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We calculate the formation enthalpies of PdHx (x = 0–3) by cluster expansion (CE) and calculations based on density functional theory. CE predicts the stable palladium hydride structures PdH, PdH2.67, and PdH2.75. The band structures and density of states indicate that the amount of hydrogen in the palladium lattice does not alter the metallic character of the palladium significantly. However, all PdHx structures with x > 1 have greater formation enthalpies than that of the given reaction path 4 PdH 2 = 2 PdH + 2 Pd + 3 H 2 and thus they are thermodynamically unstable. The shorter bond length of Pd–H and the smaller bond angle of Pd–H–Pd imply a higher cohesive energy in zincblende (ZB) PdH than that in rocksalt (RS) PdH. Bader charge analysis shows a stronger electronegativity of H atoms in ZB-PdH than that in RS-PdH. This results in a stronger Pd–H bond in ZB-PdH than that in RS-PdH. Thus ZB-PdH has lower formation enthalpy than that of RS-PdH. However, regarding the dynamic stability, we conclude that hydrogen atoms prefer to occupy the octahedral sites of the palladium lattice because of the lower zero-point energy and vibration free energy than that of occupying the tetrahedral sites.

In this paper, the composition-dependent electronic-structure and thermodynamic properties for hexagonal wurtzite (WZ) and face-centered cubic zincblende (ZB) ZnS1−xSex ternary alloys have been studied utilizing first-principles pseudopotential plane-wave self-consistent field calculations. Special quasi-random structure method was applied to generate disordered structures of 16-atom supercell for x = 0.25, 0.5 and 0.75. We obtained a direct band-gap at Г point and a shrinking tendency of the band-gap with increasing Se content in both phases. The reasons for the band-gap shrinkage have been discussed based on the analysis of electronic density of states and band offsets. Δ-sol method was applied to correct the band gaps calculated by density functional theory. The Δ-sol corrected band-gaps are in good agreement with experimental band-gaps. The thermodynamic properties were calculated by the cluster expansion method. It turns out that, the stretching forces decrease considerably and linearly with the bond length, while the bending forces are dependent insignificantly on the bond length. The phase diagrams reveal that the lattice vibration effects can influence the critical temperature Tc and the solubility of S- and Se-rich ZnS1−xSex alloys, but the influence is not obvious for ZB-ZnS1−xSex. Based on the calculated phase diagrams without lattice vibration effects, the Tc for WZ- and ZB-ZnS1−xSex are 559 K and 421 K, respectively. When the vibration enthalpy is included, the Tc values of both phases are lowered to 543 K and 418 K, respectively. And if the vibration entropy is also taken into account, the Tc values of both phases are further lowered to 529 K and 415 K, respectively. With inclusion of lattice vibration effects, the ZnS1−xSex alloy starts a phase transition from ZB to WZ at a temperature of 1600 K, and the solid solubility of Se increases with the temperature rise.

All-inorganic cesium lead halide perovskite materials, such as CsPbBr3 and CsPb2Br5, have drawn immense attention recently due to their potential applications in optoelectronics. However, unlike the extensively-studied CsPbBr3, many of the fundamental properties of CsPb2Br5 remain unclear, such as its electronic structure and photoluminescence (PL) mechanism. In this work, we developed a novel environmentally friendly chemical method to synthesize CsPb2Br5, where only water and ethanol were used as solvents. The optical property and electronic structure of CsPb2Br5 were then examined both experimentally and theoretically. For the first time we found that the band-edge emission of CsPb2Br5 was not in the visible range as reported in the literature but rather located in the ultraviolet range. By monitoring the PL property during phase conversion between CsPb2Br5 and CsPbBr3, we propose that previously-reported visible emission might be due to the involvement of CsPbBr3 during sample preparation. Overall, this study presents a novel green method for the preparation of CsPb2Br5 and clarifies the optical properties and electronic structure of CsPb2Br5.

Two-dimensional transition metal dichalcogenide materials are of great interest for the development of semiconductor devices because of their diverse properties. In this paper, we discovered the ground states of WxMo1−xS2 alloys and discussed their electronic structures and phonon properties in detail by the cluster expansion method and the first principle calculation. Three ground-state structures of WxMo1−xS2 alloys, W1/3Mo2/3S2, W1/2Mo1/2S2, and W2/3Mo1/3S2, have low order–disorder phase transition temperatures, T1/3 = 102.3 K, T1/2 = 87.8 K, and T2/3 = 102.2 K, estimated by mean field theory. Their bandgaps exhibit a nonlinear increase with increasing W content. All ground structures are dynamically stable without imagine frequency in phonon dispersion. W doping into single layer MoS2 leads low frequency optical modes hybriding with acoustic modes and lowering down the low frequency band due to larger mass of W atoms. All these may improve the developments of WxMo1−xS2 alloys devices.

The perovskite CsPbBr3 attracts great attention due to its potential in optoelectronics. However, stability remains a major obstacle to achieving its effecting application. In this work, we prepared CsPbBr3 solids through a simple reaction and investigated reversible conversion between CsPbBr3, Cs4PbBr6, and CsPb2Br5. We found that CsPbBr3 can be respectively converted to Cs4PbBr6 or CsPb2Br5 by reacting with CsBr or PbBr2. Thermodynamic analysis demonstrated that the chemical reactions above were exothermic and occurred spontaneously. Moreover, the formed Cs4PbBr6 could be converted to CsPbBr3 reversely, and then progressively converted to Cs-deficient CsPb2Br5 by extraction of CsBr with water. The CsPb2Br5 was converted to CsPbBr3 reversely under thermal annealing at 400 °C. The thermodynamic processes of these conversions between the three compounds above were clarified. Our findings regarding the conversions not only provide a new method for controlled synthesis of the ternary Cs-Pb-Br materials but also clarify the underlying mechanism for the instability of perovskites CsPbBr3.

In this work, the structural, electronic, and thermodynamic properties of wurtzite (WZ) and zincblende (ZB) CdS1-xSex alloys are investigated using the density functional theory (DFT) and the cluster expansion method. A special quasirandom structure containing 16 atoms is constructed to calculate the band structures of random alloys. The band gaps of CdS1-xSex alloys are direct and decrease as the Se content increases. The delta self-consistent-field method is applied to correct band gaps that are underestimated by DFT. The band offsets clearly reflect the variation in valence band maxima and conduction band minima, thus providing information useful to the design of relevant quantum well structures. The positive formation enthalpies of both phases imply that CdS1-xSex is an immiscible system and tends to phase separate. The influence of lattice vibrations on the phase diagram is investigated by calculating the phonon density of states. Lattice vibration effects can reduce the critical temperature Tc and i...

Bandgap engineering of ZnO is crucial towards its practical applications. Due to their wide bandgap, BexZn1−xO alloys are promising materials for making optoelectronic devices that function in the solar-blind wavelength region. In this study, a theoretical investigation of the thermodynamics of these BexZn1−xO alloys is carried out using both first-principles calculations and the cluster expansion method. The cluster expansion method is used to describe the disordered alloys. It is revealed that, for both wurtzite (WZ) and zincblende (ZB) phase BexZn1−xO alloys, the formation enthalpies of all structures are positive for the whole range of composition. This implies the occurrence of miscibility gap when BeO and ZnO form alloys. A good comparison between the density functional theory used and the effective cluster interaction fitted formation enthalpies validates the cluster expansion method in the calculation of the formation enthalpies. The phase diagram of BexZn1−xO has been derived based on the theoret...

The antiferromagnetic phase diagram of MgxNi1−xO alloy was reexamined theoretically by multicomponent cluster expansion method. It predicts antiferromagnetic state in the alloy with high Mg content, instead of a mixture of antiferromagnetic and paramagnetic states, which was deduced previously from neutron diffraction results. The theoretically found intermediate antiferromagnetic ground states at Mg contents of xMg = 0.25 and 0.75 exhibit very low neutron diffraction intensities, which leads to confusion with paramagnetic state. Our approach here reduced remarkably the computation time in searching the intermediate ground states as well as calculating the magnetic phase diagram.

In this study, the formation energy, phase diagram (with/without phonon contribution) and the relationship between bond stiffness and bond length for wurtzite (WZ) and zincblende (ZB) structures of ...