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Surface microstructures of crystals grown from molten salts
Journal of Chrystal Growth 1 (1967) 102-109 © North-Holland Publishing Co., Amsterdam
SURFACE MICROSTRUCTURES OF CRYSTALS GROWN FROM MOLTEN SALTS
ICI-IIRO SUNAGAWA Geological Survey of Japan, Hisamoto-cho 135, Kawasaki, Japan
Received 21 January 1967
Surface structures of the following crystals grown from various molten salts were studied by means ofphase contrast microscopy; emerald, corundum, hematite, spinel, magnetoplumbite, YIG, Zr0 2, ZnO, U02 and U308. Most of these crystals exhibit growth spirals. Four types of spirals were observed; I) spirals having mono-molecular step height and wide and regular spacings between the successive arms, 2) spirals having mono-molecular step height and narrow and regular spacings, 3) conical growth hills
which also consist of narrow but irregularly spaced spiral layers, and 4) minute growth hills consisting of narrow but irregularly spaced spiral layers. Their morphologies are explained on the
1. Introduction
magnetoplumbite (BaFe12O19) synthesized by J. P. Remeika and by S. Takasu, Toshiba Cent. Res. Lab., Japan; YIG (Y3Fe5O52) synthesized by S. Takasu; spine! (MgA12O4) synthesized at Mullard Res. Lab., England; zincite (ZnO) synthesized by S. Hasegawa; Zr02 synthesized by A. M. Anthony; emerald (3 BeOAI2O3 6 Si02) synthesized by Chatham, San Fransisco, U.S.A.; by R. C. Linares, Perkin-Elmer, U.S.A.; at Komatsu Diamond Industry Co., Japan. The crystals were synthesized at various temperature ranges, using various molten salts and oxides as solvents. The hematite crystals were doped with different elements of various concentrations, and the emerald crystals were synthesized in different flux mixtures.
basis of structural characteristics and growth conditions. The characteristics of growth spirals on these crystals are compared with those on the crystals ofthe same compound grown from the vapour, solution or unicomponent melt, and the difference in the growth mechanism between these methods is discussed.
The method of growing crystals of various cornpounds from molten inorganic salts and oxides, which is called the flux method, has recently been used widely in semiconductor-, ferrimagnetic- or synthetic gemstone-industries. Single crystals of a wide variety of compounds have been successfully synthesized by this method. The flux method is also interesting in geological sciences, for the crystallization process has some similarity with that in magma. The study of the growth mechanism from molten salts and oxides might give a clue to the understanding of the growth mechanism of rock-forming minerals, 2. Experimental procedure The surface structures of the crystal faces of various flux-grown compounds were studied in detail, using a phase contrast microscope and an interferometric technique, and their characteristics were compared with those of crystals grown from the vapour, hydrothermal solution and unicomponent melt, The flux-grown crystals investigated in this study are: hematite (~z-Fe2O3)synthesized by J. P. Remeika of Bell Telephone Research Lab., U.S.A.; corundum (c.-A1203) synthesized by S. Hasegawa, Tohoku Univ., Japan; by A. M. Anthony, C.N.R.S., France;
3. Observations All of the crystals listed above exhibit growth spirals on their habit faces. The observed spirals can be classifled into four categories; 1) spirals having mono-molecular step height and wide and regular spacings between the successive arms of the spiral, 2) spirals having mono-molecular step height and narrow and regular spacings, 3) conical growth hills which also consist of narrow but irregularly spaced spiral layers, and 4)
102
SURFACE
MICROSTRUCTURES
OF CRYSTALS GROWN
minute growth hills consisting of narrow but irregularly spaced spiral layers. The first type has been observed on the (0001) faces of magnetoplumbite1), hematite2), corundum and the 1). Circular, polygonal and interlaced (211) of YIG spiralsface were observed. A typical example is shown in fig. I, observed on the (0001) face of magnetoplumbite.
~
__________
FROM MOLTEN SALTS
103
tion with Burgers’ vector of one unit cell high will soon split into two oppositely oriented triangular layers which consist of RS and R*S* blocks, respectively. They have step heights equal to one half of the c axis (= 23.2 A). The minimum advancing directions 0(there are three) of the first (or lower) layer will be the maximum advancing directions of the second (or upper) layer. As a result, the upper layer will soon catch up the lower layer, and coalesce with the latter forming the layer of one unit cell high in these directions. On the other hand, in the other three directions the lower layer has the maximum advancing rate, whereas the upper layer advancing rate. AsAa cornconsequence, thethe twominimum layers will never coalesce.
I
4
~—
bination of these two will result in the formation of interlacing in the six directions in between the two sets
e
~
rn--i
_ ~
~
(~0 —0
_______
_
_______ _ __
__
-
.
.
.
.
crystal ), which is schematically shown in fig. 2. The crystal structure of magnetoplumbite consists of two sets of spinel (5) plus barium (R) blocks (RSR*S*), each set being in the position of 180°rotation around the c axis to each other. This structure suggests that the spiral layer originating from a single screw disloca-
~Th
-
-~
~
~
-
0— 0 —0
Fig I Interlaced typii. il spiral of type I observed on the (0001) face of magnetoplumbite. (a) central portion of the spiral showing oppositely orientated triangular layers, (b) interlaced pattern. Phase contrast photo. x 160.
An interlaced pattern can be clearly seen on this spiral in the six directions. The interlaced pattern is accounted for easily from the structural characteristics of the
-
-
__
3
~
x
( ~ ~
Fig. 2.
Crystal structure of magnetoplumbite BaFe
12O19. Open circle: oxygen; double circle: barium; small circle: iron.
of the above three directions. The more complicated growth patterns observed on the (0001) faces of magnetoplumbite can also be interpreted as derived from a cooperation of interlacing of many spiral layers origin-
104
1CHIRO SUNAGAWA
ating from many screw dislocations. Widely spaced spirals observed on the (211) face of YIG and (0001) faces of hematite and corundum are in circular form. The step height on YIG was estimated to be less than 50 A1), which is a small multiple of the unit length, and that on hematite and corundum is even smaller and probably less than unit cell height, judging from their visibilities on the phase contrast photomicrographs. The second type of growth spirals has been observed on the (0001) faces of hematite, corundum, zincite and the (111) face of spinel crystals. Under low magnification, they look like pyramids (hematite, corundum and spinel) or hexagonal cones (corundum, zincite) with flat side faces. An example observed on the (111) of spinel is shown in fig. 3. However if they are observed
—~ ~
I Fig. 3.
____ _______
_____
________
_______________
—
Fig. 4. High magnification phase contrast photoniicrograph of the summit of the pyramid shown in fig. 3. Spiral nature is clearly seen. Arrows show dominated spirals. x 1800.
which have mono-molecular step heights. The morphology of pyramids observed on the (0001) faces of hematite, corundum4) and on the (111) face of spinel, as well as hexagonal cones on the (0001) face of zincite7) is in good accordance with the symmetry of the faces. However, hexagonal cones observed on the (0001) face of corundum is morphologically in contradiction with the structure of the crystal. Moreover, both pyramids and hexagonal cones occur on the same surface of the corundum crystals as shown in fig. 5, though a sort of discontinuous boundary is noticed between the two
Low magnification photomicrograph of the (111) face of spinel showing type 2 spirals (pyramids). 20. ~
under high magnification, it is found that they are in fact spirals originating in most cases from single point screw dislocation (fig. 4). Extremely narrow but regularly spaced spiral layers result in the appearance of flat side faces. The narrow and constant spacings between successive layers are indicative of a spiral structure. Tn the (0001) faces of natural hematite crystals, similar pyramids were observed and found to be triangular spirals with step height of only 2.3 A5). Therefore, it can be conjectured that the pyramids and hexagonal cones observed in this study also consist of spiral layers
,~
~.
______
Fig. 5.
Phase contrast photomicrograph showing co-existence
of pyramids and hexagonal cones on the (0001) face of corundum. > 160.
SURFACE MICROSTRUCTURES
OF CRYSTALS
GROWN FROM MOLTEN SALTS
105
a
b
Fig. 6. Two examples of type 3 spirals.. (a) Circular cones observed on the (0001) face of emerald. Spirals of type 4 are also seen. x 50. (b) polygonal cones observed on the (1010) face of emerald. 20.
regions. Since the morphology of polygonal spirals originating from single dislocation and having monomolecular step height strictly follows the symmetry of the face, the occurrence of the hexagonal spirals on the (0001) face of corundum is difficult to explain, unless one assumes either the existence of an hexagonal poly-
type in corundum or epitaxial growth of foreign crystals of hexagonal symmetry. This has not been investigated yet by X-ray method, and further study is desired. The third type of spirals has been observed on the (110) and (211) faces of YIG, the (0001) face of zincite, the (Ill) face of Zr02 and the (0001) and (1OTO) faces
106
ICH1RO SUNAGAWA
of emerald. Examples are shown in fig. 6. Circular cones of this type are observed on (2 1 1 ) of YIG, (000 1 ) of zincite and emerald, and polygonal cones are on (1 10) of YIG, (1 1 1) of Zr02 and (1010) of emerald. As can be seen in these figures, some of the cones have smooth side faces on which no growth layers can be seen even under fairly high magnification, whereas others exhibit concentric parallel lines. In the latter case, the side faces often show a profile of curved steps instead of an ordinary step-wise profile of growth spirals. Fig. 7. which is an interferogram taken on the
Iig. 7.
spiral layers. They are so closely spaced that the edges of individual growth layers cannot be resolved under lower magnification. The profile of curved steps is a simple result of extremely close spaced spiral layers. This leads to a conclusion that the cones with smooth side faces such as observed on the (21 1) of YIG must also be the similar one. The fourth type spiral, which is in the form of minute growth hills, has been observed widely on most of the investigated crystals. They occur on the same surface, along with the other types of growth spirals which are
Two-beam interlerogram showing a profile of curved steps of the type 3 spirals. (0001) of emerald. x IOU.
(0001) face of emerald, clearly demonstrates this. It is also observed that these cones usually have a single summit ((1010) of emerald, (211) of Y1G, etc.), or originate from the end point of twist boundaries ((0001) of emerald, etc.), which suggests that they are spirals. Indeed, an observation of such cones on the (1010) face of emerald under very high magnification discloses that there are many faint lines, which are edges of nearly mono-molecular growth layers, in between the parallel lines observed under lower magnification. This shows that the profile of curved steps on the side faces are in fact formed by the piling of large number of very thin
the main growth centres and cover a wide area of the surface. They often align along certain lines or concentrate in a small area, though on some faces like the (1011) of hematite the whole surface is covered by large number of these minute hills alone. Two examples observed on the (1010) of emerald and (1011) of hematite are shown in fig. 8. Although they look like just dots in appearance, it is found under very high magnification that they are oval-shaped spirals. A high magnification photomicrograph of these on emerald is shown in fig. 9. This sort of minute spiral hillocks have not been observed on natural crystals of the same compound
SURFACE MICROSTRUCTURES
OF CRYSTALS GROWN
FROM MOLTEN SALTS
107
a
b
Fig. 8.
Two examples oftype 4 spirals. ta)(l0TO) face of emerald. L.arge number of small dots are the type 4 spirals. .20. (b) (I UT I) face of hematite. Phase contrast. . 240.
(hematite. emerald), and are characteristic growth features of synthetic crystals. Similar growth hillocks can often be found also on synthetic crystals grown from a hydrothermally aqueous solution such as quartz. They represent the growth from fairly high supersaturation condition, as can he infered from narrow spacings between the successive arms of the spirals. In addition to the above differences, observed between natural and synthetic crystals, the following differences were noticed between flux-grown and hydro-
thermal or vapour-grown crystals. The differences are similar to those previously6) reported for natural and synthetic emeralds: I) the morphology of the spirals observed on both (000l)and (IOTO) faces was different, 2) the spacings between the successive arms of the spirals were wide on the natural crystals and narrow on the Chatham’s emerald, 3) minute growth spirals were not found on the natural emerald, but on the synthetic one. The three differences are all infered to the different conditions of the supersaturation. Flux-
108
ICHIRO SUNAGAWA
Fig. 9.
High magnification phase contrast photomicrograph of the type 4 spirals. (1010) face of emerald. >< 1000.
grown crystals exhibit features suggesting the growth from higher conditions of supersaturation than the hydrothermal solution-grown natural crystals. Similar differences have been noticed between fluxgrown hematite and natural hematite which is grown from the vapour phase. Most of the natural hematite crystals exhibit typical spirals with step height of 2.3 A, 4.6 A, 7 A, etc. (c0 of hematite is 13,73 A) and cornposite spirals of various types, but do not exhibit the spirals of the fourth the spirals other hand, fluxgrown hematite exhibitstype. bothOn typical and minute growth hillocks, as well as triangular growth tables7) which are considered to have been formed by epitaxial settlement of the already formed crystallites on the growing (0001) face of the host crystal. In general, flux-grown hematite exhibits growth features which represent higher conditions of supersaturation than the natural hematite grown from the vapour phase. 4. Discussions Observations on the surface structures of crystal faces of a wide variety of flux-grown crystals revealed that all the crystals investigated showed some kinds of
growth spirals, starting from the typical one with wide spacings and mono-molecular step height to the spirals with very narrow spacings. Therefore, it can be coneluded with certainity that the growth of the crystals from molten salts and oxides (flux method) are performed by spiral mechanism at least at the latest stage of their crystallization. Spiral growth mechanism was at first put forward to account for the growth fromhasvapour at low 8). This been phase supported by supersaturation rate many experimental evidence, and it is known that the crystals grown from the vapour phase almost invariably exhibit growth spirals. Evidence can be found among such crystals as silicon carbide, hematite, sphalerite, biotite, phlogopite, synthetic ZnS, CdS, etc. Some of these crystals exhibit spirals of the type 1, whereas others often show type 2 or composite spirals originating from a large number of screw dislocations situated close together. That the spiral growth mechanism is applicable to the growth from solution phase has also been observed on a wide variety of crystals grown from both aqueous and non-aqueous solutions, The crystals grown from solutions on which growth spirals were
SURFACE MICROSTRUCTURES OF CRYSTALS GROWN FROM MOLTEN SALTS
observed are salol, n-alcohol, paraffine, stearic acid, n-nonatriacontane, n-propyl, npenactontanoate, nhectane, A1B2, quartz (both natural and synthetic), beryl, apatite, fluorite, sphalerite, pyrite, etc. However, some crystals grown from solutions do not exhibit growth spirals, even if they are observed carefully using the most sensitive methods. For instance, many (not all) pyrite crystals grown from a hydrothermal solution do not show growth spirals, instead they show large number of small growth islands on which no spiral layers are observed, These islands are considered to have been formed either by two-dimensional nucleation or epitaxial settlement of already formed crystallites. This shows that many crystals prepared from solution, though not all, are grown by the spiral mechanism and only a few of them are grown by the layer growth mechanism. In contrast to the above two cases, growth spirals have been observed only exceptionally among the crystals grown from a unicomponent melt. Such exceptional cases reported so far are on the crystals of salol, Hg12 and synthetic fluor-phiogopite. In the case of fluor-phlogopite, growth spirals were not found on the as-grown faces, but on the walls of elongated cavities in the crystals. No growth spirals have been observed so far on other crystals grown from the melt, This is reasonable since the growth process from the melt is different from the other methods in many respects, such as the difference in densities or number of molecular bonds between solid and liquid phases. Solid—liquid interface in the crystallization of a homogeneous phase from the melt is considered in many, though not in all cases, to be a rough surface, rather than a singular surface. For the growth to take place
109
on such rough surfaces, neither two-dimensional nucleation nor the existence of screw dislocations are necessary. There are enough sites on such surfaces to accomodate atoms or molecules from the liquid. Therefore, it is expected that neither growth spirals nor growth layers will be observed on the surfaces of many of the crystals grown from a unicomponent melt, From the above described observations and consideration, it is natural to think that the growth process from molten salts is different from the growth of a unicomponent molten phase, and is similar to the growth from vapour or solutions of low supersaturation, though the flux method represents the growth from a higher supersaturation than the latter two methods. Acknowledgments The writer expresses his thanks to the persons and organizations who supplied him with samples, and to Dr. H. Komatsu and Mr. T. Nakano for the help in observation. References 1) H. Komatsu and I. Sunagawa, Miner. J. (Tokyo) 4(1964)203. 2) I. Sunagawa, Adsorption et Croissance Cristalline (C,N.R.S.,
Paris, 1965) p. 665. 3) P. B. Braun, Philips Res. Rept. 6 (1957) 491. 4) V. A. Timofeeva, in: Rost Kristallov, Vol. 6 (Academy of Sciences USSR, Institute of Crystallography, Moscow, 1965) p. 86 (in Russian). 5)1. Sunagawa, Am. Miner. 46 (1961) 1216.
6)1. Sunagawa, Am. Miner. 49 (1964) 785: see references for earlier works. 7) K. T. Wilke, in: Rost Kristal/ov, Vol. 6 (Academy of Sciences USSR, Institute of Crystallography, Moscow, 1965) p. 75 (in Russian). 8) F. C. Frank, Discussions Faraday Soc. 5 (1949) 48.
SURFACE MICROSTRUCTURES OF CRYSTALS GROWN FROM MOLTEN SALTS
ICI-IIRO SUNAGAWA Geological Survey of Japan, Hisamoto-cho 135, Kawasaki, Japan
Received 21 January 1967
Surface structures of the following crystals grown from various molten salts were studied by means ofphase contrast microscopy; emerald, corundum, hematite, spinel, magnetoplumbite, YIG, Zr0 2, ZnO, U02 and U308. Most of these crystals exhibit growth spirals. Four types of spirals were observed; I) spirals having mono-molecular step height and wide and regular spacings between the successive arms, 2) spirals having mono-molecular step height and narrow and regular spacings, 3) conical growth hills
which also consist of narrow but irregularly spaced spiral layers, and 4) minute growth hills consisting of narrow but irregularly spaced spiral layers. Their morphologies are explained on the
1. Introduction
magnetoplumbite (BaFe12O19) synthesized by J. P. Remeika and by S. Takasu, Toshiba Cent. Res. Lab., Japan; YIG (Y3Fe5O52) synthesized by S. Takasu; spine! (MgA12O4) synthesized at Mullard Res. Lab., England; zincite (ZnO) synthesized by S. Hasegawa; Zr02 synthesized by A. M. Anthony; emerald (3 BeOAI2O3 6 Si02) synthesized by Chatham, San Fransisco, U.S.A.; by R. C. Linares, Perkin-Elmer, U.S.A.; at Komatsu Diamond Industry Co., Japan. The crystals were synthesized at various temperature ranges, using various molten salts and oxides as solvents. The hematite crystals were doped with different elements of various concentrations, and the emerald crystals were synthesized in different flux mixtures.
basis of structural characteristics and growth conditions. The characteristics of growth spirals on these crystals are compared with those on the crystals ofthe same compound grown from the vapour, solution or unicomponent melt, and the difference in the growth mechanism between these methods is discussed.
The method of growing crystals of various cornpounds from molten inorganic salts and oxides, which is called the flux method, has recently been used widely in semiconductor-, ferrimagnetic- or synthetic gemstone-industries. Single crystals of a wide variety of compounds have been successfully synthesized by this method. The flux method is also interesting in geological sciences, for the crystallization process has some similarity with that in magma. The study of the growth mechanism from molten salts and oxides might give a clue to the understanding of the growth mechanism of rock-forming minerals, 2. Experimental procedure The surface structures of the crystal faces of various flux-grown compounds were studied in detail, using a phase contrast microscope and an interferometric technique, and their characteristics were compared with those of crystals grown from the vapour, hydrothermal solution and unicomponent melt, The flux-grown crystals investigated in this study are: hematite (~z-Fe2O3)synthesized by J. P. Remeika of Bell Telephone Research Lab., U.S.A.; corundum (c.-A1203) synthesized by S. Hasegawa, Tohoku Univ., Japan; by A. M. Anthony, C.N.R.S., France;
3. Observations All of the crystals listed above exhibit growth spirals on their habit faces. The observed spirals can be classifled into four categories; 1) spirals having mono-molecular step height and wide and regular spacings between the successive arms of the spiral, 2) spirals having mono-molecular step height and narrow and regular spacings, 3) conical growth hills which also consist of narrow but irregularly spaced spiral layers, and 4)
102
SURFACE
MICROSTRUCTURES
OF CRYSTALS GROWN
minute growth hills consisting of narrow but irregularly spaced spiral layers. The first type has been observed on the (0001) faces of magnetoplumbite1), hematite2), corundum and the 1). Circular, polygonal and interlaced (211) of YIG spiralsface were observed. A typical example is shown in fig. I, observed on the (0001) face of magnetoplumbite.
~
__________
FROM MOLTEN SALTS
103
tion with Burgers’ vector of one unit cell high will soon split into two oppositely oriented triangular layers which consist of RS and R*S* blocks, respectively. They have step heights equal to one half of the c axis (= 23.2 A). The minimum advancing directions 0(there are three) of the first (or lower) layer will be the maximum advancing directions of the second (or upper) layer. As a result, the upper layer will soon catch up the lower layer, and coalesce with the latter forming the layer of one unit cell high in these directions. On the other hand, in the other three directions the lower layer has the maximum advancing rate, whereas the upper layer advancing rate. AsAa cornconsequence, thethe twominimum layers will never coalesce.
I
4
~—
bination of these two will result in the formation of interlacing in the six directions in between the two sets
e
~
rn--i
_ ~
~
(~0 —0
_______
_
_______ _ __
__
-
.
.
.
.
crystal ), which is schematically shown in fig. 2. The crystal structure of magnetoplumbite consists of two sets of spinel (5) plus barium (R) blocks (RSR*S*), each set being in the position of 180°rotation around the c axis to each other. This structure suggests that the spiral layer originating from a single screw disloca-
~Th
-
-~
~
~
-
0— 0 —0
Fig I Interlaced typii. il spiral of type I observed on the (0001) face of magnetoplumbite. (a) central portion of the spiral showing oppositely orientated triangular layers, (b) interlaced pattern. Phase contrast photo. x 160.
An interlaced pattern can be clearly seen on this spiral in the six directions. The interlaced pattern is accounted for easily from the structural characteristics of the
-
-
__
3
~
x
( ~ ~
Fig. 2.
Crystal structure of magnetoplumbite BaFe
12O19. Open circle: oxygen; double circle: barium; small circle: iron.
of the above three directions. The more complicated growth patterns observed on the (0001) faces of magnetoplumbite can also be interpreted as derived from a cooperation of interlacing of many spiral layers origin-
104
1CHIRO SUNAGAWA
ating from many screw dislocations. Widely spaced spirals observed on the (211) face of YIG and (0001) faces of hematite and corundum are in circular form. The step height on YIG was estimated to be less than 50 A1), which is a small multiple of the unit length, and that on hematite and corundum is even smaller and probably less than unit cell height, judging from their visibilities on the phase contrast photomicrographs. The second type of growth spirals has been observed on the (0001) faces of hematite, corundum, zincite and the (111) face of spinel crystals. Under low magnification, they look like pyramids (hematite, corundum and spinel) or hexagonal cones (corundum, zincite) with flat side faces. An example observed on the (111) of spinel is shown in fig. 3. However if they are observed
—~ ~
I Fig. 3.
____ _______
_____
________
_______________
—
Fig. 4. High magnification phase contrast photoniicrograph of the summit of the pyramid shown in fig. 3. Spiral nature is clearly seen. Arrows show dominated spirals. x 1800.
which have mono-molecular step heights. The morphology of pyramids observed on the (0001) faces of hematite, corundum4) and on the (111) face of spinel, as well as hexagonal cones on the (0001) face of zincite7) is in good accordance with the symmetry of the faces. However, hexagonal cones observed on the (0001) face of corundum is morphologically in contradiction with the structure of the crystal. Moreover, both pyramids and hexagonal cones occur on the same surface of the corundum crystals as shown in fig. 5, though a sort of discontinuous boundary is noticed between the two
Low magnification photomicrograph of the (111) face of spinel showing type 2 spirals (pyramids). 20. ~
under high magnification, it is found that they are in fact spirals originating in most cases from single point screw dislocation (fig. 4). Extremely narrow but regularly spaced spiral layers result in the appearance of flat side faces. The narrow and constant spacings between successive layers are indicative of a spiral structure. Tn the (0001) faces of natural hematite crystals, similar pyramids were observed and found to be triangular spirals with step height of only 2.3 A5). Therefore, it can be conjectured that the pyramids and hexagonal cones observed in this study also consist of spiral layers
,~
~.
______
Fig. 5.
Phase contrast photomicrograph showing co-existence
of pyramids and hexagonal cones on the (0001) face of corundum. > 160.
SURFACE MICROSTRUCTURES
OF CRYSTALS
GROWN FROM MOLTEN SALTS
105
a
b
Fig. 6. Two examples of type 3 spirals.. (a) Circular cones observed on the (0001) face of emerald. Spirals of type 4 are also seen. x 50. (b) polygonal cones observed on the (1010) face of emerald. 20.
regions. Since the morphology of polygonal spirals originating from single dislocation and having monomolecular step height strictly follows the symmetry of the face, the occurrence of the hexagonal spirals on the (0001) face of corundum is difficult to explain, unless one assumes either the existence of an hexagonal poly-
type in corundum or epitaxial growth of foreign crystals of hexagonal symmetry. This has not been investigated yet by X-ray method, and further study is desired. The third type of spirals has been observed on the (110) and (211) faces of YIG, the (0001) face of zincite, the (Ill) face of Zr02 and the (0001) and (1OTO) faces
106
ICH1RO SUNAGAWA
of emerald. Examples are shown in fig. 6. Circular cones of this type are observed on (2 1 1 ) of YIG, (000 1 ) of zincite and emerald, and polygonal cones are on (1 10) of YIG, (1 1 1) of Zr02 and (1010) of emerald. As can be seen in these figures, some of the cones have smooth side faces on which no growth layers can be seen even under fairly high magnification, whereas others exhibit concentric parallel lines. In the latter case, the side faces often show a profile of curved steps instead of an ordinary step-wise profile of growth spirals. Fig. 7. which is an interferogram taken on the
Iig. 7.
spiral layers. They are so closely spaced that the edges of individual growth layers cannot be resolved under lower magnification. The profile of curved steps is a simple result of extremely close spaced spiral layers. This leads to a conclusion that the cones with smooth side faces such as observed on the (21 1) of YIG must also be the similar one. The fourth type spiral, which is in the form of minute growth hills, has been observed widely on most of the investigated crystals. They occur on the same surface, along with the other types of growth spirals which are
Two-beam interlerogram showing a profile of curved steps of the type 3 spirals. (0001) of emerald. x IOU.
(0001) face of emerald, clearly demonstrates this. It is also observed that these cones usually have a single summit ((1010) of emerald, (211) of Y1G, etc.), or originate from the end point of twist boundaries ((0001) of emerald, etc.), which suggests that they are spirals. Indeed, an observation of such cones on the (1010) face of emerald under very high magnification discloses that there are many faint lines, which are edges of nearly mono-molecular growth layers, in between the parallel lines observed under lower magnification. This shows that the profile of curved steps on the side faces are in fact formed by the piling of large number of very thin
the main growth centres and cover a wide area of the surface. They often align along certain lines or concentrate in a small area, though on some faces like the (1011) of hematite the whole surface is covered by large number of these minute hills alone. Two examples observed on the (1010) of emerald and (1011) of hematite are shown in fig. 8. Although they look like just dots in appearance, it is found under very high magnification that they are oval-shaped spirals. A high magnification photomicrograph of these on emerald is shown in fig. 9. This sort of minute spiral hillocks have not been observed on natural crystals of the same compound
SURFACE MICROSTRUCTURES
OF CRYSTALS GROWN
FROM MOLTEN SALTS
107
a
b
Fig. 8.
Two examples oftype 4 spirals. ta)(l0TO) face of emerald. L.arge number of small dots are the type 4 spirals. .20. (b) (I UT I) face of hematite. Phase contrast. . 240.
(hematite. emerald), and are characteristic growth features of synthetic crystals. Similar growth hillocks can often be found also on synthetic crystals grown from a hydrothermally aqueous solution such as quartz. They represent the growth from fairly high supersaturation condition, as can he infered from narrow spacings between the successive arms of the spirals. In addition to the above differences, observed between natural and synthetic crystals, the following differences were noticed between flux-grown and hydro-
thermal or vapour-grown crystals. The differences are similar to those previously6) reported for natural and synthetic emeralds: I) the morphology of the spirals observed on both (000l)and (IOTO) faces was different, 2) the spacings between the successive arms of the spirals were wide on the natural crystals and narrow on the Chatham’s emerald, 3) minute growth spirals were not found on the natural emerald, but on the synthetic one. The three differences are all infered to the different conditions of the supersaturation. Flux-
108
ICHIRO SUNAGAWA
Fig. 9.
High magnification phase contrast photomicrograph of the type 4 spirals. (1010) face of emerald. >< 1000.
grown crystals exhibit features suggesting the growth from higher conditions of supersaturation than the hydrothermal solution-grown natural crystals. Similar differences have been noticed between fluxgrown hematite and natural hematite which is grown from the vapour phase. Most of the natural hematite crystals exhibit typical spirals with step height of 2.3 A, 4.6 A, 7 A, etc. (c0 of hematite is 13,73 A) and cornposite spirals of various types, but do not exhibit the spirals of the fourth the spirals other hand, fluxgrown hematite exhibitstype. bothOn typical and minute growth hillocks, as well as triangular growth tables7) which are considered to have been formed by epitaxial settlement of the already formed crystallites on the growing (0001) face of the host crystal. In general, flux-grown hematite exhibits growth features which represent higher conditions of supersaturation than the natural hematite grown from the vapour phase. 4. Discussions Observations on the surface structures of crystal faces of a wide variety of flux-grown crystals revealed that all the crystals investigated showed some kinds of
growth spirals, starting from the typical one with wide spacings and mono-molecular step height to the spirals with very narrow spacings. Therefore, it can be coneluded with certainity that the growth of the crystals from molten salts and oxides (flux method) are performed by spiral mechanism at least at the latest stage of their crystallization. Spiral growth mechanism was at first put forward to account for the growth fromhasvapour at low 8). This been phase supported by supersaturation rate many experimental evidence, and it is known that the crystals grown from the vapour phase almost invariably exhibit growth spirals. Evidence can be found among such crystals as silicon carbide, hematite, sphalerite, biotite, phlogopite, synthetic ZnS, CdS, etc. Some of these crystals exhibit spirals of the type 1, whereas others often show type 2 or composite spirals originating from a large number of screw dislocations situated close together. That the spiral growth mechanism is applicable to the growth from solution phase has also been observed on a wide variety of crystals grown from both aqueous and non-aqueous solutions, The crystals grown from solutions on which growth spirals were
SURFACE MICROSTRUCTURES OF CRYSTALS GROWN FROM MOLTEN SALTS
observed are salol, n-alcohol, paraffine, stearic acid, n-nonatriacontane, n-propyl, npenactontanoate, nhectane, A1B2, quartz (both natural and synthetic), beryl, apatite, fluorite, sphalerite, pyrite, etc. However, some crystals grown from solutions do not exhibit growth spirals, even if they are observed carefully using the most sensitive methods. For instance, many (not all) pyrite crystals grown from a hydrothermal solution do not show growth spirals, instead they show large number of small growth islands on which no spiral layers are observed, These islands are considered to have been formed either by two-dimensional nucleation or epitaxial settlement of already formed crystallites. This shows that many crystals prepared from solution, though not all, are grown by the spiral mechanism and only a few of them are grown by the layer growth mechanism. In contrast to the above two cases, growth spirals have been observed only exceptionally among the crystals grown from a unicomponent melt. Such exceptional cases reported so far are on the crystals of salol, Hg12 and synthetic fluor-phiogopite. In the case of fluor-phlogopite, growth spirals were not found on the as-grown faces, but on the walls of elongated cavities in the crystals. No growth spirals have been observed so far on other crystals grown from the melt, This is reasonable since the growth process from the melt is different from the other methods in many respects, such as the difference in densities or number of molecular bonds between solid and liquid phases. Solid—liquid interface in the crystallization of a homogeneous phase from the melt is considered in many, though not in all cases, to be a rough surface, rather than a singular surface. For the growth to take place
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on such rough surfaces, neither two-dimensional nucleation nor the existence of screw dislocations are necessary. There are enough sites on such surfaces to accomodate atoms or molecules from the liquid. Therefore, it is expected that neither growth spirals nor growth layers will be observed on the surfaces of many of the crystals grown from a unicomponent melt, From the above described observations and consideration, it is natural to think that the growth process from molten salts is different from the growth of a unicomponent molten phase, and is similar to the growth from vapour or solutions of low supersaturation, though the flux method represents the growth from a higher supersaturation than the latter two methods. Acknowledgments The writer expresses his thanks to the persons and organizations who supplied him with samples, and to Dr. H. Komatsu and Mr. T. Nakano for the help in observation. References 1) H. Komatsu and I. Sunagawa, Miner. J. (Tokyo) 4(1964)203. 2) I. Sunagawa, Adsorption et Croissance Cristalline (C,N.R.S.,
Paris, 1965) p. 665. 3) P. B. Braun, Philips Res. Rept. 6 (1957) 491. 4) V. A. Timofeeva, in: Rost Kristallov, Vol. 6 (Academy of Sciences USSR, Institute of Crystallography, Moscow, 1965) p. 86 (in Russian). 5)1. Sunagawa, Am. Miner. 46 (1961) 1216.
6)1. Sunagawa, Am. Miner. 49 (1964) 785: see references for earlier works. 7) K. T. Wilke, in: Rost Kristal/ov, Vol. 6 (Academy of Sciences USSR, Institute of Crystallography, Moscow, 1965) p. 75 (in Russian). 8) F. C. Frank, Discussions Faraday Soc. 5 (1949) 48.