The double perovskites Cs 2 AgBiBr 6 and Cs 2 AgBiCl 6 have been synthesized from both solid state and solution routes. X-ray diffraction measurements show that both compounds adopt the cubic double perovskite structure, space group Fm3̅ m, with lattice parameters of 11.2711(1) Å (X = Br) and 10.7774(2) Å (X = Cl). Diffuse reflectance measurements reveal band gaps of 2.19 eV (X = Br) and 2.77 eV (X = Cl) that are slightly smaller than the band gaps of the analogous lead halide perovskites, 2.26 eV for CH 3 NH 3 PbBr 3 and 3.00 eV for CH 3 NH 3 PbCl 3 . Band structure calculations indicate that the interaction between the Ag 4d-orbitals and the 3p/4porbitals of the halide ion modifies the valence band leading to an indirect band gap. Both compounds are stable when exposed to air, but Cs 2 AgBiBr 6 degrades over a period of weeks when exposed to both ambient air and light. These results show that halide double perovskite semiconductors are potentially an environmentally friendly alternative to the lead halide perovskite semiconductors.

The insulating, fully ordered, double perovskite Sr2CoOsO6 undergoes two magnetic phase transitions. The Os(VI) ions order antiferromagnetically with a propagation vector k = (1/2, 1/2, 0) below TN1 = 108 K, while the high-spin Co(II) ions order antiferromagnetically with a propagation vector k = (1/2, 0, 1/2) below TN2 = 70 K. Ordering of the Os(VI) spins is accompanied by a structural distortion from tetragonal I4/m symmetry to monoclinic I2/m symmetry, which reduces the frustration of the face centered cubic lattice of Os(VI) ions. Density functional theory calculations show that the long-range Os-O-Co-O-Os and Co-O-Os-O-Co superexchange interactions are considerably stronger than the shorter Os-O-Co interactions. The poor energetic overlap between the 3d orbitals of Co and the 5d orbitals of Os appears to be responsible for this unusual inversion in the strength of short and long-range superexchange interactions.

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Measurements of the specific heat of Dy3Al5O12 at temperatures between 1.2° and 4°K are described. A sharp λ anomaly was found at 2.49°K, and this is discussed in terms of an unusual type of antiferromagnetic ordering previously deduced from magnetic susceptibility and resonance experiments. Two different estimates are made of the mean interaction energy, and these show that about 70% of the total interaction is due to magnetic dipole-dipole coupling. An estimate of the entropy is in excellent agreement with the assumption that the Dy3+ ions may be described in terms of only one Kramers' doublet S′=½.