Hole-doping and pressure effects on the metal-insulator transition in single crystals of Y_1-xCa_xTiO₃ (0.37 ≤ x ≤ 0.41)

The measurements of electrical resistivity, magnetic susceptibility, specific heat, thermopower and powder x-ray diffraction under pressure have been performed on single crystals of Y_1-xCa_xTiO₃ (0.37 ≤ x ≤ 0.41) to investigate the hole-doping effect and pressure effect on the metal-insulator (MI) transition. In the low-temperature metallic state, the Ti moment is much reduced from S = 1/2 spin moment in the high-temperature insulating state. Upon applying pressure on the sample with x = 0.37 at temperatures below 200 K, the volume fraction of the orthorhombic phase exceeds that of the monoclinic phase, which is associated with the insulator to metal transition. The good correspondence between the x-T and P-T phase diagrams in the vicinity of the MI transition shows that the effect of hole doping by 1% Ca substitution is equivalent to the effect of application of pressure of 0.41 GPa. The phase separation temperature agrees well with the MI transition temperature, and both temperatures increase linearly with increasing x or pressure. These results indicate that the MI transition in Y _1-xCa_xTiO₃ is driven by percolation of the low-temperature orthorhombic domains.