High substitution of Fe3+ for Zr4+ in ZrV1.6P0.4O7 with small amount of FeV0.8P0.2O4 for low thermal expansion

• Phase transition of Fe-doped ZrV1.6P0.4O7 is suppressed with the content of Fe3+.
• Fe-doped ZrV1.6P0.4O7 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+.

Fe3+-doped ZrV1.6P0.4O7 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 ZrV1.6P0.4O7 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 ZrV1.6P0.4O7, which is considered responsible for the normal structure at room temperature and low thermal expansion. The thermal expansion coefficients of Fe-doped ZrV1.6P0.4O7 for Fe:Zr molar ratios from 1:9 to 4:6 are calculated to be from −4.33×10−6−4.33×10−6 to 5.2×10−7 K−15.2×10−7 K−1 by linear thermal expansion measurement. The effect of small amount of FeV0.8P0.2O4 formation with higher content of Fe3+ on thermal expansion coefficients of the samples is discussed.