Crystal structure transformation in CeRuSn seen via the atomic pair distribution function

Abstract

We report on the atomic pair distribution function determination in the ternary compound CeRuSn using high-energy synchrotron x-ray diffraction. Above room temperature CeRuSn orders in a monoclinic structure that is related to the CeCoAl type of structure by a doubling of the $c$-axis parameter. Upon cooling a very broad hysteretic structural transition has been observed in CeRuSn leading to an ill-defined structure with a main component three times larger than the $c$ parameter. The pair distribution functions collected between 90 and 315 K suggest that the local symmetry type does not change significantly during the phase transformation. However, anomalously large atomic displacement factors suggest that a wide distribution of interatomic distances exists in the sample. Limited agreement between the data at 90 K and the best fit suggests that the structure with a three times larger $c$ lattice parameter with respect to the CeCoAl type can only be regarded as an approximation to the real crystal structure of CeRuSn at low temperatures.