phase is realized in strained KZnAs (18%) and KZnSb (10%) which appears to be due to overlapping of the Zn-s and X-p orbitals. The calculated surface spectral functions further validate the non-triviality of strained KZnX (X = As, Sb), whereas strained KZnP is found to be a trivial insulator. We confirm the topological behaviour of these materials by calculating topological surface states and defining a Z2 topological invariant. Our work based on sophisticated first-principles calculations highlights that both pressure and strain can trigger topological phases in non-symmorphic trivial band insulators even with a weak spin orbit interaction. This study paves the way for realizing semi-metallic and topological insulating states in non-symmorphic ternary semiconductors, which have not been experimentally demonstrated so far. [ABSTRACT FROM AUTHOR]

We explore the effect of spin–orbit interaction (SOI) on the electronic and optical properties of CsPbCO3F using the full potential linear augmented plane wave method with the density functional theory (DFT) approach. CsPbCO3F is known for its high powder second harmonic generation (SHG) coefficient (13.4 times (d36 = 0.39 pm V−1) that of KH2PO4 (KDP)). Calculations are done for many exchange correlation (XC) potentials. After the inclusion of SOI, the calculated Tran–Blaha modified Becke–Johnson (TB-mBJ) band gap of 5.58 eV reduces to 4.45 eV in agreement with the experimental value. This is due to the splitting of Pb p-states. Importantly, the occurrence of a band gap along the H–A direction (indirect) transforms to the H–H (direct) high symmetry points/direction in the first Brillouin zone. We noticed a large anisotropy in the calculated complex dielectric function, absorption, and refractive index spectra. The calculated static birefringence of 0.1049 and 0.1057 (with SOI) is found to be higher than that of the other carbonate fluorides. From the Born effective charge (BEC) analysis we notice that the Cs atom shows a negative contribution to birefringence whereas Pb, C, and F atoms show a positive contribution. In addition, we have also calculated the nonlinear optical χ(2)ijk(−2̉;̉,̉) dispersion of a CsPbCO3F single crystal. We found that d11 = d12 = 4.35 pm V−1 at 1064 nm, which is 11.2 times higher than d36 of KDP. The origin of the highly nonlinear optical susceptibility dispersion of CsPbCO3F is explained. Overall, our results are in agreement with experiments and it is obvious from the present study that CsPbCO3F is a direct band gap, large second harmonic generation, and good phase matchable NLO crystal in the ultraviolet region. [ABSTRACT FROM AUTHOR]

First principles calculations predict the promising thermoelectric material ZnGeSb2 with a huge power factor (S2σ/τ) on the order of 3 × 1017 W m−1 K−2 s−1, due to the ultra-high electrical conductivity scaled by a relaxation time of around 8.5 × 1025Ω−1 m−1 s−1, observed in its massive Dirac state. The observed electrical conductivity is higher than the well-established Dirac materials, and is almost carrier concentration independent with similar behaviour of both n and p type carriers, which may certainly attract device applications. The low range of thermal conductivity is also evident from the phonon dispersion. Our present study further reports the gradual phase change of ZnGeSb2 from a normal semiconducting state, through massive Dirac states, to a topological semi-metal. The maximum power factor is observed in the massive Dirac states compared to the other two states. [ABSTRACT FROM AUTHOR]

Correction for ‘Topological behaviour of ternary non-symmorphic crystals KZnX (X = P, As, Sb) under pressure and strain: a first principles study’ by Atahar Parveen et al., Phys. Chem. Chem. Phys., 2018, 20, 5084–5102. [ABSTRACT FROM AUTHOR]

We have investigated the effect of hydrostatic pressure and temperature on phase stability of hydro-nitrogen solids using dispersion corrected density functional theory calculations. From our total energy calculations, ammonium azide (AA) is found to be the thermodynamic ground state of N4H4 compounds in preference to trans-tetrazene (TTZ), hydro-nitrogen solid-1 (HNS-1) and HNS-2 phases. We have carried out a detailed study on structure and lattice dynamics of the equilibrium phase (AA). AA undergoes a phase transition to TTZ at around ∼39–43 GPa followed by TTZ to HNS-1 at around 80–90 GPa under the studied temperature range 0–650 K. The accelerated and decelerated compression of a and c lattice constants suggest that the ambient phase of AA transforms to a tetragonal phase and then to a low symmetry structure with less anisotropy upon further compression. We have noticed that the angle made by type-II azides with the c-axis shows a rapid decrease and reaches a minimum value at 12 GPa, and thereafter increases up to 50 GPa. Softening of the shear elastic moduli is suggestive of a mechanical instability of AA under high pressure. In addition, we have also performed density functional perturbation theory calculations to obtain the vibrational spectrum of AA at ambient as well as at high pressures. Furthermore, we have made a complete assignment of all the vibrational modes which is in good agreement with the experimental observations at ambient pressure. Moreover, the calculated pressure dependent IR spectra show that the N–H stretching frequencies undergo red and blue-shifts corresponding to strengthening and weakening of hydrogen bonding, respectively, below and above 4 GPa. The intensity of the N–H asymmetric stretching mode B2u is found to diminish gradually and the weak coupling between NH4 and N3 ions makes B1u and B3u modes to degenerate with progression of pressure up to 4 GPa which causes weakening of hydrogen bonding and these effects may lead to a structural phase transition in AA around 4 GPa. Furthermore, we have also calculated the phonon dispersion curves at 0 and 6 GPa and no soft phonon mode is observed under high pressure. [ABSTRACT FROM AUTHOR]

A detailed variable temperature neutron total scattering study of the potential nuclear waste matrix TlTcO 4 was conducted. The long-range average structure of TlTcO 4 undergoes an orthorhombic Pnma to tetragonal I 4 1 / amd phase transition below 600 K, consistent with previous synchrotron X-ray diffraction studies. However, several anomalies were observed in the Rietveld refinements to the neutron powder diffraction data, such as large atomic displacement parameters at low temperature and a shortening of the Tc-O bond distance upon heating. Modelling the short-range local structure of both the low- and high-temperature data required a lowering of symmetry to the monoclinic P 2 1 / c model due to the stereochemical activity of the Tl + 6s 2 lone pairs. Density functional theory calculations also verified this model to have a lower ground state energy than the corresponding long-range average structure. It is concluded that at low temperatures, the Tl + 6s 2 lone pairs are 'frozen' into the structure. Upon heating, the rigid TcO 4 tetrahedra begin to rotate, as governed by the Γ 3 + and M 4 + modes. However, there is a disconnect between the two length scales, with the 6s 2 lone pair electrons remaining stereochemically active on the local scale, as observed in the neutron pair distribution function fits. The orthorhombic Pnma to tetragonal I 4 1 / amd phase transition is seemingly the result of a change in the correlation length of the Tl + 6s 2 lone pairs, leading to a larger unit cell volume due to their uncorrelated displacements.

Structural phase transitions drive several unconventional phenomena including some illustrious ferroic attributes which are relevant for technological advancements. On this note, we have investigated the ferroelastic structural transition of perovskite-type trigonal Ba 2 ZnTeO 6 across T c ∼ 150 K. With the help of Raman spectroscopy and density-functional theory (DFT)-based calculations, we report new intriguing observations associated with the phase transition in Ba 2 ZnTeO 6 . We observed the presence of a central peak (quasi-elastic Rayleigh profile), huge softening in the soft mode, hysteretic phonon behavior, and signatures of coexistent phases. The existence of a central peak in Ba 2 ZnTeO 6 is manifested by a sharp rise in the intensity of the Rayleigh profile concomitant with the huge damping (or softening) of the soft mode (at ∼31 cm -1 ) near T c , shedding light on the lattice dynamics during the phase transition. This is further corroborated by our phonon calculations that show that the soft mode (E g ) in the high-symmetry structure involving TeO 6 octahedral rotation (with Ba and Zn translation) condenses into A g and B g modes in the C 2/ m low-symmetry phase. While most of the phonon bands split below T c confirming the phase transition, we observed thermal hysteretic behavior of phonon modes, which signifies the first-order nature of the transition and the presence of coexisting phases as corroborated by our temperature-dependent X-ray diffraction and specific heat measurements.

We explore the effect of spin-orbit interaction (SOI) on the electronic and optical properties of CsPbCO 3 F using the full potential linear augmented plane wave method with the density functional theory (DFT) approach. CsPbCO 3 F is known for its high powder second harmonic generation (SHG) coefficient (13.4 times (d 36 = 0.39 pm V -1 ) that of KH 2 PO 4 (KDP)). Calculations are done for many exchange correlation (XC) potentials. After the inclusion of SOI, the calculated Tran-Blaha modified Becke-Johnson (TB-mBJ) band gap of 5.58 eV reduces to 4.45 eV in agreement with the experimental value. This is due to the splitting of Pb p-states. Importantly, the occurrence of a band gap along the H-A direction (indirect) transforms to the H-H (direct) high symmetry points/direction in the first Brillouin zone. We noticed a large anisotropy in the calculated complex dielectric function, absorption, and refractive index spectra. The calculated static birefringence of 0.1049 and 0.1057 (with SOI) is found to be higher than that of the other carbonate fluorides. From the Born effective charge (BEC) analysis we notice that the Cs atom shows a negative contribution to birefringence whereas Pb, C, and F atoms show a positive contribution. In addition, we have also calculated the nonlinear optical χ(-2ω;ω,ω) dispersion of a CsPbCO 3 F single crystal. We found that d 11 = d 12 = 4.35 pm V -1 at 1064 nm, which is 11.2 times higher than d 36 of KDP. The origin of the highly nonlinear optical susceptibility dispersion of CsPbCO 3 F is explained. Overall, our results are in agreement with experiments and it is obvious from the present study that CsPbCO 3 F is a direct band gap, large second harmonic generation, and good phase matchable NLO crystal in the ultraviolet region.

First principles calculations predict the promising thermoelectric material ZnGeSb 2 with a huge power factor (S 2 σ/τ) on the order of 3 × 10 17 W m -1 K -2 s -1 , due to the ultra-high electrical conductivity scaled by a relaxation time of around 8.5 × 10 25 Ω -1 m -1 s -1 , observed in its massive Dirac state. The observed electrical conductivity is higher than the well-established Dirac materials, and is almost carrier concentration independent with similar behaviour of both n and p type carriers, which may certainly attract device applications. The low range of thermal conductivity is also evident from the phonon dispersion. Our present study further reports the gradual phase change of ZnGeSb 2 from a normal semiconducting state, through massive Dirac states, to a topological semi-metal. The maximum power factor is observed in the massive Dirac states compared to the other two states.