High-temperature powder x-ray diffraction of yttria to melting point
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High-temperature powder x-ray diffraction of yttria to melting point V. Swamya) Max-Planck-Institut f¨ur Metallforschung, Heisenbergstrasse 5, D-70569 Stuttgart, Germany
N. A. Dubrovinskaya and L. S. Dubrovinsky Institute of Earth Sciences, Uppsala University, Norbyv¨agen 18 B, S-75236 Uppsala, Sweden (Received 23 April 1998; accepted 9 May 1998)
Powder x-ray diffraction data of yttria (Y2 O3 ) were obtained from room temperature to melting point with the thin wire resistance heating technique. A solid-state phase transition was observed at 2512 6 25 K and melting of the high-temperature phase at 2705 6 25 K. Thermal expansion data for a –Y2 O3 (C-type) are given for the range 298–2540 K. The unit cell parameter increases nonlinearly, especially just before the solid-state transition. The x-ray diffraction spectrum of the high-temperature phase is consistent with the fluorite-type structure (space group Fm3) with a refined unit ˚ at 2530 K. The sample recrystallized rapidly above cell parameter a 5.3903s6d A 2540 K, and above 2730 K, all the diffraction lines and spots disappeared from the x-ray diffraction spectrum that suggests complete melting.
I. INTRODUCTION
Yttria (Y2 O3 ) is a superrefractory oxide that finds use in a variety of applications including optics, optoelectronics, microelectronics, and display devices.1,2 Yttria is also used as an additive for liquid phase sintering of covalently bonded structural ceramics such as silicon nitride, silicon carbide, AlN, and SiAlON’s, and to stabilize the cubic structure of zirconia for applications such as oxygen gas sensor and solid-state electrolyte. Another interest in the high-temperature data of yttria arises from the potential use of its melting point as a secondary temperature standard,3 especially in view of the observations that the composition of the surrounding atmosphere has little influence on its stoichiometry at high temperatures and on its melting point.4,5 High thermal stability is the property that lends yttria (and the rare earth sesquioxides) useful in many applications. A knowledge of the real thermal behavior of this material is therefore desirable in a number of applications. In particular, characterization of thermal expansion and high-temperature phase transition is of great importance. The structural form of Y2 O3 stable up to near its melting temperature is cubic (space group Ia3, Mn2 O3 bixbyite type), known as C-Y2 O3 or a –Y2 O3 . Available thermal expansion data of C-Y2 O3 , which cover a temperature range from 110 to 2225 K, are summarized in Ref. 6. Foex and Traverse7 reported a structural transition of C-Y2 O3 to a hexagonal form at about 2570 K ˚ Y2 O3 also occurs in the with a 3.81 and c 6.08 A. a)
Present address: CSIRO Minerals, Box 312, Clayton South, Victoria 3169, Australia.
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J. Mater. Res., Vol. 14, No. 2, Feb 1999
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monoclinic C2ym space group at high pressures.8 Recently, Katagiri et al.9 observed a transit
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