Symmetry-Breaking Transitions from GdCuAs2 through GdCuAs1.15P0.85 to GdCuP2.20: Crystal Structure, Application of Landau Theory, Bonding, Magnetic and Electrical Properties

The crystal structures of GdCuAs2, GdCuAs1.15P0.85, and GdCuP2.20 have been investigated by the single crystal and powder methods. While GdCuAs2 (P4/nmm) retains the tetragonal HfCuSi2 structure (M. Brylak, M. H. M?ller, and W. Jeitschko, J. Solid State Chem. 115, 305 (1995)), GdCuAs1.15P0.85 (Pmmn, a=3.849(1), b=3.8501(1), c=9.871(3) ?) and GdCuP2.20 (Pmm2, a=5.3747(9), b=5.3830(9), c=9.7376(16) ?) undergo orthorhombic distortions. The changes are significant in GdCuP2.20: P dimers are formed in the P layer along the a direction and there is an additional (but deficient) P site on one side of the layer that links the dimers. The GdCuAs1.15P0.85 structure was predicted by the Landau theory. According to this theory the transition from GdCuAs2 to GdCuAs1.15P0.85 can be a continous one, the transition from GdCuAs1.15P0.85 to GdCuP2.20 is a first-order one. The transition to GdCuP2.20 occurs with twin formation. The electronic structure and bonding are analyzed by the extended Hückel tight-binding method. The conductivity and magnetic measurements for the arsenide and phosphide are reported.