Magnetic ground state of La2LiMoO6: A comparison with other Mo5+ (S=1/2) double perovskites

La2LiMoO6 is a double perovskite (DP) with P21/n symmetry based on the Mo5+ ion, 4d1, t2g1, S=1/2. It is isostructural with Sr2YMoO6, the magnetic ground state of which is apparently a very unusual collective spin singlet or valence-bond glass state as is the case for cubic (Fm-3m) Ba2YMoO6. Initial studies of La2LiMoO6 suggested a different ground state from the other DPs but no clear conclusions could be drawn. A more detailed study is presented here including magnetic susceptibility, heat capacity, and elastic neutron-scattering results. This DP is now well characterized as an antiferromagnet, TN=18K, via observation of magnetic Bragg peaks in neutron scattering and an anomaly in the magnetic heat capacity. The ordering wave vector is k=(1/21/20), consistent with a type I face-centered-cubic magnetic structure, and the ordered moment on Mo5+ is 0.32(11)μB, much reduced from the spin-only value of 1μB. The index, f=|ϑc|/TN∼3, indicates a low level of frustration. The heat-capacity data above TN can be interpreted in terms of a one-dimensional spin-correlation model, as can the low-temperature data which follow a T1 power law. This is consistent with an earlier suggestion. The difference with isostructural Sr2YMoO6 is attributed to differences in the local distortion of the Mo–O octahedron and the resulting orbital ordering.