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Growth of large (111) and (1̅1̅1̅) sodium bromate single crystals by Reverse Seeded Solution Growth method
Materials Letters 64 (2010) 640–642
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Growth of large (111) and (1 ̅ 1 ̅ 1 )̅ sodium bromate single crystals by Reverse Seeded Solution Growth method Ch. Snehalatha Reddy a, P.V. Raja Shekar b, K. Gopala Kishan Rao c, K. Kishan Rao a,⁎ a b c
Department of Physics, Kakatiya University, Warangal-506 009, India Department of Physics, SR Engineering College, Warangal-506 371, India Central Instrumentation Centre, Kakatiya University, Warangal-506 009, India
a r t i c l e
i n f o
Article history: Received 10 November 2009 Accepted 14 December 2009 Available online 24 December 2009 Keywords: Growth from solutions Defects Hardness
a b s t r a c t Single crystals of sodium bromate with large size (111) and ( 1 ̅ 1 ̅ 1)̅ faces were grown from solution at a constant temperature of 35 °C by a new seeding technique called Reverse Seeded Solution Growth (RSSG) method. The average lateral and vertical growth rates of crystals grown by this method are 0.8 mm/day and 0.35 mm/day respectively, which results in thick crystal platelets. The grown crystals were characterized by chemical etching, UV–Vis and Vickers microhardness studies. Anisotropy in hardness has been observed for (111) and its opposite ( 1 ̅ 1 ̅ 1 ̅) faces due to non-centrosymmetric nature of the crystal. The results show that crystals grown by RSSG method have better optical transmittance and hardness. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Sodium bromate (NaBrO3) and sodium chlorate (NaClO3) are isomorphous crystallizing in enantiomorphic point group 23. It is well known that NaBrO3 and NaClO3 of same chirality have opposite sense of optical rotation. These crystals exhibit some important properties like optical activity [1,2], ferroelectricity and piezoelectricity [3]. Although these crystals are isostructural, they grow with different morphologies. NaBrO3 crystals grow in pyramidal form predominantly bounded by two sets of {111} faces as shown in Fig. 1, whereas NaClO3 grows in cubic form with {100} faces. Due to its convenient shape, mostly NaClO3 crystals have attracted researchers to study the growth [4–6], physical and optical properties [1,7,8]. The pyramidal shape of NaBrO3 crystals is inconvenient to study some of the properties in as-grown form; hence, they have to be cut and polished. This process results in the creation of new dislocations and they affect the mechanical properties of the crystals. In view of this, we have made an attempt to grow NaBrO3 crystals with large size (111) and ( 1 ̅ 1 ̅ 1 ̅) faces by a new seeding technique called Reverse Seeded Solution Growth (RSSG) method. The aim of the present communication is to report the growth of large and transparent NaBrO3 crystals by the RSSG method. Further, these crystals were characterized by chemical etching, optical transmittance and microhardness studies and the results were compared with conventionally grown crystals, which are presented in detail in the following sections.
2.1. Solubility
⁎ Corresponding author. Tel.: + 91 9866275026. E-mail address: [email protected] (K. Kishan Rao). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.12.031
The solubility of NaBrO3 (Fluka, 99.7% purity) is determined in the temperature ranging from 25 to 60 °C in the interval of 5 °C by the following procedure. At each temperature 50 mL of slightly undersaturated solution is prepared by using three times recrystallized salt dissolved in deionized double distilled water. A crystal of known amount of weight is introduced into the growth cell [9] and observed continuously under a microscope. Initially the crystal starts dissolving and after sometime when equilibrium is reached, recovery starts. Now the seed crystal is removed and again its weight is determined. At each temperature the experiment is repeated at least four times so as to minimize errors and the solubility diagram has been prepared (Fig. 2). 2.2. Crystal growth Single crystals of NaBrO3 were grown from their aqueous solutions by slow evaporation method at a constant temperature of 35 °C. Initially, 400 mL saturated solution of NaBrO3 was prepared by dissolving known amount of salt in deionised double distilled water using the solubility diagram. After ascertaining the saturation, the filtered solution is uniformly transferred into crystallizers. Self nucleated seed crystals are allowed to grow at the base of the crystallizer (conventional growth method). Whereas in the RSSG method, the seed crystals of size 3–4 mm along the <110> direction were selected and the apex part i.e. ( 1 ̅ 1 ̅ 1 ̅) face of the seed crystal is glued and mounted on an L-shaped Perspex holder (hence termed as
C.S. Reddy et al. / Materials Letters 64 (2010) 640–642
641
Fig. 1. Crystal habit of NaBrO3.
Fig. 3. NaBrO3 crystals grown by a) conventional method and b) RSSG method.
3.2. Chemical etching Fig. 2. Solubility diagram of NaBrO3.
Reverse Seeded Method). The glue is allowed to dry for about 8 h and then these holders are carefully introduced into the solutions. Now these solutions are allowed to slowly evaporate and the rate of evaporation is controlled by covering the crystallizers with perforated lids. In a period of about 15 days good quality crystals are obtained.
3. Results and discussion 3.1. Growth rate Fig. 3 (a) and (b) shows photographs of NaBrO3 crystals grown by conventional and RSSG methods. From the photograph it can be observed that (111) faces of these crystals grown by RSSG method are more transparent than conventionally grown crystals. Microscopic studies using Magnus MLX microscope confirmed that the dominant volume of the crystals obtained by RSSG method was either free from inclusions or contained few inclusions of micron size, which are mostly confined to the edges. The average lateral and vertical growth rates of RSSG crystals are 0.8 mm/day and 0.35 mm/day respectively. This indicates that the lateral growth rate of these crystals is predominant compared to vertical growth rate. The average areas of crystals grown by conventional and RSSG methods are about 100 mm2 and 148 mm2 for (111) face, whereas 6 mm2 and 66 mm2 for ( 1 ̅ 1 ̅ 1 ̅) face respectively. From the observations made on a number of crystals, it is concluded that the crystals obtained by conventional growth method have (111) face planar area about 16 times that of ( 1 ̅ 1 ̅ 1 ̅) face, which leads to pyramidal shape of the crystal, whereas it is about 2.2 times for crystals grown by RSSG method which results in thick crystal platelets. This indicates that RSSG method yields crystals with considerably large size-parallel (111) and ( 1 ̅ 1 ̅ 1 ̅) faces of comparable area. Hence, they can be directly used for different applications without cutting and polishing. This method may be employed for crystals which grow in pyramidal shape and also with less planar area.
Chemical etching has been employed to study the distribution and density of dislocations in crystals grown by conventional and RSSG methods using a microscope. These crystals are etched in the mixture of acetic acid and formic acid. When the crystal was etched for 5 s, equilateral triangular etch pits were observed, similar to reported earlier [10]. Successive etch–polish–re-etch experiments confirmed the formation of etch pits to be at dislocations sites. Further, it is interesting to note that as these crystals are non-centrosymmetric, the atomic configuration of (111) and ( 1 ̅ 1 ̅ 1 ̅) faces is not identical. Hence the etchant which has good etching action on (111) face is incapable of etching its opposite ( 1 ̅ 1 ̅ 1 ̅) face. Similar difference in the etching action on (111) and ( 1 ̅ 1 ̅ 1 ̅) faces was observed on GaAs [11], InSb [12] and CdxHg1−xTe [13]. The etching studies made on a number of crystals grown by both methods indicate a non-uniform distribution of etch pits with an average etch pit density of about 2 × 102/cm2 at the central region of the sample and 3.5 × 103/cm2 at the edges. These results suggest that, the dislocations are more numerous at the edges than at the central region due to growth sector boundaries [14]. 3.3. UV–Vis transmission studies The optical transmission along the <111> NaBrO3 crystal has been measured in the entire UV–Vis region from 200 to 650 nm using Perkin-Elmer (Lambda 25) UV–Vis spectrophotometer at room temperature. For this study, the crystals grown by the two methods were cut and polished to have plates of 1 mm thickness. From the spectrum shown in Fig. 4, it is observed that the crystals are transparent in the entire UV–Vis region with a lower UV cut off wavelength at 260 nm. Further, the transmittance percentages in the higher wavelength region of crystals grown by conventional and RSSG method are 45% and 57% respectively, which reveal the improved transparency of crystals grown by RSSG method. The improved transmittance can be attributed to the absence of inclusions which reduces the absorption or scattering of UV–Vis radiation. 3.4. Microhardness studies Microhardness measurements on (111) and ( 1 ̅ 1 ̅ 1 ̅) NaBrO3 crystals grown by conventional and RSSG methods were made using
642
C.S. Reddy et al. / Materials Letters 64 (2010) 640–642
Fig. 5. Plot of Vickers hardness against load for NaBrO3 crystal.
Fig. 4. UV–Vis spectrum for NaBrO3 crystals.
Leitz-Wetzlar microhardness tester fitted with a Vickers diamond pyramidal indenter. Hardness values (Hv) are estimated by the expression, 2
Hv = 1:854P = d
ð1Þ
where P is the load applied in kg and d the diagonal length in µm. The variation of hardness with load for crystals grown by both the methods is shown in Fig. 5. The Hv value increases initially up to a load of 40 g and become almost load independent exhibiting the values of 183 and 190 kg/mm2 on (111) face, whereas 167 and 172 kg/mm2 on ( 1 ̅ 1 ̅ 1 ̅) face for crystals grown by conventional and RSSG methods respectively. The reason for higher hardness on both the faces in RSSG crystals appears to be due to better crystalline perfection and absence of inclusions. Further, the observed anisotropy in hardness on (111) and ( 1 ̅ 1 ̅ 1 ̅) faces is due to non-centrosymmetric nature of the crystals. The load variation can be interpreted by using Meyer's law, n
P = Ad
ð2Þ
where A is a constant and n is the Meyer's index (or work-hardening co-efficient). The value n is determined from the slope of the plots between ln P vs ln d for (111) face. From these plots, the estimated values of n are 2.46 and 2.24 for crystals grown by conventional and RSSG methods respectively. Onitsch [15] and Hanneman [16] had shown that the value of n is 1–1.6 for hard materials and more than 1.6 for soft ones. Thus the present crystals under study are moderately harder.
4. Conclusions The natural habit of sodium bromate crystal with pyramidal shape is restricted by employing a new RSSG method, which results in the growth of large size (111) and ( 1 ̅ 1 ̅ 1 ̅) faces suitable for studying various properties in as-grown form. The crystals obtained by conventional method have (111) face planar area of about 16 times that of ( 1 ̅ 1 ̅ 1 ̅) face, whereas it is about 2.2 times for crystals grown by RSSG method which resulted in thick crystal platelets with parallel faces. The average dislocation density of these crystals at the central region and edges is 2 × 102/cm2 and 3.5 × 103/cm2 respectively. Crystals grown by RSSG method possess higher transmittance and better hardness than grown by conventional method. The difference in the response of the (111) and ( 1 ̅ 1 ̅ 1 ̅) faces to etching and hardness can be attributed to the different atomic configuration on the opposite faces. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Chandrasekaran KS, Mohanlal SK. Pramana J Phys 1976;7:152–9. Xue Dongfeng, Zhang Siyuan. Chem Phys Lett 1998;287:503–8. Mason WP. Phys Rev 1946;70:529–37. Matsunaka M, Kitamura M, Sunagawa I. J Cryst Growth 1980;48:425–34. Wojciechowski K. Cryst Res Technol 1999;34:661–6. Kang Q, Duan L, Hu WR. Int J Heat and Mass Transfer 2001;44:3213–22. Sowa H. J Solid State Chem 1995;118:378–82. Radha V, Gopal ESR. J Indian Inst Sci 1968;50:26–30. Surender V, Arundhathi N, Kishan Rao K. Bull Mater Sci 2006;29:427–32. Kishan Rao K, Sirdeshmukh DB. Cryst Res Technol 1983;18:1125–8. Schell HA. Z Metallkde 1957;48:158–61. Allen JW. Phil Mag 1957;2:1475–8. Fewster PF, Cole S, Willoughby AFW, Brown M. J Appl Physics 1981;52:4568–71. Bhat HL. Prog Cryst Growth Charact 1985;11:57–87. Onitsch EM. Mikroskopia 1947;2:131–5. Hanneman M. Metallurgia Manchu 1941;23:135–40.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Growth of large (111) and (1 ̅ 1 ̅ 1 )̅ sodium bromate single crystals by Reverse Seeded Solution Growth method Ch. Snehalatha Reddy a, P.V. Raja Shekar b, K. Gopala Kishan Rao c, K. Kishan Rao a,⁎ a b c
Department of Physics, Kakatiya University, Warangal-506 009, India Department of Physics, SR Engineering College, Warangal-506 371, India Central Instrumentation Centre, Kakatiya University, Warangal-506 009, India
a r t i c l e
i n f o
Article history: Received 10 November 2009 Accepted 14 December 2009 Available online 24 December 2009 Keywords: Growth from solutions Defects Hardness
a b s t r a c t Single crystals of sodium bromate with large size (111) and ( 1 ̅ 1 ̅ 1)̅ faces were grown from solution at a constant temperature of 35 °C by a new seeding technique called Reverse Seeded Solution Growth (RSSG) method. The average lateral and vertical growth rates of crystals grown by this method are 0.8 mm/day and 0.35 mm/day respectively, which results in thick crystal platelets. The grown crystals were characterized by chemical etching, UV–Vis and Vickers microhardness studies. Anisotropy in hardness has been observed for (111) and its opposite ( 1 ̅ 1 ̅ 1 ̅) faces due to non-centrosymmetric nature of the crystal. The results show that crystals grown by RSSG method have better optical transmittance and hardness. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Sodium bromate (NaBrO3) and sodium chlorate (NaClO3) are isomorphous crystallizing in enantiomorphic point group 23. It is well known that NaBrO3 and NaClO3 of same chirality have opposite sense of optical rotation. These crystals exhibit some important properties like optical activity [1,2], ferroelectricity and piezoelectricity [3]. Although these crystals are isostructural, they grow with different morphologies. NaBrO3 crystals grow in pyramidal form predominantly bounded by two sets of {111} faces as shown in Fig. 1, whereas NaClO3 grows in cubic form with {100} faces. Due to its convenient shape, mostly NaClO3 crystals have attracted researchers to study the growth [4–6], physical and optical properties [1,7,8]. The pyramidal shape of NaBrO3 crystals is inconvenient to study some of the properties in as-grown form; hence, they have to be cut and polished. This process results in the creation of new dislocations and they affect the mechanical properties of the crystals. In view of this, we have made an attempt to grow NaBrO3 crystals with large size (111) and ( 1 ̅ 1 ̅ 1 ̅) faces by a new seeding technique called Reverse Seeded Solution Growth (RSSG) method. The aim of the present communication is to report the growth of large and transparent NaBrO3 crystals by the RSSG method. Further, these crystals were characterized by chemical etching, optical transmittance and microhardness studies and the results were compared with conventionally grown crystals, which are presented in detail in the following sections.
2.1. Solubility
⁎ Corresponding author. Tel.: + 91 9866275026. E-mail address: [email protected] (K. Kishan Rao). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.12.031
The solubility of NaBrO3 (Fluka, 99.7% purity) is determined in the temperature ranging from 25 to 60 °C in the interval of 5 °C by the following procedure. At each temperature 50 mL of slightly undersaturated solution is prepared by using three times recrystallized salt dissolved in deionized double distilled water. A crystal of known amount of weight is introduced into the growth cell [9] and observed continuously under a microscope. Initially the crystal starts dissolving and after sometime when equilibrium is reached, recovery starts. Now the seed crystal is removed and again its weight is determined. At each temperature the experiment is repeated at least four times so as to minimize errors and the solubility diagram has been prepared (Fig. 2). 2.2. Crystal growth Single crystals of NaBrO3 were grown from their aqueous solutions by slow evaporation method at a constant temperature of 35 °C. Initially, 400 mL saturated solution of NaBrO3 was prepared by dissolving known amount of salt in deionised double distilled water using the solubility diagram. After ascertaining the saturation, the filtered solution is uniformly transferred into crystallizers. Self nucleated seed crystals are allowed to grow at the base of the crystallizer (conventional growth method). Whereas in the RSSG method, the seed crystals of size 3–4 mm along the <110> direction were selected and the apex part i.e. ( 1 ̅ 1 ̅ 1 ̅) face of the seed crystal is glued and mounted on an L-shaped Perspex holder (hence termed as
C.S. Reddy et al. / Materials Letters 64 (2010) 640–642
641
Fig. 1. Crystal habit of NaBrO3.
Fig. 3. NaBrO3 crystals grown by a) conventional method and b) RSSG method.
3.2. Chemical etching Fig. 2. Solubility diagram of NaBrO3.
Reverse Seeded Method). The glue is allowed to dry for about 8 h and then these holders are carefully introduced into the solutions. Now these solutions are allowed to slowly evaporate and the rate of evaporation is controlled by covering the crystallizers with perforated lids. In a period of about 15 days good quality crystals are obtained.
3. Results and discussion 3.1. Growth rate Fig. 3 (a) and (b) shows photographs of NaBrO3 crystals grown by conventional and RSSG methods. From the photograph it can be observed that (111) faces of these crystals grown by RSSG method are more transparent than conventionally grown crystals. Microscopic studies using Magnus MLX microscope confirmed that the dominant volume of the crystals obtained by RSSG method was either free from inclusions or contained few inclusions of micron size, which are mostly confined to the edges. The average lateral and vertical growth rates of RSSG crystals are 0.8 mm/day and 0.35 mm/day respectively. This indicates that the lateral growth rate of these crystals is predominant compared to vertical growth rate. The average areas of crystals grown by conventional and RSSG methods are about 100 mm2 and 148 mm2 for (111) face, whereas 6 mm2 and 66 mm2 for ( 1 ̅ 1 ̅ 1 ̅) face respectively. From the observations made on a number of crystals, it is concluded that the crystals obtained by conventional growth method have (111) face planar area about 16 times that of ( 1 ̅ 1 ̅ 1 ̅) face, which leads to pyramidal shape of the crystal, whereas it is about 2.2 times for crystals grown by RSSG method which results in thick crystal platelets. This indicates that RSSG method yields crystals with considerably large size-parallel (111) and ( 1 ̅ 1 ̅ 1 ̅) faces of comparable area. Hence, they can be directly used for different applications without cutting and polishing. This method may be employed for crystals which grow in pyramidal shape and also with less planar area.
Chemical etching has been employed to study the distribution and density of dislocations in crystals grown by conventional and RSSG methods using a microscope. These crystals are etched in the mixture of acetic acid and formic acid. When the crystal was etched for 5 s, equilateral triangular etch pits were observed, similar to reported earlier [10]. Successive etch–polish–re-etch experiments confirmed the formation of etch pits to be at dislocations sites. Further, it is interesting to note that as these crystals are non-centrosymmetric, the atomic configuration of (111) and ( 1 ̅ 1 ̅ 1 ̅) faces is not identical. Hence the etchant which has good etching action on (111) face is incapable of etching its opposite ( 1 ̅ 1 ̅ 1 ̅) face. Similar difference in the etching action on (111) and ( 1 ̅ 1 ̅ 1 ̅) faces was observed on GaAs [11], InSb [12] and CdxHg1−xTe [13]. The etching studies made on a number of crystals grown by both methods indicate a non-uniform distribution of etch pits with an average etch pit density of about 2 × 102/cm2 at the central region of the sample and 3.5 × 103/cm2 at the edges. These results suggest that, the dislocations are more numerous at the edges than at the central region due to growth sector boundaries [14]. 3.3. UV–Vis transmission studies The optical transmission along the <111> NaBrO3 crystal has been measured in the entire UV–Vis region from 200 to 650 nm using Perkin-Elmer (Lambda 25) UV–Vis spectrophotometer at room temperature. For this study, the crystals grown by the two methods were cut and polished to have plates of 1 mm thickness. From the spectrum shown in Fig. 4, it is observed that the crystals are transparent in the entire UV–Vis region with a lower UV cut off wavelength at 260 nm. Further, the transmittance percentages in the higher wavelength region of crystals grown by conventional and RSSG method are 45% and 57% respectively, which reveal the improved transparency of crystals grown by RSSG method. The improved transmittance can be attributed to the absence of inclusions which reduces the absorption or scattering of UV–Vis radiation. 3.4. Microhardness studies Microhardness measurements on (111) and ( 1 ̅ 1 ̅ 1 ̅) NaBrO3 crystals grown by conventional and RSSG methods were made using
642
C.S. Reddy et al. / Materials Letters 64 (2010) 640–642
Fig. 5. Plot of Vickers hardness against load for NaBrO3 crystal.
Fig. 4. UV–Vis spectrum for NaBrO3 crystals.
Leitz-Wetzlar microhardness tester fitted with a Vickers diamond pyramidal indenter. Hardness values (Hv) are estimated by the expression, 2
Hv = 1:854P = d
ð1Þ
where P is the load applied in kg and d the diagonal length in µm. The variation of hardness with load for crystals grown by both the methods is shown in Fig. 5. The Hv value increases initially up to a load of 40 g and become almost load independent exhibiting the values of 183 and 190 kg/mm2 on (111) face, whereas 167 and 172 kg/mm2 on ( 1 ̅ 1 ̅ 1 ̅) face for crystals grown by conventional and RSSG methods respectively. The reason for higher hardness on both the faces in RSSG crystals appears to be due to better crystalline perfection and absence of inclusions. Further, the observed anisotropy in hardness on (111) and ( 1 ̅ 1 ̅ 1 ̅) faces is due to non-centrosymmetric nature of the crystals. The load variation can be interpreted by using Meyer's law, n
P = Ad
ð2Þ
where A is a constant and n is the Meyer's index (or work-hardening co-efficient). The value n is determined from the slope of the plots between ln P vs ln d for (111) face. From these plots, the estimated values of n are 2.46 and 2.24 for crystals grown by conventional and RSSG methods respectively. Onitsch [15] and Hanneman [16] had shown that the value of n is 1–1.6 for hard materials and more than 1.6 for soft ones. Thus the present crystals under study are moderately harder.
4. Conclusions The natural habit of sodium bromate crystal with pyramidal shape is restricted by employing a new RSSG method, which results in the growth of large size (111) and ( 1 ̅ 1 ̅ 1 ̅) faces suitable for studying various properties in as-grown form. The crystals obtained by conventional method have (111) face planar area of about 16 times that of ( 1 ̅ 1 ̅ 1 ̅) face, whereas it is about 2.2 times for crystals grown by RSSG method which resulted in thick crystal platelets with parallel faces. The average dislocation density of these crystals at the central region and edges is 2 × 102/cm2 and 3.5 × 103/cm2 respectively. Crystals grown by RSSG method possess higher transmittance and better hardness than grown by conventional method. The difference in the response of the (111) and ( 1 ̅ 1 ̅ 1 ̅) faces to etching and hardness can be attributed to the different atomic configuration on the opposite faces. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Chandrasekaran KS, Mohanlal SK. Pramana J Phys 1976;7:152–9. Xue Dongfeng, Zhang Siyuan. Chem Phys Lett 1998;287:503–8. Mason WP. Phys Rev 1946;70:529–37. Matsunaka M, Kitamura M, Sunagawa I. J Cryst Growth 1980;48:425–34. Wojciechowski K. Cryst Res Technol 1999;34:661–6. Kang Q, Duan L, Hu WR. Int J Heat and Mass Transfer 2001;44:3213–22. Sowa H. J Solid State Chem 1995;118:378–82. Radha V, Gopal ESR. J Indian Inst Sci 1968;50:26–30. Surender V, Arundhathi N, Kishan Rao K. Bull Mater Sci 2006;29:427–32. Kishan Rao K, Sirdeshmukh DB. Cryst Res Technol 1983;18:1125–8. Schell HA. Z Metallkde 1957;48:158–61. Allen JW. Phil Mag 1957;2:1475–8. Fewster PF, Cole S, Willoughby AFW, Brown M. J Appl Physics 1981;52:4568–71. Bhat HL. Prog Cryst Growth Charact 1985;11:57–87. Onitsch EM. Mikroskopia 1947;2:131–5. Hanneman M. Metallurgia Manchu 1941;23:135–40.