Perovskites

Cr, Nb doped perovskites

SrTiO3

SrTiO3 is one of the best known and most widely used perovskite materials with interesting properties primarily resulting from structure doping with aliovalent additives. Nowadays, the studies devoted to its potential use, among others, as materials for energy conversion (therein mixed ionic-electronic conductors and thermoelectric materials) are becoming increasingly important from applicational viewpoint. The modifcation of various properties of SrTiO3 material can be achieved by its carefully designed doping in one (or simultaneously both) cationic sublattices with various admixtures. We focus in our studies [1,2] on the theoretical modelling of various doping of SrTiO3 on its electronic structure properties.

Fig.1. Optimized model structures: a) undoped 3x3x3 SrTiO3 supercell, b) 0037Cr-STO (3.7% of titanium substituted with chromium) , c) 0056Cr-STO (5.6%), d) 0083Cr-STO (8.3%). Cr-O polyhedron - dark blue, Ti-O polyhedron - light blue, Sr atoms - green spheres, oxygen atoms - red spheres.

For example, detailed analysis of structural, bonding and electronic structure properties of model structures of chromium doped SrTiO3 with chromium content close to 4, 6 and 8 mol% shown [1] that chromium additive most probably replaces titanium in SrTiO3 system resulting in changes in Ti-O bonds properties and thus slight distortion of TiO6 octahedra and reduced unit cell parameters. Cr dopant in SrTiO3 structure causes also an appearance of the additional states inside band gap area with simultaneous shift of Fermi energy into this area, which should have positive influence on electrical properties of Cr-doped SrTiO3 material.

Fig.2. 1st (left) and 2nd (right) chromium coordination spheres composed of titanium octahedra.

However, substitution of Ti atoms by chromium ones causes also a creation of vacancy in oxygen sublattice and as a consequence Fermi level is shifted up into conduction band area, the effective mass of holes is significantly increased and total band gap width is also increased. Thus, one can expect that the final electrical properties of experimentally obtained Cr-doped SrTiO3 will be strongly dependent on oxygen vacancy concentration.