Thermal expansion of filled skutterudite compound GdRu₄P₁₂

We have performed X-ray diffraction and thermal expansion measurements at low temperatures of the filled skutterudite compound GdRu₄P₁₂ in order to investigate structural changes related to the antiferromagnetic phase transition at T_N=22K. No significant change in the X-ray powder diffraction pattern of GdRu₄P₁₂ was detected down to 10K. The variation of thermal expansion is continuous at T_N. Therefore, the transition is not accompanied with any crystallographic transformation. The temperature dependence of the linear thermal expansion coefficient shows large jump (1.2 × 10^-6 K^-1) at T_N. This value is comparable to that of isostructual compound SmRu₄P₁₂ at the metal-insulator transition temperature T_MI=16K.