Metal-insulator phenomena in strongly correlated oxides. The vacancy-dopedtitanate perovksites, Nd1-xTiO3 and Sm1-xTiO3

Electrical transport, both resistivity and thermopower, and heat capacity d ata are reported for two systems of cation vacancy-doped titanate perovskit es, N1-xTiO3 and Sm1-xTiO3. In the former case the range of x is from 0.33 to 0.00 and for the latter from 0.17 to 0.00. Thus, the nominal carrier con centration, n, can be varied from 0 (x = 0.33) to 1.0 (x = 0.00) electrons per Ti atom. For the Nd series two distinct metal-insulator transitions (MI T) are seen, one obtained by electron doping the charge-transfer insulator, CTI (x = 0.33), which occurs at x similar to0.20, and the other by hole do ping the Mott-Hubbard insulator, MHI (x = 0.00), which is found near x simi lar to 0.10. Beginning near the CTI composition, x = 0.30, evidence is foun d for variable range hopping from both resistivity and thermopower data. Mo tt metallization occurs near x = 0.20 and Fermi-liquid (FL) behavior (T-2 d ependence of the resistivity) is seen between x = 0.20 and 0.10. The coeffi cient of the T-2 term increases with decreasing x (increasing n) and attain s a large value, 3.0 x 10(-8) Omega -cmK(-2), for x similar to0.12, which i s more than 10 times larger than values seen in other titanate systems. Cor respondingly large values for the heat capacity gamma are observed. The x = 0.10 sample shows a remarkable resistivity temperature dependence with a m aximum at similar to 175 K and minimum at similar to 50 K followed by an up turn at the lowest temperatures. This is similar to behavior seen for Kondo or valence-fluctuating materials and is qualitatively consistent with exis ting theory, which predicts a Kondo-like transition state between the FL an d MHI regimes. The x = 0.05 and similar to0.00 are hole-doped antiferromagn etic semiconductors but thermopower data for the former shows a change from p- to n-type carriers just near 300 K, Sm1-xTiO3 is even more unusual as o nly the p-type semiconducting and the Kondo-like conducting regimes are see n. Comparisons are drawn with related Ln(1-x)A(x)TiO(3) and Ln(1-x) TiO3 se ries and the role of correlation is emphasized. (C) 2000 Academic Press.