Near-Zero Thermal Expansion and Phase Transitions in HfMg_1- Zn Mo₃O₁₂

The effects of Zn²⁺ incorporation on the phase formation, thermal expansion, phase transition, and vibrational properties of HfMg_1- Zn Mo₃O₁₂ are investigated by XRD, dilatometry, and Raman spectroscopy. The results show that (i) single phase formation is only possible for x ≤ 0.5, otherwise, additional phases of HfMo₂O₈ and ZnMoO₄ appear; (ii) The phase transition temperature from monoclinic to orthorhombic structure of the single phase HfMg_1- Zn Mo₃O₁₂ can be well-tailored, which increases with the content of Zn²⁺; (iii) The incorporation of Zn²⁺ leads to an pronounced reduction in the positive expansion of the b-axis and an enhanced negative thermal expansion (NTE) in the c-axes, leading to a near-zero thermal expansion (ZTE) property with lower anisotropy over a wide temperature range; (iv) Replacement of Mg²⁺ by Zn²⁺ weakens the Mo-O bonds as revealed by obvious red shifts of all the Mo-O stretching modes with increasing the content of Zn²⁺ and improves the sintering performance of the samples which is observed by SEM. The mechanisms of the negative and near-ZTE are discussed.