Structural complexity and magnetic phase transformations: 4f electron-interplay*

Compounds that respond to varying magnetic field (H), pressure (p), temperature (T), or composition (x) by exhibiting structural or/and magnetic phase transformations are an intriguing subject matter for materials science, chemistry, and physics communities. Here we show that unlike the parent binaries – DyCo₂ and ErCo₂ – which exhibit lone distortions at their respective Curie temperatures, T_C, a mixed pseudobinary Er_0.75Dy_0.25Co₂ demonstrates two successive magnetostructural transitions: a first order at T_C, followed by a second order below T_C. Surprisingly, the ground state phase adopts Imma symmetry instead of the commonly observed Fddd. We show that the unusual sequence of structurally and magnetically distinct phases in Er_1-xDy_xCo₂ can be judiciously controlled via 4f electrons, where Er and Dy – two next near neighbor lanthanides – each play a distinct role in defining phase stability (PRB 96, (2017) 064412).