Unveiling the Hidden Order in Elemental Gadolinium

In an earlier study at the ID22 beamline (CH-6899), we detected an unexpected distortion in the diffraction peaks of gadolinium (Gd) starting at 220 K, which continued up to the maximum tested temperature of 700 K. This range significantly exceeds gadolinium’s Curie temperature of 292 K, hinting at a distinctive underlying mechanism. Rietveld refinement revealed that a model featuring two closely related hexagonal phases effectively captured the observed peak shape variations, suggesting that Gd might experience spontaneous phase separation at elevated temperatures. Generally, phase separation driven by kinetic constraints resists reversal during temperature cycles, while that induced by thermodynamic factors can be reversible. To explore the nature and reversibility of this phase separation in elemental Gd, we plan to conduct detailed temperature cycling experiments.