Low-Temperature Spark Plasma Sintering of ZrW_{2- x}Mo _x O₈ Exhibiting Controllable Negative Thermal Expansion

Molybdenum-doped zirconium tungstate (ZrW_2-xMo_xO₈) has been widely studied because of its large isotropic coefficient of negative thermal expansion (NTE). However, low density and poor sinterability limit its production and application. In this study, relative density greater than 90% single-phase ZrW_2-xMo_xO₈ (0.0 ≤ x ≤ 1.0) sintered bodies were fabricated by spark plasma sintering (500⁻600 °C for 10 min) using ZrW_2-xMo_xO₇(OH)₂·2H₂O precursor powders as the starting material. High-temperature X-ray diffraction and thermomechanical analysis were used to investigate the change in the order⁻disorder phase transition temperature of the sintered materials; it gradually dropped from 170 °C at x = 0.0 to 78 °C at x = 0.5, and then to below room temperature at x ≥ 0.7. In addition, all sintered bodies exhibited NTE behavior. The NTE coefficient was controllable by changing the x value as follows: from -7.85 × 10^-6 °C^-1 (x = 0) to -9.01 × 10^-6 °C^-1 (x = 0.6) and from -3.22 × 10^-6 °C^-1 (x = 0) to -2.50 × 10^-6 °C^-1 (x = 1.0) before and after the phase transition, respectively. Rietveld structure refinement results indicate that the change in the NTE coefficient can be straightforwardly traced to the thermodynamic instability of the terminal oxygen atoms, which only have one coordination.