High substitution of Fe³⁺ for Zr⁴⁺ in ZrV_1.6P_0.4O₇ with small amount of FeV_0.8P_0.2O₄ for low thermal expansion

Fe³⁺-doped ZrV_1.6P_0.4O₇ with Fe:Zr molar ratios of 1:9, 2:8, 3:7 and 4:6 was synthesized to reduce phase transition and obtain low thermal expansion. It is shown that the phase transition temperature of Fe-doped ZrV_1.6P_0.4O₇ is reduced obviously with increasing the content of Fe. Fe³⁺ ion with lower valence and smaller radius than that of Zr⁴⁺ favors to extend the bond angle of V–O–V(P) close to 180° in ZrV_1.6P_0.4O₇, which is considered responsible for the normal structure at room temperature and low thermal expansion. The thermal expansion coefficients of Fe-doped ZrV_1.6P_0.4O₇ for Fe:Zr molar ratios from 1:9 to 4:6 are calculated to be from −4.33×10⁻⁶ to 5.2×10⁻⁷ K⁻¹ by linear thermal expansion measurement. The effect of small amount of FeV_0.8P_0.2O₄ formation with higher content of Fe³⁺ on thermal expansion coefficients of the samples is discussed. - Highlights: • Phase transition of Fe-doped ZrV_1.6P_0.4O₇ is suppressed with the content of Fe³⁺. • Fe-doped ZrV_1.6P_0.4O₇ shows near-zero thermal expansion (molar ratio Fe:Zr = 4:6). • Fe³⁺ substituting Zr⁴⁺ forms oxygen vacancies and decreases bond length of Zr–O. • Bond angle of V–O–V is considered to extend to 180° by the introduction of Fe³⁺