Triple molybdates K 3– x Na 1+ x M 4 (MoO 4 ) 6 ( M  = Ni, Mg, Co) and K 3+ x Li 1– x Mg 4 (MoO 4 ) 6 isotypic with II-Na 3 Fe 2 (AsO 4 ) 3 and yurmarinite: synthesis, potassium disorder, crystal chemistry and ionic conductivity

• Published in: Acta crystallographica Section B, Structural science, crystal engineering and materials Structural science, crystal engineering and materials, 2020-10, Vol.76 (5), p.913-925
• Main Authors: Gulyaeva, Oksana A., Solodovnikova, Zoya A., Solodovnikov, Sergey F., Zolotova, Evgeniya S., Mateyshina, Yuliya G., Uvarov, Nikolai F.
• Format: Article
• Language: English
• ISSN: 2052-5206; 2052-5206
• DOI: 10.1107/S2052520620010677

The triple molybdates K 3– x Na 1+ x M 4 (MoO 4 ) 6 ( M  = Ni, Mg, Co) and K 3+ x Li 1– x Mg 4 (MoO 4 ) 6 were found upon studying the corresponding ternary molybdate systems, and their structures, thermal stability and electrical conductiviplusmnty were investigated. The compounds crystallize in the space group R 3 c and are isostructural with the sodium-ion conductor II-Na 3 Fe 2 (AsO 4 ) 3 and yurmarinite, Na 7 (Fe 3+ , Mg, Cu) 4 (AsO 4 ) 6 ; their basic structural units are flat polyhedral clusters of the central M 1O 6 octahedron sharing edges with three surrounding M 2O 6 octahedra, which combine with single NaO 6 octahedra and bridging MoO 4 tetrahedra to form open three-dimensional (3D) frameworks where the cavities are partially occupied by disordered potassium (sodium) ions. The split alkali-ion positions in K 3– x Na 1+ x M 4 (MoO 4 ) 6 ( M  = Ni, Mg, Co) give their structural formulae as [(K,Na) 0.5 □ 0.5 )] 6 (Na)[ M 1][ M 2] 3 (MoO 4 ) 6 , whereas the lithium-containing compound (K 0.5 □ 0.5 ) 6 (Mg 0.89 K 0.11 )(Li 0.89 Mg 0.11 )Mg 3 (MoO 4 ) 6 shows an unexpected (Mg, K) isomorphism, which is similar to (Mn, K) and (Co, K) substitutions in isostructural K 3+ x Li 1– x M 4 (MoO 4 ) 6 ( M  = Mn, Co). The crystal chemistry of the title compounds and related arsenates, phosphates and molybdates was considered, and the connections of the cationic distributions with potential 3D ionic conductivity were shown by means of calculating the bond valence sum (BVS) maps for the Na + , Li + and K + ions. Electrical conductivity measurements gave relatively low values for the triple molybdates [σ = 4.8 × 10 −6  S cm −1 at 390°C for K 3 NaCo 4 (MoO 4 ) 6 and 5 × 10 −7  S cm −1 at 400°C for K 3 LiMg 4 (MoO 4 ) 6 ] compared with II-Na 3 Fe 2 (AsO 4 ) 3 (σ = 8.3 × 10 −4  S cm −1 at 300°C). This may be explained by a low concentration of sodium or lithium ions and the blocking of their transport by large potassium ions.