Thermodynamic properties of negative thermal expansion materials ZrW2O8 substituted for Zr site

Thermodynamic properties of negative thermal expansion materials, ZrW2O8 substituted for Zr site, were studied by X-ray diffraction and heat capacity measurements. Effect of substituted ions on lattice parameters of these materials was discussed from the viewpoint of ionic radius of substituted and host ions, and of oxygen defect. The entropy of α-to-β phase transition in Zr1−xHfxW2O8 series (x=0, 0.5 and 1) was the same value of 2.1 J K−1 mol−1. The normalized 3 1 0 diffraction peaks in Zr1−xHfxW2O8 (x=0 and 0.5) and Zr0.98Sc0.02W2O8−y, as an indicator of the order parameter, followed the universal curve against (T−Ttrs)/Ttrs, where Ttrs is the transition temperature. These results suggest that these materials have the same order–disorder phase transition mechanism. The abrupt decrease in transition temperatures of Zr1−xMxW2O8−y (M=Sc, In and Y), compared to that of Zr1−xHfxW2O8, can be explained by the decrease of ordering in α-phase due to oxygen defect.