Structural modulation and multipolar ordering in the metal-nonmetal compounds Pr(Ru_1-xRh_x)₄P₁₂

We present a X-ray diffraction study of Pr(Ru_1-xRh _x)₄P₁₂ (x = 0, 0.05), which exhibit a metal-nonmetal transition at about 50-60 K. Superlattice reflections which are related to [Ru,Rh] and P atom displacements could be observed below the transition temperature. In PrRu₄P₁₂, these displacements are found to increase gradually as the temperature decreases, in agreement with previous studies. In Pr(Ru_0.95Rh_0.05)₄P ₁₂, the [Ru_0.95Rh_0.05] and P atom displacements first increase with decreasing temperature below 50 K, but nearly cancel at lowest temperatures. We discuss this unexpected structural behavior by considering the Pr 4f electron states in the Pr(Ru_1-xRh _x)₄P₁₂ compounds.