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

Two hybrid rare-earth double perovskites, (CH3NH3)2KGdCl6 and (CH3NH3)2KYCl6, 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 R3̅m symmetry, as found previously for (MA)2KBiCl6, and lattice parameters of a = 7.7704(5) Å and c = 20.945(2) Å for (MA)2KGdCl6 and a = 7.6212(12) Å and c = 20.742(4) Å for (MA)2KYCl6. Both phases exhibit a rhombohedral-to-cubic phase transition on heating to ∼435 K for (MA)2KYCl6 and ∼375 K for (MA)2KGdCl6. 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)2KGdCl6, could exhibit magnetic ordering at low temperatures.