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Growth, morphology and mechanism of rare earth vanadate crystals under mild hydrothermal conditions
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Journal of Crystal Growth 306 (2007) 94–101 www.elsevier.com/locate/jcrysgro
Growth, morphology and mechanism of rare earth vanadate crystals under mild hydrothermal conditions K. Byrappaa,, C.K. Chandrashekara, B. Basavalingua, K.M. LokanathaRaib, S. Anandab, M. Yoshimurac a Department of Geology, University of Mysore, Manasagangotri, Mysore 570 006, India Department of Chemistry, University of Mysore, Manasagangotri, Mysore 570 006, India c Materials Structures Laboratory, Center for Materials Design, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226, Japan b
Received 18 July 2006; received in revised form 20 January 2007; accepted 8 March 2007 Communicated by T. Hibiya Available online 20 April 2007
Abstract Single crystals of RVO4 (R ¼ Y,Gd) doped with optically active element like Nd have been obtained under mild hydrothermal conditions (T ¼ 240 1C, P80 bars). A detailed mechanism of the crystallization process, which helps in the considerable reduction of the PT conditions of the growth of these high-melting (m.p.41800 1C) crystals has been formulated. The crystals obtained have been subjected to morphological, X-ray powder diffraction and infrared spectroscopic studies. r 2007 Elsevier B.V. All rights reserved. PACS: 81.10.h; 42.70.Hj; 64.70Dv; 81.10.Aj Keywords: A1. Morphological stability; A2. Hydrothermal crystal growth; A2. Single crystal growth; B1. Rare earth compounds; B1. Vanadates; B2. Phosphors; B3. Solid state lasers
1. Introduction Growth of laser crystals is an attractive field in solid-state science owing to their technological significance. There exists a large number of solid-state laser host crystals from the family of phosphates, vanadates, fluorides, tungstates, etc. Amongst them, the yttrium orthovanadate-based laserdiode pumped solid-state lasers are the frontier materials and carry a wide variety of applications in science and technology because of its high stability, compactness, high efficiency and long life time [1–3]. Nd:YVO4 crystals have been employed as highly efficient laser-diode pumped microlasers [4], an efficient phosphors [5], very attractive polarizer materials [6] and low-threshold laser hosts [7], etc. In comparison with the conventional Nd: YAG crystal, Nd:YVO4 offers many advantages such as larger absorption co-efficient and gain cross section [8–10]. Corresponding author. Tel.: +91 821 2419720; fax: +91 821 2419720.
E-mail address: [email protected] (K. Byrappa). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.03.055
The rare earth vanadates are known as the high-melting materials (melting point 41800 1C) with low solubility and obviously their synthesis by any technique usually insists upon higher temperature conditions. That is one of the main reasons why most of the earlier workers have obtained these laser crystals by flux or melt methods [11–14]. In spite of its excellent physical properties, the high-tech applications have not been realized due to the crystal growth difficulties. One of the major problems encountered in the growth of RVO4 crystals is the presence of oxygen imperfections (color centers and inclusions), which are introduced during the crystal growth processes. Although YVO4 melts congruently [15], vanadium oxides vaporize incongruently, causing changes in Y/V ratio and oxygen stoichiometry in the melt. These undesired effects could generate additional phases and oxygen defects in theYVO4 crystals grown especially from the melt [16]. Efforts to eliminate these defects did not yield significant success in the flux and melt techniques. Rubin and Vanuitert [11] were the first to obtain large bulk crystals
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of Nd:YVO4 using the Czochralski technique [17]. But problems related to the oxygen deficiency and inclusions could not be eliminated. This has been emphasized in numerous reports of Czochralski growth [17–19], and other melt growth methods like Verneuil [20], modified floating zone [21,22], laser-heated pedestal growth [23,24] and Bridgman [18]. However, there are not many reports on the hydrothermal synthesis of these crystals [25–27]. This is probably connected with the higher temperature and pressure conditions involved in the growth of these crystals and also to the strong belief that their growth under mild hydrothermal conditions will introduce water into the structure or to the interstices. Infact, several problems have been encountered in the growth of rare earth vanadates by the above-said conventional methods. The instability of pentavalent vanadium at higher temperatures and the loss of oxygen through surface encrustation by the reaction of the melt with the crucible material further complicate the growth processes. The unstable melt behaviour of YVO4 leads to the appearance of zoning in these crystals. Recently, it has been observed that YVO4 crystals are very difficult to grow with a stoichiometric or near-stoichiometric composition [28]. Further, the most critical aspect is the appearance of metastable phases and also some precipitates within the YVO4 crystals [29]. The detection of correct Y–V–O stoichiometry in YVO4 crystals is difficult [30]. Moreover, since the temperature involved in the flux growth is about 1300 1C, and in melt growth above 1800 1C, there is a high probability of vanadium attacking the container. In order to overcome most of the difficulties encountered in the melt and high-temperature solution techniques, our group for the first time has proposed the hydrothermal technique as a solution [25,26]. Since the experiments are carried out in a closed system, the loss of oxygen could be readily prevented. Here, the authors report the hydrothermal growth of Nd:RVO4 crystals and made a detailed study of the crystallization mechanism under hydrothermal conditions and their characterization through morphological, X-ray diffraction and infrared spectroscopic studies. 2. Experimental procedure The experiments on the hydrothermal growth of Nd:RVO4 (R ¼ Y, Gd) crystals were carried out using
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general purpose autoclaves made of SS316, provided with teflon liners of capacity 30 ml. The use of teflon liners helps in overcoming the entry of inclusions from the autoclave material into the crystals. The starting materials such as R2O3 (Y2O3 and Gd2O3, Analar grade, s.d Fine Chem Ltd., India), V2O5 (Analar grade, s.d Fine Chem Ltd., India), Nd2O3 (Analar grade, Rankem, India) were taken in an appropriate molar proportion in Teflon liners. Table 1 gives the experimental details. A suitable mineralizer of known concentration was added into the teflon liner and the entire mixture was stirred well till a clear homogeneous and relatively less viscous solution was obtained. The temperature was kept constant at 240 1C and the pressure 80 bars were maintained through appropriate % fill. The pH of the growth medium was measured before and after each experimental run. In the present work, the crystallization was carried out in all the experiments through spontaneous nucleation. Hence, an effort was made to reduce the number of nucleation centres by increasing the temperature of the crystallization reactor slowly at the rate of 10 1C/h up to 100 1C and beyond at the rate of 5 1C/h. The experimental temperature was held at 240 1C for a period of 3 days without any fluctuations in the temperature. From the fourth day onwards the temperature fluctuation to the tune of 710 to 720 1C was introduced periodically in order to reduce the number of nucleation centres. By this means, small crystallites, which had developed earlier, dissolve leaving only bigger crystallites to grow. It was observed that as the size of the crystal increases, the crystal quality reduces [27]. The pressure was measured in all the initial experimental runs using a Bourdon Gauge in order to fix the quantity of starting materials for 80 bars, and in the subsequent routine experiments the volume of the starting materials was adjusted to obtain a calculated pressure. The authors have extended the experimental duration to 20 days in order to check whether the crystal size improves with the duration however, to our surprise, the crystal size remained the same without any size increase. The reason for this is that the solution might have lost its ionic strength and hence the crystal never grows further, but instead, the grown crystals remain in equilibrium state with the solution. This also states that the crystal growth occurs only for a few days and then ceases as soon as the solution
Table 1 Experimental conditions for the growth of Nd:RVO4 crystals, where, R ¼ Y,Gd (R : V2O5 ¼ 1:2 molar ratio), where R ¼ Y2O3, Gd2O3, Nd2O3 ¼ 2–10 wt%) Sl. no.
Molar concentration of mineralizers
1
3 M HCl
2 M HNO3
2 3 4 5
4 M NH4OH 1.5 M HCl 3 M HCl 2 M HCl
1 M HNO3 3 M HNO3 1.5 M HNO3 2 M HNO3
Temperature ¼ 240 1C, Duration ¼ 8 days.
P (bar)
% fill
pH
Crystal size (mm)
Remarks
70
60
0.18
4.6
70 100 70 80
60 80 60 60
0.05 0.05 0.06 0.13
2.2 3.0 4.6 2.2
Well-developed plate-like rhombohedral thicker crystals Unfacetted flaky habit crystals Undeveloped crystals Irregular-shaped thicker crystals Very fine crystals
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looses its ionic strength. The authors have also carried out some experiments at lower temperatures as low as 100 1C using HNO3+HCl as mineralizer. The crystals obtained were small (o1 mm) and transparent with an octahedral morphology and honey yellow colour. If we can improve the size of the crystals in such experiments, the growth of these Nd:RVO4 can be accomplished in a larger scale and in an almost economic way. There is also a possibility of growing these Nd:RVO4 nanocrystals for some special applications like bio-imaging and biosensors because of its excellent behaviour as a phosphor with size reduction to a nanoscale. In that case, the experiments quoted by the present authors at 100 1C would be very useful to prepare such nanoparticles and also nanocomposites like zeolites encapsulated with these nanoparticles. Even the use of a continuous-flow hydrothermal reactor has been attempted under supercritical hydrothermal conditions. The run products were characterized using powder X-ray diffractometer (Rigaku Miniflex, Japan) and FTIR (JASCO FTIR 460 plus, Japan). The following reaction series proposed are the possible reactions for the formation of Nd:RVO4 (R ¼ Y, Gd) from V2O5 and R2O3. V2 O5 þ 10HCl ! 2VCl4 þ 5H2 O þ Cl2 m;
(1)
R2 O3 þ 3HCl ! RðOHÞ3 þ RCl3 ;
(2)
R2 O3 þ 6HNO3 ! 2RðNO3 Þ3 þ 3H2 O;
(3)
V2 O5 þ 6HNO3 ! 2H3 VO4 þ 3N2 O5 m;
(4)
2VCl4 þ 4HNO3 ! 2VOCl3 þ 2HCl þ H2 m þ 2N2 O5 m;(5) RCl3 þ H3 VO4 ! RVO4 þ 3HCl;
(6)
VOCl3 þ RðOHÞ3 ! RVO4 þ 3HCl;
(7)
2H3 VO4 þ 2RðNO3 Þ3 ! 2RVO4 þ 3H2 O þ 3N2 O5 m;
(8)
H3 VO4 þ RðNO3 Þ3 ! RVO4 þ 3HNO3 ;
(9)
H3 VO4 þ RðOHÞ3 ! RVO4 þ 3H2 O:
(10)
It is presumed that the formation of a stable complex, Nd:RVO4 at 240 1C and 80 bars pressure must have undergone several intermediate stages of salvation as depicted in the above reactions, forming stable and unstable solvated complexes, at different temperatures (room temperature to 240 1C) and pressure between 1 and 80 bars.
3. Results and discussion 3.1. Morphology control Understanding the crystal morphology for any material with a device potential is very important. The crystal habit is governed by the chemical kinetics rather than equilibrium. Crystal morphology depends on various factors like degree of supersaturation, pH of the solution, solvent type, etc. The present authors have studied the influence of these parameters on the growth morphology of Nd:RVO4 crystals. The defects as seen by macro- and micromorphology of crystals depend upon the growth parameters. The morphology of a crystal is the result of the relative growth rates of its different faces and the general rule being that the faces, which grow slowest, are expressed in the crystal habit. The growth rates of the various crystal faces are determined by intermolecular interactions between molecules in the crystal as well as by a number of external parameters such as solvent, degree of supersaturation, duration, temperature and foreign materials. Any one of these may lead to dramatic modifications in crystal morphology. The crystals obtained in all the experiments were subjected to a systematic morphological analysis. The crystals usually exhibit plate-like, pseudo-cubic and rhombohedral habits. The size of the crystals varies from 0.1 to 4.0 mm. The characteristic photographs of the crystals obtained are shown in Fig. 1. with schematic diagrams in Fig. 2. The experiments with Nd2O3 up to 2 wt%, solvent such as 3 M HCl+2 M HNO3 in 3:1 ratio with a resulting pH of 0.18 yield Nd:RVO4 crystals with a characteristic platelike morphology, honey yellow colour having vitreous lustre (Fig. 1a). The commonly observed faces are (1 0 0), (0 1 0), (0 0 1). The growth rate is (0 0 1)4(1 0 0) (Fig. 2a). The experiments with Nd2O3 up to 5 wt%, solvent such as NH4OH+conc. HNO3 in 4:1 ratio with a resulting pH of 0.05 and also the clear homogeneous solution, Nd:RVO4 crystals with flaky or micaceous morphology. The crystals were unfaceted and show typical flaky habit (Fig. 1b). The commonly observed faces are (1 0 1), (0 1 1), (1 0 1) and (0 1 1) (Fig. 2b). The experiments with Nd2O3 up to 5 wt%, solvent such as 1.5 M HCl+3 M HNO3 in 3:1 ratio with a resulting pH of 0.05, yield Nd:RVO4 crystals with not so welldeveloped crystal shapes and transparent crystals having vitreous lustre (Fig. 1c) The commonly observed faces are (0 0 1), (1 1 0), (0 1 0) and (1 1 0). The growth rate is (0 0 1)4(0 1 0)4(1 1 0) (Fig. 2c). The experiments with Nd2O3 up to 5 wt%, solvent such as 3 M HCl+1.5 M HNO3 in 2:1 ratio with a resulting of pH0.06, yield Nd:YVO4 crystals with a characteristic irregular elongation of the c-axis (Fig. 1d). The experiments with Nd2O3 up to 3 wt%, solvent such as pH0.13 (2 M HCl+2 M HNO3), yield Nd:YVO4 very fine crystals (Fig. 1e).
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Fig. 1. Characteristic photographs of Nd:RVO4 crystals: (a) pH 0.18 (3 M HCl+2 M HNO3), (b) pH 0.05 (NH4OH+conc. HNO3 in 4:1 ratio), (c) pH 0.05 (1.5 M HCl+3 M HNO3 in 3:1 ratio), (d) pH 0.06(3 M HCl+1.5 M HNO3) and (e) pH 0.13 (2 M HCl+2 M HNO3).
Fig. 2. Schematic diagrams of Nd:RVO4 crystals: (a) schematic diagram of Fig. 1a, (b) schematic diagram of Fig. 1b and (c) schematic diagram of Fig. 1c.
It is true that the growth rate is much lower in the hydrothermal technique compared to the melt technique. However, under hydrothermal conditions the problems associated with the melt technique could be easily solved for rare earth vanadates. Further, because of the lower experimental temperature conditions, the diffusion can be controlled and the defects can be minimized. In the present experimental conditions, the authors have even tested 100 1C to synthesize the title compounds without any structural water in the products. However, the only disadvantage at the moment by this technique is the lower growth rate. If we can overcome the problem of ionic strength loss of the solution during the growth experiments by some means, it is possible to increase the size of these crystals significantly. The present authors are working in this direction.
Fig. 3. XRD pattern of Nd:YVO4 crystals.
Fig. 4. XRD pattern of Nd:GdVO4 crystals.
3.2. X-ray powder diffraction X-ray powder diffraction patterns of Nd:YVO4 (Fig. 3) and Nd:GdVO4 (Fig. 4) showed that the resultant products are homogeneous in composition. The cell parameters were calculated for the representative samples and are similar to the reported values [31]. Table 2 gives the cell parameters of rare earth vanadates obtained in the present work. The
crystallinity of the products was good in all the samples including the one synthesized at 100 1C. 3.3. FTIR The infrared spectra for the representative samples of Nd:YVO4 (Fig. 5) and Nd:GdVO4 (Fig. 6) were recorded
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Table 2 The cell parameters for rare earth vanadates obtained under mild hydrothermal conditions Compound
a (A˚)
c (A˚)
V (A˚3)
YVO4 Nd0.05Y0.95VO4 GdVO4 Nd0.05Gd0.95VO4
7.123 7.127 7.201 7.223
6.292 6.297 6.35 6.359
319.24 319.85 329.18 331.76
Fig. 5. FTIR spectrum of Nd:YVO4 crystals.
solvent–solute interaction, complexation and related physico-chemical aspects of crystallization. A probable mechanism for the formation of RVO4 from V2O5 and R2O3 has been proposed here. The present authors have attempted a detailed mechanism for the generation of RVO4 from V2O5 and R2O3 for the first time to RVO4 crystal of our knowledge, because this explains crystalline structure of the molecule. This mechanism also depicts the spatial arrangement of various atoms (oxygen, yttrium/gadolinium and vanadium) in a molecule. The mechanism proposed here is on the theoretical basis. V2O5 reacts with 8 moles of HCl forming a transient intermediate V2OCl8 with the elimination of 4 moles of water [32]. This disappropriates forming VOCl3 with the elimination of VCl3 and 1 mole of chlorine. VOCl3 further reacts with 1 mole of HCl forming hydroxy vanadium chloride, which reacts with 1 more mole of HCl forming VCl5 and a mole of water. VCl5 and VCl3 react together forming 2 moles of VCl4, out of which one is nucleophilic and other is electrophilic (Scheme 1). Rare earth oxide (R2O3) reacts with 2 moles of HCl forming dihydroxy rare earth chloride, which isomerizes to new dihydroxy rare earth chloride via cyclic oxonium intermediate. This rearranged molecule further reacts with oe more molecule of HCl forming R(OH)3 and RCl3 (Scheme 2).
Table 3 Infrared shifts of rare earth vanadates in cm1
Fig. 6. FTIR spectrum of Nd:GdVO4 crystals.
Composition
VO3 4
n (V–O)
d (O–V–O)
n (VQO)
Nd0.05Y0.95VO4 Nd0.05Gd0.95VO4
864 856
970 938
573 572
1384 1384
in the range 4000–400 cm1 to check the presence of (OH), which is detrimental to any laser devices. The infrared spectra show more fineness and multiple splitting in the region of lower wave numbers. The most interesting feature of FTIR spectra is the absence of (OH) bands in the region 3000–3500 cm1. This shows that water has not been incorporated into the interstices of vanadates crystals grown under the present experimental conditions, since the anhydrous rare earth vanadate crystal structure does not accommodate the OH molecules either interstitially or substitutionally. Table 3 gives the absorption bands for rare earth vanadates. 3.4. Hydrothermal crystal growth mechanism The study of complexation process with reference to the solvent–solute interaction is of great importance to understand the crystallization of rare earth vanadates. Reports on such studies are seldom found in the literature for rare earth vanadates. Hence, an attempt was made to study the
Scheme 1.
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Scheme 2.
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further with another molecule of HNO3 forming VOCl3 with liberation of N2O5 and HCl (Scheme 5). Rare earth chloride reacts with H3VO4 forming rare earth vanadate (RVO4) with the elimination of 3 moles of HCl via cyclic intermediate (Scheme 6). VOCl3 reacts with rare earth hydroxide forming rare earth vanadate (Scheme 7). H3VO4 reacts with one molecule of rare earth nitrate forming a transient intermediate dinitro dihydroxy rare earth vanadate with the elimination of HNO3 molecule. This intermediate liberates a molecule of N2O5 forming dihydroxy rare earth vanadate. This vanadate reacts with eliminated HNO3 molecule forming mononitro hydroxy rare earth vanadate. Two moles of this vanadate react together with the formation of RVO4 liberating a molecule of N2O5 and H2O (Scheme 8). H3VO4 reacts with rare earth nitrate with the formation of RVO4 and 3 moles of HNO3 eliminated (Scheme 9). H3VO4 reacts with rare earth hydroxide with the formation of RVO4 and 3 moles of water eliminated (Scheme 10). The above scheme of hydrothermal reaction mechanism indicates that the Schemes 1–5 show the formation of
Scheme 3.
Scheme 4.
Nitration of rare earth oxide (R2O3) with 6 moles of HNO3 lead to the formation of R(NO3)3 with the elimination of 3 moles of water (Scheme 3). Nitration of V2O5 with 6 moles of HNO3 gives two molecules of H3VO4 by eliminating 3 moles of N2O5 (Scheme 4). Electrophilic VCl4 formed in the first step reacts with one molecule of HNO3 forming an intermediate nitrotrichloro vanadate. This in turn reacts with another molecule of HNO3 forming VOCl3 with the elimination of a molecule of N2O5 and H2. Nucleophilic VCl4 reacts with one molecule of HNO3 forming nitrotetrachloro vanadate with the elimination of a HCl molecule. This vanadate reacts
Scheme 5.
Scheme 6.
Scheme 7.
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crystallization have helped in understanding the hydrothermal growth processes for these rare earth vanadates. The study of mechanism helps to understand the formation of intermediate complexes and the pathway of the formation of RVO4. Acknowledgements The authors wish to acknowledge the financial support by the Department of Science and Technology, New Delhi. Also the authors acknowledge the help extended by Dr. B. Nirmala and Dr. B.V. Suresh Kumar, Department of Studies in Geology, University of Mysore, India. References Scheme 8.
Scheme 9.
Scheme 10.
intermediates of hydroxides and nitrates of rare earth vanadate. The pH and temperature mainly play an important role here in the formation of intermediates. The Schemes 6–10 give the formation of RVO4 from the above intermediates. These intermediates because of their low stability constants combine to form a more stable complex, RVO4. Such studies help us to monitor the crystallization process at every stage in order to obtain good-quality crystals without any major defects. 4. Conclusions A systematic study of the growth of rare earth vanadates indicates that these compounds can be obtained at lower PT conditions using the hydrothermal technique. The quality of crystals obtained by the hydrothermal technique is good. The morphology and growth rate of rare earth vanadates vary with reference to the degree of supersaturation, the concentration of the mineralizer, pH of the solution, percent fill in the liner. The use of a suitable solvent brings down the crystallization temperature drastically. Further, the study of solvent–solute interaction and the relevant chemical reactions leading to the scheme of
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Journal of Crystal Growth 306 (2007) 94–101 www.elsevier.com/locate/jcrysgro
Growth, morphology and mechanism of rare earth vanadate crystals under mild hydrothermal conditions K. Byrappaa,, C.K. Chandrashekara, B. Basavalingua, K.M. LokanathaRaib, S. Anandab, M. Yoshimurac a Department of Geology, University of Mysore, Manasagangotri, Mysore 570 006, India Department of Chemistry, University of Mysore, Manasagangotri, Mysore 570 006, India c Materials Structures Laboratory, Center for Materials Design, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226, Japan b
Received 18 July 2006; received in revised form 20 January 2007; accepted 8 March 2007 Communicated by T. Hibiya Available online 20 April 2007
Abstract Single crystals of RVO4 (R ¼ Y,Gd) doped with optically active element like Nd have been obtained under mild hydrothermal conditions (T ¼ 240 1C, P80 bars). A detailed mechanism of the crystallization process, which helps in the considerable reduction of the PT conditions of the growth of these high-melting (m.p.41800 1C) crystals has been formulated. The crystals obtained have been subjected to morphological, X-ray powder diffraction and infrared spectroscopic studies. r 2007 Elsevier B.V. All rights reserved. PACS: 81.10.h; 42.70.Hj; 64.70Dv; 81.10.Aj Keywords: A1. Morphological stability; A2. Hydrothermal crystal growth; A2. Single crystal growth; B1. Rare earth compounds; B1. Vanadates; B2. Phosphors; B3. Solid state lasers
1. Introduction Growth of laser crystals is an attractive field in solid-state science owing to their technological significance. There exists a large number of solid-state laser host crystals from the family of phosphates, vanadates, fluorides, tungstates, etc. Amongst them, the yttrium orthovanadate-based laserdiode pumped solid-state lasers are the frontier materials and carry a wide variety of applications in science and technology because of its high stability, compactness, high efficiency and long life time [1–3]. Nd:YVO4 crystals have been employed as highly efficient laser-diode pumped microlasers [4], an efficient phosphors [5], very attractive polarizer materials [6] and low-threshold laser hosts [7], etc. In comparison with the conventional Nd: YAG crystal, Nd:YVO4 offers many advantages such as larger absorption co-efficient and gain cross section [8–10]. Corresponding author. Tel.: +91 821 2419720; fax: +91 821 2419720.
E-mail address: [email protected] (K. Byrappa). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.03.055
The rare earth vanadates are known as the high-melting materials (melting point 41800 1C) with low solubility and obviously their synthesis by any technique usually insists upon higher temperature conditions. That is one of the main reasons why most of the earlier workers have obtained these laser crystals by flux or melt methods [11–14]. In spite of its excellent physical properties, the high-tech applications have not been realized due to the crystal growth difficulties. One of the major problems encountered in the growth of RVO4 crystals is the presence of oxygen imperfections (color centers and inclusions), which are introduced during the crystal growth processes. Although YVO4 melts congruently [15], vanadium oxides vaporize incongruently, causing changes in Y/V ratio and oxygen stoichiometry in the melt. These undesired effects could generate additional phases and oxygen defects in theYVO4 crystals grown especially from the melt [16]. Efforts to eliminate these defects did not yield significant success in the flux and melt techniques. Rubin and Vanuitert [11] were the first to obtain large bulk crystals
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of Nd:YVO4 using the Czochralski technique [17]. But problems related to the oxygen deficiency and inclusions could not be eliminated. This has been emphasized in numerous reports of Czochralski growth [17–19], and other melt growth methods like Verneuil [20], modified floating zone [21,22], laser-heated pedestal growth [23,24] and Bridgman [18]. However, there are not many reports on the hydrothermal synthesis of these crystals [25–27]. This is probably connected with the higher temperature and pressure conditions involved in the growth of these crystals and also to the strong belief that their growth under mild hydrothermal conditions will introduce water into the structure or to the interstices. Infact, several problems have been encountered in the growth of rare earth vanadates by the above-said conventional methods. The instability of pentavalent vanadium at higher temperatures and the loss of oxygen through surface encrustation by the reaction of the melt with the crucible material further complicate the growth processes. The unstable melt behaviour of YVO4 leads to the appearance of zoning in these crystals. Recently, it has been observed that YVO4 crystals are very difficult to grow with a stoichiometric or near-stoichiometric composition [28]. Further, the most critical aspect is the appearance of metastable phases and also some precipitates within the YVO4 crystals [29]. The detection of correct Y–V–O stoichiometry in YVO4 crystals is difficult [30]. Moreover, since the temperature involved in the flux growth is about 1300 1C, and in melt growth above 1800 1C, there is a high probability of vanadium attacking the container. In order to overcome most of the difficulties encountered in the melt and high-temperature solution techniques, our group for the first time has proposed the hydrothermal technique as a solution [25,26]. Since the experiments are carried out in a closed system, the loss of oxygen could be readily prevented. Here, the authors report the hydrothermal growth of Nd:RVO4 crystals and made a detailed study of the crystallization mechanism under hydrothermal conditions and their characterization through morphological, X-ray diffraction and infrared spectroscopic studies. 2. Experimental procedure The experiments on the hydrothermal growth of Nd:RVO4 (R ¼ Y, Gd) crystals were carried out using
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general purpose autoclaves made of SS316, provided with teflon liners of capacity 30 ml. The use of teflon liners helps in overcoming the entry of inclusions from the autoclave material into the crystals. The starting materials such as R2O3 (Y2O3 and Gd2O3, Analar grade, s.d Fine Chem Ltd., India), V2O5 (Analar grade, s.d Fine Chem Ltd., India), Nd2O3 (Analar grade, Rankem, India) were taken in an appropriate molar proportion in Teflon liners. Table 1 gives the experimental details. A suitable mineralizer of known concentration was added into the teflon liner and the entire mixture was stirred well till a clear homogeneous and relatively less viscous solution was obtained. The temperature was kept constant at 240 1C and the pressure 80 bars were maintained through appropriate % fill. The pH of the growth medium was measured before and after each experimental run. In the present work, the crystallization was carried out in all the experiments through spontaneous nucleation. Hence, an effort was made to reduce the number of nucleation centres by increasing the temperature of the crystallization reactor slowly at the rate of 10 1C/h up to 100 1C and beyond at the rate of 5 1C/h. The experimental temperature was held at 240 1C for a period of 3 days without any fluctuations in the temperature. From the fourth day onwards the temperature fluctuation to the tune of 710 to 720 1C was introduced periodically in order to reduce the number of nucleation centres. By this means, small crystallites, which had developed earlier, dissolve leaving only bigger crystallites to grow. It was observed that as the size of the crystal increases, the crystal quality reduces [27]. The pressure was measured in all the initial experimental runs using a Bourdon Gauge in order to fix the quantity of starting materials for 80 bars, and in the subsequent routine experiments the volume of the starting materials was adjusted to obtain a calculated pressure. The authors have extended the experimental duration to 20 days in order to check whether the crystal size improves with the duration however, to our surprise, the crystal size remained the same without any size increase. The reason for this is that the solution might have lost its ionic strength and hence the crystal never grows further, but instead, the grown crystals remain in equilibrium state with the solution. This also states that the crystal growth occurs only for a few days and then ceases as soon as the solution
Table 1 Experimental conditions for the growth of Nd:RVO4 crystals, where, R ¼ Y,Gd (R : V2O5 ¼ 1:2 molar ratio), where R ¼ Y2O3, Gd2O3, Nd2O3 ¼ 2–10 wt%) Sl. no.
Molar concentration of mineralizers
1
3 M HCl
2 M HNO3
2 3 4 5
4 M NH4OH 1.5 M HCl 3 M HCl 2 M HCl
1 M HNO3 3 M HNO3 1.5 M HNO3 2 M HNO3
Temperature ¼ 240 1C, Duration ¼ 8 days.
P (bar)
% fill
pH
Crystal size (mm)
Remarks
70
60
0.18
4.6
70 100 70 80
60 80 60 60
0.05 0.05 0.06 0.13
2.2 3.0 4.6 2.2
Well-developed plate-like rhombohedral thicker crystals Unfacetted flaky habit crystals Undeveloped crystals Irregular-shaped thicker crystals Very fine crystals
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looses its ionic strength. The authors have also carried out some experiments at lower temperatures as low as 100 1C using HNO3+HCl as mineralizer. The crystals obtained were small (o1 mm) and transparent with an octahedral morphology and honey yellow colour. If we can improve the size of the crystals in such experiments, the growth of these Nd:RVO4 can be accomplished in a larger scale and in an almost economic way. There is also a possibility of growing these Nd:RVO4 nanocrystals for some special applications like bio-imaging and biosensors because of its excellent behaviour as a phosphor with size reduction to a nanoscale. In that case, the experiments quoted by the present authors at 100 1C would be very useful to prepare such nanoparticles and also nanocomposites like zeolites encapsulated with these nanoparticles. Even the use of a continuous-flow hydrothermal reactor has been attempted under supercritical hydrothermal conditions. The run products were characterized using powder X-ray diffractometer (Rigaku Miniflex, Japan) and FTIR (JASCO FTIR 460 plus, Japan). The following reaction series proposed are the possible reactions for the formation of Nd:RVO4 (R ¼ Y, Gd) from V2O5 and R2O3. V2 O5 þ 10HCl ! 2VCl4 þ 5H2 O þ Cl2 m;
(1)
R2 O3 þ 3HCl ! RðOHÞ3 þ RCl3 ;
(2)
R2 O3 þ 6HNO3 ! 2RðNO3 Þ3 þ 3H2 O;
(3)
V2 O5 þ 6HNO3 ! 2H3 VO4 þ 3N2 O5 m;
(4)
2VCl4 þ 4HNO3 ! 2VOCl3 þ 2HCl þ H2 m þ 2N2 O5 m;(5) RCl3 þ H3 VO4 ! RVO4 þ 3HCl;
(6)
VOCl3 þ RðOHÞ3 ! RVO4 þ 3HCl;
(7)
2H3 VO4 þ 2RðNO3 Þ3 ! 2RVO4 þ 3H2 O þ 3N2 O5 m;
(8)
H3 VO4 þ RðNO3 Þ3 ! RVO4 þ 3HNO3 ;
(9)
H3 VO4 þ RðOHÞ3 ! RVO4 þ 3H2 O:
(10)
It is presumed that the formation of a stable complex, Nd:RVO4 at 240 1C and 80 bars pressure must have undergone several intermediate stages of salvation as depicted in the above reactions, forming stable and unstable solvated complexes, at different temperatures (room temperature to 240 1C) and pressure between 1 and 80 bars.
3. Results and discussion 3.1. Morphology control Understanding the crystal morphology for any material with a device potential is very important. The crystal habit is governed by the chemical kinetics rather than equilibrium. Crystal morphology depends on various factors like degree of supersaturation, pH of the solution, solvent type, etc. The present authors have studied the influence of these parameters on the growth morphology of Nd:RVO4 crystals. The defects as seen by macro- and micromorphology of crystals depend upon the growth parameters. The morphology of a crystal is the result of the relative growth rates of its different faces and the general rule being that the faces, which grow slowest, are expressed in the crystal habit. The growth rates of the various crystal faces are determined by intermolecular interactions between molecules in the crystal as well as by a number of external parameters such as solvent, degree of supersaturation, duration, temperature and foreign materials. Any one of these may lead to dramatic modifications in crystal morphology. The crystals obtained in all the experiments were subjected to a systematic morphological analysis. The crystals usually exhibit plate-like, pseudo-cubic and rhombohedral habits. The size of the crystals varies from 0.1 to 4.0 mm. The characteristic photographs of the crystals obtained are shown in Fig. 1. with schematic diagrams in Fig. 2. The experiments with Nd2O3 up to 2 wt%, solvent such as 3 M HCl+2 M HNO3 in 3:1 ratio with a resulting pH of 0.18 yield Nd:RVO4 crystals with a characteristic platelike morphology, honey yellow colour having vitreous lustre (Fig. 1a). The commonly observed faces are (1 0 0), (0 1 0), (0 0 1). The growth rate is (0 0 1)4(1 0 0) (Fig. 2a). The experiments with Nd2O3 up to 5 wt%, solvent such as NH4OH+conc. HNO3 in 4:1 ratio with a resulting pH of 0.05 and also the clear homogeneous solution, Nd:RVO4 crystals with flaky or micaceous morphology. The crystals were unfaceted and show typical flaky habit (Fig. 1b). The commonly observed faces are (1 0 1), (0 1 1), (1 0 1) and (0 1 1) (Fig. 2b). The experiments with Nd2O3 up to 5 wt%, solvent such as 1.5 M HCl+3 M HNO3 in 3:1 ratio with a resulting pH of 0.05, yield Nd:RVO4 crystals with not so welldeveloped crystal shapes and transparent crystals having vitreous lustre (Fig. 1c) The commonly observed faces are (0 0 1), (1 1 0), (0 1 0) and (1 1 0). The growth rate is (0 0 1)4(0 1 0)4(1 1 0) (Fig. 2c). The experiments with Nd2O3 up to 5 wt%, solvent such as 3 M HCl+1.5 M HNO3 in 2:1 ratio with a resulting of pH0.06, yield Nd:YVO4 crystals with a characteristic irregular elongation of the c-axis (Fig. 1d). The experiments with Nd2O3 up to 3 wt%, solvent such as pH0.13 (2 M HCl+2 M HNO3), yield Nd:YVO4 very fine crystals (Fig. 1e).
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Fig. 1. Characteristic photographs of Nd:RVO4 crystals: (a) pH 0.18 (3 M HCl+2 M HNO3), (b) pH 0.05 (NH4OH+conc. HNO3 in 4:1 ratio), (c) pH 0.05 (1.5 M HCl+3 M HNO3 in 3:1 ratio), (d) pH 0.06(3 M HCl+1.5 M HNO3) and (e) pH 0.13 (2 M HCl+2 M HNO3).
Fig. 2. Schematic diagrams of Nd:RVO4 crystals: (a) schematic diagram of Fig. 1a, (b) schematic diagram of Fig. 1b and (c) schematic diagram of Fig. 1c.
It is true that the growth rate is much lower in the hydrothermal technique compared to the melt technique. However, under hydrothermal conditions the problems associated with the melt technique could be easily solved for rare earth vanadates. Further, because of the lower experimental temperature conditions, the diffusion can be controlled and the defects can be minimized. In the present experimental conditions, the authors have even tested 100 1C to synthesize the title compounds without any structural water in the products. However, the only disadvantage at the moment by this technique is the lower growth rate. If we can overcome the problem of ionic strength loss of the solution during the growth experiments by some means, it is possible to increase the size of these crystals significantly. The present authors are working in this direction.
Fig. 3. XRD pattern of Nd:YVO4 crystals.
Fig. 4. XRD pattern of Nd:GdVO4 crystals.
3.2. X-ray powder diffraction X-ray powder diffraction patterns of Nd:YVO4 (Fig. 3) and Nd:GdVO4 (Fig. 4) showed that the resultant products are homogeneous in composition. The cell parameters were calculated for the representative samples and are similar to the reported values [31]. Table 2 gives the cell parameters of rare earth vanadates obtained in the present work. The
crystallinity of the products was good in all the samples including the one synthesized at 100 1C. 3.3. FTIR The infrared spectra for the representative samples of Nd:YVO4 (Fig. 5) and Nd:GdVO4 (Fig. 6) were recorded
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Table 2 The cell parameters for rare earth vanadates obtained under mild hydrothermal conditions Compound
a (A˚)
c (A˚)
V (A˚3)
YVO4 Nd0.05Y0.95VO4 GdVO4 Nd0.05Gd0.95VO4
7.123 7.127 7.201 7.223
6.292 6.297 6.35 6.359
319.24 319.85 329.18 331.76
Fig. 5. FTIR spectrum of Nd:YVO4 crystals.
solvent–solute interaction, complexation and related physico-chemical aspects of crystallization. A probable mechanism for the formation of RVO4 from V2O5 and R2O3 has been proposed here. The present authors have attempted a detailed mechanism for the generation of RVO4 from V2O5 and R2O3 for the first time to RVO4 crystal of our knowledge, because this explains crystalline structure of the molecule. This mechanism also depicts the spatial arrangement of various atoms (oxygen, yttrium/gadolinium and vanadium) in a molecule. The mechanism proposed here is on the theoretical basis. V2O5 reacts with 8 moles of HCl forming a transient intermediate V2OCl8 with the elimination of 4 moles of water [32]. This disappropriates forming VOCl3 with the elimination of VCl3 and 1 mole of chlorine. VOCl3 further reacts with 1 mole of HCl forming hydroxy vanadium chloride, which reacts with 1 more mole of HCl forming VCl5 and a mole of water. VCl5 and VCl3 react together forming 2 moles of VCl4, out of which one is nucleophilic and other is electrophilic (Scheme 1). Rare earth oxide (R2O3) reacts with 2 moles of HCl forming dihydroxy rare earth chloride, which isomerizes to new dihydroxy rare earth chloride via cyclic oxonium intermediate. This rearranged molecule further reacts with oe more molecule of HCl forming R(OH)3 and RCl3 (Scheme 2).
Table 3 Infrared shifts of rare earth vanadates in cm1
Fig. 6. FTIR spectrum of Nd:GdVO4 crystals.
Composition
VO3 4
n (V–O)
d (O–V–O)
n (VQO)
Nd0.05Y0.95VO4 Nd0.05Gd0.95VO4
864 856
970 938
573 572
1384 1384
in the range 4000–400 cm1 to check the presence of (OH), which is detrimental to any laser devices. The infrared spectra show more fineness and multiple splitting in the region of lower wave numbers. The most interesting feature of FTIR spectra is the absence of (OH) bands in the region 3000–3500 cm1. This shows that water has not been incorporated into the interstices of vanadates crystals grown under the present experimental conditions, since the anhydrous rare earth vanadate crystal structure does not accommodate the OH molecules either interstitially or substitutionally. Table 3 gives the absorption bands for rare earth vanadates. 3.4. Hydrothermal crystal growth mechanism The study of complexation process with reference to the solvent–solute interaction is of great importance to understand the crystallization of rare earth vanadates. Reports on such studies are seldom found in the literature for rare earth vanadates. Hence, an attempt was made to study the
Scheme 1.
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Scheme 2.
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further with another molecule of HNO3 forming VOCl3 with liberation of N2O5 and HCl (Scheme 5). Rare earth chloride reacts with H3VO4 forming rare earth vanadate (RVO4) with the elimination of 3 moles of HCl via cyclic intermediate (Scheme 6). VOCl3 reacts with rare earth hydroxide forming rare earth vanadate (Scheme 7). H3VO4 reacts with one molecule of rare earth nitrate forming a transient intermediate dinitro dihydroxy rare earth vanadate with the elimination of HNO3 molecule. This intermediate liberates a molecule of N2O5 forming dihydroxy rare earth vanadate. This vanadate reacts with eliminated HNO3 molecule forming mononitro hydroxy rare earth vanadate. Two moles of this vanadate react together with the formation of RVO4 liberating a molecule of N2O5 and H2O (Scheme 8). H3VO4 reacts with rare earth nitrate with the formation of RVO4 and 3 moles of HNO3 eliminated (Scheme 9). H3VO4 reacts with rare earth hydroxide with the formation of RVO4 and 3 moles of water eliminated (Scheme 10). The above scheme of hydrothermal reaction mechanism indicates that the Schemes 1–5 show the formation of
Scheme 3.
Scheme 4.
Nitration of rare earth oxide (R2O3) with 6 moles of HNO3 lead to the formation of R(NO3)3 with the elimination of 3 moles of water (Scheme 3). Nitration of V2O5 with 6 moles of HNO3 gives two molecules of H3VO4 by eliminating 3 moles of N2O5 (Scheme 4). Electrophilic VCl4 formed in the first step reacts with one molecule of HNO3 forming an intermediate nitrotrichloro vanadate. This in turn reacts with another molecule of HNO3 forming VOCl3 with the elimination of a molecule of N2O5 and H2. Nucleophilic VCl4 reacts with one molecule of HNO3 forming nitrotetrachloro vanadate with the elimination of a HCl molecule. This vanadate reacts
Scheme 5.
Scheme 6.
Scheme 7.
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crystallization have helped in understanding the hydrothermal growth processes for these rare earth vanadates. The study of mechanism helps to understand the formation of intermediate complexes and the pathway of the formation of RVO4. Acknowledgements The authors wish to acknowledge the financial support by the Department of Science and Technology, New Delhi. Also the authors acknowledge the help extended by Dr. B. Nirmala and Dr. B.V. Suresh Kumar, Department of Studies in Geology, University of Mysore, India. References Scheme 8.
Scheme 9.
Scheme 10.
intermediates of hydroxides and nitrates of rare earth vanadate. The pH and temperature mainly play an important role here in the formation of intermediates. The Schemes 6–10 give the formation of RVO4 from the above intermediates. These intermediates because of their low stability constants combine to form a more stable complex, RVO4. Such studies help us to monitor the crystallization process at every stage in order to obtain good-quality crystals without any major defects. 4. Conclusions A systematic study of the growth of rare earth vanadates indicates that these compounds can be obtained at lower PT conditions using the hydrothermal technique. The quality of crystals obtained by the hydrothermal technique is good. The morphology and growth rate of rare earth vanadates vary with reference to the degree of supersaturation, the concentration of the mineralizer, pH of the solution, percent fill in the liner. The use of a suitable solvent brings down the crystallization temperature drastically. Further, the study of solvent–solute interaction and the relevant chemical reactions leading to the scheme of
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