Preparation of Pure Alkali Halide Crystals and Some of Their Properties J. M. Peech, D. A. Bower, and R. O. Pohl Citation: Journal of Applied Physics 38, 2166 (1967); doi: 10.1063/1.1709847 View online: http://dx.doi.org/10.1063/1.1709847 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/38/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Hardness of pure alkali halides J. Appl. Phys. 44, 982 (1973); 10.1063/1.1662382 Electric Field Gradient at a (100) Surface of Some Alkali Halide Crystals J. Chem. Phys. 48, 1780 (1968); 10.1063/1.1668911 Some Properties of Alkali Halide Crystals Am. J. Phys. 29, 182 (1961); 10.1119/1.1937716 Dislocation Etch Techniques for Some Alkali Halide Crystals J. Appl. Phys. 29, 1768 (1958); 10.1063/1.1723043 A New Method of Preparing Strongly Luminescent Thallium Activated Alkali Halides and Some Properties of These Phosphors J. Chem. Phys. 16, 241 (1948); 10.1063/1.1746851 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 134.153.184.170 On: Sat, 22 Nov 2014 11:44:38 2166 H. SOBOL AND A. L. EICHENBAUM of several hundred volts. His experiments were performed with a system where (:J near the exit aperture was much lower than in the bulk of the cathode. Electron extraction from a low (:J region took place and the bulk of the plasma was used only as a transport media to the aperture vicinity. The extraction is in an almost ion-free region and nearly follows a! law. The experimental results therefore cannot be explained by the present theory. A model wherein (:J is variable is necessary for this case. tions has been presented. Extracted current as a function of extractor potential has been computed. The theory, when applied to the separate emitter cathode, gives results that agree fairly well with experiments. More definitive experiments must be carried out with the CE cathode to determine the values of {3 and T accurately and additional theoretical work is needed to explain the case for the variable {3. IV. CONCLUSIONS The authors are pleased to acknowledge the work of C. B. Davis in programming the equations for potential distribution. The assistance of L. Seministow in performing the experiments is gratefully acknowledged. Current extraction from plasma cathodes has been treated theoretically by space-charge analyses. Stability data for simple space-charge mode potential distribu- ACKNOWLEDGMENTS VOLUME 38, NUMBER 5 JOURNAL OF APPLIED PHYSICS APRIL 1967 Preparation of Pure Alkali Halide Crystals and Some of Their Properties* J. M. PEECH,t D. A. BOWER, AND R. O. POHL Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, N(!"/J.J York (Received 17 October 1966) This paper describes some of the techniques that have been used successfully in this laboratory for the purification and preparation of a number of alkali halide crystals, particularly KCl. Measurements of the optical absorption, ionic conductivity, and thermal conductivity, are employed to test the purity of the crystals. Surface contamination of the zone-refined salt upon exposure to air, during handling between the different steps of preparation, appears to be a major source of contamination as judged by the ionic conductivity. A special apparatus has, therefore, been designed to permit both zone refining and subsequent seed pulling of the crystal to be carried out without changing the protective atmosphere. I. INTRODUCTION ITH increasing sophistication of the experimental studies on the physical properties of crystalline solids, the demand for large single crystals of greater chemical and physical perfection and better controlled doping has grown markedly over the years. This paper summarizes some of the work carried out at Cornell University on the preparation of alkali halide crystals. Because alkali halides are relatively soft, we have found that alkali halide crystals grown by the Kyropoulos seed-pulling method showed considerably fewer dislocations and small-angle grain boundaries than crystals grown by gradient methods, unless extremely large containers were used for the latter technique. We have, therefore, utilized the seed-pulling technique and describe here two types of furnaces that we have used in this work. W * This work was supported by the U.S. Atomic Energy Commission and the Advanced Research Projects Agency. t Present address: Division of Engineering and Applied Physics, Pierce Hall, Harvard University, Cambridge, Massachusetts. Of the various methods used to purify the alkali halide salt prior to crystal growing in these furnaces, we have found two to be particularly effective. The first method consists of treating the alkali halide under its halogen gas and has been used for the chlorides and bromides and, with somewhat less success, for the iodides, either below or above the melting point of the alkali halides. This method has been found very effective for the removal of oxygen-containing impurities,l,2 which are frequent contaminants. The second method is zone refining, which we have used for KCI. During the last four years, several investigators2- S have advocated the use of zone refining as a method for the preparation of some high-purity alkali halides, notably KCI, which are especially low in multivalent M. V. Klein, Phys. Rev. 122, 1393 (1961). T. M. Strinivasan and W. D. Compton, Phys. Rev. 137, A264 (1965). 3 H. Griindig, Z. Physik 158, 577 (1960); Z. Physik 182, 477 (1965) . 4 H. Kanzaki, K. Kido, and T. Ninomiya, J. Appl. Phys. 33, 482 (1962). • H. Griindig and E. Wassermann, Z. Physik 176, 293 (1963). 6 R. W. Warren, Rev. Sci. lnstr. 36, 731 (1965). 1 2 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 134.153.184.170 On: Sat, 22 Nov 2014 11:44:38 PRE PAR A T ION AND PRO PER TIE S 0 F PUR E A L K A L I HAL IDE CRY S TAL S ion impurities. Typically, these authors have studied the properties of single crystals of zone-refined KCI that were cleaved from local singular regions of the zone refined ingots. This Bridgman-like growth of these crystals is, however, a random process, influenced at least by the surface irregularities of the sides of the boat or the tube containing the salt, so that much of the zone-refined ingot is usually polycrystalline. Single crystals, obtained from zone-refined ingots, therefore, seldom exceed 2 cm in any dimension and usually exhibit many small-angle grain boundaries. To overcome these limitations, we have seed-pulled crystals from zone-refined material using several different methods. It soon became apparent that exposure of the salt to air during transfer of the salt from the zone-refining apparatL's to the crystal growing furnace resulted in serious ~ urface contamination of the salt. This led to develcpment of a special apparatus in which zone r . fining and subsequent crystal growing could be carried out without changing the protective atmosphere. This paper describes, in some detail, the apparatus that has been used in this laboratory for the purification and preparation of alkali halide crystals and presents the results obtained with KCl. II. DESCRIPTION OF THE APPARATUS For seed pulling under an inert-gas protective atmosphere, furnaces of rather conventional design have been used? A large stainless steel cylindrical highvacuum chamber, completely water-cooled and fitted with a quartz observation window, contains both the graphite heating element and the molten salt in a crucible. It has been found necessary to heat the supposedly spectroscopically pure-graphite8 heating element alternately in vacuum and in chlorine gas at 1400°C before use in the crystal growing furnace. The same Cb treatment is given to the pyrolytic graphite crucibles.9 The seed crystal is fused with a 02-H2 torch to a heavy platinum wire, which is clamped to the watercooled stainless steel pulling rod. The pulling rod is rotated at about 1 rpm while it is raised at the rate of about 2.5 cm per hour. Another small crystal is secured to a second stainless steel rod which can be manipulated from the outside in such a manner as to reach all parts of the surface of the melt. This "dipping rod" is used for the removal of specks of graphite, floating on top of the melt, which may have entered the salt during prior purification processes. Under the protective atmosphere of high-purity argon gas,lO which had been further cleaned by a 7 Shop drawing may be obtained upon request from the Non· Metal Crystal Growth Facility of the Materials Science Center at Cornell University. 8 Graphite was obtained from Ultra Carbon Crop. g Pyrolytic graphite crucibles were obtained from High Temperature Materials, Inc. 10 Argon was obtained from Matheson Co. Typical mass spectroscopic analysis of this gas gave: C02<1 ppm, O2<5 ppm, H2<1 ppm, CO<l ppm, N 2 <5 ppm, CH.<2 ppm, H20<5 ppm. 2167 2" IT-··--·ft"":...,....,..~ 1.500~ IT ,i MAIN TEFL.ON BODY 1.125- ljl~ ~,~.~'~~~~~~~~~ 5 PIECE TEFLON GASKET ASSEMBLY TEFLON PLUG FIG. 1. Halogen-resistant high-vacuum seal mounted in Teflon plug. number of techniques,!! the crystals grown in the stainless steel furnace described above were always found to exhibit ultraviolet absorption bands in the region of 200 mJL with peak absorption constants of 0.1 to 0.5 cm-!. Assuming that this band is caused by OH- present in the crystals, this absorption corresponds!2 to a number density of OH- between 0.3X 1016 and 1.5X 1016 cm-3 • This ultraviolet absorption can be decreased by at least a factor of 10 if the crystals are grown in a furnace under a protective atmosphere containing their respective halogen gas. Such a furnace was first conceived by H. Kappel in this laboratory. It consists of a vertical flat-bottomed quartz tube, 5 to 7 cm in diam and 50 cm long which contains the crucible. The lower end of the ~ube is inserted into a wirewound heating element. To achieve the desired temperature distribution in the crucible a 2-cm hole is drilled in the bottom of the heatin~ element and a metal rod is inserted as a "heat sink" for the bottom of the crucible. The seed crystal is fastened to the water-cooled quartz pulling rod by means of a graphite chuck which, in turn, is attached to the rod with a snug push fit.13 The protective atmosphere used in this furnace consists of about 0.2 atm of C12 and 0.8 atm of Ar. The use of a higher partial pressure of C12 results in the formation and entrapment of gas bubbles in the boule. The technical innovation of this design is a halogen resistant high-vacuum seal for the quartz pulling rod. It consists of a Teflon Wilson seal mounted in a Teflon plug which is, in turn, vacuum-sealed with a flange to the top of the quartz tube. The Teflon plug containing this seal is shown in Fig. 1. The two procedures used for purification of the salt i.e., halogen treatment and zone refining, have bee~ II We presently pass the argon through a TiZr filter at 700°C and then through activated charcoal at -80°C. 12 C. K. Chau, M. V. Klein, and B. Wedding, Phys. Rev. Letters17,521 (1965); and B. Fritz, F. Luty, andJ. Anger Z. Physik 174,240 (1963). ' 13 It has recently been found possible to fuse the seed crystal directly onto the pulling rod with a OrH2 torch since KCI " containing OR-, wets quartz. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 134.153.184.170 On: Sat, 22 Nov 2014 11:44:38 2168 PEECR, BOWER, AND ,(,' j' / .' .r~ / ~.) PORL in Fig. 2. An additional furnace (not shown in the figures), which extends over the full length of the boat as well as the T section of the outer tube, is placed around the outer tube and is heated to 700°C first under Cb and then under vacuum. A fresh atmosphere of Cb is then added (while the furnace is at 500°C) and the temperature of the furnace is increased to 900°C to melt the salt in the boat. This additional furnace is then removed, and the zone refining is commenced. The Cl2 atmosphere is exchanged for a Cb-Ar mixture after 10 passes, after which 40 more passes are made under the same protective atmosphere that is to be used in growing the crystal. III. EXPERIMENTAL RESULTS FIG. 2. Schematic drawing of the combined apparatus for zone refining and seed pulling under halogen atmosphere. The above configuration of the apparatus is for zone refining. The numbered components of the apparatus are: (1) mechanical drive for the zone-refining heater, (2) outer quartz tube, (3) inner quartz tube containing the boat, (4) pyrolytic graphite boat, (5) quartz rod for pulling the boat, (6) inner quartz liner, (7) flange for Teflon plug, (8) Teflon plug with Wilson seal, (9) pumping line, (10) mobile heater for zone refining, (11) seed crystal, (12) graphite chuck, (13) 50/60 quartz inner ground joint, (14) 50/60 Pyrex outer ground joint, (15) Teflon plug with Wilson seal, (16) quartz seed pulling rod, (17) tower for the pullingrod mechanical drive. described sufficiently elsewhere2- 4 and will not be repeated here. In the present work, it has been found desirable to perform the zone refining and the subsequent seed pulling in a single apparatus without changing the atmosphere. Such an apparatus is shown in Figs. 2 and 3. In Fig. 2, a 20-in.-Iong pyrolytic graphite boat (4) is positioned for the zone refining inside an inner quartz tube (3) which protects the outer quartz tube (2) against KCI spillage. In this configuration, the inner quartz liner (6) (not to be confused with the inner quartz tube) collects the evaporated salt, during zone refining, which would otherwise condense on the walls of the outer tube as well as on the seed crystal (11) in the chuck (12). After completion of the zone refining, the boat inside the inner quartz tube (3) and the liner (6) are pulled by a quartz rod (5) to the "T" region of the outer tube so as to position the clean end of the ingot directly below the seed-crystal pulling rod (16) as shown in Fig. 3. In this position, the ingot is remelted by means of a U-shaped furnace (22), with a heat sink at the bottom, which surrounds the outer tube. Both the quartz rod (15) and the pulling rod (16) are brought through a vacuum seal in a Teflon plug as shown in Fig. 1. This part of the apparatus thus closely resembles the quartz furnace described above. Prior to zone refining, the graphite chuck (13), the boa t (4), the inner quartz tube (3) and the quartz liner (6) are Cl2 treated at lO00°C. The boat is then filled with KCI, pretreated by bubbling Cb through the molten salt, and is placed inside the inner quartz tube which is positioned in the outer quartz tube as shown A. Ionic Conductivity With the exception of KCI crystals cleaved directly from zone-refined ingots, we found that virtually all crystals, including commercial crystals, displayed an extrinsic ionic conductivity at 250°C in the range indicated by the vertical bar in Fig. 4. In particular, curve Q11 was obtained from a crystal pulled under Cb from untreated commercial salt in the quartz furnace. Similar results were obtained for crystals pulled in the stainless steel furnace. Curve OR gives the ionic conductivity for high-purity KCI prepared by Butler and co-workersl4 at the Oak Ridge National Laboratory. This KCI crystal was pulled from salt which was purified chemically but which was not zone refined. Curve Z is typical of the ionic conductivity of crystals that have been cleaved directly from zone-refined ingots. FIG. 3. Schematic drawing of the apparatus illustrated in Fig. 2 except that the configuration shown here is for crystal growing. The numbered components of the apparatus that are not already described in Fig. 2 are: (18) stainless steel capillary attached to cooling water source, (19) platinum pin for securing chuck on pulling rod, (20) molten salt of clean end of ingot, (21) zone-refined ingot which is not remelted, (22) U-shaped heater for remelting clean end of ingot. 14 C. T. Butler, J. R. Russell, R. B. Quincy, Jr., and D. E. LaValle, J. Chern. Phys. 45, 968 (1966). [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 134.153.184.170 On: Sat, 22 Nov 2014 11:44:38 PRE PAR A T ION AND PRO PER TIE S 0 F PUR E A L K A L I HAL IDE CRY S TAL S It had been hoped that crystals pulled from such zonerefined salts would also have a low ionic conductivity. This, however, was not true. Curve ZK represents the ionic conductivity of crystals grown from zone-refined salt in either stainless steel or quartz furnace. The ionic conductivity of these crystals was in all cases no better than that measured for crystals grown from commercial salt. This is in agreement with the results of Grtindig. 5 The ionic conductivity of his zone-refined ingot was virtually identical to that shown by our curve Z. Curve G2 is the conductivity of crystals regrown by Grtindig from his zone-refined KCl using the Bridgman technique. The following three experiments show that the drastic deterioration of the quality of the regrown crystals was caused by surface impurities, which were presumably collected on the zone-refined salt during transfer from the zone-refining apparatus to the crystal-growing furnace, and which were then distributed through the bulk of the remelted salt. In the first experiment, a KCl ingot was zone-refined and its ionic conductivity was measured after 1, 30, and 31 passes. For each set of measurements, the ingot was removed from the zone refiner and samples were cleaved from the ingot. During this process, the ingot had to be exposed to air. After each measurement, the ingot was returned to the boat with the pieces of the ingot ordered in their original position. The ionic conductivity for the clean end of the ingot, at 250°C, was found to be 3XlO- lO , 9XI0-13 , and 4XIo-11 Q-1 cm- 1 after 1, 30, and 31 passes, respectively. The ionic conductivity of the ingot, after 31 passes, was found to be of the same order of magnitude as the conductivity of a typical KCl crystal regrown from the zone-refined salt. This experiment indicated that the contamination of the ingot, after the 30th pass, was due to either handling of the ingot during cleaving, or exposure to air, or both. In the second experiment, a zone-refined ingot of KCl was prepared using the same experimental procedure as that just described in the first experiment. After 30 passes, air was deliberately admitted into the zJne-refining tube and, after a few minutes, the air was replaced by a fresh Cb atmosphere. An additional pass was then made, and the ionic conductivity of the ingot was found to be 2.5X 10-11 Q-1 cm-1 at 250°C. One may safely assume that the ionic conductivity of the clean end of the ingot, after 30 passes, should have been the same as that found for the ingot after 30 passes in the first experiment. In this case, however, the high ionic conductivity cannot be attributed to contamination resulting from handling and cleaving of the ingot. In the third experiment, a comparison was made of the ionic conductivity of two KCl crystals grown under different conditions from zone-refined salt. Crystal ZKl was grown under argon in a stainless steel furnace from an ing1t having ionic conductivity of 1.2 X 10-12 Q-1 cm-1 500"K 600"K 10~9 ~h ~, '" Qc\l, ZK 'In 6 hA q, 66 :0- 10 ·0 '"o.o. 0 ~ ... 0 ~ 10- 11 ° - OR .. ... .... 0 0 "c '" 0 u 10- 12 ~ G2 "0 u 2169 • ZG 0 0 0 u °'6 c oS 0 '60 Z "0 '/, KCI 0 0 IO~13 0 0 IO~ 14L-_--'-_--'--_---'-_ _"--_--'--_-'--_----! 15 16 17 18 , IT FI3. 4. Ionic conductivity of KCI crystals obtained by the different methods described in the text: Curve Z: KCI crystal cleaved from clean end of zone-refined ingot. Curve ZG: KCI crystal grown in apparatus shown in Figs. 2 and 3. Curve ZK: typical KCI crystal grown in this laboratory from zone-refined salt in either stainless steel or quartz furnace. Curve Qll: KCI crystal pulled under Cl, from untreated commercial salt in a quartz furnace. Curve G 1: KCI crystal grown by Griindig from commercial salt using the Bridgman technique. Curve G2: KCI crystal grown by Griindig from his zone-refined salt using the Bridgman technique. Curve OR: KCI crystal grown at Oak Ridge National Laboratory from chemically purified salt using the Kyropoulos technique. at 250°C, with a minimum exposure of the ingot to air, and crystal ZK2 was grown under C12 in a quartz furnace from an ingot having ionic conductivity of 2X 10-13 Q-1 cm-1 at 250°C, but, in this instance, the ingot had been cleaved into smaller pieces, thus exposing a greater surface area to air. The ionic conductivities of crystal ZKl and crystal ZK2 at 250°C were found to be 6X 10-11 and 9X 10-11 Q-1 em-I, respectively. Thus, contrary to our expectation that crystal ZK2 grown under C12 should be purer, the ionic conductivity of crystal ZK2 was found to be slightly higher than that of crystal ZKl grown under argon. All evidence from these three experiments indicates that surface impurities, resulting from exposure of the zone-refined salt to air, constituted a major source of contamination. Several crystals were then grown using the apparatus, described in the last section, in which zone refining and the subsequent seed pulling could be accomplished [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 134.153.184.170 On: Sat, 22 Nov 2014 11:44:38 PEECH, 2170 170 , 90 180 2CO BOWER, AND miL 10 2 E u ;:: 0 ~ c 10 0 u 1 KCI c 0 (l 0 en n <! 10 0 0 '! Ci 0 10- 1 ZG3 10- 2 L-_-L-_.l.-_.l.-_-'--_--'---_-'-_-L._.....J 74 72 70 68 Photon 66 6.4 Energy, 62 60 58 eV FIG. 5. Optical absorption of the different KC\ crystals described in the text: Curve 1: KCl crystal exhibiting OH- impurities. Curve 2: KCl crystal, exhibiting iodine impurities, measured by Mahr. Curve 3: KCl crystal measured by Tomiki; sample was cleaved directly from a zone-refined ingot. Curve ZG3: KCl crystal grown using the apparatus shown in Figs. 2 and 3. without changing the atmosphere. Curve ZG is representative of the ionic conductivity of crystals grown using this technique. It can be seen from Fig. 4 that the ionic conductivity given by curve ZG is markedly lower than that of any of the aforementioned seedpulled crystals. Extrapolating from the measurements of Kelting and Witt,15 who correlated the ionic conductivity and the CaH-ion concentration as determined by chemical analysis, we estimate the total multivalent ion number density for boule ZG to be 6-16X101s cm-s. B. Optical Absorption The optical absorption measurements were made using a modified Beckman IR7 infrared spectrophotometer, Cary spectrophotometers models 14 and 15, and a vacuum single-beam ultraviolet spectrophotometer which was built by P. L. Hartman and co-workers in this laboratory. The results of optical measurements made on KCI crystals, prepared by different methods, 11 H. Kelting and H. Witt, Z. Physik 126, 697 (1949). POHL are shown in Fig. 5. Curve 1 shows an OH- absorption peak which is typical for KCI crystals, grown under argon. Curve 2, measured previously by Mahr,16 shows the absorption of a KCI crystal grown from Cb-treated reagent grade KCl. The absorption peak near 6.6 eV had been ascribed by Mahr to iodine, the concentration of which he estimated to be about 1015 cm-s. For measuring optical absorption by KCI crystals grown in the combined zone-refining and seed-pulling apparatus, shown in Figs. 2 and 3, a boule having a length of 7.5 cm and a cross section of lOX 10 mm was used. For future reference here and in the discussion of the thermal conductivity results, this boule will be designated ZG3. The thickest sample cleaved from ZG3 gave a path length of 5.9 cm. The optical absorption by this sample was measured from 2.5 to 15.0 JI- and from 175 to 250 mJl-, and no absorption was observed except at the fundamental exciton edge with its onset at about 180 mJl-. This absorption is plotted as curve ZG3 in Fig. 5 from measurements using both the Cary model 15 and the vacuum spectrophotometers. The slit width for the Cary model 15 did not exceed 0.5 mm, so that the incident light had a bandwidth of less than 0.3 mJl- over the entire range of wavelength measured (250-178 mJl-). The data obtained with the vacuum spectrophotometer were not analyzed according to the method originally suggested by Moser and UrbachP The data points, given by the equation, absorption constant X sample thickness= 1, consistently fell on the lower branch of the inverted S-shaped absorption curve which was obtained for each sample thickness measured. The curve ZG3 was, therefore, fitted through the steepest portion of these inverted S-shaped curves. Curve 3 in Fig. 5 shows the optical absorption of KCI crystals obtained directly from zonerefined ingots as reported by Tomiki. 18 The salt, in this case, was chemically purified and vacuum-distilled before zone refining. Assuming that the results of Tomiki represent the optical absorption by a bromine-free KCl crystal, we would estimate19 the concentration of Br in boule ZG3 to be 6-10X 1016 bromine ions cm-s. This is not unreasonable, since zone refining is not expected to be effective for removing Br impurities from KCl. C. Thermal Conductivity In Fig. 6 we compare the thermal conductivity of KCI crystals grown by the three different methods. The thermal conductivity shown by curve A is typical of that obtained for crystals grown under argon as protective atmosphere from either vacuum dried KCI powder or pure KCI that had been purified in a separate step by heating the powder in Cb gas close to the H. Mahr, Phys. Rev. 125, 1510 (1962). F. Moser and F. Urbach, Phys. Rev. 102, 1519 (1956). 18 T. Tomiki, J. Phys. Soc. Japan 21, 403 (1966). 19 K. Kobayashi and T. Tomiki, J. Phys. Soc. Japan IS, 1982 (1960); 16, 1417 (1961). 16 17 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 134.153.184.170 On: Sat, 22 Nov 2014 11:44:38 2171 PREPARATION AND PROPERTIES OF PURE ALKALI HALIDE CRYSTALS melting point. 20 The curve labeled Qll shows the thermal conductivity of a crystal grown straight from commercial KCl powder under C12 protective atmosphere. A noticeable improvement in the thermal conductivity below lOoK is shown by this curve which has been published previously.21 The top curve (labeled ZG3) gives the conductivity of a sample cleaved from boule ZG3, which was grown in the combined zonerefining and seed-pulling apparatus shown in Figs. 2 and 3. The thermal conductivity of this crystal is about 5% higher than that of crystal Qll at 80 oK, 15% higher around the maximum, and 10% higher in the boundary region, allowing for the larger size of crystal ZG3. The Casimir scattering length in this sample is still about 15% smaller than that predicted by theory22 depending somewhat on how one chooses to fit the low~temperature data. It is believed, however, that this discrepancy is caused partly by accidental plastic deformation of the soft KCl crystal which occurred during either cleaving or mounting of the sample in the cryostat. The thermal conductivity shown by curve L was obtained on boule ZG3 before it was cleaved. For these measurements, the sides of the boule had been shaved off very gently to produce a uniform cross section of 12.75X13.0 mm2. The boule was then sandblasted in the same way as all the other samples. From the T 31ine, which the data approach at the lowest temperatures, a Casimir scattering length was obtained that was only 8% smaller than that predicted by theory. The discrepancy between the results of thermal conductivity obtained on the large and the small samples emphasizes the importance of physical imperfections in these rather soft crystals. From the small improvement in thermal conductivity resulting from the zone refining, we conclude that the thermal conductivity of crystal ZG3 is indeed that of pure KCl and that the relatively low maximum in the conductivity is a result of the strong isotopic scattering in KC1.23 From the closeness of the two curves for '0 For example of data obtained on samples prepared by either of these methods, we refer to C. T. Walker and R. O. Pohl, Phys. Rev. 131, 1433 (1963); or J. W. Schwartz and C. T. Walker, Phys. Rev. Letters 16, 97 (1966). 'I W. D. Seward, in Proceedings of Ihe Ninth International Conference on Low Temperature Phy~ics, Columbus, Ohio, 1964, J. G. Dannt et al., Eds. (Plenum Press, Inc., New York, 1965), p. 1130; also D. W. Seward and V. Narayanamurti, Phys. Rev. 148,463 (1966). "For a detailed discussion of the boundary effect, we refer to a study of the boundary effect on isotopically pure LiF by P. D. Thacher, Ph.D. thesis, Cornell University, 1965, Cornell University Materials Science Center Report No. 369, and submitted to Phys. Rev. 23 G. A. Sl<Lck, Phys. Rev. 105,829 (1957). 10 '" '" ~ 0> "0 E u '0 ~ ,., > g ~ -g o "\o 0 0 ~o 0 E Q; .c f- .1 ~ __ ~ ______- L_ _ _ _- L_ _ _ _ ~ ______ 10 Temperature, ~ __ ~ 100 Degree K FIG. 6. Thermal conductivity of KCI crystals pulled from the melt under different conditions. Curve A: Starting material heated in Cb below the melting point before transfer to the stainless steel furnace; the crystal was pulled under argon atmosphere and had a cross section of 5X5 mm'. Curve Qll: Crystal pulled from the melt under Cb protective atmosphere without any prior purification; the cross section of the crystal was 5.1 X 5.1 mm'. Curve ZG3: KCI zone-refined and pulled in the same apparatus without change of the Cb atmosphere; the cross section of the crystal was 6.78X5.63 mm'. Curve L: Same boule as ZG3, but having a larger cross section: 12.7X 13.0 mm'. crystals ZG3 and Qll, it follows that, for all practical purposes, straight seed-pulling under C12 atmosphere produces sufficiently pure crystals for investigations of the thermal conductivity of pure KCl. It should be interesting, however, to study the influence of zone refining on the conductivity of materials of higher isotopic purity, like, for instance, NaI or even KI. ACKNOWLEDGMENTS The techniques described in this paper are based to a large extent on work done earlier in this laboratory by Hans Kappel, John Lombardi, and Ulrich Wittel, to whom we are greatly indebted. Thanks are also due to Eleftherious Logothetis for measuring the spectra in the vacuum ultraviolet spectrophotometer, to Professor Herbert Mahr for helpful discussion concerning their interpretation, and finally to Richard Castonguay for his assistance in the measurements using the Cary model 15 spectrophotometer. The financial support of the US Atomic Energy Commission and the Advanced Research Projects Agency is gratefully acknowledged. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 134.153.184.170 On: Sat, 22 Nov 2014 11:44:38