Synthesis and Characterization of Sr₂Cu(W_1–<i>x</i>Mo_<i>x</i>)O₆: A Quasi-Two-Dimensional Magnetic System

Abstract

Here, we report the synthesis of the double-perovskite system Sr2Cu(W1–xMox)O6, with x = 0.0–1.0, where the Mo-containing members have not been synthesized before. This system provides an example of a quasi-two-dimensional S = 1/2 square lattice, with the possibility of magnetic frustration. The compounds were synthesized under ambient pressure for 0 ≤ x ≤ 0.6 and under high pressure (HP) for 0.7 ≤ x ≤ 1.0. A small percentage of impurities, primarily Sr(W,Mo)O4, was present in all samples and increased for x ≥ 0.5. Tetragonal distortion of the compounds decreases with increasing x, and correspondingly, a first-order tetragonal-cubic cooperative Jahn–Teller ordering of the 3d hole on the CuII into dx2–y2 orbitals below a TJT ≈ 920 °C in Sr2CuWO6 is lowered to a TJT ≈ 500 °C in Sr2CuMoO6. The samples were all insulators, but the band gap was found to decrease with increasing Mo content due to the weaker Mo 4d–O 2p hybridization compared to that of W 5d–O 2p. Ordering of the CuII holes into dx2–y2 orbitals creates magnetic interactions between spins S = 1/2 with antiferromagnetic 180° Cu–O–(W,Mo)–O–Cu parameters (−J2) and ferromagnetic 90° Cu–O–(W,Mo)–O–Cu parameters (+J1) in the (001) planes. Magnetic susceptibility showed no long-range magnetic order in any sample down to 5 K.