Crystallographic and magnetic structures of the polymorphisms of ${\mathrm{Pr}}_5{\mathrm{Si}}_2{\mathrm{Ge}}_2$ compound: the tetragonal $t\text{−}{\mathrm{Pr}}_5{\mathrm{Si}}_2{\mathrm{Ge}}_2$ and the monoclinic $m\text{−}{\mathrm{Pr}}_5{\mathrm{Si}}_2{\mathrm{Ge}}_2$, are investigated by neutron powder diffraction at different temperatures. The $t\text{−}{\mathrm{Pr}}_5{\mathrm{Si}}_2{\mathrm{Ge}}_2$ crystallizes in the ${\mathrm{Zr}}_5{\mathrm{Si}}_4$-type structure with the space group $P{4}_1{2}_{1}2$ down to $4\mathrm{K}$. Long-range magnetic ordering takes place at $T_C=52\mathrm{K}$, and the magnetic structure can be modeled with the magnetic space group $P{4}_{1}2_1{}^{\prime }{2}^{\prime }$. 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 $m\text{−}{\mathrm{Pr}}_5{\mathrm{Si}}_2{\mathrm{Ge}}_2$ crystallizes in the ${\mathrm{Gd}}_5{\mathrm{Si}}_2{\mathrm{Ge}}_2$-type structure with the space group $P{112}_1∕a$ down to $4\mathrm{K}$. Long-range magnetic ordering occurs at $T_C=40\mathrm{K}$ and the magnetic structure can be modeled with the magnetic space group $P112_1{}^{\prime }∕a^{\prime }$. 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 $t\text{−}{\mathrm{Pr}}_5{\mathrm{Si}}_2{\mathrm{Ge}}_2$ and $m\text{−}{\mathrm{Pr}}_5{\mathrm{Si}}_2{\mathrm{Ge}}_2$. 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.

• Received 25 April 2007

DOI:https://doi.org/10.1103/PhysRevB.76.064425