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Crystal growth of lead fluorohalides
Journal of Crystal Growth 37 (1977) 187—188 © North-Holland Publishing Company
CRYSTAL GROWTH OF LEAD FLUOROHALIDES A.F. CORSMIT and G.J. DIRKSEN Solid State Uhemistry Department, State University Utrecht, Utrecht, The Netherlands Received 7 December 1975; revised manuscript received 18 July 1976
The experimental procedures for the dehydration of starting materials, the application of zone refining in synthesis and the use of the Bridgman method in crystal growth of lead fluorohalides are described. Large optically clear cystals were obtained.
In the course of a study of the photodecomposition of the lead halides the luminescence properties and electrical conductivity of these compounds have attracted much attention during the past years in our
luminescence [5] and was estimated to be about 1 at% or less. We paid careful attention to the dehydration of all starting materials. PbC12 and PbBr2 were zone
laboratory. Within this program it seemed interesting to prepare lead halides with a pronounced layered structure [11,viz. PbFX (X = Cl or Br) to investigate the anisotropy of the properties mentioned above, These compounds have a layered tetragonal structure, with cell dimensions a = 4.10 A, c = 7.22 A and a = 4.19 A, c = 7.62 A for PbFC1 (Matlockite) and PbFBr, respectively [2]. Synthesis of PbFX by wet methods [3] was excluded because of the possibility of incorporation of 0H ions. We found that slowly cooling molten equimolar amounts of PbF2 and PbX2, even if carefully weighed, resulted in a large number of approximately parallel oriented crystal platelets with well developed 001 faces. The platelets were isolated from each other by thin air layers due to shrinking of the melt during solidification. To grow large crystals this habit must be modified. We therefore decided to use the method of zone refining starting from an excess of one of the components. However, in this technique there is the possibility of the formation of nonst6ichiometric compositions (PbF1~X1+~ or PbF1+~X1~)depending on the excess component PbX 2 or PbF2, respectively, despite the small regions of solid solubility of PbF2 or PbX2 in PbFX as appears from the phase diagrams given by Sandonnini [4]. A deviation from stoichiornetry was in fact observed in the crystals obtained by measuring their
refined by a method developed for lead halides in this laboratory and described elsewhere [6]. PbF2 of Merck Optipur quality was prepared according to the method described by Jones [7]. IR absorption in the region around 3~iwas absent after these treatments. This indicates the absence, or at least a very low concentration, of OH— ions. Equimolar amounts of pretreated PbF2 and PbX2 plus a 10% excess by weight of PbX2 (also to compensate for evaporation losses) were placed in a platinum 10% rhodium boat, which was put into an unsealed quartz ampoule. After evacuating, the ternperature was raised to 300°C for 24 h. Next dry nitrogen (obtained from boiling liquid nitrogen) was admitted at atmospheric pressure and the temperature. increased to 20°above the melting point of PbF2 for half an hour to complete the mixing of the constituents. After cooling to room temperature the ampoule was evacuated again, flushed with dry nitrogen and sealed off at a slightly reduced pressure. The ampoule was then transferred to a conventional zone refining apparatus equipped with 1 theresistance contents heating. After 5 to 8 passes at 1 cm h of the platinum boat showed in the middle a polycrystalline mass of large, optically clear pieces of PbFX (see fig. 1). At the ends, a mass of lamellar crystals was formed, slightly coloured by impurities. According to atomic absorption analysis the crystals —
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187
188
A.F. Corsmit, G.J. Dirksen
/ Crystal growth of lead fluorohalides
Fig. 1. PbFBr crystals, cleaved along (001) (grid 1 mm).
contained neither rhodium nor platinum, For electrical conductivity measurements monoand trivalent ion-doped crystals were necessary. We prepared Ag, Tl, La and Bi-doped crystals with a typical dope concentration in the crystals of 100 ppm. To obtain these doped crystals, large pieces, selected from the middle of the boat, were placed with about twice the desired amount of doping material into an elongated slightly conical platinum— 10% rhodium crucible, which was again placed in a quartz ampoule and submitted to a similar drying procedure as described above, before sealing off. The ampoule was then treated in a convential Bridgman furnace, the thermocouple readings of which had been checked against the melting point of the relevant lead fluorohalide. The measured values agreed with those values given by Sandonnini [4], viz., 601°Cfor PbFC1 and 561°Cfor PbFBr. The upper part of the furnace was maintained 20 C above, the
lower part 20°Cbelow the melting point of the relevant fluorohalide. Separate temperature regulators were used, fed by a constant voltage unit. The lowering rate was 1—3 mm h’. Optically clear crystals were obtained with an area of 1 .5 cm2 and 2 mm thick. References [1] J. Schoonman, G.J. Dirksen and G. Blasse, J. Solid State Chem. 7(1973)245. . [2] W. Nieuwenkamp and J.M. Bijvoet, Z. Krist. 81(1931) 469; W. Nieuwenkamp and J.M. Bijvoet. Z. Krist. 82 (1932) 157. [3] G. Starck, Z. Anorg. Allg. Chem. 70(1911)173. [4] C. Sandonnini,Gazz.Chem. Ital.41 II (1911) 144. [5] A.J.H. Eijkelenkamp, J. Solid State Chem, to be published. [6] B. Willemsen, J. Solid State Chem. 3 (1971) 567. [7] D.A. Jones, Proc. Phys. Soc. (London) B68 (1955) 165.
CRYSTAL GROWTH OF LEAD FLUOROHALIDES A.F. CORSMIT and G.J. DIRKSEN Solid State Uhemistry Department, State University Utrecht, Utrecht, The Netherlands Received 7 December 1975; revised manuscript received 18 July 1976
The experimental procedures for the dehydration of starting materials, the application of zone refining in synthesis and the use of the Bridgman method in crystal growth of lead fluorohalides are described. Large optically clear cystals were obtained.
In the course of a study of the photodecomposition of the lead halides the luminescence properties and electrical conductivity of these compounds have attracted much attention during the past years in our
luminescence [5] and was estimated to be about 1 at% or less. We paid careful attention to the dehydration of all starting materials. PbC12 and PbBr2 were zone
laboratory. Within this program it seemed interesting to prepare lead halides with a pronounced layered structure [11,viz. PbFX (X = Cl or Br) to investigate the anisotropy of the properties mentioned above, These compounds have a layered tetragonal structure, with cell dimensions a = 4.10 A, c = 7.22 A and a = 4.19 A, c = 7.62 A for PbFC1 (Matlockite) and PbFBr, respectively [2]. Synthesis of PbFX by wet methods [3] was excluded because of the possibility of incorporation of 0H ions. We found that slowly cooling molten equimolar amounts of PbF2 and PbX2, even if carefully weighed, resulted in a large number of approximately parallel oriented crystal platelets with well developed 001 faces. The platelets were isolated from each other by thin air layers due to shrinking of the melt during solidification. To grow large crystals this habit must be modified. We therefore decided to use the method of zone refining starting from an excess of one of the components. However, in this technique there is the possibility of the formation of nonst6ichiometric compositions (PbF1~X1+~ or PbF1+~X1~)depending on the excess component PbX 2 or PbF2, respectively, despite the small regions of solid solubility of PbF2 or PbX2 in PbFX as appears from the phase diagrams given by Sandonnini [4]. A deviation from stoichiornetry was in fact observed in the crystals obtained by measuring their
refined by a method developed for lead halides in this laboratory and described elsewhere [6]. PbF2 of Merck Optipur quality was prepared according to the method described by Jones [7]. IR absorption in the region around 3~iwas absent after these treatments. This indicates the absence, or at least a very low concentration, of OH— ions. Equimolar amounts of pretreated PbF2 and PbX2 plus a 10% excess by weight of PbX2 (also to compensate for evaporation losses) were placed in a platinum 10% rhodium boat, which was put into an unsealed quartz ampoule. After evacuating, the ternperature was raised to 300°C for 24 h. Next dry nitrogen (obtained from boiling liquid nitrogen) was admitted at atmospheric pressure and the temperature. increased to 20°above the melting point of PbF2 for half an hour to complete the mixing of the constituents. After cooling to room temperature the ampoule was evacuated again, flushed with dry nitrogen and sealed off at a slightly reduced pressure. The ampoule was then transferred to a conventional zone refining apparatus equipped with 1 theresistance contents heating. After 5 to 8 passes at 1 cm h of the platinum boat showed in the middle a polycrystalline mass of large, optically clear pieces of PbFX (see fig. 1). At the ends, a mass of lamellar crystals was formed, slightly coloured by impurities. According to atomic absorption analysis the crystals —
.
187
188
A.F. Corsmit, G.J. Dirksen
/ Crystal growth of lead fluorohalides
Fig. 1. PbFBr crystals, cleaved along (001) (grid 1 mm).
contained neither rhodium nor platinum, For electrical conductivity measurements monoand trivalent ion-doped crystals were necessary. We prepared Ag, Tl, La and Bi-doped crystals with a typical dope concentration in the crystals of 100 ppm. To obtain these doped crystals, large pieces, selected from the middle of the boat, were placed with about twice the desired amount of doping material into an elongated slightly conical platinum— 10% rhodium crucible, which was again placed in a quartz ampoule and submitted to a similar drying procedure as described above, before sealing off. The ampoule was then treated in a convential Bridgman furnace, the thermocouple readings of which had been checked against the melting point of the relevant lead fluorohalide. The measured values agreed with those values given by Sandonnini [4], viz., 601°Cfor PbFC1 and 561°Cfor PbFBr. The upper part of the furnace was maintained 20 C above, the
lower part 20°Cbelow the melting point of the relevant fluorohalide. Separate temperature regulators were used, fed by a constant voltage unit. The lowering rate was 1—3 mm h’. Optically clear crystals were obtained with an area of 1 .5 cm2 and 2 mm thick. References [1] J. Schoonman, G.J. Dirksen and G. Blasse, J. Solid State Chem. 7(1973)245. . [2] W. Nieuwenkamp and J.M. Bijvoet, Z. Krist. 81(1931) 469; W. Nieuwenkamp and J.M. Bijvoet. Z. Krist. 82 (1932) 157. [3] G. Starck, Z. Anorg. Allg. Chem. 70(1911)173. [4] C. Sandonnini,Gazz.Chem. Ital.41 II (1911) 144. [5] A.J.H. Eijkelenkamp, J. Solid State Chem, to be published. [6] B. Willemsen, J. Solid State Chem. 3 (1971) 567. [7] D.A. Jones, Proc. Phys. Soc. (London) B68 (1955) 165.