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 similar to 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 (Delta L-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.