Subtle lattice distortion-driven phase transitions in layered ACu₄As₂ (A = Eu, Sr)

The compounds composed of transition metal cations and pnictide anions provide a rich platform for studying novel physical phenomena. Here we report on the observation of a phase transition at ∼ 70 K and 145 K in layered compound EuCu₄As₂ and SrCu₄As₂, respectively. from both the transport and heat capacity. The thermal expansion measurements show that the variation of the lattice parameters (ΔL_b/L_ab) around T_P is much less than that for a typical crystalline phase transition. Our experimental results reveal that the transition in EuCu₄As₂ and SrCu₄As₂ should be driven by subtle structural-distortion.