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.