Synthesis and Characterization of the Rare-Earth Hybrid Double Perovskites: (CH3NH3)(2)KGdCl6 and (CH3NH3)(2)KYCl6

Two hybrid rare-earth double perovskites, (CH3NH3)(2)KGdCl6 and (CH3NH3)(2)KYCl6, have been synthesized by a solution evaporation method and their structures determined by variable temperature single-crystal X-ray diffraction. The diffraction results show that at room temperature both perovskites adopt a rhombohedral structure with R (3) over barm symmetry, as found previously for (MA)(2)KBiCl6, and lattice parameters of a = 7.7704(5) angstrom and c = 20.945(2) angstrom for (MA)(2)KGdCl6 and angstrom = 7.6212(12) angstrom and c = 20.742(4) angstrom for (MA)(2)KYCl6. Both phases exhibit a rhombohedral-to-cubic phase transition on heating to similar to 435 K for (MA)(2)KYCl6 and similar to 375 K for (MA)(2)KGdCl6. Density functional calculations on the rhombohedral phase indicate that both materials have large direct band gaps, are mechanically stable, and, in the case of (MA)(2)KGdCl6, could exhibit magnetic ordering at low temperatures.