X-Ray Study of Crystal Distortions in RMn₂ Compounds (R: Y, Pr, Nd, Sm and Gd)

Abstract

Powder X-ray diffraction measurements on intermetallic compounds RMn₂ (R: Y, Pr, Nd, Sm and Gd) have been made in a temperature range from 69 to 300 K. Below T_N, every sample shows a large volume expansion, being as large as 4.6% for YMn₂. In addition, PrMn₂ and NdMn₂ having the hexagonal C14 type of crystal structure are distorted into monoclinic below T_N. On the other hand, SmMn₂ having the same C14 type structure is distorted into orthorhombic. On the basis of a theory of magnetoelastic coupling the directions of magnetization of these compounds below T_N were determined. The cubic C15 type YMn₂ shows a very small and probably rhombohedral distortion below T_N, whereas the same C15 type GdMn₂ shows no symmetry change.