The Metal-Insulator Transition in Y_<1-x>Ca_xTiO_3 Caused by Phase Separation

概要

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In order to explore the origin of the metal-insulator (M-I) transition, the precise crystal structures of the hole-doped Mott insulator system, Y_<1-x>Ca_XTiO_3 (x = 0.37, 0.39 and 0.41), are studied for the temperature range between 20 K and 300 K by synchrotron radiation (SR) X-ray powder diffraction. For both Y_<0.63>Ca_<0.37>TiO_3 and Y_<0.61>Ca_<0.39>TiO_3 compositions, the orthorhombic (Pbnm) - monoclinic (P2_1,/n) structural phase transition occurs at approximately 230 K, which is much higher than their individual M-I transition temperatures, i.e., 60 K and 130 K, respectively. For these compositions, significant phase separation (low-temperature orthorhombic + monoclinic) is also found in the vicinity of the M-I transition temperature. On the other hand, Y_<0.59>Ca_<0.41>TiO_3, which does not exhibit M-I transition and preserves a metallic behavior down to 1.5 K, is in a two phase state from 20 K to 300 K. It is concluded that the existence of the phase separation causes the M-I transition in Y_<1-x>Ca_xTiO_3, and the low-temperature orthorhombic phase contributes to the metallic property of this system.
2002-09-15