Perovskite metallic conductivity

At the other end of the conduction spectrum, many oxides have conductivities dominated by electron and positive hole contributions to the extent that some, such as Re03, SnOa and tire perovskite LaCrOs have conductivities at the level of metallic conduction. High levels of p-type semiconduction are found in some transition metal perovskites especially those containing alio-valent ions. Thus the lanthairum-based perovskites containing transition metal ions, e.g. LaMOs (M-Cr, Mn, Fe, Co, Ni) have eirlranced p-type semiconduction due to the dependence of the transition metal ion valencies on the ambient... 

The syntheses of several simple AB03 compounds by Art Sleight led him to the first example of oxide superconductors possessing the ordered perovskite structure. It was already known that BaPbOg, a distorted perovskite with pseudo-orthorhombic symmetry, exhibited metallic conducting properties to very low temperatures, but... 

None of the A2B04 oxides seems to exhibit a true metallic behavior down to the lowest temperatures. Most of these oxides show activated conduction and even those phases that have been considered to be metallic (e.g., La2Cu04) exhibit conductivities of the order of 10 ohm-1 cm-1. This is much less than the conductivities found in metallic oxides of perovskite structure (e.g., La NiC>3 or LaSrCo206 with cr of 103 ohm-1 cm-1). It is not clear whether the absence of true metallic conductivity in A2B04 oxides has something to do with localization in two dimensions 85, 86). 

Tungsten bronzes are inert materials M WOy (0 < x < 1) with defect perovskite structures Figure 5.23). Their colour depends on x golden for x a 0.9, red for x 0.6, violet for x a 0.3. Bronzes with x > 0.25 exhibit metallic conductivity owing to a band-like structure associated with W(V) and W(VI) centres in the lattice those with x < 0.25 are semiconductors (see Section 5.8). Similar compounds are formed by Mo, Ti and V. ... 

The relatively simple band stmcture of a cubic perovskite becomes more complex when the electron spin is taken into account. Calculations show that the mainly tj and e bands split into two components, one for spin-up electrons and one for spin-down species. The original two bands become four. An example is provided by the perovskite CaMnOj, containing Mn (d which is an AFM semiconductor. The lower-energy t band should contain three electrons with parallel spins. As the tj state can hold up to six electrons, it is only half full in CaMnOj, thus predicting metallic conductivity. However, because of electron correlation (Hund s coupling between electrons, which results in Hund s mles for electron occupation of similar orbitals), both the t and e orbital sets are split into two sub-bands, one for spin-up (t) electrons and one for spin-down (f) electrons. The simplified density of states diagram (Figure 8.11) shows that the valence band, derived mainly from O 2p states. 

Several perovskites exhibit metallic conductivity, typical examples being LaTi03, LaNi03, SrV03, AM0O3 (A = Ca, Sr, Ba), Re03, and A O. Metallic conductivity in similar perovskite oxides is due to a strong cation-anion-cation interaction [21,24,34]. 

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