Coupling between Orbital and Lattice Degrees of Freedom in Y_1-xCa_xTiO₃ (0 < x ≤ 0.75): A Resonant X-ray Scattering Study

We have studied the orbitally ordered states in Y1-xCaxTiO3 (0 < x < 0.75) by resonant X-ray scattering (RXS) near the Ti K-edge (E similar to 5 keV) and by powder X-ray diffraction measurements. The RXS intensities at the 1s -> 4p transition energy (main-edge) gradually decrease with increasing x and remain finite even in the metallic phase for x > 0.4. In contrast, the intensities at the 1s -> 3d transition energy (pre-edge) decrease rapidly with increasing x up to 0.15 and disappear at x similar to 0.4. The x dependences of the Jahn-Teller distortion is consistent with those of the RXS intensities at pre-edge. Namely, the RXS intensity at the pre-edge energy reflects the orbital ordered state. From our results, we conclude that the magnetic order in Y1-xCaxTiO3 becomes unstable at x similar to 0.15 when the orbital order is weakened, and the metallic phase appears in x > 0.4 when the order of orbitals melts.