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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 theoretical calculations. It turns out that the inclusion of phonon contributions into the cluster expansion Hamiltonian affects markedly the substituent solubility of Be- and Zn-rich alloys. When lattice vibrations are considered, the solubility limits of Be in WZ-ZnO and Zn in WZ-BeO at 2000 K increase from 5.9% to 12.7% and from 0.7% to 3.8%, respectively, while the solubility limit of Be in ZB-ZnO reduces from 5.7% to 0.4% and that of Zn in ZB-BeO increases from 1.3% to 32.4%. A phase transition of BexZn1−xO from wurtzite to zincblende is predicted to occur around 1000 K.

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Divalent IIA metals such as Be, Mg, Ca, Sr, Ba and transition IIB metals such as Zn, Cd were investigated as possible n-type dopants into the Cu 2 O theoretically by using the first-principles calculations based on density functional theory. By systematical analyses of the lattice parameters, the bond length, the electronic structure, the local density of states and the defect formation energy for various doping systems, it is revealed that Ca, Sr, Ba and Be are more suited for n-type doping into Cu 2 O as shallow donors, compared to Mg which introduces a relatively deep donor level in Cu 2 O. Meanwhile, Zn and Cd can hardly be doped into Cu 2 O due to the positive formation energy of relevant defects.

VO2 is a metal oxide that has a thermally-induced phase-transition. In the vicinity of 341 K, VO2 undergoes a reversible transition from the high-temperature metal phase to the low-temperature insulator phase. Associated with the metal-insulator transition (MIT), there are drastic changes in its optical, electrical and magnetic characteristics. These make VO2 an attractive material for various applications, such as optical and/or electrical switches, smart glass, storage media, etc. Thus, the reversible metal-insulator phase transition in VO2 has long been a research hotspot. However, the metal-insulator transition mechanism in VO2 has been a subject of debate for several decades, and yet there is no unified explanation. This paper first describes changes of the crystal structure and the energy band structure during VO2 phase transition. With regard to the crystal structure, VO2 transforms from the low-temperature monoclinic phase VO2(M) into the high-temperature stable rutile phase VO2(R), and in some special cases, this phase transition process may also involve a metastable monoclinic VO2(B) phase and a tetragonal VO2(A) phase. In respect of the energy band structure, VO2 undergoes a transition from the low-temperature insulator phase into a high-temperature metal phase. In the band structure of low-temperature monoclinic phase, there is a band gap of about 0.7 eV between d// and * bands, and the Fermi level falls exactly into the band gap, which makes VO2 electronically insulating. In the band structure of high-temperature rutile phase, the Fermi level falls into the overlapping portion of the * and d// bands, which makes VO2 electronically metallic. Next, this paper summarizes the current research status of the physical mechanism underlying the VO2 MIT. Three kinds of theoretical perspectives, supported by corresponding experimental results, have been proposed so far, which includes electron-correlation-driven MIT, Peierls-like structure-driven MIT, and MIT driven by the interplay of both electron-correlation and Peierls-like structural phase transition. It is noted that recent reports mostly focus on the controversywhether VO2 is a Mott insulator, and whether the structural phase transition and the MIT accurately occur simultaneously in VO2. Finally, the paper points out the near-future development direction of the VO2 research.