Raman study of gaseous bubble inclusions in bismuth germanate and bismuth germanium silicon oxide single crystals
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R. Kesavamoorthy Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, India
C.V. Kannan Crystal Growth Centre, Anna University, Chennai-600 025, India
P. Santhanaraghavan Department of Physics, MIT Campus, Anna University, Chennai-600 044, India
P. Ramasamy Crystal Growth Centre, Anna University, Chennai-600 025, India (Received 22 February 2002; accepted 18 November 2002)
Gaseous bubble inclusions in bismuth germanate (BGO) and bismuth germanium silicon oxide (BGSO) crystals were studied by means of Raman spectroscopy at room temperature. Their Raman spectra in the range from 60 to 70 cm−1 showed three peaks for the rotational Raman modes of O2 and N2. Vibrational Raman modes of O2 and N2 were also recorded for BGO and BGSO crystals. It was found that all the rotational and vibrational modes were blue shifted from those of free molecules due to the hydrostatic pressure in the bubbles. Internal pressure in the bubbles was estimated from the rotational and vibrational Raman mode frequencies. O2 gas pressure in the bubble was estimated as 140 GPa, and N2 gas pressure, as 31 GPa. The pressure coefficient of the vibrational mode frequency of O2 (0.368 cm−1/GPa for O2 vibrational mode of 1580 cm−1) and N2 (0.322 cm−1/GPa for N2 vibrational mode of 2331 cm−1) was also obtained from the blue shift and the calculated bubble pressure.
I. INTRODUCTION
Bismuth germanate (Bi4Ge3O12; BGO) as a promising scintillator has attracted much attention because of its excellent scintillation properties. The requirement for single crystals of bismuth germanate for use as a scintillator, notably in high-energy physics calorimeters, is enormously high. These features make the material useful for application in nuclear medicine (computerized tomography system), in high-energy physics (large electromagnetic calorimeters), in ␥-ray spectroscopy, and in many other applications.1 Gas bubbles, voids, or cavities are often incorporated in most Czochralski-grown oxide single crystals, and gas bubble entrapment is one of the most serious problems. The first discussion of void formation was presented by Nassau and Broyer,2 and they reported that gas bubbles were due to the segregation of a)
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J. Mater. Res., Vol. 18, No. 4, Apr 2003 Downloaded: 14 Mar 2015
dopant impurities and/or basic constituents, associated with the cellular growth caused by constitutional supercooling. It was also demonstrated that the fluid flow convection modes in the melt associated with crystal rotation are also the primary determinants of gas bubble entrapment in the crystals.3 Nucleation of bubbles would occur at the crystal–melt interface when the concentration of the gas constituents exceeds its solubility in the melt.2 The possible sources of dissolved gas are the dissociation of ambient atmosphere, the presence of gaseous impurities in the starting chemicals, or the dissociation of the starting chemicals into gaseous prod
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