Structural and magnetic properties ofLa1−xPrxMnO3+δ(0⩽x⩽1.0)

Abstract

X-ray, neutron powder diffraction (NPD), and magnetic measurements were performed in order to investigate the effect of Pr substitution for La on crystallographic structure, on character of magnetic ordering, and on transition temperature in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3+\ensuremath{\delta}}$ manganites series $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}1.0)$. Strong dependence of structural and magnetic properties on average $A$-site ionic radius was found. The magnetic phase diagram, characterizing Pr concentration dependence of the phase transition temperature, was constructed, based on magnetic and on NPD results. In the compounds with $xl0.7$, the ferromagneticlike behavior with ferromagnetic moment aligned along the $b$ axis was observed in the temperature interval below the Curie temperature, ${T}_{\mathrm{C}}$, varying in the range of $113--153\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, down to Mn ions canted spin arrangement transition temperature, ${T}_{\mathrm{CANT}}$, varying in the range of $73--115\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. It was found that ${T}_{\mathrm{C}}$ changes nonmonotonously with Pr content, it decreases rapidly for $xl0.3$ and for $xg0.6$, while is practically independent on $x$ for $0.3lxl0.6$. The Mn magnetic moments increase from 3.2 for $x=0$ to 3.6 for $x=0.3$ before decreasing to $1.7\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}∕\mathrm{Mn}$ for $x=1.0$ and form a canting structure with canting angle decreasing from 32\ifmmode^\circ\else\textdegree\fi{} for $x=0$ to 16\ifmmode^\circ\else\textdegree\fi{} for $x=0.3$ and then increasing to 70\ifmmode^\circ\else\textdegree\fi{} for $x=1.0$. The NPD and x-ray diffraction data indicate that Pr doping in $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$ is accompanied by changes of both the lattice parameters and crystal structure. The compound with $x=0$ was shown to have a rhombohedral crystal structure, while the samples with $0.1\ensuremath{\leqslant}x\ensuremath{\leqslant}0.6$ have orthorhombic symmetry with a small Jahn-Teller distortion (or pseudocubic) which continuously transforms towards highly distorted orthorhombic structure for $xg0.6$. From ac magnetic susceptibility, studied under hydrostatic pressure up to $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, it was found that ${T}_{\mathrm{C}}$ and ${T}_{\mathrm{CANT}}$ change linearly with an applied pressure. The results are interpreted taking into account the competition of ferromagnetic double-exchange and antiferromagnetic superexchange interactions. This competition leads to a low temperature disordered system with spin canted features as a result of a decrease of the average $A$-site cation radius followed by decrease of Mn-O bond lengths, Mn-O-Mn bond angles, and the static Jahn-Teller distortion of the $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra.