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

Electrical transport, both resistivity and thermopower, and heat capacity data are reported for two systems of cation vacancy-doped titanate perovskites, 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 concentration, 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 (MIT) 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 doping the Mott-Hubbard insulator, MHI (x = 0.00), which is found near x similar to 0.10. Beginning near the CTI composition, x = 0.30, evidence is found for variable range hopping from both resistivity and thermopower data. Mott metallization occurs near x = 0.20 and Fermi-liquid (FL) behavior (T-2 dependence of the resistivity) is seen between x = 0.20 and 0.10. The coefficient of the T-2 term increases with decreasing x (increasing n) and attains a large value, 3.0 x 10(-8) Omega -cmK(-2), for x similar to0.12, which is more than 10 times larger than values seen in other titanate systems. Correspondingly large values for the heat capacity gamma are observed. The x = 0.10 sample shows a remarkable resistivity temperature dependence with a maximum at similar to 175 K and minimum at similar to 50 K followed by an upturn at the lowest temperatures. This is similar to behavior seen for Kondo or valence-fluctuating materials and is qualitatively consistent with existing theory, which predicts a Kondo-like transition state between the FL and MHI regimes. The x = 0.05 and similar to0.00 are hole-doped antiferromagnetic 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 only the p-type semiconducting and the Kondo-like conducting regimes are seen. Comparisons are drawn with related Ln(1-x)A(x)TiO(3) and Ln(1-x) TiO3 series and the role of correlation is emphasized. (C) 2000 Academic Press.