Dielectric Properties of Spinel, Garnet and Perovskite Oxides

Abstract

We suggest a definition of the effective ionic charges q*i of different pairs of ions connected in tetrahedral, octahedral and dodecahedral polyhedra in such oxide crystals as spinels, garnets and perovskites. The electronic polarizability αi of cations and the O2— atomic core polarizability are calculated by 〈r2k〉 according to Kirkwood. The polarizability caused by O2— valence electrons is determined from the empirical data relating α(O2—) to the amount of spin-uncompensated cation electrons. The static limits of electronic and ionic components of permittivity ε* are calculated by using q'i and αiaccording to Born-Szigeti and Lorentz-Lorenz relations, adapted to crystalline structures studied. The agreement of the calculated values with experimental data, obtained from the frequency dependences of ε1 and ε2, is illustrated for a large number of ferrimagnetic and low magnetic mono- and polycrystals. The decrease of the electronic and ionic components of ε1 (0) as a result of the substitution of magnetically active ions by diamagnetic ones is attributed to an increase of the degree of ionicity of the crystals. The optical spectra ε1 (ω) and ε2 (ω) , calculated by the model of noninteracting quasiharmonic oscillators, which takes into account lattice and electronic resonances, allow to restore the spectra of the reflection coefficient R(ω), which are also in good agreement with the experiment.