Synthesis, Structure, and Conductivity of Alluaudite‐Related Phases in the Na 2 MoO 4 –Cs 2 MoO 4 –CoMoO 4 System

Phase formation study of the Na 2 MoO 4 –Cs 2 MoO 4 –CoMoO 4 system resulted in new cesium‐containing alluaudite‐related phases. The solid solution Na 4–2 x ‐ y Cs y Co 1+ x (MoO 4 ) 3 (0 ≤ x, y ≤ 0.30), based on the alluaudite‐type Na 4–2 x Co 1+ x (MoO 4 ) 3 , and triple molybdate Na 10 (Cs 4‐ x Na x )Co 5 (MoO 4 ) 12 (0 ≤ x ≤ 0.30) were found, and their structures were solved. In the structure of Na 3.21 Cs 0.37 Co 1.21 (MoO 4 ) 3 ( a = 13.0917(8) Å, b = 13.5443(8) Å, c = 7.1217(4) Å, space group C 2/ c , β = 112.331(2), Z = 4), the cesium ions partially substitute the Na + in the channels running along the c ‐axis. The structure of Na 10 (Cs 3.77 Na 0.23 )Co 5 (MoO 4 ) 12 ( a = 13.6572(3) Å, b = 11.5063(3) Å, c = 27.9898(5) Å, space group Pbca , Z = 4) was proved to be the aristotype for the pseudo orthorhombic Na 25 Cs 8 R 5 (MoO 4 ) 24 ( R = Fe, Sc, In). The compounds contain alluaudite‐like layers of MoO 4 tetrahedra and pairs of edge‐shared (Co, Na)O 6 or ( R , Na)O 6 and NaO 6 octahedra, which are connected by bridging MoO 4 tetrahedra to form 3D frameworks differing from the alluaudite type. The frameworks contain channels along the c ‐axis filled by Cs + and Na + ions. Bond valence sum (BVS) maps show that the alluaudite‐related molybdates can have a 2D sodium‐ion conductivity at elevated temperatures in contrast to the alluaudite‐type cathode material Na 2+2 x Fe 2‐ x (SO 4 ) 3 with a 1D conductivity. The measured ionic conductivity of Na 4–2 x Co 1+ x (MoO 4 ) 3 , Na 4–2 x ‐ y Cs y Co 1+ x (MoO 4 ) 3 , and Na 10 Cs 4 Co 5 (MoO 4 ) 12 reaches 10 –3 –10 –2 S cm –1 at 500 °C.