Five double perovskites, each containing a transition-metal ion with a 5d1 configuration, have been studied to better understand the surprising diversity of magnetic ground states seen in these isoelectronic compounds. Ba2ZnReO6 adopts the cubic double perovskite structure and magnetically orders below 16 K, with a canted ferromagnetic structure and a saturated magnetization of ∼0.24 μB/Re. X-ray magnetic circular dichroism indicates a substantial orbital moment of approximately 0.4 μB/Re that opposes the spin moment. The structures of Ba2NaOsO6 (canted ferromagnet, T C = 7 K) and Ba2LiOsO6 (antiferromagnet, T N = 8 K) are reinvestigated using time-of-flight neutron powder diffraction and found to crystallize with the cubic double perovskite structure. No evidence for a structural distortion can be found in either compound down to 10 K. Ba2CdReO6 is also cubic at room temperature but undergoes a structural transition upon cooling below ∼180 K to a tetragonal structure with I4/m symmetry that involves compression of the Re–O bonds that are parallel to the c-axis. Sr2LiOsO6 shows a similar tetragonal distortion at room temperature and maintains that structure down to 10 K. Surprisingly, the Os-centered octahedron in Sr2LiOsO6 is distorted in the opposite direction, exhibiting an elongation of the Os–O bonds along the c-axis. Differences in the distortions of the octahedra lead to different magnetic ground states, antiferromagnetic (T N = 4 K) for Ba2CdReO6 and spin glass (T g = 30 K) for Sr2LiOsO6. Theoretical modeling shows that the varied magnetic behaviors of double perovskites containing 5d1 ions are closely tied to crystallographic distortions. These distortions remove the degeneracy of the 5d t2g orbitals, leading to changes in orbital occupation that ultimately determine which of the several competing magnetic ground states is favored.

Precipitation of Cs2InBr5·H2O from HBr(aq) solutions containing Pb2+ ions results in powders that exhibit narrowband green photoluminescence (λmax = 521 nm, fwhm = 89.9 meV). Synchrotron powder X-ray diffraction reveals trace amounts of Cs4PbBr6 and CsPbBr3 that cannot be detected by laboratory powder diffraction measurements. Broadening of the CsPbBr3 diffraction peaks suggests crystals that are tens of nanometers in size. Evidence for the presence of CsPbBr3 can also be seen in diffuse reflectance spectra. Cathodoluminescence imaging shows that luminescence originates from small nanometer-sized regions. Taken together, these observations point to CsPbBr3 nanocrystal inclusions as the source of photoluminescence. Heating these samples to temperatures at or above 80 °C triggers a reversible dehydration process that leads to an irreversible change in the photoluminescence from green to blue (λmax ≈ 480 nm, fwhm = 278 meV), accompanied by significant changes in the microstructure. Cathodoluminescence imaging indicates that the blue emission occurs over much larger micron-sized regions of the sample. The position of blue PL is similar to other hybrid lead bromide compounds where the emission has been assigned to 3P1 → 1S0 transitions on [PbBr4]2– ions. Based on the emission wavelength and cathodoluminescence imaging, the blue emission is assigned to isolated [PbBr4]2– ions that substitute for [InBr5·H2O]2– ions in the parent hydrate phase. This work provides new insight on the spontaneous formation of halide perovskite nanocrystals in an inert matrix, one that does not rely on the use of organic solvents and is stable in ambient atmospheres.

Two novel ternary compounds from the pseudobinary CH3NH3X–AgX (X = Br, I) phase diagrams are reported. CH3NH3AgBr2 and CH3NH3Ag2I3 were synthesized via solid state sealed tube reactions and the crystal...