  We report on the structural, magnetic, and electronic properties of two new
double-perovskites synthesized under high pressure; Pb2CaOsO6 and Pb2ZnOsO6.
Upon cooling below 80 K, Pb2CaOsO6 simultaneously undergoes a metal--insulator
transition and develops antiferromagnetic order. Pb2ZnOsO6, on the other hand,
remains a paramagnetic metal down to 2 K. The key difference between the two
compounds lies in their crystal structure. The Os atoms in Pb2ZnOsO6 are
arranged on an approximately face-centred cubic lattice with strong
antiferromagnetic nearest-neighbor exchange couplings. The geometrical
frustration inherent to this lattice prevents magnetic order from forming down
to the lowest temperatures. In contrast, the unit cell of Pb2CaOsO6 is heavily
distorted up to at least 500 K, including antiferroelectric-like displacements
of the Pb and O atoms despite metallic conductivity above 80 K. This distortion
relieves the magnetic frustration, facilitating magnetic order which in turn
drives the metal--insulator transition. Our results suggest that the phase
transition in Pb2CaOsO6 is spin-driven, and could be a rare example of a Slater
transition.
