The Study Of Synthesis And Properties Of Negative Thermal Expansion Solid Solutions And Their Ceramic Zr_1-xRE_xW_2-yMo_yO_8-x/2 (RE=Sc, Yb, Er, Eu)

Cubic ZrW₂O₈ has received considerable attention in the past several years because of its isotropic negative thermal expansion over a wide temperature range. This makes the investigation of structurally related phases Zr_1-xRE_xW_2-yMo_yO_8-x/2 solid solutions an attractive goal.In this paper, Zr_1-xRE_xW_2-yMo_yO_8-x/2 partial solid solutions with RE³⁺ = Sc³⁺ Yb³⁺, Er³⁺ and Eu³⁺ of different substitution have been synthesized by solid-state reaction. The XRD patterns of these solid solutions were indexed usingα-ZrW₂O₈ as structural model and their lattice parameters were found to be described by Vegard's rule, i.e., the lattice parameter decreases with increase in substitution fraction of RE³⁺. Using this method, the limited solubility of Sc³⁺, Yb³⁺ and Er³⁺ are determined to be about 6.3 mol%, 4.4 mol% and 3.3 mol%, respectively. The phase transition of solid solutions were investigated by variable temperature XRD, it is found that the phase transition temperature, Tc, drop drastically with increasing the substituted RE content. In the meantime, a novel method for evaluating limited solubility of lanthanide ions in Zr_1-xEuxW₂O_8-x/2 solid solutions has been developed based on Tc^x to be 1.5 mol%. The factors affected solid solutions'lattice parameter include concentration of RE³⁺ ions, ionic radius difference (Δr), valence difference (Δz) and order parameter (η'), the expressions for the lattice parameters of those solid solutions is aα= 9.1478 +x (0.782Δrk– 0.129) + 0.0090η'(?).To investigate the mechanics and electrics of solid solutions, Zr_1-xErxW₂O_8-x/2 and Zr_1-xYb_xW_1.6Mo_0.4O_8-x/2 ceramic pellets were prepared. The microstructure and elements distribution of ceramic pellets was observed by SEM and EDX, it is found that the ceramic pellet compose with grain, grain boundary, segregation phase and pore, and the partial substitution of Ln³⁺ for Zr⁴⁺ was effective for promoting grain growth, resulting in an increase of density. Due to the volatilization of WO₃ with MoO₃ and decomposing of grain, there are some impurities in pellets, which distribute mostly in the surface and segregation phase of ceramic pellet.The conductivity of ceramic pellets has been investigated by impedance spectroscopy. The result indicates that Zr_1-xEr_xW₂O_8-x/2 and Zr_1-xYb_xW_1.6Mo_0.4O_8-x/2 shows clearly oxide ionic conductibility. Their ionic conductivity at 400℃is estimated to be about 10^-4 S·cm^-1, which is comparable to that observed for ceria based electrolytes. The conductivity increased with Ln³⁺ content, that is obviously caused by the increase in amount of oxygen vacancy since the Ln-substitution here is aliovalent in nature. The resistance of segregation phase is large and dominating factor for the conductivity of ceramic pellets, which can be decreased by substitution of multi-Ln³⁺ for Zr⁴⁺.