The double-phase transition of ErFe4Ge2. An XRPD study

High-quality powder XRD data of the compound ErFe4Ge2 collected in the ESRF beam line BM16, are presented for the entire magnetically ordered regime (TN=44 K). The data analysis reveals the occurrence of a double symmetry breaking at the magnetic transition. This experiment has allowed us to distinguish between structural and magnetic satellites, both present in the neutron patterns, and to demonstrate the interdependence of structural and magnetic transitions. The high-temperature (HT) phase disproportionates by a first-order transition into two distinct phases: P42/mnm (Tc, TN=44 K)→Cmmm (majority LT phase)+Pnnm (minority IT Phase) which coexist in proportions varying with temperature down to 4 K. The phase diagram comprises three temperature regions: (a) the HT range with T>TN for the tetragonal P42/mnm phase; (b) the IT (intermediate temperature) range, 20 K<T<TN, where the two phases coexist in strongly variable proportions and the Pnnm phase reaches its highest concentration (∼31%) around 30 K and (c) the LT (low temperature) range, 1.5-20 K, where the Cmmm phase is dominating (up to 95%). We suggests that this phenomenon is the result of competing magneto-elastic mechanisms involving the Er crystal field anisotropy, the Er-Er, Er-Fe and the Fe-Fe exchange interactions and their coupling with the lattice strains