Perovskite Structures d Bands

We turn now to a very important class of materials that have the formula /IBC3, with the C frequently oxygen. Strontium titanate is a familiar example and one we shall use for illustrative purposes. Titanium is in the D4 column of the Solid State Table, having four electrons beyond its argonlike core. Strontium has two electrons outside its kryptonlike core, so we may think of the six valence electrons as having been transferred to the three oxygen atoms to form a simple ionic system. As we shall see, however, the titanium d states form the lowest conduction band and are important in the bonding properties as well. 

The electronic structure of SrTi03 is now very well understood. Accurate APW band calculations for this compound, and for a scries of related compounds that we shall discuss, were carried out by Mattheiss (1972b). Experimental studies of these compounds continue to be made sec, for example, Perlosa and Michel-Calendini (1978). 

Mattheiss interpreted the bands in terms of an LCAO de.scription and we shall follow that interpretation. His results for wave numbers in a [100] direction are shown in Fig. 19-3, for four of the compounds he treated. Let us consider the SrTiO, bands the Sr core stales are not of interest, but wc shall include the oxygen s states, which lie some ten electron volts below. We refer to the oxygen p states as the valence bands they arc completely occupied. The transition-metal d 

The perovskite structure of SrTi03 may be understood as a simple cubic lattice of titanium ions with oxygen ions at the center of every cube edge. The Sr ions re,side at the cube centers. Na.WOj has the same structure but with variable numbers of the cube centers occupied. 

Wlten the counting of electrons leads to many d electrons per ion, a nniltiplet structure may form. KM11F3 is such a case (as are KFcF, KCoFj, and KC11F3). 

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Both elements form dioxides, M02, that have distorted rutile structures in which there are M M bonds. Re03 is an important oxide but Tc03 is not known to exist. The structure of red Re03, shown in Fig. 18-D-l, is also adopted by Cr03, W03, and others and is very similar to the perovskite structure. Re03 has metallic type electrical conductivity due to delocalization of the Re 1 electrons in a conduction band of the solid. 

HMAFM in oxides with double perovskite structure was proposed by Warren Pickett for the first time in 1998. Band structure calculations based on local spin-density approximations (LSDA), predicted La2MnV06 double perovskite as a promising candidate for exhibiting HMAFM. The calculations were based on the assumption that both Mn and V ions are trivalent and more importantly, that the Mn (d ) occurs in a low spin configuration (t2g" Cg ). The band structure of La2MnVOg show that only the minority 2g bands of both V and Mn contribute to the DOS near the The antiparallel alignment of low spin Mn + (tzg Ttzg i) and states would yield a net zero... 

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