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 ( T-N = 44 K). The data analysis reveals the occurrence of a double symmetry breaking at the magnetic transition. This experiment has allowed us to dist inguish between structural and magnetic satellites, both present in the neu tron patterns, and to demonstrate the interdependence of structural and mag netic transitions. The high-temperature (HT) phase disproportionates by a f irst-order transition into two distinct phases: P4(2)/mnm (T-c, T-N = 44 K) --> Cmmm (majority LT phase) + Pnnm (minority IT Phase) which coexist in p roportions varying with temperature down to 4 K, The phase diagram comprise s three temperature regions: (a) the HT range with T > T-N for the tetragon al P4(2)/mnm phase; (b) the IT (intermediate temperature) range, 20 K < T < T-N, where the two phases coexist in strongly variable proportions and the Pnnm phase reaches its highest concentration( approximate to 31%) around 3 0 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 o f competing magneto-elastic mechanisms involving the Er crystal field aniso tropy, the Er-Er, Er-Fe and the Fe-Fe exchange interactions and their coupl ing with the lattice strains. (C) 2000 Elsevier Science B.V. All rights res erved.