Meta–Insulator Phenomena in Strongly Correlated Oxides. The Vacancy-Doped Titanate Perovksites, Nd1−xTiO3 and Sm1−xTiO3
✍ Scribed by G. Amow; N.P. Raju; J.E. Greedan
- Publisher
- Elsevier Science
- Year
- 2000
- Tongue
- English
- Weight
- 312 KB
- Volume
- 155
- Category
- Article
- ISSN
- 0022-4596
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✦ Synopsis
Electrical transport, both resistivity and thermopower, and heat capacity data are reported for two systems of cation vacancy-doped titanate perovskites, Nd 1؊x TiO 3 and Sm 1؊x TiO 3 . 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& 0.20, and the other by hole doping the Mott+Hubbard insulator, MHI (x ؍ 0.00), which is found near x&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 coe7cient of the T 2 term increases with decreasing x (increasing n) and attains a large value, 3.0؋10 ؊8 -cm K ؊2 , for x&0.12, which is more than 10 times larger than values seen in other titanate systems. Correspondingly large values for the heat capacity are observed. The x ؍ 0.10 sample shows a remarkable resistivity temperature dependence with a maximum at &175 K and minimum at &50 K followed by an upturn at the lowest temperatures. This is similar to behavior seen for Kondo or valence-6uctuating 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 &0.00 are hole-doped antiferromagnetic semiconductors but thermopower data for the former shows a change from p-to n-type carriers just near 300 K. Sm 1؊x TiO 3 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 TiO 3 series and the role of correlation is emphasized. 2000 Academic Press