Magnetic and crystal structures of the polymorphic Pr5Si2Ge2 compound

Crystallographic and magnetic structures of the polymorphisms of Pr5Si2Ge2 compound: the tetragonal t-Pr5Si2Ge2 and the monoclinic m-Pr5Si2Ge2, are investigated by neutron powder diffraction at different temperatures. The t-Pr5Si2Ge2 crystallizes in the Zr5Si4-type structure with the space group P4(1)2(1)2 down to 4 K. Long-range magnetic ordering takes place at T-C=52 K, and the magnetic structure can be modeled with the magnetic space group P4(1)2(1)'2('). 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-Pr5Si2Ge2 crystallizes in the Gd5Si2Ge2-type structure with the space group P112(1)/a down to 4 K. Long-range magnetic ordering occurs at T-C=40 K and the magnetic structure can be modeled with the magnetic space group P112(1)'/a'. 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 Pr5Si2Ge2 is readily understood based on the correlation between the crystal structures of t-Pr5Si2Ge2 and m-Pr5Si2Ge2. 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.