Synthesis and Characterization of Sr₂Cu(W_1–xMo_x)O₆: A Quasi-Two-Dimensional Magnetic System

Here, we report the synthesis of the double-perovskite system Sr 2 Cu(W 1– x Mo x )O 6, 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)O 4, 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 3 d hole on the Cu II into d x 2 –y 2 orbitals below a T JT ≈ 920 °C in Sr 2 CuWO 6 is lowered to a T JT ≈ 500 °C in Sr 2 CuMoO 6 . The samples were all insulators, but the band gap was found to decrease with increasing Mo content due to the weaker Mo 4 d –O 2 p hybridization compared to that of W 5 d –O 2 p . Ordering of the Cu II holes into d x 2 –y 2 orbitals creates magnetic interactions between spins S = 1 / 2 with antiferromagnetic 180° Cu–O–(W,Mo)–O–Cu parameters (− J 2 ) and ferromagnetic 90° Cu–O–(W,Mo)–O–Cu parameters (+ J 1 ) in the (001) planes. Magnetic susceptibility showed no long-range magnetic order in any sample down to 5 K.