Crystal structure and negative thermal expansion of solid solution Yb2W3-xMoxO12

A new series of solid solutions Yb2W3-xMoxO12 (0.5 ≤ x ≤ 2.5) were successfully synthesized by the high-temperature solid state method. Their crystal structure and negative thermal expansion properties were studied using high-temperature X-ray powder diffraction and the Rietveld method. All samples of rare earth tungstates and molybdates were found to 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 (TEC) of Yb2W3-xMoxO12 were determined as −18.3×10−6 K−1 for x = 0.5 and −19.6×10−6 K−1 for x = 2.5 comparing to −19.1×10−6 K−1 and −18.3×10−6 K−1 for unsubstituted Yb2W3O12 and Yb2Mo3O12 in the identical temperature range of 200–800°C. High temperature X-ray diffraction data and bond length analysis suggest that the difference between W—O and Mo—O bond is responsible for the change of TECs after the element substitution in this series of solid solutions.