INFRARED-REFLECTIVITY STUDY OF ZRO2-HFO2 SOLID-SOLUTIONS

Infrared-reflectance spectra of the solid solutions Zr1-xHfxO2 (0 less-than-or-equal-to x less-than-or-equal-to 1) have been measured at room temperature in the frequency region 50-4000 cm-1 by Fourier-transform infrared spectroscopy in order to study the composition effect on the vibrational properties of Zr1-xHfxO2. At most, twelve infrared-active modes of 5A(u) and 7B(u) symmetry were identified in the reflectance spectra of Zr1-xHfxO2. The spectra of Zr1-xHfxO2 varied systematically with x, with changes in the frequency, linewidth, and intensity of these modes. Particularly, the B(u) modes at 585 and 606 cm-1 for ZrO2 considerably stiffened in a nonlinear way, indicating a pronounced stiffening around x = 0.8 as x increases. It was found that the lattice parameters a, b, and c of Zr1-xHfxO2 decrease linearly with x whereas beta initially increases up to x = 0.6 and then decreases towards that of the end member HfO2. Hence, it was concluded that the stiffening of all other modes except a B(u) mode is associated with the bond reinforcement evidenced by the reduced cell volume of Zr1-xHfxO2 as the Hf4+ ions of smaller radius are substituted for the Zr4+ ions. It was also concluded that the anomalous feature around x = 0.8 in the frequency, linewidth, and intensity of the modes as a function of x can be reconciled with two-mode behavior, which has been characterized by discontinuities at either x = 0.6 or 0.3 in the x dependence of seven Raman mode frequencies for ZrxHf1-xO2 [C. Carlone, Phys. Rev. B 45, 2079 (1992)].