  In order to explore the origin of the Metal-Insulator (M-I) transition, the
precise crystal structures of the hole-doped Mott insulator system, Y1-XCaXTiO3
(x=0.37, 0.39 and 0.41), are studied for the temperature range between 20K and
300K by the synchrotron radiation (SR) X-ray powder diffraction. For both
Y0.63Ca0.37TiO3 and Y0.61Ca0.39TiO3 compositions, the orthorhombic (Pbnm) -
monoclinic (P21/n) structural phase transition occurs at around 230K, which is
much higher than their own M-I transition temperatures, i.e. 60K and 130K,
respectively. For these compositions, the significant phase separation
(low-temperature orthorhombic + monoclinic) is also found in the vicinity of
the M-I transition temperature. On the other hand, Y0.59Ca0.41TiO3, which dose
not exhibit M-I transition and preserve a metallic behavior down to 1.5K, is in
two phases state from 20K to 300K. It is concluded that the existence of the
phase separation causes the M-I transition in Y1-xCaxTiO3, and the
low-temperature orthorhombic phase contributes to the metallic property of this
system.
