Crystals of three potassium-cadmium molybdates found previously in the K2MoO4–CdMoO4 system are grown and their structures are determined. The structure of new type rhombohedral K3.90Cd1.05(MoO4)3 contains an open 3D framework of CdO6 octahedra and bridging MoO4 tetrahedra, and their voids accommodate potassium atoms with CN 6-9. The monoclinic β-form of double molybdate K4–2xCd1+x(MoO4)3 (0.26 ≤ x ≤ 0.38) is isostructural to alluaudite, while the unit cell of its triclinic (low-temperature) α-form is three times larger in volume and is related to the β-form cell as aα = (aβ + bβ)/2, bα = (aβ – bβ)/2 – cβ, cα = (bβ – aβ)/2 – 2cβ. Both structures consist of layers of edge-sharing (Cd, K)O6 octahedra linked by MoO4 tetrahedra into sublayer frameworks whose voids contain disordered potassium ions with CN 6 and 8. High-energy migration barriers of K+ and Cd2+ ions theoretically estimated by the bond valence method in the studied structures exclude the noticeable cationic conductivity in these compounds.
Solodovnikov et al. (Wed,) studied this question.