Spin and orbital disordering by hole doping inPr1−xCaxVO3

Abstract

High-resolution powder x-ray diffraction and single-crystal neutron diffraction were used to investigate the crystal structure and magnetic ordering of the compound $\mathrm{P}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{a}}_{x}\mathrm{V}{\mathrm{O}}_{3}$ ($0\ensuremath{\le}x\ensuremath{\le}0.3$), which undergoes an insulator-to-metal transition for $x\ensuremath{\sim}0.23$. Since the ionic radii of $\mathrm{P}{\mathrm{r}}^{3+}$ and $\mathrm{C}{\mathrm{a}}^{2+}$ are almost identical and structural disorder is minimal, $\mathrm{P}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{a}}_{x}\mathrm{V}{\mathrm{O}}_{3}$ is a good model system for the influence of hole doping on the spin and orbital correlations in transition metal oxides. The end member $\mathrm{PrV}{\mathrm{O}}_{3}$ is a Mott-Hubbard insulator, which exhibits a structural phase transition at ${T}_{\mathrm{S}}=180\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ from an orthorhombic to a monoclinic structure with space groups Pbnm and $P{2}_{1}/b$, respectively. This transition is associated with the onset of orbital ordering and strong Jahn-Teller distortions of the $\mathrm{V}{\mathrm{O}}_{6}$ octahedra. Antiferromagnetic $C$-type order with vanadium moments oriented in the $ab$ plane is observed below ${T}_{\mathrm{N}}=140\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. 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 $\mathrm{P}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{a}}_{x}\mathrm{V}{\mathrm{O}}_{3}$ phase diagram, Ca doping reduces both the orbital and magnetic transition temperatures so that ${T}_{\mathrm{S}}=108\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}}=95\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ 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) ${\ensuremath{\mu}}_{\mathrm{B}}$ per vanadium atom could still be observed on the vanadium site below $T\ensuremath{\sim}60\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. We discuss these observations in terms of doping-induced spin-orbital polaron formation.