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The B-site ordered double perovskite Ba2CaOsO6 was studied by d.c. magneticsusceptibility, powder neutron diffraction and muon spin relaxation methods.The lattice parameter is a = 8.3619(6) A at 280K and cubic symmetry (Fm-3m) isretained to 3.5K with a = 8.3426(5) A. Curie-Weiss susceptibility behavior isobserved for T > 100K and the derived constants are C = 0.3361(1)emu-K/mole andT_CW = -156.2(3) K, in excellent agreement with literature values. This Curieconstant is much smaller than the spin-only value of 1.00 emu-K/mole for a 5d^2Os6+ configuration, indicating a major influence of spin-orbit coupling.Previous studies had detected both susceptibility and heat capacity anomaliesnear 50K but no definitive conclusion was drawn concerning the nature of theground state. While no ordered Os moment could be detected by powder neutrondiffraction, muon spin relaxation (muSR) data show clear long-livedoscillations indicative of long-range magnetic order below T_C = 50K. Anestimate of the ordered moment on Os6+ is ~0.2 mu_B, based upon a comparisonwith muSR data for Ba2YRuO6 with a known ordered moment of 2.2 mu_B. Theseresults are compared with those for isostructural Ba2YReO6 which contains Re5+,also 5d^2, and has a nearly identical unit cell constant, a = 8.3628(2) A. Incontrast, Ba2YReO6 shows T_CW = -616 K, and a complex spin-disordered and,ultimately, spin-frozen ground state below 50 K, indicating a much higher levelof geometric frustration than in Ba2CaOsO6. A comparison is made to recenttheory on d^2 double perovskites.

Long-Range Magnetic Order in the 5d^2 Double Perovskite Ba2CaOsO6: Comparison with Spin-Disordered Ba2YReO6