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

La2LiMoO6 is a double perovskite (DP) with P2(1)/n symmetry based on the Mo5+ ion, 4d(1), t(2g)(1), 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, T-N = 18 K, via observation of magnetic Bragg peaks in neutron scattering and an anomaly in the magnetic heat capacity. The ordering wave vector is k = (1/2 1/2 0), consistent with a type I face-centered-cubic magnetic structure, and the ordered moment on Mo5+ is 0.32(11) mu(B), much reduced from the spin-only value of 1 mu(B). The index, f = vertical bar theta vertical bar/T-N similar to 3, indicates a low level of frustration. The heat-capacity data above T-N can be interpreted in terms of a one-dimensional spin-correlation model, as can the low-temperature data which follow a T-1 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.