High-pressure phase transformation of BeGa 2 O 4
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High-pressure phase transformation of beryllium gallium oxide (BeGa2C>4) has been studied. Applying high pressure at elevated temperatures to the original hexagonal BeGa2 O 4 (/3-Si 3 N 4 -type structure), a high-pressure modification with orthorhombic structure (olivine-type structure) was obtained, i.e., o-BeGa 2O 4. Lattice parameters of the new phase were determined to be a = 0.5698, b = 0.9759, and c = 0.4551 nm. The pressure and temperature ranges where the high-pressure phase was observed are 3.5 to 7.5 GPa and 800 to 1600 °C, respectively. A tentative pressure-temperature phase diagram for BeGa 2 O 4 was proposed. Transformation is not straightforward; decomposition of the original phase into single oxides and their recombination to form o-BeGa 2O4 are necessary. This process seems to apply in both ways, formation and decomposition of the high-pressure phase. The stability of the high-pressure phase is explained in terms of the total molar volume for the phase, the result of summing up molar volumes of constituent compounds. This is the first known report on transformation of y8-Si 3 N 4 -type structure into a denser structure.
I. INTRODUCTION Inorganic compounds with the general formula AB2 X 4 have been classified into various types of crystal structure.1"3 Even when the composition is restricted to X = oxygen, several crystal types such as spinel (MgAl 2 O 4 ), olivine (MgSi 2 O 4 ), phenacite (Be 2 Si0 4 ), barium aluminate (BaAl 2 O 4 ), calcium ferrite (CaFe 2 O 4 ), etc. can be found. Moreover, complex oxides containing beryllium and a rare-earth element, as in the case of BeY 2 O 4 , present a unique crystal structure belonging to the monoclinic system.4 In an early report on solid solubility in the system BeAl 2 O 4 -BeR 2 O 4 (where R = Cr, Fe, and Ga), Tabata et al. determined the crystal structure of the end member, BaGa 2 O 4 , to be hexagonal in the space group P6 3 /m or P6 3 . 5 - 6 Considering the similarity in ionic radii for the 6-fold coordinated Cr +3 (0.062 nm) and Ga + 3 (0.062 nm),7 '8 this result was quite surprising because no structural change was observed when substituting Cr ions for Al ions in the orthorhombic structure of BeAl 2 O 4 . 9 A contemporary and independent work by Schweizer and Miiller-Buschbaum10 assigned the space group P63/m to BeGa 2 O 4 , establishing its isostructural character to the hexagonal /3-Si 3 N 4 . u > 12 The latter is now a well-known compound that has attracted the attention of many researchers and design engineers due to its high potential for use in high-temperature structural ceramics. 1314 Within the hexagonal lattice, Be and Ga ions are statistically distributed at the tetrahedrally
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)Author to whom all correspondence should be addressed. J. Mater. Res., Vol. 8, No. 7, Jul 1993
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coordinated Si sites of /3-Si 3 N 4 , whereas O ions are located at N sites. As indicated by x-ray diffraction studies (XRD) and density measurements,9 the hexagonal BeGa 2 O 4 , hereinafter h-BeGa2 O 4 , poss
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