Magnetic and crystal structures of the polymorphicPr5Si2Ge2compound

Abstract

Crystallographic and magnetic structures of the polymorphisms of ${\mathrm{Pr}}_{5}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$ compound: the tetragonal $t\text{\ensuremath{-}}{\mathrm{Pr}}_{5}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$ and the monoclinic $m\text{\ensuremath{-}}{\mathrm{Pr}}_{5}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$, are investigated by neutron powder diffraction at different temperatures. The $t\text{\ensuremath{-}}{\mathrm{Pr}}_{5}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$ crystallizes in the ${\mathrm{Zr}}_{5}{\mathrm{Si}}_{4}$-type structure with the space group $P{4}_{1}{2}_{1}2$ down to $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Long-range magnetic ordering takes place at ${T}_{C}=52\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and the magnetic structure can be modeled with the magnetic space group $P{4}_{1}2_{1}{}^{\ensuremath{'}}{2}^{\ensuremath{'}}$. The net magnetic moment occurs exclusively along the $c$ direction. The lattice parameters change continuously around ${T}_{C}$ with a small negative magnetovolume effect, indicating a second-order phase transition. The $m\text{\ensuremath{-}}{\mathrm{Pr}}_{5}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$ crystallizes in the ${\mathrm{Gd}}_{5}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$-type structure with the space group $P{112}_{1}∕a$ down to $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Long-range magnetic ordering occurs at ${T}_{C}=40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and the magnetic structure can be modeled with the magnetic space group $P112_{1}{}^{\ensuremath{'}}∕{a}^{\ensuremath{'}}$. The net magnetic moment lies on the $ab$ plane, with the main component along the $a$ axis. No other magnetic transition is observed below ${T}_{C}$ for both the compounds, and the largest shrinking of lattice parameter upon cooling through ${T}_{C}$ occurs along the direction with the largest net magnetic moment component. The relatively stable existence at room temperature of the polymorphic ${\mathrm{Pr}}_{5}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$ is readily understood based on the correlation between the crystal structures of $t\text{\ensuremath{-}}{\mathrm{Pr}}_{5}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$ and $m\text{\ensuremath{-}}{\mathrm{Pr}}_{5}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$. In the scenario of the Ruderman-Kittel-Kasuya-Yosida interaction model, the complex noncollinear magnetic structures of the compounds can be attributed to a competition of different Pr-Pr exchange interactions due to the different chemical environments around Pr atoms on different sites and to the broad range of the Pr-Pr distances.