  The B-site ordered double perovskite Ba2CaOsO6 was studied by d.c. magnetic
susceptibility, powder neutron diffraction and muon spin relaxation methods.
The lattice parameter is a = 8.3619(6) A at 280K and cubic symmetry (Fm-3m) is
retained to 3.5K with a = 8.3426(5) A. Curie-Weiss susceptibility behavior is
observed for T > 100K and the derived constants are C = 0.3361(1)emu-K/mole and
T_CW = -156.2(3) K, in excellent agreement with literature values. This Curie
constant is much smaller than the spin-only value of 1.00 emu-K/mole for a 5d^2
Os6+ configuration, indicating a major influence of spin-orbit coupling.
Previous studies had detected both susceptibility and heat capacity anomalies
near 50K but no definitive conclusion was drawn concerning the nature of the
ground state. While no ordered Os moment could be detected by powder neutron
diffraction, muon spin relaxation (muSR) data show clear long-lived
oscillations indicative of long-range magnetic order below T_C = 50K. An
estimate of the ordered moment on Os6+ is ~0.2 mu_B, based upon a comparison
with muSR data for Ba2YRuO6 with a known ordered moment of 2.2 mu_B. These
results 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. In
contrast, Ba2YReO6 shows T_CW = -616 K, and a complex spin-disordered and,
ultimately, spin-frozen ground state below 50 K, indicating a much higher level
of geometric frustration than in Ba2CaOsO6. A comparison is made to recent
theory on d^2 double perovskites.
