Subsolidus phase relations and structures of solid solutions in the systems K2MoO4–Na2MoO4–MMoO4 (M = Mn, Zn)

Abstract

Study of phase formation in the system K2MoO4–Na2MoO4 at 500 °C showed the existence of the only double molybdate K2-xNaxMoO4 (0.4 ≤ x ≤ 1) of the glaserite-related type. Using these data, the subsolidus phase relations in the systems K2MoO4–Na2MoO4–MMoO4 (M = Mn, Zn) were studied at 470–560 °C and the extended solid solutions based on the alluaudite-type Na4−2xM1+x(MoO4)3 were found. In the structure of K1.71Na1.71Mn1.29(MoO4)3 (sp. gr. C2/c, Z = 4, a = 13.0595(3) Å, b = 13.7892(3) Å, c = 7.2973(2) Å, β = 111.339(1)°, R = 0.0134), the potassium ions partially replace Na+ in the channels running along c-axis. The structure of K1.84Na2.11Zn1.02(MoO4)3 (sp. gr. P1̅, Z = 2, a = 7.3258(8) Å, b = 9.4128(10) Å, c = 9.4302(10) Å, α = 93.104(3)°, β = 104.573(3)°, γ = 104.432(3)°, R = 0.0220) is isotypic with alluaudite-related α-K4Zn(WO4)3 and seems to be a result of stabilizing an unknown triclinic modification of K4Zn(MoO4)3 by substitution of sodium for potassium with the strong differentiation of alkali and zinc cations in the structure. In the structure of K1.5Na0.5Mn2(MoO4)3 (sp. gr. Pcca, Z = 8, a = 10.6160(7) Å, b = 12.2714(8) Å, c = 18.0275(11) Å, R = 0.0155) of the α-K2Mn2(MoO4)3 type, one of three initial potassium positions is jointly occupied by (Na, Mn); a similar phenomenon occurs in isostructural Rb3Na3Sc2(MoO4)6. The structure contains zigzag-like ribbons of edge-sharing MnO6 octahedra linked with separate MnO6 octahedra and МоО4 tetrahedra into 3D containing channels along the c-axes filled by K+ ions. BVS maps show possible one-dimensional sodium-ion conductivity for the structures of K1.71Na1.71Mn1.29(MoO4)3 and Rb3Na3Sc2(MoO4)6.