Spin and orbital disordering by hole doping in Pr_(1-x)Ca_xVO_3

摘要

High-resolution powder x-ray diffraction and single-crystal neutron diffraction were used to investigate the crystal structure and magnetic ordering of the compound Pr_(1-x)Ca_xVO_3 (0 ≤ x ≤ 0.3), which undergoes an insulator-to-metal transition for x ~ 0.23. Since the ionic radii of Pr~(3+) and Ca~(2+) are almost identical and structural disorder is minimal, Pr_(1-x)Ca_xVO_3 is a good model system for the influence of hole doping on the spin and orbital correlations in transition metal oxides. The end member PrVO_3 is a Mott-Hubbard insulator, which exhibits a structural phase transition at T_s = 180K from an orthorhombic to a monoclinic structure with space groups Pbnm and P2_i/b, respectively. This transition is associated with the onset of orbital ordering and strong Jahn-Teller distortions of the VO_6 octahedra. Antiferromagnetic C-type order with vanadium moments oriented in the ab plane is observed below T_N = 140 K. Upon cooling, the vanadium moments induce a progressive magnetic polarization of the praseodymium sublattice, resulting in a ferrimagnetic structure with coexisting modes (C_x, F_y) and (F_x, C_y). In the insulating range of the Pr_(1-x)Ca_xVO_3 phase diagram, Ca doping reduces both the orbital and magnetic transition temperatures so that T_s = 108 K and T_n = 95 K for x = 0.20. The Jahn-Teller distortions and ordered vanadium moments also decrease upon doping. In a metallic sample with x = 0.30, Jahn-Teller distortions and long-range orbital ordering are no longer observable, and the average crystal structure remains orthorhombic down to low temperature. However, broadening of some lattice Bragg reflections indicate a significant increase in lattice strain. Antiferromagnetic short-range order with a weak ordered moment of 0.14(3) μB per vanadium atom could still be observed on the vanadium site below T ～ 60 K. We discuss these observations in terms of doping-induced spin-orbital polaron formation.