Formation process and composition-dependent properties in non-centrosymmetric Ta2O5 phase with Zr substitution

(ZrxTa1−x)2O5−x was prepared by a solid-state reaction of ZrO2 and Ta2O5, and the L′-Ta2O5 phase was obtained by cooling the H-Ta2O5 phase. High-temperature x-ray diffraction measurements showed that the starting materials, ZrO2 and low-temperature L-Ta2O5, formed the high-temperature H-Ta2O5 phase when heated above 1360 °C. Upon cooling, this phase sequentially transformed into L″-Ta2O5, the high-temperature L′-Ta2O5 phase, and L′-Ta2O5 phases. As the Zr content, x, decreased, the transition from the H-Ta2O5 phase to the L″-Ta2O5 phase slowed. The temperature dependence of the dielectric constant revealed a maximum value, which is attributed to the phase transition from L′-Ta2O5 to L″-Ta2O5. This transition temperature decreases by approximately 50 °C for every 0.01 increase in the x value. The L′-Ta2O5 phase exhibited negative volumetric thermal expansion (NTE) behavior near the phase transition temperature. As x decreased, the NTE coefficient increased from −1.09 × 10−6/K (77–127 °C) for x = 0.10 to −2.06 × 10−5/K (327–427 °C) for x = 0.05. The substitution of Zr into Ta2O5 stabilized the non-centrosymmetric L′-Ta2O5 phase and controlled the phase transition temperature and thermal expansion behavior.