Crystal structure and negative thermal expansion properties of solid solution Er2W3-xMoxO12

A series of solid solutions Er₂W_3?xMo_xO₁₂ (0.5≤x≤2.5) were successfully synthesized by the solid state method. Their crystal structures and negative thermal expansion properties were studied by high temperature X-ray powder diffraction and the Rietveld method. All samples with rare earth tungstates and molybdates crystallize in the same orthorhombic structure with space group Pnca, and show the negative thermal expansion phenomena related to transverse vibration of bridging oxygen atoms in the structure. Thermal expansion coefficients (TECs) of Er₂W_3?xMo_xO₁₂ were determined as ?16.2×10^?6 K^?1 for x=0.5 and ?16.5×10^?6 K^?1 for x=2.5 while ?20.2×10^?6 K^?1 and ?18.4×10^?6 K^?1 for unsubstituted Er₂W₃O₁₂ and Er₂Mo₃O₁₂ in the identical temperature range of 200?800 °C. High temperature XRD data and bond length analysis suggest that the difference between W—O and Mo—O is responsible for the change of TECs after the element substitution in the series of solid solutions.