SYNTHESIS, STRUCTURE, AND MAGNETIC-PROPERTIES OF A NOVEL MIXED-VALENCE COPPER(I II) PHOSPHATE, CU2PO4/

Via phase compatibility studies, a novel mixed-valence copper(I/II) ph osphate, Cu2PO4, has been isolated from a direct reaction of (Cu2O)-O- I, (CuO)-O-II, and P2O5 in fused silica. The single-crystal X-ray diff raction shows that the title compound crystallizes in a triclinic (P ( 1) over bar) unit cell, with lattice dimensions a = 6.145(2) Angstrom, b = 9.348(2) Angstrom, c = 6.009(1) Angstrom, alpha = 96.46(2)degrees , beta = 100.16(2)degrees, gamma = 73.97(2)degrees, V = 325.8(1) Angst rom(3): Z = 4. The structure has been refined by the least-squares met hod to R = 0.019, R(w) = 0.030, and GOF = 1.43 for 128 variables. The four copper atoms in each asymmetric unit adopt three distorted coordi nation geometries that are consistent with the corresponding electroni c states, e.g., square pyramidal Cu(1)O-II(5), octahedral Cu(2)O-II(6) , and linear Cu(3,4)O-I(2). A low-dimensional framework exists consist ing of arrays of nearly parallel CuO2 units which are separated by the nonmagnetic, closed-shell P5+ cation in PO4 tetrahedra. Closely space d CuO2 chains and a relatively short Cu-I-Cu-I distance, e.g., 2.737 A ngstrom for Cu(3)-Cu(3), are attributed to the bond strength of the cr oss-linked PO4 tetrahedra. In the extended Cu(I/II)-O framework, short linkages of Cu-I-O-Cu-II-O-Cu-I and Cu-II-O-Cu-II, composed of regula r Cu-O bonds (1.86-1.99 Angstrom), are interconnected through long Cu- II-O bonds (2.36-2.74 Angstrom). The magnetic measurements indicate th at the Cu-O framework exhibits a spin 1/2 ground state and an antiferr omagnetic ordering with a broad susceptibility maximum between 95 and 105 K. The results of stoichiometric synthesis, thermal analysis, and bond valence sum calculations of the title compound are also discussed .