Structural transformations in R3Cu4Sn4 (RÂ =Â Ho, Er, Tm) intermetallic compounds

The family of ternary intermetallic compounds R3T4X4 (R = rare-earth; T = d-element; X = Si, Ge, Sn) comprises over 100 members that have a generic orthorhombic structure (space group Immm, #71) at room temperature. We have observed a monoclinic ↔ orthorhombic crystallographic transformation in Ho3Cu4Sn4, Er3Cu4Sn4 and Tm3Cu4Sn4 by high-resolution synchrotron X-ray and neutron powder diffraction. We show that the temperature at which this transformation occurs scales linearly with the ionic radius of the rare-earth. No such transformation was observed in Dy3Cu4Sn4, down to 1.7 K, consistent with this scaling.