AN AB-INITIO PERTURBED ION STUDY OF THE BALIF3 AND BALIH3 INVERTED PEROVSKITE STRUCTURES

This work describes an ab initio perturbed ion (aiPI) study for the de termination of the relative stability of normal and inverted perovskit e structures ABX(3) (A = Li+, K+, Rb+, Cs+; B = Mg2+, Ca2+, Sr2+, Ba2; X = F-, H-). Large Slater-type orbitals for representing atomic cent ers have been used. The inverted BaLiF3 system and ternary hydride BaL iH3 and SrLiH3 perovskites have a lower lattice energy than normal str uctures. The differences between ionic radii of the different alkaline s, alkaline earth cations, and fluorine and hydride anions are capable of explaining the relative stability of normal and inverted structure s. The values of the bulk modulus reveal that the inverted structure i s less compressible than the normal one for the BaLiF3 structure. A lo cal geometry optimization has been carried out for BaLiF3 and BaLiH3 i n order to minimize the effective energy using a large cluster model o f 173 ions. Vibrational frequencies (v) associated with the breathing vibrational modes, a(1g), in both A and B sites for these pure systems have been characterized. For the corresponding doped (Ni2+ for Ba2+) crystal structure, this property has been characterized at the 12-fold site, revealing an increment of the v value from the pure to the dope d BaLiF3 structure; an opposite trend is observed for the BaLiH3 syste m. These substitutions present favorable defect reaction energies in b oth crystal lattices, but the energy change calculated for the direct reaction between NiF2 and BaLiF3 is positive for Ni2+ entering the Ba2 + site. The numerical results are analyzed and compared with experimen tal data, the geometrical distances obtained by computer simulation be ing in agreement with the reported experimental values. The validity o f the methodology is discussed.