A First Principles Study Of Photovoltaic And Multiferroic Energy Storage Materials Under High-pressures

Pressure has a considerable effect on properties of the matter. High-pressure often decreases the distance between atoms, changes the atom’s valence states, induces charge transfer between atoms, and thereby promotes the probability of the special chemical reactions. Therefore, designing new materials with novel properties applying the high-pressure technology has become an essential method. This paper takes the high pressure physics as the pivot, which mainly includes two parts. The first part is the research work of photovoltaic energy storage materials under high pressure. The basic properties and the research methods of photovoltaic energy-storage materials are introduced. To search and design new photovoltaic semiconductor material has important significance. The second part is aimed to design new types of magnetoelectric materials and analysis their magnetic and ferroelectric coupling mechanism. The main results were listed as following:The new phase of the AgIn S₂ in the I-III-VI compounds of the research hotspots of photovoltaic energy storage material is predicted using the CALYPSO method through the first-principle calculation. Previous experimental studies found that the CuInS₂ will transform to a new phase when the pressure exceeds 9.5 GPa, with some additional changes of properties. To the best of our knowledge, there is no report on the crystal and electronic structure of the new phase. Through calculations the result shows that the structural phase transition from its ambient pressure with the tetragonal structure（space group I-42d） to a high pressure phase with a cubic structure（space group Fd-3m） at 1.25 GPa, which is in accordance with the experimental conclusion, but the pressure of phase transformation in AgInS₂ is higher than 9.5eV in CuInS₂. In order to obtain more accurate band gap of AgInS₂ the result we calculated using HSE06 method is about 1.2eV, which is closer to the ideal value（1.4eV） than the tetragonal phase. In the whole Brillouin, no imaginary phonon frequencies zone indicates the phase is dynamically stable. At the same time, the bonding behavior is examined with the calculation of the electron localization function and the charge density.The ground-state properties of DyNiO₃ at high oxygen pressure with different Hubbard-U dependence have been investigated by using density functional theory within generalized gradient approximation plus the Hubbard-U parameter（GGA + U） method. The ground state structural, electronic, magnetic and ferroelectric properties of DyNiO₃ are calculated, and the origin of ferroelectricity is analyzed. The result shows after considering spin polarized and antiferromagnetic configuration, the symmetry lowers from P21/n to polar space group P21. The electronic structures of DyNiO₃ reveal that it has a direct band gap of0.715 eV. The charge ordering is characterized by a nickel charge disproportionation, The local magnetic moment of Ni1 and Ni2 are 1.703μB and 0.710μB, respectively. The coupling of Ni-3d and O-2p is found in the PDOS. Our calculations of electronic charge density and Born effective charges demonstrate an ionic nature of Ni-O bonds in DyNiO₃. The calculated spontaneous polarization is mainly 6.78μC/cm2 along the b-direction. The resulting DyNiO₃ exhibits intrinsic ferroelectricity which is caused by a charge order from site-centered towards bond-centered.The ground state structural, electronic, magnetic and ferroelectric properties of DyNiO₃ are calculated by using density functional theory. The calculations reveal that the crystal structure is changed at the high-pressure, the lattice parameters and volume of DyNiO₃ decreases as the pressure increases from 0GPa to 10 GPa. The symmetry enhance from P21 to polar space group P21/n at 10 GPa, and the band gap get wider from 0.715 eV to1.25 eV yielded an indirect band gap. With the pressure increasing from 0GPa to 10 GPa, the local magnetic moment of the Ni1 atoms remains the same, but decreases in the Ni2 atoms, that causes the change in the magnetic structure. The local magnetic moment becomes zero when the pressure exceeds 6GPa. This kind of magnetic structure eliminates the bond-centered charge order in DyNiO₃, which leds to the disappearance of spontaneous polarization.