Symmetry-breaking transitions in HoCuAs2-xPx and ErCuAs2-xPx (x=0-2): Crystal structure, application of Landau theory, magnetic and electrical properties

摘要

Crystal structures of compounds undergoing symmetry-breaking transitions have been investigated by the Xray single crystal and powder methods. While the phases HoCuAs2 through HoCuAs1.33P0.67 and ErCuAs2 through ErCuAsP retain the tetragonal HfCuSi2 structure (P4mm space group), the compounds HoCuAsP through HoCuAs0.33P1.67 and ErCuAs0.67P1.33 through ErCuAs0.33P1.67 undergo orthorhombic distortions to the GdCuAs1.15P0.85 structure (Pmmn space group). Further distortions follow in the phosphides: HoCuP2 adopts a larger orthorhombic cell (Cmma, a = 5.273(3), b = 5.305(3). c = 9.645(5) Angstrom) and ErCuP2 has a monoclinic cell with a doubled parameter, the b parameter in the monoclinic cell (P2(1), a = 3.737(3). b = 19.239(15), c = 3.728(3) Angstrom, beta = 90.09(1)degrees). Zigzag chains are formed in the phosphorus laver of ErCuP2. According to Landau theory the transitions from HoCuAs1.33P0.67 to HoCuAsP and from ErCuAsP to ErCuAs0.67P1.33 can be continuous, and the transitions from HoCuAs0.33P1.67 to HoCuP2 and from ErCuAs0.33P1.67 to ErCuP2 are necessarily first-order. The results of magnetic and electrical measurements for HoCuAs2 and HoCuP2 are reported. Due to the magnetic moments localized on Ho atoms both compounds order antiferromagnetically at low temperatures. They exhibit metallic conductivity. [References: 18]