Structural phase transition in Tb₅(Si_0.6Ge_0.4)₄ at low temperature

Herein we report on crystallographic details of a structural phase transition measured with powdered Tb₅(Si_0.6Ge_0.4)₄ at low temperatures (150 and 130 K). The pseudo-binary compound is monoclinic at room temperature (P112₁/a). Changes of reflection intensities and positions in the diffraction patterns indicate a structural phase transition between 100 K and 150 K, where the compound orders in an orthorhombic structure (Pnma). This is in accordance with a magnetic phase transition from paramagnetic to ferromagnetic state (T_C = 150 K), where the compound exhibits the giant magnetocaloric effect. Split-up crystallographic sites of the monoclinic unit cell degenerate to sites of higher symmetry in the orthorhombic structure. Changes in the corresponding bond lengths are listed in detail. It turned out that both phases coexist at the selected temperatures but the phase content changes significantly. The observed phase transition is very similar to that reported for Gd₅Si₂Ge₂, that is a shear movement of pairs of nearly rigid slabs of atoms along a direction by 0.84 Å and the tremendous increase of selected (Si/Ge)-(Si/Ge) distances from 2.64 to 3.59 Å in between those slabs. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)