Mechanochemical Synthesis, Structure, and Ionic Conductivity of K₂NaYCl₆ and KNa₂YCl₆

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K2NaYCl6 and KNa2YCl6 were prepared by a mechanochemical synthesis route from NaCl, KCl, and YCl3 as starting materials. The crystal structures and properties were characterized by X-ray powder diffraction, thermogravimetric and differential thermal analyses, energy-dispersive X-ray spectroscopy, and electrochemical impedance spectroscopy. At room temperature, both samples are isostructural to P21/c-Ba2LaRuO6. At high temperature (T < 500 °C), only K2NaYCl6 exhibits a phase transition from the room-temperature form, which crystallizes in the monoclinic system, space group P21/c (No. 14) with a = 7.2447(4) Å, b = 7.5518(4) Å, c = 12.6454(8) Å, β = 124.557(3)°, V = 569.76(6) Å3, and Z = 2 to the high-temperature form, which crystallizes in the cubic system, space group Fm3̅m (No. 225) with a = 10.6903(1) Å, V = 1221.72(1) Å3, and Z = 4. The HT-K2NaYCl6 (cF40) polymorph is isostructural to K2NaAlF6. The structures are monoclinic and cubic double perovskites A2BB′X6 in which the octahedrally coordinated Na and Y atoms are well-ordered at the B and B′ sites, respectively. The A site is occupied by K and K/Na atoms in K2NaYCl6 and KNa2YCl6, respectively. The P21/c ⇄ Fm3̅m phase transition mechanism was accurately determined and is illustrated in a video. K2NaYCl6 and KNa2YCl6 exhibit ionic conductivities of 8 × 10–8 and 4.9 × 10–9 S cm–1 at 75 °C, respectively.