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Growth of cadmium boracite single crystals by chemical vapor transport
Journal of Crystal Growth 33 (1976) 361—364 © North-Holland Publishing Company
LETTERS TO THE EDITOR GROWTH OF CADMIUM BORACITE SINGLE CRYSTALS BY CHEMICAL VAPOR TRANSPORT Tatsuo TAKAHASHI and Osamu YAMADA RCA Research Laboratories, Inc., P.O. Box 5151, Tokyo International 100-31, Japan Received 14 January 1976
A new and simple method ~ogrow Cd
3B7O13X (X = Cl or Br) single crystals by chemical vapor transport was developed. In an evacuated quartz ampoule, CdO and CdX2 were placed at one end and B203 was placed at the other end. The ampoule was heated in a two-zone furnace, keeping the Cd compound end at the higher temperature (TH) and the B203 end at the lower temperature (TL). Single crystals of polyhedral shape (max. edge length ~-5 mm) were found to grow at the middle section of the ampoule. The optimum temperatures were TH = 850°C,TL = 800°C for Cd—Cl boracite and TH = 820°C,TL = 770°Cfor Cd—Br boracite, respectively. The crystals were found to have twin lamellar Structure. The lamellae were parallel to the {i l0} planes of the high temperature cubic phase. DTA measurements showed an anomaly at a temperature below the transition temperature to the cubic structure. Such an anomaly seems to suggest the occurrence of a higher order phase transition at that temperature.
The compounds having a chemical formula 2~to Cd2~, Me3B7O13X, Me = divalent metal from Mg X = Cl, Br or I are known to be isomorphous with a mineral boracite Mg3 B7013C1 11—4]. The high temperature phase has a cubic T~—F43cstructure and the low temperature phase has an orthorhombic C~~—Pca structure [5]. The transition temperature to the cubic phase (Ttr) varies widely with chemical composition and ranges from 64 K for Ni—I boracite to 798 K for Cd—Cl boracite [4]. Extensive investigations of the physical properties of boracite compounds were made and some boracites were found to be ferroelectric and ferromagnetic simultaneously at low temperatures [1,6—10]. Because of these interesting properties of boracite compounds, numerous attempts were made to synthesize boracites in the past [1—41. Most recently, Schmid [4] successfully grew single crystals of various boracites by an elaborate chemical transport technique. In his method, MeO, MeX2 and B203 were placed in three quartz pans separately and these pans were vacuum-sealed in a quartz tube. Upon heating of the system, boracite crystals were found to grow mostly in the B2O3 pan. The reaction was thought to take place between MeO transported via vapor phase and B203 transported via liquid film. Since this method requires a rather complicated glass blowing technique, 361
we attempted to develop a more simple chemical vapor transport technique. In our method, MeO and MeX 2 are transported from the high temperature to a lower temperature, and B2O3 from the low temperature to a higher temperature. Reaction takes place to form Me3 B7O13X single crystals at the intermediate ternperature zone. In principle, the technique can be applied to any of the boracite compounds. We have established an optimum condition for the growth of Cd—X boracites (X Cl or Br) that is described below along with metallographic observations and differential thermal analysis (DTA) measurements of the resulting crystals. In an evacuated quartz ampoule (inner diameter 12—16 mm), CdO and CdX2, either anhydrate or hydrate, were placed at one end and B2O3 was placed at the other end. The ampoule was constricted at the middle and quartz wool was used to prevent each constituent from mixing as shown schematically in fig. 1. The amounts of reagents were typically 15 rnmol of CdO, 3 mmol of CdX2 or CdX2nH2O and 24 mmol of B203. All the reagents used were of c.p. grade. No special precaution was taken to dehydrate the B2 03. The ampoule was heated in a two-zone furnace. The Cd compound end was kept at a higher temperature (T11) and the B2O3 end was kept at a lower tempera-
F. Takahashi, 0. Yainada / Growth of cadmium horacite single cr~’stals/,v Cvi
362
I8O~2OOmm~—k
5O~6O~--
~artz
are [4]:
----
MeO(s) + 2 HX(g)
~
__________
~‘
-
-
~ CdG
MeX
2 (g) + H70(g),
eO(s)+ X2 (g)=MeX2(g)+~ 02(g), M for T,1 -~ TL
I B203
Cd X2 or CdX2-nH2O
=
B703(Q)
+
6 HX(g)
=
2 BX3 (g) + 3 H20 (g),
B~O3(2)+3X2(g)=2BX2(g)+~O2 (Tb)
B2O3(2)+2HX(g)~~OX)3(g)+H7O,
ITH)
Fig. J. Packing arrangement inside the quartz ampoule for Cd—X boracite crystal growth. Approximate dimensions are shown.
B203
(2) + X2 (g)
=
B703 (2) + H20 (g) for TL -~ TH;
~ BOX)3 (g) + ~ =
2 HBO2 (g)
ture (TL). After heating for one week, good sized
crystals of polyhedral shape (max. edge length ~5 mm) were found to grow at the middle section of the quartz ampoule. In fig. 2, such Cd—Br crystals are shown in the “as grown” situation. The optimum temperatures were TH = 850°Cand TL = 800°C for Cd—-Cl horacite; and TH = 820°Cand T~= 770°Cfor Cd—Br boracite. Under these conditions, no borate crystals were formed as by-products. As can he seen from fig. 2, the crystals were found to grow at the middle section well apart from the positions of the initial MeO, MeX2 and B203 charges. This clearly indicates that CdO is transported from the TH zone to a lower temperature, and B203 is transported from the TL zone to a higher temperalure via the vapor phase. Possible reactions involved
_____________________________________________________________________
_____________ ___________
_______
Br hmaciic crvstak em%% It at in the ampoule: 2.
temperature ~ ne
(2)
=
2 Me3 B7013X (s)
for T11 > T> TL.
Smce it is difficult to use completely water free reagents, a partial pressure of H20 exists even when anhydrate is used. The use of hydrate instead of anhydrate produced little difference in growth rate or crystal habit. In boracite crystals, the orthorhombic ---cubic transformation involves only the changes in Me and halogen atom positions, and the B—O network remains almost unchanged [5]. The lattice parameters hardly change by the transformation and the X-ray diffraction pattern of the low temperature phase is almost identical to that of the high temperature cubic phase. These characteristics suggest that the mechanism of the transformation is a diffusionless one. The martensitic transformation in various alloys [II] and the ferroelectric t ransforination in BaTiO [12,13] and Rochelle salt [14], are some other examples of a diffusionless transformation. Therefore, the microstructure of a boracite crystal is expected to have features which are commonly observed in those materials. Indeed, we
_____________
lie. 2. Cd
5 MeO(s) + MeX2 (g) + 7 B203
tile Interlllcdiate
observed the twin lamellar structure in the Cd boracite crystals. The structure is quite similar to the twinned structure in ln---Tl alloys [15] and the domain structures in ferroelectric BaTiO3 [12,13] and Rochelle salt [141. In figs. 3 and 4, the twin lamellar structures in the Cd—Cl and Cd—Br boracite crystals are shown. In both crystals, the bkmellae are parallel to the f 11 0~
T. Takahashi, 0. Yamada / Growth of cadmium boracite single crystals by CVT
1t14_j*
~
363
—
I ig. 3. Twin lamellar structure of a (100) cut Cd—Cl horaeite crystal observed with transmitted light; ~400.
planes of the high temperature cubic crystal. In fig. 3, the two sets of twins, one parallel to (110) and another parallel to (110) coexist. The thickness of a twin is of the order of a few microns in the Cd—Cl boracite crystal but it is usually much wider in the Cd—Br boracite crystal. Moreover, the twin interface in the Cd—Cl boracite crystal is not quite straight indicating that the habit plane index is not exactly (110). Such irrationality of a habit plane is also commonly observed in martensite plates [11]. The effect of temperature on twin lamellar structure is illustrated in fig. 5. The fine structure of twin lamellae at room temperature (fig. 5a) is almost unchanged even at T~-~ Ttr, but its contrast becomes less distinct at higher temperatures
-
~
(a)
--
.
~
.
.
.
I % .
.
~
(b)
Fig. 5. Twin lamellar structure of a (110) cut Cd—Cl boraeite crystal observed with transmitted light; )< 100. (a) At room temperature; notice an antiphase boundary at upper left. (b) At near Ttr; an interface between the low temperature phase and the high temperature phase is moving from left to right.
--
_______
~ -
~
I is. 4. Twin Iainellar structure of a (110) cut Cd—Br horaeitc crystal observed with transmitted light; X400.
(fig. Sb). As the phase boundary propagates, the cubic phase with no twin markings is generated. On cooling, the process reverses restoring an almost identical twin lamellar structure. A Rigaku Denki Thermoflex unit was used to re-
364
1 Takahashi, 0. Yamada / Growth of cadmium horacite single
cord DTA curves of Cd horacite crystals. The peak temperature Ttr was determined on heating. It ranged from 51 5 to 523°Cfor the Cd—Cl boracite crystals, and from 430 to 435°Cfor the Cd—Br horacite crystals. The reported Tir values are 525°Cfor Cd—Cl horacite [21 and 459—464°Cfor Cd-—Br horacite [4]. respectively. The reported Ttr value for Cd—Br horacite was determined visually under a polarizing microscope. Since the change in birefringence at Ttr in Cd-- Br horacite is small, the detection of transformation by (hrect observation is admittedly very difficult [4]. Hence, the Ttr determined by the DTA measurements is believed to be more reliable. No sample weight change was observed during heating up to Tmax = 700°C.In addition to the endothermic peak at Ttr, there appeared an inflection point at Tinf ~ T~for both crystals. A typical example of the DTA curve that exhibits such an inflection, as well as an endothermic peak at Tir, is shown in fig. 6. TlItf ranged from 298 to 31 5°Cfor the Cd—Cl horacite crystal and from 248 to 270°C for the Cd—Br horacite crystal. Although no change in microstructure was observed in the vicinity of T~~t (cf. above), the presence of such an anomaly in a DTA curve seems to suggest the existence of a higher order phase transformation.
crystals
hi’ CVT
formation is so small that it cannot be detected by an ordinary X-ray technique. A specially-built X-ray strain meter [16] is required to detect such a transformation directly. A more definitive evidence for such a higher order transformation will be obtained by a heat capacity measurement. A simple chemical vapor transport method to grow Cd 3B7O13X (X = Cl or Br) boracite crystals was developed. Single crystals of up to 5 mm on an edge with a polyhedral shape were successfully grown. Metallographic observations revealed that these crystals have a twin lamellar structure from room temperature to ~ The DTA curves showed an anomaly at a temperature below Ttr, indicating a possible higher order phase transformation at that temperature. The authors wish to thank hO. Johnson and F. Okanioto for critical reading of the manuscript.
References ]1[ J.W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. V (Longmans, Green and Co., New York, 1924) p. 137. [2] F. Jomi, J. Phys. Chem. 63(1959) 1750.
For example, an orthorhombic to tetragonal transformation could conceivably take place without any major
131
change in the twin lamellar structure. However, the change in lattice parameter during the phase trans-
141 H. Schmid, J. Phys. Chem. Solids 26 (1965) 973. IS] T. Ito, N. Morimoto and R. Sadanaga, Acta Cryst. 4
F. Heide, G. Walter and R. Urlau, Naturwissenschaften
48 (1961) 97.
(1951) 310. F. Ascher, H. Sehmid and D. Tar, Solid State Commun. 2 (1964) 45. 17] F. Ascher, 11. Rieder, I-I. Schmid and H. Stosscl, J. App!. Phys. 37(1966)1404. [8] J. Kobayashi, Fl. Schmid and F. Ascher, Phys. Status Solidi 26 (1968) 277.
161 --
--
[9] G.Quezeland H. Schmid, Solid State Commun. 6
-
00
200
300
-
-
j
-
400
50~
600
Ier~pe,ot~re ‘C
lig. 6. DTA curve for a (110) cut Cd—Cl boracite crystal. Standard: ~-Ai 2O3, heating rate: 20°C/mm.
~
[10] F. Smuti-iy and 1. Fousek, Phys. Status Solidi 40(1970) K13. 588. New iii M. Cohen, Phase Transformations in 951)p. Solids (Wiley, York;Merz, Chapman Condon, I [12J W.J. Phys. and Rev.Hall, 95 (1954) 690. 1131 J.A. Hooton and W.J. Merz, Phys. Rev. 98(1955)409. 1141 T. Mitsui and J. luruichi, Phys. Rev. 90(1953)193. [15] J.S. Bowles, C.S. Barrett and F. Guttman, Trans. AIME 188 (1950) 1478. 1161 J. Kobayashi, N. Yamada and T. Azumi, Rev. Sci. Instr. 39 (1968) 1647.
LETTERS TO THE EDITOR GROWTH OF CADMIUM BORACITE SINGLE CRYSTALS BY CHEMICAL VAPOR TRANSPORT Tatsuo TAKAHASHI and Osamu YAMADA RCA Research Laboratories, Inc., P.O. Box 5151, Tokyo International 100-31, Japan Received 14 January 1976
A new and simple method ~ogrow Cd
3B7O13X (X = Cl or Br) single crystals by chemical vapor transport was developed. In an evacuated quartz ampoule, CdO and CdX2 were placed at one end and B203 was placed at the other end. The ampoule was heated in a two-zone furnace, keeping the Cd compound end at the higher temperature (TH) and the B203 end at the lower temperature (TL). Single crystals of polyhedral shape (max. edge length ~-5 mm) were found to grow at the middle section of the ampoule. The optimum temperatures were TH = 850°C,TL = 800°C for Cd—Cl boracite and TH = 820°C,TL = 770°Cfor Cd—Br boracite, respectively. The crystals were found to have twin lamellar Structure. The lamellae were parallel to the {i l0} planes of the high temperature cubic phase. DTA measurements showed an anomaly at a temperature below the transition temperature to the cubic structure. Such an anomaly seems to suggest the occurrence of a higher order phase transition at that temperature.
The compounds having a chemical formula 2~to Cd2~, Me3B7O13X, Me = divalent metal from Mg X = Cl, Br or I are known to be isomorphous with a mineral boracite Mg3 B7013C1 11—4]. The high temperature phase has a cubic T~—F43cstructure and the low temperature phase has an orthorhombic C~~—Pca structure [5]. The transition temperature to the cubic phase (Ttr) varies widely with chemical composition and ranges from 64 K for Ni—I boracite to 798 K for Cd—Cl boracite [4]. Extensive investigations of the physical properties of boracite compounds were made and some boracites were found to be ferroelectric and ferromagnetic simultaneously at low temperatures [1,6—10]. Because of these interesting properties of boracite compounds, numerous attempts were made to synthesize boracites in the past [1—41. Most recently, Schmid [4] successfully grew single crystals of various boracites by an elaborate chemical transport technique. In his method, MeO, MeX2 and B203 were placed in three quartz pans separately and these pans were vacuum-sealed in a quartz tube. Upon heating of the system, boracite crystals were found to grow mostly in the B2O3 pan. The reaction was thought to take place between MeO transported via vapor phase and B203 transported via liquid film. Since this method requires a rather complicated glass blowing technique, 361
we attempted to develop a more simple chemical vapor transport technique. In our method, MeO and MeX 2 are transported from the high temperature to a lower temperature, and B2O3 from the low temperature to a higher temperature. Reaction takes place to form Me3 B7O13X single crystals at the intermediate ternperature zone. In principle, the technique can be applied to any of the boracite compounds. We have established an optimum condition for the growth of Cd—X boracites (X Cl or Br) that is described below along with metallographic observations and differential thermal analysis (DTA) measurements of the resulting crystals. In an evacuated quartz ampoule (inner diameter 12—16 mm), CdO and CdX2, either anhydrate or hydrate, were placed at one end and B2O3 was placed at the other end. The ampoule was constricted at the middle and quartz wool was used to prevent each constituent from mixing as shown schematically in fig. 1. The amounts of reagents were typically 15 rnmol of CdO, 3 mmol of CdX2 or CdX2nH2O and 24 mmol of B203. All the reagents used were of c.p. grade. No special precaution was taken to dehydrate the B2 03. The ampoule was heated in a two-zone furnace. The Cd compound end was kept at a higher temperature (T11) and the B2O3 end was kept at a lower tempera-
F. Takahashi, 0. Yainada / Growth of cadmium horacite single cr~’stals/,v Cvi
362
I8O~2OOmm~—k
5O~6O~--
~artz
are [4]:
----
MeO(s) + 2 HX(g)
~
__________
~‘
-
-
~ CdG
MeX
2 (g) + H70(g),
eO(s)+ X2 (g)=MeX2(g)+~ 02(g), M for T,1 -~ TL
I B203
Cd X2 or CdX2-nH2O
=
B703(Q)
+
6 HX(g)
=
2 BX3 (g) + 3 H20 (g),
B~O3(2)+3X2(g)=2BX2(g)+~O2 (Tb)
B2O3(2)+2HX(g)~~OX)3(g)+H7O,
ITH)
Fig. J. Packing arrangement inside the quartz ampoule for Cd—X boracite crystal growth. Approximate dimensions are shown.
B203
(2) + X2 (g)
=
B703 (2) + H20 (g) for TL -~ TH;
~ BOX)3 (g) + ~ =
2 HBO2 (g)
ture (TL). After heating for one week, good sized
crystals of polyhedral shape (max. edge length ~5 mm) were found to grow at the middle section of the quartz ampoule. In fig. 2, such Cd—Br crystals are shown in the “as grown” situation. The optimum temperatures were TH = 850°Cand TL = 800°C for Cd—-Cl horacite; and TH = 820°Cand T~= 770°Cfor Cd—Br boracite. Under these conditions, no borate crystals were formed as by-products. As can he seen from fig. 2, the crystals were found to grow at the middle section well apart from the positions of the initial MeO, MeX2 and B203 charges. This clearly indicates that CdO is transported from the TH zone to a lower temperature, and B203 is transported from the TL zone to a higher temperalure via the vapor phase. Possible reactions involved
_____________________________________________________________________
_____________ ___________
_______
Br hmaciic crvstak em%% It at in the ampoule: 2.
temperature ~ ne
(2)
=
2 Me3 B7013X (s)
for T11 > T> TL.
Smce it is difficult to use completely water free reagents, a partial pressure of H20 exists even when anhydrate is used. The use of hydrate instead of anhydrate produced little difference in growth rate or crystal habit. In boracite crystals, the orthorhombic ---cubic transformation involves only the changes in Me and halogen atom positions, and the B—O network remains almost unchanged [5]. The lattice parameters hardly change by the transformation and the X-ray diffraction pattern of the low temperature phase is almost identical to that of the high temperature cubic phase. These characteristics suggest that the mechanism of the transformation is a diffusionless one. The martensitic transformation in various alloys [II] and the ferroelectric t ransforination in BaTiO [12,13] and Rochelle salt [14], are some other examples of a diffusionless transformation. Therefore, the microstructure of a boracite crystal is expected to have features which are commonly observed in those materials. Indeed, we
_____________
lie. 2. Cd
5 MeO(s) + MeX2 (g) + 7 B203
tile Interlllcdiate
observed the twin lamellar structure in the Cd boracite crystals. The structure is quite similar to the twinned structure in ln---Tl alloys [15] and the domain structures in ferroelectric BaTiO3 [12,13] and Rochelle salt [141. In figs. 3 and 4, the twin lamellar structures in the Cd—Cl and Cd—Br boracite crystals are shown. In both crystals, the bkmellae are parallel to the f 11 0~
T. Takahashi, 0. Yamada / Growth of cadmium boracite single crystals by CVT
1t14_j*
~
363
—
I ig. 3. Twin lamellar structure of a (100) cut Cd—Cl horaeite crystal observed with transmitted light; ~400.
planes of the high temperature cubic crystal. In fig. 3, the two sets of twins, one parallel to (110) and another parallel to (110) coexist. The thickness of a twin is of the order of a few microns in the Cd—Cl boracite crystal but it is usually much wider in the Cd—Br boracite crystal. Moreover, the twin interface in the Cd—Cl boracite crystal is not quite straight indicating that the habit plane index is not exactly (110). Such irrationality of a habit plane is also commonly observed in martensite plates [11]. The effect of temperature on twin lamellar structure is illustrated in fig. 5. The fine structure of twin lamellae at room temperature (fig. 5a) is almost unchanged even at T~-~ Ttr, but its contrast becomes less distinct at higher temperatures
-
~
(a)
--
.
~
.
.
.
I % .
.
~
(b)
Fig. 5. Twin lamellar structure of a (110) cut Cd—Cl boraeite crystal observed with transmitted light; )< 100. (a) At room temperature; notice an antiphase boundary at upper left. (b) At near Ttr; an interface between the low temperature phase and the high temperature phase is moving from left to right.
--
_______
~ -
~
I is. 4. Twin Iainellar structure of a (110) cut Cd—Br horaeitc crystal observed with transmitted light; X400.
(fig. Sb). As the phase boundary propagates, the cubic phase with no twin markings is generated. On cooling, the process reverses restoring an almost identical twin lamellar structure. A Rigaku Denki Thermoflex unit was used to re-
364
1 Takahashi, 0. Yamada / Growth of cadmium horacite single
cord DTA curves of Cd horacite crystals. The peak temperature Ttr was determined on heating. It ranged from 51 5 to 523°Cfor the Cd—Cl boracite crystals, and from 430 to 435°Cfor the Cd—Br horacite crystals. The reported Tir values are 525°Cfor Cd—Cl horacite [21 and 459—464°Cfor Cd-—Br horacite [4]. respectively. The reported Ttr value for Cd—Br horacite was determined visually under a polarizing microscope. Since the change in birefringence at Ttr in Cd-- Br horacite is small, the detection of transformation by (hrect observation is admittedly very difficult [4]. Hence, the Ttr determined by the DTA measurements is believed to be more reliable. No sample weight change was observed during heating up to Tmax = 700°C.In addition to the endothermic peak at Ttr, there appeared an inflection point at Tinf ~ T~for both crystals. A typical example of the DTA curve that exhibits such an inflection, as well as an endothermic peak at Tir, is shown in fig. 6. TlItf ranged from 298 to 31 5°Cfor the Cd—Cl horacite crystal and from 248 to 270°C for the Cd—Br horacite crystal. Although no change in microstructure was observed in the vicinity of T~~t (cf. above), the presence of such an anomaly in a DTA curve seems to suggest the existence of a higher order phase transformation.
crystals
hi’ CVT
formation is so small that it cannot be detected by an ordinary X-ray technique. A specially-built X-ray strain meter [16] is required to detect such a transformation directly. A more definitive evidence for such a higher order transformation will be obtained by a heat capacity measurement. A simple chemical vapor transport method to grow Cd 3B7O13X (X = Cl or Br) boracite crystals was developed. Single crystals of up to 5 mm on an edge with a polyhedral shape were successfully grown. Metallographic observations revealed that these crystals have a twin lamellar structure from room temperature to ~ The DTA curves showed an anomaly at a temperature below Ttr, indicating a possible higher order phase transformation at that temperature. The authors wish to thank hO. Johnson and F. Okanioto for critical reading of the manuscript.
References ]1[ J.W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. V (Longmans, Green and Co., New York, 1924) p. 137. [2] F. Jomi, J. Phys. Chem. 63(1959) 1750.
For example, an orthorhombic to tetragonal transformation could conceivably take place without any major
131
change in the twin lamellar structure. However, the change in lattice parameter during the phase trans-
141 H. Schmid, J. Phys. Chem. Solids 26 (1965) 973. IS] T. Ito, N. Morimoto and R. Sadanaga, Acta Cryst. 4
F. Heide, G. Walter and R. Urlau, Naturwissenschaften
48 (1961) 97.
(1951) 310. F. Ascher, H. Sehmid and D. Tar, Solid State Commun. 2 (1964) 45. 17] F. Ascher, 11. Rieder, I-I. Schmid and H. Stosscl, J. App!. Phys. 37(1966)1404. [8] J. Kobayashi, Fl. Schmid and F. Ascher, Phys. Status Solidi 26 (1968) 277.
161 --
--
[9] G.Quezeland H. Schmid, Solid State Commun. 6
-
00
200
300
-
-
j
-
400
50~
600
Ier~pe,ot~re ‘C
lig. 6. DTA curve for a (110) cut Cd—Cl boracite crystal. Standard: ~-Ai 2O3, heating rate: 20°C/mm.
~
[10] F. Smuti-iy and 1. Fousek, Phys. Status Solidi 40(1970) K13. 588. New iii M. Cohen, Phase Transformations in 951)p. Solids (Wiley, York;Merz, Chapman Condon, I [12J W.J. Phys. and Rev.Hall, 95 (1954) 690. 1131 J.A. Hooton and W.J. Merz, Phys. Rev. 98(1955)409. 1141 T. Mitsui and J. luruichi, Phys. Rev. 90(1953)193. [15] J.S. Bowles, C.S. Barrett and F. Guttman, Trans. AIME 188 (1950) 1478. 1161 J. Kobayashi, N. Yamada and T. Azumi, Rev. Sci. Instr. 39 (1968) 1647.