Phase formation in the systems Ag 2MoO 4– MO–MoO 3 ( M=Ca, Sr, Ba, Pb, Cd, Ni, Co, Mn) and crystal structures of Ag 2M2(MoO 4) 3 ( M=Co, Mn)

Abstract

Phase equilibria in the systems Ag 2MoO 4– MMoO 4 ( M=Ca, Sr, Ba, Pb, Ni, Co, Mn) and subsolidus phase relations in the systems Ag 2MoO 4– MO–MoO 3 ( M=Ca, Pb, Cd, Mn, Co, Ni) were investigated using XRD and thermal analysis. The systems Ag 2MoO 4– MMoO 4 ( M=Ca, Sr, Ba, Pb, Ni) belong to the simple eutectic type whereas in the systems Ag 2MoO 4– MMoO 4 ( M=Co, Mn) incongruently melting Ag 2 M 2(MoO 4) 3 ( M=Co, Mn) were formed. In the ternary oxide systems studied no other compounds were found. Low-temperature LT-Ag 2Mn 2(MoO 4) 3 reversibly converts into the high-temperature form of a similar structure at 450–500°C. The single crystals of Ag 2Co 2(MoO 4) 3 and LT-Ag 2Mn 2(MoO 4) 3 were grown and their structures determined (space group P 1 ̄ , Z=2; lattice parameters are a=6.989(1) Å, b=8.738(2) Å, c=10.295(2) Å, α=107.67(2)°, β=105.28(2)°, γ=103.87(2)° and a=7.093(1) Å, b=8.878(2) Å, c=10.415(2) Å, α=106.86(2)°, β=105.84(2)°, γ=103.77(2)°, respectively) and refined to R( F)=0.0313 and 0.0368, respectively. The both compounds are isotypical to Ag 2Zn 2(MoO 4) 3 and contain mixed frameworks of MoO 4 tetrahedra and pairs of M 2+O 6 octahedra sharing common edges. The Ag + ions are disordered and located in the voids forming infinite channels running along the a direction. The peculiarities of the silver disorder in the structures of Ag 2 M 2(MoO 4) 3 ( M=Zn, Mg, Co, Mn) are discussed as well as their relations with analogous sodium-containing compounds of the structural family of Na 2Mg 5(MoO 4) 6. The phase transitions in Ag 2 M 2(MoO 4) 3 ( M=Mg, Mn) of distortive or order–disorder type are suggested to have superionic character.