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We have studied the Ilmenite, Perovskite and LiNbO 3 type crystal structures of ternary oxide MgSnO 3 by first-principles methods using density functional theory (DFT) and beyond. We conclude that MgSnO 3 in LiNbO 3 type is both mechanically and dynamically stable, whereas Ilmenite and Perovskite crystal structures are mechanically stable but dynamically unstable. Vibrational stability in MgSnO 3 warrants some distortion in the octahedra caused due to the strength of bonding between the atom pairs Sn–O in LiNbO 3 type crystal structure. Similarly, Ilmenite and Perovskite crystal structures require a higher number of Mg–Sn and (Mg–Mg, Sn–Sn, O–O and Mg–O) bonds respectively. Ilmenite and LiNbO 3 type crystal structures can be used as window layers featuring a large bandgap of 5.22 eV and 3.88 eV respectively, along with a lower absorption coefficient and reflectivity. Likewise, Perovskite crystal structure with a bandgap of 2.55 eV can be deployed as an absorber layer that traps green light of the solar irradiation in tandem solar cells. Perovskite crystal structure has the lowest charge carrier effective masses among all the structures in MgSnO 3. LiNbO 3 type crystal structure shows a high hardness of 56.5 GPa. It should be tested experimentally in applications requiring super-hard materials. [ABSTRACT FROM AUTHOR]

We employ first principles methods based on density functional theory and beyond to study Zn x Cd 1-x Te, 0 ≤ x ≤ 1 , alloys in the zinc blende (B3) crystal structure. Cluster Expansion and Monte Carlo formalisms were deployed to provide a phase diagram determining consolute temperature of 387 K at 40% Zn concentration. Opto-electronic properties are computed with the hybrid HSE06 functional for disordered Zn x Cd 1-x Te alloys, which were simulated using special quasi-random structures. Bowing effects in the bandgap and effective carrier masses were observed in alloying which can be attributed to local geometrical distortions as portrayed by bond length distributions. Downward bowing in the electronic bandgap will be beneficial in photovoltaic applications through increased net photocurrent. Absorption coefficients show robust absorption in Zn x Cd 1-x Te as indicated by substantial optical absorption throughout all Zn concentrations, aided by low reflectivity that ensures a high absorption. Presence of short-range order in Zn 0.25 Cd 0.75 Te was observed through clustering and anti-clustering among different cationic species of Zn and Cd. It has a value of 0.11 at 1000 K compared to −0.79 at 400 K. The bandgap of Zn 0.25 Cd 0.75 Te at 1000 K was found to be slightly higher, 0.05 eV, than at 400 K, consistent with the value of short-range order. Thus, short-range order and possibly composition can be used in the design and synthesis of the appropriate solar cell, thereby improving the performance in this system. • T-x phase diagram calculation of Zn x Cd 1-x Te shows consolute temperature of 387 K. • Opto-electronic properties are computed with the hybrid HSE06 functional. • Downward bowing in the electronic bandgap and effective charge carrier masses. • Zn 0.25 Cd 0.75 Te has the best band gap of 1.49 eV for absorber layer in solar cells. • Short-range order observed in Zn 0.25 Cd 0.75 Te through clustering and anti-clustering. [ABSTRACT FROM AUTHOR]

Abstract: The adsorption and diffusion behavior of a Ga adatom on the GaAs (001)‐c(4×4)-heterodimer surface were studied by employing ab initio density functional theory (DFT) computations in the local density approximation. Structural and bonding features of the c(4×4)-heterodimer reconstruction surface were examined. A comparison with the c(4×4)-ss reconstruction was performed. Minimum energy sites (MES) on the c(4×4)-heterodimer surface were located by mapping the potential energy surface for a Ga adatom. Barriers for diffusion of a Ga adatom between the neighboring MES were calculated by using top hopping- and exchange-diffusion mechanisms. We proposed two unique diffusion pathways for a Ga adatom diffusing between the global minimums of two neighboring unit cells. Signature differences between electronic structures of top hopping- and exchange‐diffusion mechanisms were studied for relevant atoms. We observed a higher diffusion barrier for exchange mechanism compared to top hopping. [Copyright &y& Elsevier]

We have studied the disordered rocksalt, orthorhombic, and disordered wurtzite phases of the ternary nitride semiconductor MgSnN 2 by first-principles methods using density functional theory (DFT) and beyond. The results imply that MgSnN 2 is mechanically and dynamically stable in all three phases. However, pCOHP analysis suggests that the disordered rocksalt structure has antibonding states below the Fermi level between −5 eV and −2 eV, as compared to the bonding states in the other two phases, indicative of its thermodynamic metastability. Computed lattice constant and electronic band-gap values of 4.56 Å and 2.69 eV for MgSnN 2 in the disordered rocksalt structure compare well with experimentally reported values of 4.48 Å and 2.3 eV, respectively. Furthermore, band gaps were computed for MgSnN 2- x O x (x = 0.5, 1.0, 1.5, 2.0) to elucidate the role of possible oxygen impurities. Band-gap bowing is suggested to occur upon alloying with oxygen. Of the three phases, the disordered rocksalt structure shows the lowest charge carrier effective masses. Moreover, the absorption coefficient and reflectivity of this phase make it promising for use as the absorber layer of tandem solar cells in the higher energy region of the visible portion of the solar spectrum. The other two phases, disordered wurtzite and orthorhombic, might be utilized as the window layer of solar cells owing to their larger band-gap values of 4.36 eV and 4.86 eV, respectively. • A computational study inspired by recent experimental synthesis of disordered rocksalt MgSnN 2 has been carried out. • The calculated lattice constant of 4.56 Å matches an experimental result of 4.48 Å. • The calculated indirect band gap of 2.69 eV is comparable to an experimental direct gap of 2.3 eV. • Band gaps of MgSnN 2- x O x (x = 0.5, 1.0, 1.5) are 0 eV, whereas that of MgSnO 2 is 1.01 eV. • Rocksalt MgSnN 2 is mechanically and dynamically stable, thermodynamically metastable, and is suitable as a short-wavelength photovoltaic absorber material. [ABSTRACT FROM AUTHOR]

• In computed phase diagram of CdSe x Te 1−x , consolute temperature = 325 K. • In CdSe x Te 1−x alloy, zincblende-wurtzite phase transition seen at x = 0.5–0.6. • Band gap lowest at 45% Se concentration to be around 1.41 eV due to bowing. • Significant clustering effects are observed in the Te/Se sublattice of CdSe 0.5 Te 0.5. • Negative bowing in effective hole mass contributing to high photo-current. We employ first principles methods based on density functional theory and beyond to study CdSe x Te 1−x alloys in the zincblende and wurtzite structures. From the cluster expansion formalism, we provide a detailed phase diagram showing a consolute temperature of 325 K, above which miscibility may be achieved. In the random solid solution, a zincblende-to-wurtzite phase boundary is found to range from Se concentrations of x = 0.5–0.6, in agreement with experiment, owing to increasing ionic character of the Cd-anion bonds. Disordered CdSe x Te 1−x configurations are modeled using special quasirandom structures, for which optoelectronic properties are computed with the hybrid HSE06 functional. Alloying is shown to cause strong bowing effects in the band gap and effective electron/hole masses, which we attribute to local structural distortions as illustrated by analysis of bond length distributions. Downward bowing in the band gap and effective hole mass of the zincblende structure is highlighted for its potential benefits in photovoltaics through increased net photocurrent. Absorption coefficients and reflectivity are also reported, showing promising results in zincblende CdSe x Te 1−x as indicated by substantial optical absorption throughout all Se concentrations. Lastly, we identify the presence of short-range order in CdSe x Te 1−x characterized by clustering among like atoms in order to minimize strain. The degree of clustering, which may be tuned by temperature, also controls the magnitude of the band gap. Therefore, we propose both composition and short-range order as effective tools to be utilized in the design and synthesis of improved solar cell absorber layer materials. [ABSTRACT FROM AUTHOR]

We study the phase stability, mechanical properties, and electronic structure of two quasi-binary ceramic systems, Ti 1-x Sc x N and Ti 1-x Y x N (0 ≤ x ≤ 1), using first principles methods based on density functional theory, cluster expansion formalism, and Monte Carlo techniques. Owing to the similarity in ionic radii and electronegativities of their respective transition metals, strong exothermic mixing of TiN and ScN is predicted, with four ordered intermetallic phases lying on the convex hull: TiScN 2 , TiSc 8 N 9 , TiSc 9 N 10 , and Ti 3 Sc 2 N 5. These structures form layered rocksalt-type configurations to minimize strain energy while maximizing occupation of bonding states. The fully-detailed phase diagram including these predicted ground states and known end members is constructed, revealing an upper consolute temperature of 660 K. In contrast to Ti 1-x Sc x N, the mismatched properties of TiN and YN lead to large structural distortions and positive strain energies. As a result, endothermic mixing with significant upward bowing in the formation energy is observed at intermediate concentrations, with the consolute temperature of 7225 K being predicted from the phase diagram of Ti 1-x Y x N. TiN, ScN, and YN are found to display hardness values of 23.4, 25.1, and 20.6 GPa respectively, in good agreement with experimental data. The intermetallic phase Ti 3 Sc 2 N 5 is predicted to exhibit an exceptionally high hardness of 27.3 GPa. From analysis of projected electronic density of states and Crystal Orbital Hamiltonian Populations, we attribute enhanced hardness to strong nitrogen p and metal d hybridization, being related to 3 d e g occupation, and decreased tendency towards shearing, being related to minimal 3 d t 2g occupation. These features extend to the case of random solutions, which we model using special quasirandom structures, showing a maximum hardness at a valence electron concentration of 8.4. Based on these findings, we suggest Ti 1-x Sc x N alloys for implementation in hard coating applications. • Phase stability of Ti 1-x Sc x N and Ti 1-x Y x N (0 ≤ x ≤ 1) alloy systems were investigated. • Novel intermetallics in Ti 1-x Sc x N exhibited strong stability and high hardness up to H V = 27.3 GPa. • Hardness in Ti 1-x Sc x N is maximized at intermediate Sc concentration at VEC 8.4–8.6. • In contrast to Ti 1-x Sc x N, solubility in Ti 1-x Y x N is achieved only at very high temp (> 7225 K). • Enhanced hardness of Ti 1-x Sc x N is attributed to strong metal d and nitrogen p hybridization. [ABSTRACT FROM AUTHOR]