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Morphology of gallium orthophosphate crystals grown under hydrothermal conditions
P r o g r e s s in Crystal Growth a n d Characterization Progress in Crystal Growth and Characterization of Materials of Materials (2000) 243-251 http ://www.elsevier.com/locate/pcrysgrow
PERGAMON
Morphology of gallium orthophosphate crystals grown under hydrothermal conditions A . 1 . M o t c h a n y a, P . P . C h v a n s k i " a n d N . I . L e o n y u k b
aRussian Research Institute for the Synthesis of Minerals (VNIISIMS) Alexandrov 601600, Vladimir District,Russian Federation bDepartment of Crystallography and Crystallochemistry, Geological Faculty Moscow State University, Moscow 119899, Russian Federation
Abstract
Gallium orthophosphate crystals with sizes of 5 mm and more have been obtained by spontaneous nucleation flora hydrothermal 5.6 - 15 M orthophosphoric acid solutions. Preferable concentrations of solvent were found in the range of 11 - 12 M, the temperature difference should not be over 6-10°C at the heating rate of 4 - 5 °C/day. Morphological investigations are carried out using optical and polarizing rnieroscopies. GaPO4 crystals, like quartz and berlinite, tends to grow with well developed { 1Ot1}, { O ~ } , { 1 ~0}, { 1 ~ } and {O'i2} faces, and they were divided into three habit types. Effect of orthophosphoric acid concenlration on the crystal habit has been studied.
PACS: 81.10.Dn Keywords: Hydrothermal synthesis, Crystal growth, orthophosphate
Crystal morphology,
CONTENTS
1. Introduction 2.Experimental 3. Results and discussions 3.1. Spontaneous crystallization
3.2. Morphology of crystals 4. Summary References
0960-8974/00/$ - see front matter © 2000 Published by Elsevier Science Ltd. Pll: S0960-8974(00)00034-6
Gallium
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1. INTRODUCTION
Gallium orthophosphate GaPO4 crystals belong to the 32 class symmetry with quartz (Si02) and berlinite (A1PO4) P3121 (P3221) space group. In the case of the GaPO4 structure, silicon positions are tmiformly occupied with gallium and phosphorus atoms. It makes it possible to compare structural and physical characteristics of quartz, berlinite and gallium orthophosphate [1-3] (Table 1). In contrast to quartz and berlinite, GaPO4 crystals have a rather higher coupling coefficient (K) and twice as much piezoelectric modulus up to high temperatures (about 1000°C). For example, the K values for AT cutting crystals are 8.5, 11,0 and 18.4% for quartz, berlinite and gallium orthophosphate respectively [4]. Additionally, they possess a high chemical and radiation stability. For this reason, gallium orthophosphate crystals offer some advantages for piezoelectric and acousto-electric applications. To date, more and more research has been concentrated on these crystals which, like berlinite, can be grown under hydrothermal conditions both in the field of negative solubility coefficient (T < 335°C) and positive one (T > 335°C) [5-11].
Table 1. Some Structural and Physical Characteristics of Quartz and GaPO4 Crystals Lattice Crystal
GaPO4
Space
d*,
group
A
P312[
1.67
constants
(A)
P3121
Density
Hardness
refraction
(g/em 3)
(Moh)
transition
a
c
4.889
11.034
(P3221) SiO2
Index of
no
ne 1.603
3.570
- 6.5
2.655
7
(average) 1.61
4.9138
5.4052
1.535
1.544
573°C
(P3221) d*- internuclear distances Si-O2 and average internuclear distances Ga-O2, P-O2
In this work, the morphology of gallium orthophosphate crystals has been studied depending on the conditions of hydrothermal spontaneous crystallization from 5.6-15 M orthophosphoric acid solutions.
2.
EXPERIMENTAL
Gallium orthophosphate crystals as well as berlinite have, as a rule, negative
solubility in
hydrothermal solutions. Therefore, it is possible to grow them in the regions of both negative (T < 335 °C) [12] and positive (T> 335 °C) [13] solubility,
A. 1". Motchany et al. / Prog. Crystal Growth and Charact. 40 (2000) 243-251
245
At the first stage, small crystals were obtained by the increasing temperature method that was suggested, for the first time, by the author [14] for the hydrothermal growth of berlinite crystals. For this process, preferable temperature and pressure ranges, solvent composition and other factors have been found for the spontaneous crystallization of gallium orthophosphate. As a result, a preference was given to aqueous solutions of the orthophosphoric acid of 5.6 - 12 M concentration. Solutions were previously saturated by dissolving the gallium oxide powder in orthophosphorie
acid. In hydrothermal
synthesis, the
supersaturation was created and kept at a suitable level by adding gallium oxide "tablets". These tables were pressed Ga203 powder and annealed at a temperature of 1000° C during 24 hours. Hydrothermal synthesis was carried out using steel reaction vessels lined with fluoroplastic with an internal volume of 250 ml. The temperature was slowly increased from 150 to 260 °C at a rate of 3 - 10 °C/day. Temperature differences between the zones of dissolution (the vessel's upper part) and spontaneous crystallization (the lower part ) was kept at 6 - 20 ° C. The space factor of an vessel was changed in the interval of 80 to 93 % of its "free" volume. Temperature is measuared in both zones by two external thermocouples with an accuracy of 4- 2 ° C. After the experimental runs, in order to avoid dissolution of spontaneous crystals, the autoclaves were cooled down very fast with running water. Gallium orthophosphate crystals were carefully washed, dried and sorted by fraction. Those with dimensions more than 5 mm were used for further charges in GaPO4 crystal growth processes. Morphological investigations are carried out using optical and polarizing microscopy. Lattice cell parameters were determined by a X-Ray powder diffraction method.
3. RESULTS AND DISCUSSIONS 3.1. Spontaneous crystallization
Fine-crystalline gallium orthophosphate differed in fraction size was synthesized in runs involving spontaneous crystallization using on the method of increasing temperature. In order to obtain crystals with dimension not less than 5 mm, that it is necessary to use them as an initial charge in further experiments, fractional distribution of these crystals of sizes were studied under different growth conditions. It depends on the solvent concentration, the temperature differences between zones of dissolution and growthl the increasing temperature rate and on the final level of temperature. For example the yield of spontaneous crystals having dimension more than 5 mm is practically invariant in the range of the withan autoclave space factor from 80 to 93 %. Increasing the temperature gradient to 15 - 20 °C at a constant concentration of orthophosphoric acid yields large crystals. An increase in the heating rate upto 10 °C/day reduces amount of the spontaneous by grown crystals with sizes of 5 mm and more. Best results were obtained in the using heating rates of 4 - 5 °C/day. The total yield of crystals increases with both an increase in the concentration of orthophosphoric
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acid and the final synthesis temperature. However, crystal sizes reduces as soon as acid concentration exceeds 12 M. Thus, the optimal concentration of H3PO4 solvent should be in the range of 11 - 12 M, the temperature difference is not over 6 - 10 °C at the heating rate of 4 - 5 °C/day. Just under these conditions, it is possible to obtain a maximum number of spontaneous by grown crystals with desirable sizes for use as a charge for developing crystal growth experiments. The lattice cell parameters of grown GaPO4 crystal are the followings: a = 4.901(1) A and c = 11.040(1) A, and they correspond to earlier results [3, 11].
3.2. Morphology of crystals
GaPO4 crystals, like quartz and berlinite, tends to grow with developed { 10il },{ 01il }, { 10i0 }, { 10i2 } and { 0112 } faces as a result of similarity of their structures and crystallochemical peculiarities of the [SiO4] 4- and [PO4]3-tetrahedra [15]. The GaPO4 crystals grown can visually be divided into three habit types: (1) crystals having approximately equal rhombohedral faces r{ 10il } and z{ 0 lil }; (2) crystals with strongly developed positive rhombohedrun r; (3) crystals facetted by rhombohedra and pinacoid e {0001 } as a smooth face. Goniometric measurements allowed one to determine the following simple forms: rhombohedra { 10il } and { 01il }, prism { 10i0 } and pinacoid {0001 }. Also, it was found that ~ { 10i2 } and re' { 0112 } rhombohedra are stable growth faces-for all the gallium orthophosphate crystals. Fig.1 shows the first example of these crystals with positive r and z rhombohedra. The second type of GaPO4 crystals have strongly developed positive rhombohedron, but the negative one plays a secondary role,,The {0001 }faces-are well developed in crystals belongnig to the third morphological type (Fig.2).
a
Fig. 1. First type of GaPO4crystals (a, x6) and their ideal habit (b).
A. L Motchany et al. / Prog. Crystal Growth and Charact. 40 (2000) 243-251
247
Thus, as opposed to quartz and berlinite, gallium orthophosphate crystals, as a rule, have well developed e {0001 } pinacoidal faces in the region of their retrograde solubility. At the same time, quartz is always facetted with m { 1010 } prism, but these faces are seldom observed in A1PO4 and GaPO4 crystals. Positive r { 10il } rhornbohedron are always the most slowly growing simple form in gallium orthophosphate crystals. Generally, in the case of the first habit type, there is the following correlation between the crystal growth rate of the r, z, n, m and e faces: V{lo Tll < V(ol 711<~V{lo 121< V(lo 10}~ V(O0Oll.
a
Fig. 2. Third type ofGaPO4crystals (a, x6) and their ideal habit (b).
The morphology of GaPO4 crystals in many respects is determined by the conditions of hydrothermal crystallization. Morphological peculiarities of these crystals grown in various intervals of orthophosphoric acid concentration were studied by the authors of refs. [6,8,12,16]. Here, the main attention is paid to the morphology of spontaneous by grown crystals obtained from hydrothermal solutions in a wide range of H3PO4 concentration, from 5.6 M to 12 M. Variations of in the crystal growth conditions resulted in the appearance or disappearance of some of the simple forms. In the case of 5.6-7 M H3PO4 concentrations, the first and second types of GaPO4 crystals were grown. If the concentration of H3PO4 was less than 8 M, m {10i0} prism faces are absent in these crystals, or this prism was represented as a narrow hexagonal belt. The facetting of gallium orthophosphate crystals is characterized mainly by r { 10il } and z { 01il } rhombohedra, but rhombohedral n { 10i2 } and n' { 0112 } faces rarely appeared. Increase in the concentration of orthophosphoric acid in solutions always promotes the development of e {0001 } as well as m { 1010 } faces in GaPO4 crystals. Therefore, an increase
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in the concentration of solvent causes variations between the growth rates of the r { 10il } and z { 0111 } rhombohedra as well as the m { 1010 } prism and e {0001 } pinacoid: the growth rates of prism and pinacoid slow down and these faces appear in the crystals. Growth surfaces of GaPO4 crystals are similar to the same faces of quartz and berlinite, and it is an easy way for identification of the simple forms in gallium orthophosphate crystals. A specific peculiarity of these crystals is displayed during a "post-growth". Etching occurs following cooling of the reaction vessel after crystallization process because of negative solubility of this material. These etch pits have trigonal symmetry in pinacoidal faces (Fig.3), and r rhombohedron faces are covered with round asymmetric knobs (m planar symmetry). Prism faces have strongly developed growth striations parallel to the prism and rhombohedron edges (planar symmetry --2).
Fig. 3. "Post-grown'etching holes in a pinacoidal face of QaPO4 crystals (x8).
Most of the crystals are twinned during the process of spontaneous nucleation, and there are twins like those in quartz and berlinite crystals: Brazilian, Dauphine Leydolt twinning laws. "Post-grown" etching allows one to reveal twinn boundaries and to identify the type of defects in GaPO4 crystals. For example, it was found that the 7t (1012) face is the Brazilian fiat-twinning face in gallium orthophosphate crystals. It also follows from crystallochemical analysis of the Brazilian twinning law in quartz [17], because { 1012 } faces have the same structure what they hare in GaPO4 crystals.
]
~
4. SUMMARY
Galli~n orthophosphate crystals with sizes of 5 mm and more have been obtained by spontaneous nucleation from hydrothermal 5.6-15 M orthophosphoric acid solutions. Recommended concentrations of solvent were found in the range of 11 - 12 M, the temperature difference should not be over 6 - 10 °C at the heating rate of 4 - 5 °C/day. Morphoiogical investigations are carried out using optical and polarizing
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microseopies. GaPO4 crystals, like quartz and berlinite, tend to grow with well developed { 10il }, { 0111}, {10i0} , {I0i2} and {01i2} faces, and they were divided into three habit types. The effect of orthophosphorie acid concentration on the crystal habit has been studied.
REFERENCES 1. E. Philippot, A. Goiffon, A. Ibanez and M. Pintard. Structure deformations and existence of the c~ 13 transition in M X O 4
quartz -
like materials. J. of Solid State Chemistry, VoL 110 (1994) N 2, pp.356 - 362.
2. A.Goiffon, G.Baule, R.Astier. Crystallochimie des phases GaPO4, A1AsO4 et GaAsO4. Etude Compar6e des structures de tupe quartz - ct. Rev. Chim. Miner., (1983) Vol. 20 (1983) N 3, pp. 338 - 350. 3. E.Philippot, D.Palmier, M.Pintard and A.Goiffon. A general survey of quartz and quartz- like materials: packing distortions, temperature, and pressure effects. J. of Solid State Chemistry, Vol. 123 (1996) N 1, pp. 1-13. 4. P.Yot, D.Palmier, O.Cambon, A.Goiffon, E.Philippot. Crystal growth and characterization of an ct - quartz /
- like piezoelectric materials, gallium orthophosphate. Ann. Chim. Sci. Mat., Vol. 22 (1997) N8, pp. 679 682. 5. U. Krauss and G. Lehmann. EPR of Fe 3+ in low quartz isomorphs A(III)B(V)O4. Ztsehr. Naturforsch. A., Vol. 30 (1997) N1, pp.28 - 34. 6. H. T. Chai, E.Buehler and J.J.Flynn. Hydrotherrnal crystal growing process and apparature. European Patent Specification N 0057773.26.02. 1986. 7. S.Hirano, K.Miwa, S.Naka. Hydrothermal synthesis of gaUium orthophosphate crystals. J. Crystal Growth, Vol. 79 (1986) N1/3, pp. 215 - 218. 8. A.Coiffon, J.C.Jumas, R.Astier and E. Philippot. Solubilit6s des phases M HI xVo4 (M = A1, Ga; X = P, As) darts les solutions d'aeide H3X04 sous conditions hydrothermales: application/t la crislallog6n6se de l'ars6niate d'aluminium A1AsO4. J. Crystal Growth, Vol. 71 (1985) N 3, pp. 763 - 770. 9. A.A.Sternberg, G.S.Mironova, O.V.Zvereva and M.V.Molomina. Structural analogeus of the ct - quartz: aluminium and gallium orthophosphates. Rost Kristallov, vol.17 (1988), pp.142 - 149 (in Russian). 10. E.Philippot, A.Ibanez, A.Goiffon, M.Cochez, A.Zarka, B.Capelle, J.Schwartzel and J.Detaint. A quartz - like material: gallium phosphate (GaPO4); crystal growth and characterization. J. Crystal Growth, Vol. 130 (1993) N2, pp. 195-208. 11. O.Baumgartner, A.Preisinger, H.Krempl and H.Mang. Die Kristallstruktur von GaPO4 bei 20 °C, 500 °C und 750 °C. Ztschr. Kristallogr., Vo1.168 (1984) N 1-4, pp. 83 - 91. 12. G.Engel, H.Klapper, P.Krempl and H.Mang. Crowth twinning in quartz - homeotypic gallium orthophosphate crystals. J. Crystal Growth, Vol. 94(1989) N 3, pp. 597 - 606.
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13. A.S.Shigarov, O.V.Zvereva and Yu.M.Mininzon. Solybility of gallium orthophosphate
GaPO4 under
hydrothermal conditions. Kristailografiya, VoL 39 (1994) N 4, pp. 717 - 719 (in Russian). 14. J.M.Stanley. Hydrothermal synthesis of large aluminum phosphate crystals. Ind. and Eng. Chem., Vol. 46 (1954) N 8, pp. 1684 - 1689. 15. H.Strunz. lsotypie yon Berlinit und Quartz. Ztsehr. Kristallogr., Vol.103 (1941) N 4, pp. 228 - 229. 16. E.Philippot, A.Goiffon and A.Ibanez. Comparative crystal habit study of quartz and MPO4 isomorphous compounds (M = A1, Ga). J. Crystal Growth, (1996) Vol. 160 (1996) N 2, pp. 268 - 278. 17. L.I.Tsinober and V.G.Balakirev. Stryctural simulation of the Brazilian twin boundary in a-quartz crystals. Doklady Akademii nauk SSSR. Vol.259 (1981) N4, pp. 846 - 850 ( in Russian).
i1 Dr. A. Motchany
Dr. P, Chvanski
Prof. N.I. Leonyuk
Dr. A. I. Motchany graduated from Physical Department of Dnepropetrovsk State University in 1985, works at Hydrothermal Synthesis Department of All-Russian Research Institute for the Synthesis of Raw Materials (VNIISIMS) as a senior scientific worker. Now he pass training of the Chair of Crystallography and Crystal Chemistry of Geologikal Department of Moscow State University. The theme of his dissertation is " Hydrothermal synthesis of structural analogs of a - quartz". Dr. A. Motehany is specializing in present time - growth and characterization of orthophosphate crystals (berlinite and gallium orthophosphate). Dr. A. Motehany is the author of about 25 research articles.
Dr. P. Chvanski graduated from Geological Department of Novocherkask Polytechnic institute obtained his Ph.D. (Crystallography and Crystal Physicals degree from the Moscow State University in 1989. Since 1991 he is appointed as a head of laboratory of calcite synthesis. In1996 he beeame the head of hydrothermal synthesis depai'tment Dr. P. Chvanski works at VNIISIMS since 1980. He takes part in realisation of scientific experiments on development of a laboratory and industrial technique for hydrothermal growth of berlinite, caleite, and quartz crystals. Dr. P. Chvanski is the author of about 30 research articles and patents.
A. L Motchany et al. / Prog. Crystal Growth and Charact. 40 (2000) 243-251
Prof. N.I. Leonvuk Graduated from Lomonosov Moscow State University (LMSU) Ph.D. (Crystallography and Crystallo - Physics), LMSU Dr.Sc. (Chemistry), LMSU Professor, LMSU Scientific in,rest: Growth and characterization of single crystals Long-term course of lectures on the crystal growth and crystal morphology for Teaching: students- crystallographers of the 3rd, 4th, 5th and 6th academic years at the LMSU Scientific works: about 400 including 2 books and 10 patents. 1969t97219851988-
25
PERGAMON
Morphology of gallium orthophosphate crystals grown under hydrothermal conditions A . 1 . M o t c h a n y a, P . P . C h v a n s k i " a n d N . I . L e o n y u k b
aRussian Research Institute for the Synthesis of Minerals (VNIISIMS) Alexandrov 601600, Vladimir District,Russian Federation bDepartment of Crystallography and Crystallochemistry, Geological Faculty Moscow State University, Moscow 119899, Russian Federation
Abstract
Gallium orthophosphate crystals with sizes of 5 mm and more have been obtained by spontaneous nucleation flora hydrothermal 5.6 - 15 M orthophosphoric acid solutions. Preferable concentrations of solvent were found in the range of 11 - 12 M, the temperature difference should not be over 6-10°C at the heating rate of 4 - 5 °C/day. Morphological investigations are carried out using optical and polarizing rnieroscopies. GaPO4 crystals, like quartz and berlinite, tends to grow with well developed { 1Ot1}, { O ~ } , { 1 ~0}, { 1 ~ } and {O'i2} faces, and they were divided into three habit types. Effect of orthophosphoric acid concenlration on the crystal habit has been studied.
PACS: 81.10.Dn Keywords: Hydrothermal synthesis, Crystal growth, orthophosphate
Crystal morphology,
CONTENTS
1. Introduction 2.Experimental 3. Results and discussions 3.1. Spontaneous crystallization
3.2. Morphology of crystals 4. Summary References
0960-8974/00/$ - see front matter © 2000 Published by Elsevier Science Ltd. Pll: S0960-8974(00)00034-6
Gallium
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1. INTRODUCTION
Gallium orthophosphate GaPO4 crystals belong to the 32 class symmetry with quartz (Si02) and berlinite (A1PO4) P3121 (P3221) space group. In the case of the GaPO4 structure, silicon positions are tmiformly occupied with gallium and phosphorus atoms. It makes it possible to compare structural and physical characteristics of quartz, berlinite and gallium orthophosphate [1-3] (Table 1). In contrast to quartz and berlinite, GaPO4 crystals have a rather higher coupling coefficient (K) and twice as much piezoelectric modulus up to high temperatures (about 1000°C). For example, the K values for AT cutting crystals are 8.5, 11,0 and 18.4% for quartz, berlinite and gallium orthophosphate respectively [4]. Additionally, they possess a high chemical and radiation stability. For this reason, gallium orthophosphate crystals offer some advantages for piezoelectric and acousto-electric applications. To date, more and more research has been concentrated on these crystals which, like berlinite, can be grown under hydrothermal conditions both in the field of negative solubility coefficient (T < 335°C) and positive one (T > 335°C) [5-11].
Table 1. Some Structural and Physical Characteristics of Quartz and GaPO4 Crystals Lattice Crystal
GaPO4
Space
d*,
group
A
P312[
1.67
constants
(A)
P3121
Density
Hardness
refraction
(g/em 3)
(Moh)
transition
a
c
4.889
11.034
(P3221) SiO2
Index of
no
ne 1.603
3.570
- 6.5
2.655
7
(average) 1.61
4.9138
5.4052
1.535
1.544
573°C
(P3221) d*- internuclear distances Si-O2 and average internuclear distances Ga-O2, P-O2
In this work, the morphology of gallium orthophosphate crystals has been studied depending on the conditions of hydrothermal spontaneous crystallization from 5.6-15 M orthophosphoric acid solutions.
2.
EXPERIMENTAL
Gallium orthophosphate crystals as well as berlinite have, as a rule, negative
solubility in
hydrothermal solutions. Therefore, it is possible to grow them in the regions of both negative (T < 335 °C) [12] and positive (T> 335 °C) [13] solubility,
A. 1". Motchany et al. / Prog. Crystal Growth and Charact. 40 (2000) 243-251
245
At the first stage, small crystals were obtained by the increasing temperature method that was suggested, for the first time, by the author [14] for the hydrothermal growth of berlinite crystals. For this process, preferable temperature and pressure ranges, solvent composition and other factors have been found for the spontaneous crystallization of gallium orthophosphate. As a result, a preference was given to aqueous solutions of the orthophosphoric acid of 5.6 - 12 M concentration. Solutions were previously saturated by dissolving the gallium oxide powder in orthophosphorie
acid. In hydrothermal
synthesis, the
supersaturation was created and kept at a suitable level by adding gallium oxide "tablets". These tables were pressed Ga203 powder and annealed at a temperature of 1000° C during 24 hours. Hydrothermal synthesis was carried out using steel reaction vessels lined with fluoroplastic with an internal volume of 250 ml. The temperature was slowly increased from 150 to 260 °C at a rate of 3 - 10 °C/day. Temperature differences between the zones of dissolution (the vessel's upper part) and spontaneous crystallization (the lower part ) was kept at 6 - 20 ° C. The space factor of an vessel was changed in the interval of 80 to 93 % of its "free" volume. Temperature is measuared in both zones by two external thermocouples with an accuracy of 4- 2 ° C. After the experimental runs, in order to avoid dissolution of spontaneous crystals, the autoclaves were cooled down very fast with running water. Gallium orthophosphate crystals were carefully washed, dried and sorted by fraction. Those with dimensions more than 5 mm were used for further charges in GaPO4 crystal growth processes. Morphological investigations are carried out using optical and polarizing microscopy. Lattice cell parameters were determined by a X-Ray powder diffraction method.
3. RESULTS AND DISCUSSIONS 3.1. Spontaneous crystallization
Fine-crystalline gallium orthophosphate differed in fraction size was synthesized in runs involving spontaneous crystallization using on the method of increasing temperature. In order to obtain crystals with dimension not less than 5 mm, that it is necessary to use them as an initial charge in further experiments, fractional distribution of these crystals of sizes were studied under different growth conditions. It depends on the solvent concentration, the temperature differences between zones of dissolution and growthl the increasing temperature rate and on the final level of temperature. For example the yield of spontaneous crystals having dimension more than 5 mm is practically invariant in the range of the withan autoclave space factor from 80 to 93 %. Increasing the temperature gradient to 15 - 20 °C at a constant concentration of orthophosphoric acid yields large crystals. An increase in the heating rate upto 10 °C/day reduces amount of the spontaneous by grown crystals with sizes of 5 mm and more. Best results were obtained in the using heating rates of 4 - 5 °C/day. The total yield of crystals increases with both an increase in the concentration of orthophosphoric
246
A. £ Motchany et al. / Prog. Crystal Growth and Charact. 40 (2000) 243-251
acid and the final synthesis temperature. However, crystal sizes reduces as soon as acid concentration exceeds 12 M. Thus, the optimal concentration of H3PO4 solvent should be in the range of 11 - 12 M, the temperature difference is not over 6 - 10 °C at the heating rate of 4 - 5 °C/day. Just under these conditions, it is possible to obtain a maximum number of spontaneous by grown crystals with desirable sizes for use as a charge for developing crystal growth experiments. The lattice cell parameters of grown GaPO4 crystal are the followings: a = 4.901(1) A and c = 11.040(1) A, and they correspond to earlier results [3, 11].
3.2. Morphology of crystals
GaPO4 crystals, like quartz and berlinite, tends to grow with developed { 10il },{ 01il }, { 10i0 }, { 10i2 } and { 0112 } faces as a result of similarity of their structures and crystallochemical peculiarities of the [SiO4] 4- and [PO4]3-tetrahedra [15]. The GaPO4 crystals grown can visually be divided into three habit types: (1) crystals having approximately equal rhombohedral faces r{ 10il } and z{ 0 lil }; (2) crystals with strongly developed positive rhombohedrun r; (3) crystals facetted by rhombohedra and pinacoid e {0001 } as a smooth face. Goniometric measurements allowed one to determine the following simple forms: rhombohedra { 10il } and { 01il }, prism { 10i0 } and pinacoid {0001 }. Also, it was found that ~ { 10i2 } and re' { 0112 } rhombohedra are stable growth faces-for all the gallium orthophosphate crystals. Fig.1 shows the first example of these crystals with positive r and z rhombohedra. The second type of GaPO4 crystals have strongly developed positive rhombohedron, but the negative one plays a secondary role,,The {0001 }faces-are well developed in crystals belongnig to the third morphological type (Fig.2).
a
Fig. 1. First type of GaPO4crystals (a, x6) and their ideal habit (b).
A. L Motchany et al. / Prog. Crystal Growth and Charact. 40 (2000) 243-251
247
Thus, as opposed to quartz and berlinite, gallium orthophosphate crystals, as a rule, have well developed e {0001 } pinacoidal faces in the region of their retrograde solubility. At the same time, quartz is always facetted with m { 1010 } prism, but these faces are seldom observed in A1PO4 and GaPO4 crystals. Positive r { 10il } rhornbohedron are always the most slowly growing simple form in gallium orthophosphate crystals. Generally, in the case of the first habit type, there is the following correlation between the crystal growth rate of the r, z, n, m and e faces: V{lo Tll < V(ol 711<~V{lo 121< V(lo 10}~ V(O0Oll.
a
Fig. 2. Third type ofGaPO4crystals (a, x6) and their ideal habit (b).
The morphology of GaPO4 crystals in many respects is determined by the conditions of hydrothermal crystallization. Morphological peculiarities of these crystals grown in various intervals of orthophosphoric acid concentration were studied by the authors of refs. [6,8,12,16]. Here, the main attention is paid to the morphology of spontaneous by grown crystals obtained from hydrothermal solutions in a wide range of H3PO4 concentration, from 5.6 M to 12 M. Variations of in the crystal growth conditions resulted in the appearance or disappearance of some of the simple forms. In the case of 5.6-7 M H3PO4 concentrations, the first and second types of GaPO4 crystals were grown. If the concentration of H3PO4 was less than 8 M, m {10i0} prism faces are absent in these crystals, or this prism was represented as a narrow hexagonal belt. The facetting of gallium orthophosphate crystals is characterized mainly by r { 10il } and z { 01il } rhombohedra, but rhombohedral n { 10i2 } and n' { 0112 } faces rarely appeared. Increase in the concentration of orthophosphoric acid in solutions always promotes the development of e {0001 } as well as m { 1010 } faces in GaPO4 crystals. Therefore, an increase
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in the concentration of solvent causes variations between the growth rates of the r { 10il } and z { 0111 } rhombohedra as well as the m { 1010 } prism and e {0001 } pinacoid: the growth rates of prism and pinacoid slow down and these faces appear in the crystals. Growth surfaces of GaPO4 crystals are similar to the same faces of quartz and berlinite, and it is an easy way for identification of the simple forms in gallium orthophosphate crystals. A specific peculiarity of these crystals is displayed during a "post-growth". Etching occurs following cooling of the reaction vessel after crystallization process because of negative solubility of this material. These etch pits have trigonal symmetry in pinacoidal faces (Fig.3), and r rhombohedron faces are covered with round asymmetric knobs (m planar symmetry). Prism faces have strongly developed growth striations parallel to the prism and rhombohedron edges (planar symmetry --2).
Fig. 3. "Post-grown'etching holes in a pinacoidal face of QaPO4 crystals (x8).
Most of the crystals are twinned during the process of spontaneous nucleation, and there are twins like those in quartz and berlinite crystals: Brazilian, Dauphine Leydolt twinning laws. "Post-grown" etching allows one to reveal twinn boundaries and to identify the type of defects in GaPO4 crystals. For example, it was found that the 7t (1012) face is the Brazilian fiat-twinning face in gallium orthophosphate crystals. It also follows from crystallochemical analysis of the Brazilian twinning law in quartz [17], because { 1012 } faces have the same structure what they hare in GaPO4 crystals.
]
~
4. SUMMARY
Galli~n orthophosphate crystals with sizes of 5 mm and more have been obtained by spontaneous nucleation from hydrothermal 5.6-15 M orthophosphoric acid solutions. Recommended concentrations of solvent were found in the range of 11 - 12 M, the temperature difference should not be over 6 - 10 °C at the heating rate of 4 - 5 °C/day. Morphoiogical investigations are carried out using optical and polarizing
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microseopies. GaPO4 crystals, like quartz and berlinite, tend to grow with well developed { 10il }, { 0111}, {10i0} , {I0i2} and {01i2} faces, and they were divided into three habit types. The effect of orthophosphorie acid concentration on the crystal habit has been studied.
REFERENCES 1. E. Philippot, A. Goiffon, A. Ibanez and M. Pintard. Structure deformations and existence of the c~ 13 transition in M X O 4
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like materials. J. of Solid State Chemistry, VoL 110 (1994) N 2, pp.356 - 362.
2. A.Goiffon, G.Baule, R.Astier. Crystallochimie des phases GaPO4, A1AsO4 et GaAsO4. Etude Compar6e des structures de tupe quartz - ct. Rev. Chim. Miner., (1983) Vol. 20 (1983) N 3, pp. 338 - 350. 3. E.Philippot, D.Palmier, M.Pintard and A.Goiffon. A general survey of quartz and quartz- like materials: packing distortions, temperature, and pressure effects. J. of Solid State Chemistry, Vol. 123 (1996) N 1, pp. 1-13. 4. P.Yot, D.Palmier, O.Cambon, A.Goiffon, E.Philippot. Crystal growth and characterization of an ct - quartz /
- like piezoelectric materials, gallium orthophosphate. Ann. Chim. Sci. Mat., Vol. 22 (1997) N8, pp. 679 682. 5. U. Krauss and G. Lehmann. EPR of Fe 3+ in low quartz isomorphs A(III)B(V)O4. Ztsehr. Naturforsch. A., Vol. 30 (1997) N1, pp.28 - 34. 6. H. T. Chai, E.Buehler and J.J.Flynn. Hydrotherrnal crystal growing process and apparature. European Patent Specification N 0057773.26.02. 1986. 7. S.Hirano, K.Miwa, S.Naka. Hydrothermal synthesis of gaUium orthophosphate crystals. J. Crystal Growth, Vol. 79 (1986) N1/3, pp. 215 - 218. 8. A.Coiffon, J.C.Jumas, R.Astier and E. Philippot. Solubilit6s des phases M HI xVo4 (M = A1, Ga; X = P, As) darts les solutions d'aeide H3X04 sous conditions hydrothermales: application/t la crislallog6n6se de l'ars6niate d'aluminium A1AsO4. J. Crystal Growth, Vol. 71 (1985) N 3, pp. 763 - 770. 9. A.A.Sternberg, G.S.Mironova, O.V.Zvereva and M.V.Molomina. Structural analogeus of the ct - quartz: aluminium and gallium orthophosphates. Rost Kristallov, vol.17 (1988), pp.142 - 149 (in Russian). 10. E.Philippot, A.Ibanez, A.Goiffon, M.Cochez, A.Zarka, B.Capelle, J.Schwartzel and J.Detaint. A quartz - like material: gallium phosphate (GaPO4); crystal growth and characterization. J. Crystal Growth, Vol. 130 (1993) N2, pp. 195-208. 11. O.Baumgartner, A.Preisinger, H.Krempl and H.Mang. Die Kristallstruktur von GaPO4 bei 20 °C, 500 °C und 750 °C. Ztschr. Kristallogr., Vo1.168 (1984) N 1-4, pp. 83 - 91. 12. G.Engel, H.Klapper, P.Krempl and H.Mang. Crowth twinning in quartz - homeotypic gallium orthophosphate crystals. J. Crystal Growth, Vol. 94(1989) N 3, pp. 597 - 606.
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13. A.S.Shigarov, O.V.Zvereva and Yu.M.Mininzon. Solybility of gallium orthophosphate
GaPO4 under
hydrothermal conditions. Kristailografiya, VoL 39 (1994) N 4, pp. 717 - 719 (in Russian). 14. J.M.Stanley. Hydrothermal synthesis of large aluminum phosphate crystals. Ind. and Eng. Chem., Vol. 46 (1954) N 8, pp. 1684 - 1689. 15. H.Strunz. lsotypie yon Berlinit und Quartz. Ztsehr. Kristallogr., Vol.103 (1941) N 4, pp. 228 - 229. 16. E.Philippot, A.Goiffon and A.Ibanez. Comparative crystal habit study of quartz and MPO4 isomorphous compounds (M = A1, Ga). J. Crystal Growth, (1996) Vol. 160 (1996) N 2, pp. 268 - 278. 17. L.I.Tsinober and V.G.Balakirev. Stryctural simulation of the Brazilian twin boundary in a-quartz crystals. Doklady Akademii nauk SSSR. Vol.259 (1981) N4, pp. 846 - 850 ( in Russian).
i1 Dr. A. Motchany
Dr. P, Chvanski
Prof. N.I. Leonyuk
Dr. A. I. Motchany graduated from Physical Department of Dnepropetrovsk State University in 1985, works at Hydrothermal Synthesis Department of All-Russian Research Institute for the Synthesis of Raw Materials (VNIISIMS) as a senior scientific worker. Now he pass training of the Chair of Crystallography and Crystal Chemistry of Geologikal Department of Moscow State University. The theme of his dissertation is " Hydrothermal synthesis of structural analogs of a - quartz". Dr. A. Motehany is specializing in present time - growth and characterization of orthophosphate crystals (berlinite and gallium orthophosphate). Dr. A. Motehany is the author of about 25 research articles.
Dr. P. Chvanski graduated from Geological Department of Novocherkask Polytechnic institute obtained his Ph.D. (Crystallography and Crystal Physicals degree from the Moscow State University in 1989. Since 1991 he is appointed as a head of laboratory of calcite synthesis. In1996 he beeame the head of hydrothermal synthesis depai'tment Dr. P. Chvanski works at VNIISIMS since 1980. He takes part in realisation of scientific experiments on development of a laboratory and industrial technique for hydrothermal growth of berlinite, caleite, and quartz crystals. Dr. P. Chvanski is the author of about 30 research articles and patents.
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Prof. N.I. Leonvuk Graduated from Lomonosov Moscow State University (LMSU) Ph.D. (Crystallography and Crystallo - Physics), LMSU Dr.Sc. (Chemistry), LMSU Professor, LMSU Scientific in,rest: Growth and characterization of single crystals Long-term course of lectures on the crystal growth and crystal morphology for Teaching: students- crystallographers of the 3rd, 4th, 5th and 6th academic years at the LMSU Scientific works: about 400 including 2 books and 10 patents. 1969t97219851988-
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