Investigations on the Stoichiometry of Lead Tungstate Melt Anil K. Chauhan* Crystal Technology Section, Technical Physics & Prototype Engineering Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 4 755-758
Received March 26, 2004
ABSTRACT: The melting behavior of lead tungstate was investigated with an aim to perform quantitative analysis of the evaporation losses taking place during the growth of this material. The analysis was carried out using differential thermal analysis of the melts kept for known durations at 1150 °C. It was observed that the stoichiometric charge started to lose the lead rich component through evaporation at this temperature. The measured rate of evaporation was found uniform and independent of the composition of the material. The composition of the leftover material was successfully related to the duration of keeping the material in the molten state. It has been demonstrated that by adding a calculated amount of lead oxide in the leftover material one can reattain the stoichiometric composition. It was also observed that off-stoichiometric composition undergoes two phase transformations at 935 and 880 °C, which are detrimental for the melt growth of crystals from these compositions. Introduction Having several excellent properties,1 lead tungstate has been proven to be a very important material for high-energy physics experiments and has been selected as a detector material for some of the internationally collaborated experiments.2,3 For such purposes, one requires large crystals that can be grown by melting methods. However, the major problem associated with the growth of lead tungstate by these methods is the stoichiometric deviation due to the selective loss of one component from the melt. This deviation affects optical and other important properties of the crystal.4-8 The problem has persisted for a long time, and some efforts have been reported to analyze5,9,10 and empirically compensate the evaporating compound. However, the exact composition of the evaporated material has not been known till now, and the amount of evaporation has also not been quantitatively analyzed yet. An easy method was required to calculate the off-stoichiometry of the melt, and results of our efforts in this direction are reported in this paper where the off-stoichiometry of lead tungstate is related to the amount of time; the charge was kept in the molten state. It is established that adding calculated amounts of lead oxide can compensate for these stoichiometric deviations. Experimental Section Differential Thermal Analysis (DTA) Experiments. The stoichiometric composition of lead tungstate was prepared by mixing lead oxide and tungsten oxide of 4 N purity in a 1:1 molar ratio. This powder was mixed using a ball-mill for 20 min and then treated in a sealed platinum crucible at 800 °C for 24 h in a resistant heating furnace. The resultant material was again crushed to a fine powder and remixed, and the same heat treatment was repeated. The powder X-ray diffraction (XRD) pattern of such prepared material was recorded, and formation of the mono-phase PbWO4 was confirmed by comparing this with the data available in the literature.11 * To whom correspondence should be addressed. Telephone: +91-22-25590439. Fax: +91-22-25505151. E-mail: akchau@ magnum.barc.ernet.in.
Figure 1. Recorded DTA pattern for the stoichiometric composition of lead tungstate material. For the DTA measurements, 171.7 mg of stoichiometric powder was taken in a small experimental platinum crucible, with a surface area of 19.63 mm2 (diameter 5 mm) and a depth of 5 mm. The patterns were recorded using a thermal analyzer SETRAM model 92, using Al2O3 as the standard taken in an identical crucible. The samples were heated in an atmosphere close to air, with a rate of 10 °C per minute up to 1150 °C, and also cooled with the same rate. The first experiment was carried out using the stoichiometric mixture, and the results have been reproduced in Figure 1. A clear melting and freezing of the material were observed at an identical temperature 1127.2 °C, and no other peak was seen in the plot. After the experiment, the crucible was taken out and kept inside a bigger Pt crucible of 50 mm × 50 mm, which already contained another small crucible with a known amount of stoichiometric material (for comparison). This assembly was kept inside a resistance furnace. Because in a typical growth assembly it is quite usual that some part of the melt remains about 20-25 °C higher due to temperature gradients, the soaking temperature was finalized to be 1150 °C. The furnace was heated to this temperature with a rate as fast as 250 °C/h to minimize the evaporation in the heating and cooling cycle. The samples were kept at this temperature for 15 h initially.
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Figure 3. Variation of (a) evaporation losses and (b) melting temperatures of the lead tungstate, with respect to the soaking time of melt at 1150 °C.
Figure 2. Plots for the DTA patterns of lead tungstate material after soaking at 1150 °C for (a) 15, (b) 45, (c) 75, (d) 150, and (e) 300 h. All of the plots share the same temperature profile shown in panel a. The furnace was cooled with the same rate of 250 °C/h to room temperature. The weight losses in both of the crucibles were measured and found to have in the same ratio. The recorded DTA pattern for this material is shown in Figure 2a. An endothermic phase transformation at 887 °C was observed in the heating cycle, which was shifted to 847 °C in the cooling cycle. The melting point of the material was also lowered slightly to 1122.2 °C, and the sharpness of the melting peak was also reduced as compared to Figure 1 in the heating cycle. The same material was used for the repetition of measurement for different soaking time durations viz. 30, 30, 75, and 150 h. The recorded DTA patterns for the total soaking times of 45, 75, 150, and 300 h are shown in Figure 2b-e. It is clear from the plots that in the DTA pattern recorded after 45 h of soaking, a peak at 935 °C was observed. The intensity of this peak was found to be increased in all of the subsequent experiments, and at the same time, the sharpness of the higher temperature peak, its relative intensity, and its position continue to decline. The position of the third peak did not follow any certain trend or behavior; however, at all of the times, its position in the cooling cycle was found to be shifted about 40 °C to that of the heating cycle. The weight loss in the crucible after each cycle was carefully measured, and the results are presented in Table 1. These data were plotted with respect to soaking time and are shown in plot a of Figure 3. The plot is a straight line and hence suggests that the rate of evaporation from the exposed surface of the melt is uniform. From these measurements, the average rate of evaporation per unit time per unit area was calculated and
Figure 4. Lead oxide deficiency from stoichiometric composition with respect to the soaking time at 1150 °C. found to be 2.012 × 10-2 mg/mm2 h. The evaporated material was also analyzed for its composition, and the ratio of Pb/W was found to be 2.2, which was quite close to the reported data.10 The melting points of the leftover materials were calculated from their respective DTA patterns, using their cooling cycle freezing temperatures, and are presented in Table 1 as well as plotted in Figure 3b. The compositions corresponding to these melting temperatures were evaluated using the tungsten rich side of the phase diagram,12 after it was expanded and digitized. The obtained composition data are also given in Table 1 and plotted in Figure 4 with respect to the soaking time at 1150 °C. The powder XRD pattern taken for the leftover material in the crucible is reproduced in Figure 5. This pattern was found
Table 1. sr. no.
soaking time (h)
weight of material (mg)
evaporation (mg)
melting temp (°C)
weight loss (%)
composition of PbO in % mole
% mole deficiency to stoichiometry
1 2 3 4 5 6
0 15 45 75 150 300
171.7 168.5 153.2 142.9 113.3 65.7
0 5.9 18.6 28.8 58.4 105.4
1127.2 1122.2 1116.4 1108.3 1099.8 1089.1
0 3.44 10.83 16.77 34.01 61.79
50.00 47.96 46.53 44.67 43.29 41.67
0 2.04 3.47 5.33 6.71 8.33
Stoichiometry of Lead Tungstate Melt
Crystal Growth & Design, Vol. 4, No. 4, 2004 757 Table 2. Data for the Compositions (in Percentage) of Lead Oxide in Leftover Charge with Respect to Soaking Time at 1150 °C
Figure 5. XRD pattern (Cu KR radiation) of the leftover charge after 100 h of melt treatment at 1150 °C. The peaks marked with E do not match with the standard pattern for PbWO4. to have all of the peaks for the PbWO4 phase,11 along with a set of extra peaks at angles 24.76, 26.76, 40.28, 42.28, 46.04, 63.76, 64.28, and 68.72 (marked with E) in Figure 5. Crystal Growth Experiments. To check the effect of observed off-stoichiometry on the growth process, about 200 g of stoichiometric material was prepared and taken in a platinum crucible of 50 mm d × 50 mm L and kept at 1150 °C for 100 h prior to the growth process. The growth geometry was prepared using active after heater to minimize the axial gradient.13 Growth attempts were made on the indigenously developed, software controlled, and fully automated crystal puller described elsewhere.14 A uniform pull rate of 2 mm/h was employed throughout the experiment. The crystal growth was completed successfully; however, in the cooling cycle, the crystal got cracked at around 935 °C. During subsequent experiments, the cooling rate was lowered to as low as 10 °C/h but the cracking could not be avoided. Most of the times, cracking took place only around this temperature. The powder XRD pattern recorded for the cracked crystal was also found to be similar to that shown in Figure 5. To reattain the stoichiometry of this material, the amount of lead oxide deficit was calculated using Figure 4 and added to the crucible material. It was ground very fine, mixed thoroughly, and given a heat treatment at 800 °C for 50 h. The recorded XRD pattern of the resultant material was found to match with the pattern for stoichiometric composition.11 The DTA pattern of this material also contained only one sharp peak at 1127 °C and did not show any of the extra peaks at 935 and 880 °C. A similar exercise was repeated for several other compositions, deliberately produced after keeping the melt at 1150 °C for different durations. In all of the cases, stoichiometry was restored. Comprehensive data for the leftover compositions corresponding to various soaking times were also generated by interpolating Figure 4, and this is presented in the Table 2.
Results The following important observations were made during this study. (i) A selective loss of a lead rich component took place from the melt, and its evaporation rate was found to be uniform and independent of the composition of the leftover material. (ii) The amount of lead oxide deficiency was calculated using the melting point of the leftover material and was related to the soaking time of the melt. (iii) The stoichiometric compound was successfully prepared from the leftover
time (h)
composition
time (h)
composition
5 10 15 20 25 30 35 40 45
49.33 48.99 47.96 47.73 47.49 47.25 47.02 46.77 46.52
50 60 70 80 90 100 110 120 130
46.23 45.61 44.98 44.58 44.39 44.22 44.03 43.85 43.67
time (h)
composition
140 150 175 200 225 250 275 300
43.48 43.31 43.02 42.75 42.49 42.22 41.95 41.67
material by adding a calculated amount of lead oxide (using Figure 4) and a subsequent heat treatment. (iv) A fairly intense peak at 935 °C and a low intensity peak at around 880 °C were found in all DTA experiments of nonstoichiometric composition during the heating cycle. The 880 °C peak always got shifted about 40 °C in the cooling cycle. (v) The crack free single crystals could not be obtained using lead deficient compositions as the starting material. The crystals, however, were mostly grown crack free but cracked while cooling. A slow cooling rate also could not overcome this cracking. Discussion The DTA peak at 935 °C started developing only after a few hours of the soaking treatment, and no peak corresponding to this temperature was observed in the stoichiometric composition. Also, as the soaking time increases, the sharpness and position of the higher temperature peak started to decrease. Because no melting or freezing was observed at 1127 °C in the subsequent DTA experiments, it was a clear indication of the fact that the PbWO4 phase was forming a solution with the other unknown phase, and we followed the S + L (solid + liquid) region of the phase diagram. In view of these factors, this DTA peak may be associated with the eutectic temperature. Few lead deficient phases (Pb7W8O32-x and Pb7.5W8O32) were also found reported in the literature,15-17 but the recorded powder XRD data of the leftover material did not match with either of them completely. The low intensity, 880 °C peak could not be explained on the basis of available phase diagrams. Also, a consistent and considerably high temperature shift (∼40 °C) was observed in the cooling cycle, which could not be associated with the melting and solidification of some possible compounds10 in this system. Therefore, this peak may be due to any structural phase transformation in the material that was taking place at around this temperature and needs further investigations. This study also revealed some useful information on the cracking behavior of the lead tungstate crystals in the cooling cycle. If the growth experiments were started with slightly off-stoichiometry, a different phase would be developed, which undergoes exothermic phase transformations at two different temperatures, 935 and 840 °C, while cooling. Because cracking of the crystal was taking place at around 935 °C, it may be associated with the exothermic reaction taking place at around this temperature. As being a poor conductor of heat, crystal may not be able to sustain the higher stresses exerted
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by the production of heat of solidification in the interior of the crystal. Acknowledgment. I am grateful to Drs. V. C. Sahni, J. V. Yakhmi, Sangeeta, and S. C. Sabharwal for their support and help provided during this work. I am also highly indebted to Dr. Ratikanta Mishra of the Applied Chemistry Division for helping with the DTA measurements, without which this work would not have been possible. References (1) Nikl, M. Phys. Status Solidi A 2000, 178, 595-620. (2) CMS. Technical Proposal, CERN/LHCC 94-38 LHCC/P1, 1994. (3) ALICE. Technical Proposal CERN/LHCC 95-71, Dec 15, 1995. (4) Sabharwal, S. C.; Sangeeta; Desai, D. G.; Karandikar, S. C.; Chauhan, A. K.; Sangiri, A. K.; Keswani, K. S.; Ahuja, M. N. J. Cryst. Growth 1996, 169, 304-308. (5) Yang, C.; Chen, G.; Shi, P. J. Lumin. 2001, 93, 249-253. (6) Senguttuvan, N.; Mohan, P.; Babu, S. M.; Subramanian, C. J. Cryst. Growth 1998, 191, 130-134.
Chauhan (7) Tanji, K.; Ishii, M.; Usuki, Y.; Kobayashi, M.; Hara, K.; Takano, H.; Senguttuvan, N. J. Cryst. Growth 1999, 204, 505-511. (8) Senguttuvan, N.; Mohan, P.; Babu, S. M.; Ramasamy, P. J. Cryst. Growth 1998, 183, 391-397. (9) Burachas, S.; Beloglovski, S.; Makov, I.; Saveliev, Y.; Vassilieva, N.; Ippolitov, M.; Manako, V.; Nikulin, S.; Vassiliev, A.; Apanasenko, A.; Tamulaitis, G. Nucl. Instrum. Methods Phys. Res., Sect. A 2002, 486, 83-88. (10) Burachas, S.; Beloglovski, S.; Makov, I.; Saveliev, Y.; Vassilieva, N.; Ippolitov, M.; Manako, V.; Nikulin, S.; Vassiliev, A.; Apanasenko, A.; Tamulaitis, G. J Cryst. Growth 2002, 242, 367-374. (11) Powder Diffraction File no. 86-0843, JCPDS-ICDD, 2001. (12) Chang, L. L. Y. J. Am. Ceram. Soc. 1971, 54, 357-358. (13) Chauhan, A. K. J. Cryst. Growth 2003, 254, 418-422. (14) Chauhan, A. K. Cryst. Growth Des. 2004, 4, 135-139. (15) Moreau, J. M.; Galez, P. H.; Peigneux, J. P.; Korzhik, M. V. J. Alloys Compd. 1996, 238, 46-48. (16) Moreau, J. M.; Gladyshevskii, R. E.; Gelez, P. H.; Peigneux, J. P.; Korzik, M. V. J. Alloys Compd. 1999, 284, 104-107. (17) Powder Diffraction File no. 86-0844, JCPDS-ICDD, 2001.
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