CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 6 1042-1043
Articles Growth and Luminescence Properties of NaIn(WO4)2 Crystal A. E. Kokh,† N. G. Kononova,† T. B. Bekker,*,† S. Yu. Stonoga,† P. P. Fedorov,‡ S. Kh. Batygov,‡ and M. D. Skorochvatov# Institute of Geology and Mineralogy SB RAS, 630090 NoVosibirsk, Russia, General Physics Institute RAS, Moscow, Russia, and Russian Research Centre ”KurchatoV Institute”, Moscow, Russia ReceiVed July 28, 2006; ReVised Manuscript ReceiVed March 3, 2007
ABSTRACT: Crystals of NaIn(WO4)2 that are 0.25 cm3 have been grown in two configurations by the top-seeded solution growth (TSSG) method. X-ray excited luminescence spectra of the grown crystals were studied at different temperatures. At room temperature, the light output of NaIn(WO4)2 was shown to be 65% of that of Bi4Ge3O12, while at liquid-nitrogen temperature it increases by 10-fold. 1. Introduction Scintillation properties of indium-containing crystals attract considerable interest for registration of low energy (less than 10 MeV) electron neutrino, particularly in solar neutrino spectrometry,1 investigation of geo-neutrino generated by radioactive decay of K40, etc. The availability of NaIn(WO4)2 for this purpose was shown in ref 2. Also, the application of NaIn(WO4)2 as an active laser material was patented,3 and according to measurements of NaIn(WO4)2 powder this crystal is a suitable material for Raman lasers.4 Scintillation properties of powders of some indium-containing tungstates prepared by solid-phase synthesis were investigated in ref 2. NaIn(WO4)2 was found to have promising scintillation characteristics at room temperature. The aim of this work was to grow bulk crystals of NaIn(WO4)2 for the development and study of scintillation detectors and lasers based on this crystal. NaIn(WO4)2 crystallizes in monoclinic symmetry belonging to the P2/c space group.5 It melts incongruently at 1000 °C according to the phase diagram published in ref 6. So the crystal cannot be grown directly from the stoichiometric melt. Growth of lilac-colored NaIn(WO4)2 needle crystals by spontaneous nucleation in melt solution was reported in ref 5. Sodium bitungstate Na2W2O7 was chosen as a solvent, while crystallization occurred at temperatures between 1100 and 750 °C. 2. Experimental Procedures and Results Crystal Growth. In2O3 (99.999%), WO3 (99.99%), and Na2CO3 (99.999%) were chosen as starting reagents. We observed full degassing * Corresponding author. Tel./fax: +7(383)3333947.
[email protected]. † Institute of Geology and Mineralogy SB RAS. ‡ General Physics Institute RAS. # Russian Research Centre ”Kurchatov Institute”.
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and sintering of the charge material at 800 °C. Then, the temperature was raised to 1000 °C, and the melted mixture was allowed to remain for 20 h for homogenization. Homogeneity of the melt solution was determined when the solution was transparent. First, compositions and temperatures of the primary area of NaIn(WO4)2 phase crystallization in a NaIn(WO4)2-Na2W2O7 binary system have been found by a visual-polythermic analysis method.7 We used the following mole ratio of the starting components for crystal growth experiments: Na2O/In2O3/WO3 ) 10:1:22. First experiments with crystallization on platinum wire were performed to obtain polycrystalline samples of NaIn(WO4)2 suitable for preparing of nonoriented monocrystal seeds. Platinum crucibles 40 mm in diameter and 40 mm in height filled with 100 g (15 mL) of the melt solution were used. Crystal growth was carried out by the top-seeded solution growth (TSSG) technique in two configurations. The first one consists of continuous rotation and pulling of the seed (configuration of the Czochralski method) in the heat field with vertical and radial temperature gradients of about 10 K/cm. In this way, bulk NaIn(WO4)2 crystals 1-1.5 cm in diameter and up to 20 g in weight were obtained (Figure 1a). Rotation and pulling rates were 3 rpm and 0.2 mm/day, respectively. In the second configuration, crystals were grown without rotation and pulling of the seed in temperature gradients of about 1 K/cm. The growth took place in the medium of the melt solution (configuration of the Kyropulos method). In that case, well-faceted polycrystals about 20 g in weight were grown (Figure 2a). In both cases, the rate of temperature decrease was 0.5-2 K/day. The temperature range of crystallization and the starting temperature of crystallization were up to 100 and 920 °C, respectively. In both configurations, the volume of the monocrystal area was about 0.25 cm3 (Figures 1b and 2b); however, Kyropulos configuration seems to be more promising for growing large-sized and high-quality crystals. The habit planes of the crystals are pinacoids {100}, {010} and rhombic prisms {210}, {011} (see Figure 2c). Also, crystals show perfect micalike cleavage on (100). 2.2. Luminescence Measurements. The luminescence spectra of the crystals were studied at room and liquid-nitrogen temperatures. Luminescence was excited by X-ray radiation from a cuprum anode at 20 kV. Measurements were carried out on a KSVU-23 spectrometer without spectra sensitivity compensation. To estimate the light output
10.1021/cg060502+ CCC: $37.00 © 2007 American Chemical Society Published on Web 04/07/2007
Growth and Properties of NaIn(WO4)2 Crystals
Figure 1. Photographs of NaIn(WO4)2 crystal grown by the Czochralski method (a) and its section 5 mm in thickness (b).
Crystal Growth & Design, Vol. 7, No. 6, 2007 1043
Figure 4. Luminescence spectra of NaIn(WO4)2 at room (300 K) (a) and liquid-nitrogen (100 K) (b) temperatures. of NaIn(WO4)2 crystal is 65% of that of Bi4Ge3O12. This result completely agrees with data obtained for powder samples in ref 2. Luminescence spectra for NaIn(WO4)2 at 300 and 100 K are presented in Figure 4. A wide emission band with a maximum at 480490 nm observed in the spectra is due to the WO42- complex.8 It was found that at 100 K the luminescence intensity increases by 10-fold in comparison with one at 300 K. This phenomenon of considerable thermal quenching of the NaIn(WO4)2 luminescence shows the significance and potential of this crystal for registration of low-energy neutrino. Afterglow was observed in NaIn(WO4)2 samples during several seconds in both cases (100 and 300 K). This indicates carriers trapping due to crystal defects. It might be a cause of weak afterglow in NaIn(WO4)2 powder observed in ref 2.
3. Conclusion Figure 2. Photographs of NaIn(WO4)2 crystal grown by the Kyropulos method (a) and its section 5 mm in thickness (b); (c) morphological scheme of NaIn(WO4)2 crystal.
Crystal growth of NaIn(WO4)2 with a monocrystal volume of about 0.25 cm3 was carried out by the TSSG technique in Czochralski (with rotation and pulling) and Kyropulos (without rotation and pulling) configurations. The X-ray excited luminescence spectra of the grown crystals showed that the light output of NaIn(WO4)2 is 65% of that of Bi4Ge3O12, while at liquid-nitrogen temperature it increases by 10-fold. References
Figure 3. Luminescence spectra of BGO (a) and NaIn(WO4)2 (b) crystals at room temperature (300 K). of the grown crystals, the spectra of Bi4Ge3O12 (BGO) crystals were measured at the same conditions. The size of BGO samples used for luminescence measurements was about 7 × 9 × 5 mm3. Figure 3 shows room-temperature spectra of NaIn(WO4)2 and BGO crystals. Since the spectra of NaIn(WO4)2 and BGO are similar, the light output of these crystals may be directly compared. It was shown that the light output
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