  The magnetic ground state in the double perovskite system Sr2-xCaxCoOsO6
changes from an antiferromagnet (x = 0), to a spin glass (x = 1), to a
ferrimagnet (x = 2) as the Ca content increases. This crossover is driven by
chemical pressure effects that control the relative strength of magnetic
exchange interactions. The synthesis, crystal structure, and magnetism of
SrCaCoOsO6 and Ca2CoOsO6 are investigated and compared with Sr2CoOsO6. Both
compounds adopt a monoclinic crystal structure with rock salt ordering of Co2+
and Os6+ and a-a-b+ octahedral tilting, but the average Co-O-Os bond angle
evolves from 158.0(3) degrees in SrCaCoOsO6 to 150.54(9) degrees in Ca2CoOsO6
as the smaller Ca2+ ion replaces Sr2+. While this change may seem minor it has
a profound effect on the magnetism, changing the magnetic ground state from
antiferromagnetic in Sr2CoOsO6 (TN1 = 108 K, TN2 = 70 K), to a spin glass in
SrCaCoOsO6 (Tf1 = 32 K, Tf2 = 13 K), to ferrimagnetic in Ca2CoOsO6 (TC = 145
K). In the first two compounds the observation of two transitions is consistent
with weak coupling between the Co and Os sublattices.
