Synthesis, Structure, and Magnetic Properties of a Novel Mixed-Valence Copper(I/II) Phosphate, Cu2PO4

Abstract

Via phase compatibility studies, a novel mixed-valence copper(I/II) phosphate, Cu{sub 2}PO{sub 4}, has been isolated from a direct reaction of Cu{sub 2}{sup I}O, Cu{sup II}O, and P{sub 2}O{sub 5} in fused silica. The single-crystal X-ray diffraction shows that the title compound crystallizes in a triclinic (P1) 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) {angstrom}{sup 3}; Z =4. The structure has been refined by the least-squares method to R = 0.019, R{sub w} = 0.030, and GOF = 1.43 for 128 variables. The four copper atoms in each asymmetric unit adopt three distorted coordination geometries that are consistent with the corresponding electronic states, e.g., square pyramidal Cu(1){sup II}O{sub 5}, octahedral Cu(2){sup II}O{sub 6}, and linear Cu(3,4){sup I}O{sub 2}. A low-dimensional framework exists consisting of arrays of nearly parallel CuO{sub 2} units which are separated by the nonmagnetic, closed-shell P{sup 5+} cation in PO{sub 4} tetrahedra. Closely spaced CuO{sub 2} chains and a relatively short Cu{sup I}-Cu{sup I} distance, e.g., 2.737 {angstrom} for Cu(3)-Cu(3), are attributed to the bond strength of the cross-linked PO{sub 4} tetrahedra. In the extended Cu(I/II)-Omore » framework, short linkages of Cu{sup I}-O-Cu{sup II}-O-Cu{sup I} and Cu{sup II}-O-Cu{sup II}, composed of regular Cu-O bonds (1.86-1.99 {angstrom}), are interconnected through long Cu{sup II}-O bonds (2.36-2.74 {angstrom}). The magnetic measurements indicate that the Cu-O framework exhibits a spin 1/2 ground state and an antiferromagnetic 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.« less