Akio Shigemi

Enthalpy of Formation of Various Phases and Formation Energy of Point Defects in Perovskite-Type NaNbO3 by First-Principles Calculation

In order to clarify the thermodynamic relations of various phases in NaNbO3, the enthalpies of formation for the cubic (Pm3m), tetragonal (P4/mbm), orthorhombic (Ccmm, Pnmm, Pbma) and rhombohedral (R3c) phases in NaNbO3 were calculated using a plane-wave pseudopotential method. The NaNbO3 phase with a lower symmetry has a smaller enthalpy of formation. The enthalpy of formation for the rhombohedral (R3c) phase is the smallest. Moreover, in order to quantitatively evaluate the formation energies of point defects in NaNbO3, first-principles pseudopotential calculations using plane-wave basis functions were performed. Relaxation of first- and second-neighbor atoms around a vacancy was considered in a 40-atom supercell. The formation energies of point defects were calculated as a function of the atomic chemical potentials of constituent elements. The formation energy of Na vacancy was the lowest under the oxidation limit of NaNbO3 and that of O vacancy was the lowest under the reduction atmosphere. The formation energy of Nb vacancy was the highest under both oxygen-rich and -poor conditions. This result agrees with the empirical rule that the B site defect in the perovskite-type oxide does not exist.

Evaluations of Phases and Vacancy Formation Energies in KNbO3 by First-Principles Calculation

In order to clarify thermodynamic relationships of the various phases of KNbO3, enthalpies of formation for cubic (Pm3m), tetragonal (P4mm), orthorhombic (Bmm2) and rhombohedral (R3m) phases of KNbO3 were calculated using a plane-wave pseudopotential method within a density functional formalism. The KNbO3 phase with the lowest symmetry was found to have the lowest enthalpy of formation. Moreover, we quantitatively evaluated the formation energies of neutral vacancies in KNbO3 as functions of the atomic chemical potentials of the constituent elements by the use of the same procedure. Relaxation of the first- and the second-neighbor atoms around the vacancy was considered in a 40-atom supercell. The formation energy of a K vacancy was found to be the lowest under an oxidizing atmosphere and that of an O vacancy was found to be the lowest under a reducing atmosphere. The formation energy of a Nb vacancy was the highest under both oxygen-rich and -poor conditions. These results are in agreement with the empirical rule that B site defects in perovskite-type oxide do not exist. These results are discussed on the basis of the band structure of KNbO3.

Crystallographic phase stabilities and electronic structures in AgNbO3 by first-principles calculation

Silver niobate (AgNbO3) assumes several phases that crystallise in the perovskite structure similar to sodium niobate (NaNbO3). In order to investigate the phase stability of AgNbO3, the formation enthalpies of both physically the realised (real) cubic , tetragonal (P4/mbm) and orthorhombic (Cmcm, Pbcm) phases as well as the virtual phases, the orthorhombic (Pc21 b) and rhombohedral (R3cR) phases, which are observed in NaNbO3, have been obtained using a plane-wave pseudopotential method. Although the orthorhombic (Pbcm) phase has the lowest formation enthalpy among the real phases, the virtual rhombohedral (R3cR) phase has the lowest symmetry as well as the lowest formation enthalpy among all phases. We thus speculate that the rhombohedral (R3cR) phase may be stable at extremely low temperatures provided that it is accessible kinetically. In addition, the electronic structures of the various phases of AgNbO3 are calculated within the generalised gradient approximation with the Perdew–Burke–Ernzerhof correction (GGA-PBE). The valence band top was found to consist of predominately localised Ag 4d and O 2p orbitals constituting the O 2p–Nb 4d bonding orbital, while the bottom of the conduction band was found to mainly consist of the anti-bonding orbital of Nb 4d–O 2p under the Ag 5s orbital.

First-principles study of defect formation in the photovoltaic semiconductor Cu2SnS3 for comparison with Cu2ZnSnS4 and CuInSe2

The formation energies of neutral Cu, Sn, and S vacancies in monoclinic Cu2SnS3 were calculated by first-principles pseudopotential calculations using plane-wave basis functions in typical points in a schematic ternary phase diagram of a Cu–Sn–S system. The formation energy of a Cu atom vacancy in Cu2SnS3 under the Cu-poor condition has been calculated to be 0.23 eV, which is considerably smaller than those of Sn and S vacancies in Cu2SnS3. The results have been compared with those in Cu2SnZnS4 and CuInSe2 calculated with the same version of program code. The formation energy of a Cu atom vacancy in Cu2SnS3 under the Cu-poor condition is smaller than those for Cu2SnZnS4 (0.40 eV) and CuInSe2 (0.50 eV). The results indicate that Cu vacancies are easily formed in Cu2SnS3 under the Cu-poor condition as is the case with Cu2ZnSnS4 and CuInSe2. In this respect, Cu2SnS3 has the appropriate character of a light-absorbing material for thin-film solar cells, as is the case with Cu2ZnSnS4 and CuInSe2.

Surface Stabilities of Various Crystal Faces of CuInSe2 and Related Compounds by First-Principles Calculation

The typical crystal surfaces of CuInSe2 (CISe) and related compounds were studied using density functional theory (DFT) methods. We evaluated energies and surface structures of (112), (), (110), (102), (100), (00), (001), and (00) surfaces on CISe. For CISe, the (112) surface had the lowest energy among these surfaces, and the (110) and (102) surfaces had slightly higher energies than the (112) surface. These surface atoms coordinate with the three surrounding atoms. We found that the (112) surface is most stable for CISe, and this result is consistent with the experimental results showing that the (112) surface is most frequently observed in polycrystalline CISe thin films. We also evaluated the surface energies of CuGaSe2 (CGSe), CuAlSe2 (CASe), CuInS2 (CIS), CuInTe2 (CIT), and AgInSe2 (AISe). For CGSe, CIS, and CIT, the (112) surface had the lowest energy as in the case of CISe. However, for CASe and AISe, the (110) surface had the lowest energy.

First-principles study of defect formation in the photovoltaic semiconductors Cu2GeS3 and Cu2ZnGeS4 for comparison with Cu2SnS3, Cu2ZnSnS4, and CuInSe2

The formation energies of neutral Cu, Ge, and S vacancies in monoclinic Cu2GeS3 and those of neutral Cu, Zn, Ge, and S vacancies in kesterite-type Cu2ZnGeS4 were evaluated by first-principles pseudopotential calculations using plane-wave basis functions. The calculations were performed at typical points in a schematic ternary phase diagram of a Cu–Ge–S system for Cu2GeS3 and in Cu–(Zn1/2Ge1/2)–S and Cu29S16–ZnS–GeS2 pseudoternary phase diagrams for Cu2ZnGeS4. The results have been compared with those for Cu2SnS3, Cu2ZnSnS4, and CuInSe2 calculated with the same version of the CASTEP program code. The results indicate that Cu vacancies are easily formed in Cu2GeS3 and Cu2ZnGeS4 under the Cu-poor condition as in the cases of Cu2SnS3, Cu2ZnSnS4, and CuInSe2, suggesting that Cu2GeS3 and Cu2ZnGeS4 are also preferable p-type absorber materials for thin-film solar cells. Desirable preparation conditions of these thin films for photovoltaic application are discussed using the calculated formation energies of antisite defects.

Enthalpies of Formation and Electronic Structures of Various Phases in Lead-Free Piezoelectric (K 0.5 Bi 0.5 )TiO 3 Evaluated by First-Principles Calculation

We calculated enthalpies of formation and evaluate electronic structures of various phases in (K_0.5Bi_0.5)TiO₃ using a plane-wave pseudopotential method within a density functional formalism. In (K_0.5Bi_0.5)TiO₃, the enthalpy of formation for a tetragonal (I4mm) phase was found to be lower than those of a cubic (Fm3 macrm) and a rhombohedral (R3m) phases. The result agreed with the experimental result that the tetragonal phase was stable at low temperatures. In the electronic structure of (K_0.5Bi_0.5)TiO₃, a valence band mainly consisted of O 2p, Ti 3d and Bi 6p orbitals and the conduction band mainly consisted of Ti 3d, Bi 6p and O 2p antibonding orbitals. The features of the bondings between K and O, Bi and O, and Ti and O are discussed.