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Synthesis and Growth of Fresnoite (Ba2tisi2o8)

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Synthesis and Growth of Fresnoite (Ba2tisi2o8) JOURNAL OF RESEARC H of the Notionol Bureau of Sta ndards - A. Physics and Chemistry Vol. 74A, Na. 2, March- April 1970 ) Synthesis and Growth of Fresnoite (Ba 2TiSi 20 S) from a Ti0 2 Flux and Its Relation to the System BaTi0 3-Si0 2 > C. R. Robbins Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234 > (December 4, 1969) Crystals of Ba2 Ti Si,O. (syntheti c fresnoite) up to 5 mm in longest dimension have been grown by slow cooling of a TiO" ri ch liquid of initial composition IBaO : ITiO, : lSiO, . The syntheti c crystals are essentially identi cal to th eir mineral equivalent in morphology, cleavage, optical properties and unit cell dimension s. X·ray powder diffraction data previously reported for th e compound BaTiSiO.-, ("barium sphene") is that of Ba, TiSi, O •. Apparently th e compound BaTiS iO ., has never been syn ­ thesized and the system BaTiO,,-SiO, is not binary. Key word s: Ba,TiSi,0 8; BaTiSiO.-,; crystal growth; fresnoite; phosph or; pi ezoelectri c; system BaTiO,, -SiO,; TiO, solvent. 1. Introduction Table 1. X -ray and opticaL crystallographic data for fresnoite (Ba, Ti Si2 0 . ) a Crystals of Fresnoite (Ba2 TiSi 2 0 8) were found by Alfors, Stinson, Matthews, and Pabst [J] 1 in a study of the minerals of eastern Fresno County, California. X-RAY DATA /' The mineral occurs as subhedral to euhedral tetragonal I crystals, elongated slightly in the [001] direction, with System: Tetragonal forms {001}, {1l0} and one distinct cleavage, (001). Space Group: P4/mbn, P4bm or P4b2 Composition was established [1] to be Ba2TiSi2 0 g Unit Cell: a = 8.52 ± 0.01 A, c = 5.210 ± 0.005 A with traces of impurities by arc emission spectro­ c/a = 0.611 5 graphic analyses. The crystals ranged in size from Density: 4.43 ± 0.02 (measured) 0.1 to 3 mm with an average longest dimension of 4.45 (cal culated) '" 0.3 mm, and exhibited pleochroism in transmitted CelJ content: Z = 2 ( polarized light (from light yellow to colorless), anoma­ lous blue interference colors and pal!;; yellow fluores- OPTICAL DATA cence under ultraviolet light (2652 A). The optical and x-ray data [1] are summarized in table 1. Alfors Optic sign: Uniaxial (-) Refractive indices: w=1.775±0.003 et aI., also prepared synthetic Baz TiSi2 0 s in formula proportions (5! hr at 1350 0c) and obtained a sintered to = 1.765 ± 0.003 compact whose x-ray diffraction pattern was identical Pleochroism: 0= colorless with that of the mineral. E= yellow The crystal structure of Ba2TiSizOs was determined by Masse, Grenier, and Durif [2], using synthetic crys­ tals, and independently by Moore [3], using a natural a Data of [1] for mineral type specimen from eastern Fresno crystal from the type locality. Masse et al. reported County, California. growth of crystals from a stoichiometric oxide mixture ) melted at about 1350 °C and cooled at 40 o~ per hour. 11 percent for (hko), 13 percent for (okl ) were ob­ They obtained a cell of a=8.52, c=5.21 A for their tained in reference [2], while an overall R value of synthetic crystals and Moore reported a = 8.52 ± 0.01, 11.1 percent (all reflections) was obtained in reference c = 5.210 ± 0.005 A for the mineral crystal. Both struc­ [3]. Similarities of the structure of Ba2 TiSi20 s to tures were assigned to space group P4bm; R values of that of hardystonite (Ca2ZnSi207) [2] and that of melilite (Ca, Na)z(Mg, AI)Si20 7 [3] were noted. In the structure of Ba2 TiSb08, Si20 7 groups are linked to I It alic ized fi gures ill bnlckels indicat.e the lite rature referenceb at the end of this paper. square pyramidal Ti05 groups to form sheets that are 229 parallel to (001) and are held together by Ba ions. An index of refraction of 1. 775. They assigned to them the interesting feature of the structure is the presence of formula BaTiSi05 and the congruent melting point Ti in five-fold coordination in the sq\;lare pyramid, with 1400 0C. They noted [5] that the refractive index of four Ti-O distances of 2.00 ± 0.04 A and a fifth Ti-O the glass of mole ratio IBaO: ITi02 : ISi02 was greater distance of 1.66 ± 0.08 A for the pyramidal oxygen than that of the average index of the crystals (1.86 as , atom [3]. compared with 1.77). This suggested a negative Ba2 TiSi20 g was prepared as a crystalline powder coefficient of thermal expansion for the crystals but 7 by Blasse [4] by heating mixtures of BaC03 , Ti02 the coefficient was found to be of the order of 50 X 10- and Si02 in oxygen at 1000 to 1100 °C. He obtained a per °C. Polymorphism was not observed. Rase and unit cell of a = 8.52, c = 5.21 A (in agreement with Roy [5] reported unindexed x-ray powder data for 'I [1, 2, 3]) and reported a blue-green fluorescence BaTiSi05 and suggested a structural similarity to I under excitation with 254 mm radiation and with sphene (CaTiSi05 ). Their x-ray data were subsequently cathode rays. The spectral energy distribution of indexed by ASTM personnel (BaTiSi05 , card 11-150) emission, the diffuse reflection spectra, the excitation on the basis of a tetragonal cell (a=8.525, c=5.22 A), spectra, the decay time of fluorescence (3' 1O - 3 s.) the cell later obtained by [1, 2, 3] for fresnoite, and the temperature dependence of the phosphor Ba2TiShOs. were studied. The exceptionally efficient fluorescent The x-ray powder diffraction data for BaTiSi05 properties of Ba2 TiSi20 g at room temperature were ([5], ASTM indexing) and for Ba2 TiSi20 H [1] are /1 attributed to the presence in the structure of the listed in table 2 and shown schematically in figure 2. pyramidal Ti05 group. It may be concluded from comparison of the two The initial objectives of the present study were: (1) patterns that the data are essentially identical, and the to grow small single crystals of Ba2 TiSi20 g for other two independent studies were of the same compound. physical property measurements, and (2) to prepare The composition Ba2 TiSi20 g has been established by crystals of the compound BaTiSi05 (reported by Rase chemical analyses [1] and by single crystal x-ray and Roy [5] in their study of the system BaTi03-Si02 ) structure determination [2, 3 ] , and is therefore the for x-ray analysis and comparison with crystals of composition of the phase obtained by Rase and Roy synthetic sphene (CaTiSi05 ) previously described by [5] in their study of the system BaTiO:1·Si02 and Robbins [6], and Brower and Robbins [7]. described by them as BaTiSi05 , the barium analog of sphene. Apparently BaTiSi05 has not yet been synthesized and the system BaTi03·Si02 is not 2. Phase Relations binary. The liquidus surface of the phase diagram for the system (fig. 1, [5]) must be that of a cut in the The phase diagram for the ternary system BaO­ ternary system BaO-TiOt -Si02. The congruent melting Ti02-Si02 in the region of the composition Ba2 TiSi20 s point of 1400°C given for BaTiSi05 then represents a has not been reported. The work of Masse et a1. [2] point on this liquidus surface at which the compound / indicated congruent melting for Ba2TiSi 20 s near Ba2 TiSi 20 s would start to crystallize from a Ti02-rich -1 1350°C. liquid of mole ratio lBaO: 1Ti0 2: ISi02. The large In the course of the work on their phase diagram for observed difference between index of refraction of the binary system BaTi03 -Si0 (fig. 1) Rase and Roy 2 glass and average index of crystals (1.86 versus 1.77) [5] obtained tabular, biaxially positive crystals show­ [5] apparently resulted from comparison of a glass of ing bluish interference colors and having an average composition IBaO: 1TiO z: ISi0 2 with crystals of com­ position 2BaO . 1TiO t • 2Si02. Hex . BoTiO, s s + Liquid 3. Experimental Procedure Cristobo 1ile Liquid + Liqu id On the basis of the preceding conclusions two 14 70" ternary compositions, 2BaO: 1Ti0 2: 2Si0 2 (fresnoite) and lBaO: 1Ti0 2: ISiOz ("barium sphene"), were prepared in 20 g amounts from high purity BaC03 , Ti02 • Tridymi1e oTiSi05 + and Si02, and thoroughly mixed mechanically. The + Liquid Liquid mixtures in 40 ml platinum crucibles were heated in a I 1260' platinum wound muffle furnace in air to near 1425 °C, held for 1 hr and cooled at 3 deg per hour to 1000 °C Tr idymite + and then at 10°C per hour to room temperature. Cub ic BoTiO! s s BoTiSi20 7 1000 + - Products obtained were examined by light microscopy, 965' BaTiSi 0 5 x-ray powder diffractometry and single crystal x-ray 0' i m u; <f) o~ Tr idymite u; precession methods. I ~ ~ + ~ i= SaTi Si 30 9 "' "' as I 20 40 60 80 4. Results and Discussion Mole % Crystals of Ba2TiSi20 g (fresnoite) up to 5 mm in F IGU RE 1. Phase equilibrium diagram for the system BaTi03·SiO" Rase and Roy [5]. longest dimension were obtained from the experiment " I 230 TABLE 2. X-ray powder diffraction data for N BaTiSiO, and Ba,TiSi ,OH Bo Ti Si05 2 N_ Bo.TiSiO," Ba,TiSi,OH" ON ) ~ ~N -N'" 0 d( A ) I hkl d(lq 1 hkl 6 i<'i=-N ~ - " I 6.0 10 110 I II i I I I I 5.20 15 001 5.20 20 001 N B0 TiSi 0.
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