A Raman spectral characterization of various crystalline mixtures in the ZrO2-TiO2 and HfO2-TiO2 systems

ZrTiO 4 can be used as a dielectric resonator material in microwave devices [1]. Since HfTiO4 is isostructural to ZrTiO4, it may also be used in electrical application similar to those of ZrTiO4. Furthermore, because the coefficient of thermal expansion is low for HfO2-TiO~ ceramics whose HfO2 concentration is greater than 50 tool %, hafnium titanates have potentially useful refractory applications [2-4]. The isostructural high-temperature phases of ZrTiO4 and Hf-l'iO4 belong to the space group PbcnD~ 4 with two formula units per unit cell [2, 5]. Both titanium and zirconium atoms are randomly distributed on equivalent crystal sites. Phase diagrams have been reported for both the ZrO2-TiO2 system and the HfOz-TiO2 system [4-7]. These two titanate systems are very similar except that phase equilibrium is more difficult to achieve in the HfO2-TiO2 system. This condition is especially true in the region between approximately 10 and 30mol % TiO2 [7]. This letter will investigate the comparative structural changes that occur in materials prepared at various high temperatures with air-quenching for the ZrO2-TiO2 and the HfO2-TiO2 systems, using Raman spectroscopy. Structural/spectral effects due to nonequilibrium behaviour in the HfO2-TiO2 system will also be discussed. Samples containing the desired oxide compositions were prepared by a coprecipitation method from aqueous solutions containing the appropriate concentrations of the metal chlorides. The Ti(IV) ioncontaining solutions were prepared by mixing ice-cold TIC14 and deionized water. The resulting solutions had to be used immediately or kept refrigerated to prevent spontaneous hydrolysis of the solution to titania. The titanium- and zirconium/hafnium-containing solutions were then mixed. The solid products were precipitated using ammonium hydroxide. Most of the resulting precipitates were fired in platinum crucibles for 6h at approximately 1650°C, and were then airquenched to room temperature. Portions of the 75 mol % HFO2-25 mol % TiO2 precipitate were also fired at 1000°C for 20h, at 1300°C for 6h, and at 1390 ° C for 10 h. The crystalline products were analysed by wet chemical analysis to verify their ZrO2/HfO2 and TiO2 contents. Powder X-ray diffraction patterns were measured for the quenched samples at room temperature with an X-ray diffractometer using CuKc~ radiation and a graphite monochromator. The observed d-spacings and relative intensities were compared with literature values reported for monoclinic ZrO2, monoclinic HfO 2, TiO 2 (rutile), HfTiO4 and ZrTiO4 [8-12]. Scanning electron micrographs were obtained in the backscatter mode. Raman spectra were measured for the powdered polycrystalline samples using a double-grating spectrometer with an argon ion laser an an excitation source. Scattered radiation was collected at 90 ° to the direction of the incident beam. Samples were sealed in capillary tubes for the spectral measurement, and their spectra were obtained at room temperature. The instrument was calibrated with distilled indene, and the bands were reproducible to + 1 cm J. Infrared absorption spectra were measured using a dispersive double-beam infrared spectrophotometer with an airpurging device. These spectra were obtained at room temperature using KBr pellets. The powder X-ray diffraction data for all crystalline mixtures in the ZrO2-TiO2 system that were prepared by firing the samples at 1650°C clearly show the progressive development of the ZrTiO4 phase as one approaches the composition containing 50mo1% TiO2. The second phase in the non-stoichiometric compositions is either TiO 2 (rutile) or monoclinic ZrO2 depending upon the compositional concentrations. Solid-solution behaviour is noted from the X-ray diffraction pattern both ofZrO 2 in the prepared sample containing 75 tool % ZrO2 and of TiO 2 in the prepared sample containing 25 mol % ZrO2. Shifts are noted in their d-spacings with respect to those of pure ZrO2 and TiO2. This trend is consistent with observations made from the phase diagram that zirconium