Thermal Expansion of Filled Skutterudite Compound SmRu4P12

Abstract

The compound SmRu4P12 crystallized in the cubic filled skutterudite-type structure with space group Im 3 (Th, No. 204) has attracted much interest because it exhibits a metal–insulator (M–I) transition at TMI 1⁄4 16:5K. PrRu4P12 is also known to undergo a M–I transition at TMI 60K. The M–I transition in PrRu4P12 has been considered to result from a charge-density-wave (CDW) transition caused by the perfect three dimensional nesting of the Fermi surface. The mechanism of M–I transition in SmRu4P12 has been, however, believed to be different from that in PrRu4P12. The M–I transition of SmRu4P12 has magnetic origin because the magnetic susceptibility clearly shows an anomaly at M–I transition, unlike PrRu4P12. 2) The specific heat at zero field shows a -shaped peak anomaly at the M–I transition; this is a second order transition. Recent work has revealed that this M–I transition occurs in fact in two successive steps. The specific heat exhibits a double peak in magnetic field. In 9 T two anomalies clearly appear at 13K (TN) and 17K (TMI). The temperature derivative of the electrical resistivity d ðTÞ=dT and of the magnetization dMðTÞ=dT also exhibit two anomalies at the same positions as the specific heat peaks. The lattice constant and the magnetic properties show Sm ion in this compound to be trivalent with J 1⁄4 5=2. The magnetic entropy estimated at zero field reaches R ln 4 at TMI. 5) This indicates that the crystalline electric field (CEF) ground state is 67 quartet (4 f , J 1⁄4 5=2) in the cubic point group Th, which is consistent with the theoretical prediction. 67 is the same as 8 quartet in Oh. 8) 67 has both magnetic and orbital degree of freedom. The existence of a double anomaly can be ascribed to two successive transitions on cooling: orbital ordering, then magnetic ordering such as in CeB6. 9) The temperature dependence of the magnetization MðTÞ shows an upturn at TMI and a round peak near 15K below TMI. 6) TMIðHÞ slightly increases (H-dependence) with increasing magnetic field H. This behavior is consistent with the upturn in MðTÞ considering thermodynamic relations for second order transition. On the other hand, MðTÞ shows a steep drop below TN and TN monotonically decreases with H. Such behavior in the phase diagram is often observed in compounds which show antiferro-quadrupolar (AFQ) ordering. The possibility of AFQ ordering in SmRu4P12, therefore, has been argued due to these reasons. On the other hand, recent ultrasonic measurements suggest a breakdown of the time reversal symmetry below TMI. 11,12) This result and group-theoretical considerations give a new scenario for the mechanism of M–I transition, which is due to the coupling between dipole moment and octupole moment. In spite of much discussion as stated above, the nature of M–I transition in SmRu4P12 is still an enigma. In this paper, we report thermal expansion measurements of SmRu4P12 in applied magnetic field, which have been performed to investigate the properties of the successive phase transitions at TN and TMI. Single-phase polycrystalline SmRu4P12 was prepared at high temperature and high pressure using a wedge-type cubic-anvil high-pressure apparatus. The pressure-cell assembly is similar to that used for the synthesis of black phosphorus. The compound was prepared by reaction of stoichiometric amounts of each metal and red phosphorus powder at 4GPa. The reaction temperature and time were 1100 C and 30min, respectively. The sample was characterized by powder X-ray diffraction using CuK 1 radiation. The sample was shaped into a cylindrical shape with 3.4mm diameter and 4.1mm length (L). The coefficient of linear thermal expansion 1⁄4 ð1=LÞðdL=dTÞ in the magnetic field up to 8 T was measured using a sensitive three-terminal capacitance cell, with a detection limit L=L 10 . We show the relative change of thermal expansion L=L in SmRu4P12 as a function of temperature at various fields (Fig. 1). Vertical dotted lines indicate transition temperatures of TMI 1⁄4 16:5K and TN 14K at zero field, determined by 1⁄4 ð1=LÞðdL=dTÞ. Experimental values are normalized to 22K. Below TMI, the sample length contracts, L=L 1:0 10 , with decreasing temperature at zero