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Novel solvents for the single crystal growth of germanate phases by the flux method
Journal of Crystal Growth 426 (2015) 25–32
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Novel solvents for the single crystal growth of germanate phases by the flux method V.A. Ivanov a, M.O. Marychev a,n, P.V. Andreev a, I. Koseva b, P. Tzvetkov b, V. Nikolov a a b
N.I. Lobachevsky State University of Nizhni Novgorod, Nizhni Novgorod 603950, Russia Bulgarian Academy of Science, Institute of General and Inorganic Chemistry, BU-1113 Sofia, Bulgaria
art ic l e i nf o
a b s t r a c t
Article history: Received 23 March 2015 Received in revised form 27 April 2015 Accepted 29 April 2015 Communicated by: V. Fratello Available online 9 May 2015
A series of alkali-borate (Na2O B2O3, Na2O 1.5B2O3, Na2O 2B2O3 and Li2O B2O3), and a series of alkalimolybdate (Na2O 1.5MoO3, Na2O 2MoO3 and Li2O MoO3) solvents were studied with a view to find out the suitable conditions for growing single crystals from germanate phases by the flux method. The ternary systems solvent-CaO–GeO2 were investigated and crystallization temperature and crystalized phase were determined. As a main result the concentration and temperature regions of crystallization of calcium germanates (Ca5Ge3O11, CaGeO3) and of some alkali germanates (Li2GeO3, Li2CaGeO4, Na2CaGe6O14) were experimentally determined for a first time. The conditions found for Ca2GeO4 growth are significantly more favorable than those known so far. Additionally during this study, two novel unknown phases were obtained. X-ray diffractograms of the novel phases are presented. The obtained conditions are a basis for successful single crystal growth by the flux method for all mentioned germanate compounds. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Phase diagrams A1. Solvents A1. X-ray diffraction A2. Growth from high temperature solutions A2. Single crystal growth B1. Oxides
1. Introduction Solid-state lasers emitting in the spectral region of 1.1–1.6 μm find increasing applications in medicine, ecology, telecommunications, instrument-building, etc. [1–3]. Several single crystals doped with ions of the transition elements emit in this region. Of major importance among them are single crystals doped with Cr4þ . The laser media most often used so far are Cr4þ doped Mg2SiO4 (forsterite) and Y3Al5O12 (YAG) [4,5]. These media have, however, several drawbacks, e.g., low quantum yield due to non-emitting transitions; presence of Cr3 þ in the matrix and limited doping capacity with Cr4þ . Last but not least are the problems of crystal growth related to the high melting temperatures of these two compounds [6,7]. Recently, a number of compounds have been studied as potential substitutes for Mg2SiO4 and Y3Al5O12. Very few of these studies have, however, yielded positive results. A major group of compounds with potential application are the Cr4 þ doped compounds with olivine structure. Unfortunately, most of these compounds display drawbacks such as incongruent melting, phase transitions, and non-radiative emission. For example, single crystal growth of Mg2GeO4 is accompanied by significant evaporation of GeO2 [8]; CaMgSiO4 melts incongruently [9]; Ca2SiO4 undergoes several polymorphous transitions [10]. n Correspondence to: Faculty of Physics, N.I. Lobachevsky State University of Nizhni Novgorod, Gagarin Avenue, 23, build. 3, off. 403, Nizhni Novgorod 603950, Russia. Tel.: þ 7 903 604 14 12. E-mail address: [email protected] (M.O. Marychev).
http://dx.doi.org/10.1016/j.jcrysgro.2015.04.042 0022-0248/& 2015 Elsevier B.V. All rights reserved.
The only material with olivine structure (besides forsterite) that has provided single crystals of satisfactory size and quality and has demonstrated laser emission of high efficiency is Cr4 þ :Ca2GeO4 [11,12]. As a representative of the olivines, this compound also undergoes a phase transition, but at a higher temperature (1450 1C). The low-temperature modification with olivine structure is very suitable to be doped with Cr4 þ ; germanium is situated in the required tetrahedral environment and Cr4 þ and Ge4 þ are of the same valence state and have close ionic radii (0.041 и 0.039 nm, respectively). Moreover, the large ionic radius of Ca2þ does not allow its substitution by Cr3þ and Cr2þ , which are undesirable for this application [13]. Attempts have been made to grow single crystals of Cr4þ :Ca2GeO4 by the flux method at temperatures lower than 1450 1C (the phase transition temperature). The reported studies show that the use of CaCl2 as a solvent is not appropriate due to the intense evaporation of CaCl2 and the formation of hygroscopic phases that enter in the crystal and destroying it [14,15]. When CaF2 is used as a solvent, crystal growth should be carried out at a temperature above 1350 1C, where solvent evaporation takes place; moreover, the process should take place in an iridium crucible in a nitrogen atmosphere [16]. The use of a lithium-molybdate solvent is also reported, but no details on solution chemical composition are given [17]. The main aim of the present work was to find out a novel, more appropriate solvent for the growth of single crystals of Ca2GeO4 by studying a series of alkali-borate and alkali-molybdate solvents. During the studies, appropriate conditions for the growth of other germanates with potential applications, such as Ca5Ge3O11, CaGeO3,
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Li2GeO3, Li2CaGeO4, and Na2CaGe6O14, were also found. Additionally, as a result of the studies, two novel phases, unknown so far, were discovered.
2. Experimental First a group of alkali-borate compounds (Na2O B2O3, Na2O 1.5B2O3, Na2O 2B2O3 and Li2O B2O3) were examined as solvents. It is known that these solvents are characterized by relatively low melting temperatures and high dissolving capacity of a number of compounds of different compositions. The number of solvents was limited to the compositions shown above, because both the viscosity of solvents with a higher B2O3 content and the volatility of solvents with a higher alkali oxide content increased. The second group of solvents included alkali-molybdate compounds (Na2O 1.5MoO3, Na2O 2MoO3 and Li2O MoO3). These solvents are also characterized by low melting temperatures, but their dissolving capacity is lower than that of the alkali-borate solvents. On the other hand, the viscosity of the solutions based on the molybdate solvents is significantly lower, which considerably facilitates the mass transfer during crystal growth. As a result, irrespective of the lower concentrations of dissolved solids in this case, the crystals grow with a considerably higher rate and display a better quality. The compositions of the alkali-molybdate solvents were also limited, as shown above, because of the sharp decrease in solubility at higher MoO3 contents and the increased evaporation at higher alkali oxide contents (established in the preliminary experiments). The crystallization temperature was limited to 1150 1C, i.e. a temperature of solution homogenization was about 1200 1C. The evaporation of the solutions at a crystallization temperature above 1150 1C during the prolonged flux process would lead to poorly controlled growth conditions and low reproducibility of the results. The concentration borders are shown on the figures. The main purpose of the study was to find out the temperature and concentration regions of crystallization of lithium-, sodiumand calcium-germanate phases with potential application as solidstate laser media (matrices). Germanates containing Mg and Zn, as well as Al, Ga, Sc and In will be the subject of future studies. The ternary systems solvent-CaO–GeO2 were subjected to spontaneous crystallization as homogeneous solutions upon lowering their temperature down to the crystallization temperature (saturation temperature). The following starting reagents were used: Li2CO3
(99.99%), Na2CO3 (99.6%), H3BO3 (99.9%), MoO3 (99.9%), CaCO3 (99.5%) and GeO2 (99.999%). The reagents in a total amount of 20–25 g were weighed with a precision of7 0.01 g. They were mechanically mixed and were transferred to a platinum crucible of 3 cm diameter and 3 cm height. Depending on the composition and the temperature of the final solution, the depth of the solution was about 1 cm. A vertical Kanthal resistance furnace capable of temperatures up to 1250 1C with programmable temperature control and precision of 0.1 1С (controller-programmer type EUROTHERM 2704) was used. After decomposition of the carbonates and the boric acid, the solution was homogenized for 1–3 h at a temperature exceeding the assumed crystallization temperature by about 50 1C. An indication that a homogeneous solution was obtained was the solidification of the solution as a transparent glassy mass without any nondissolved particles to be seen when the platinum probe was removed and observed under microscope ( 100). The “sticking” of the platinum probe to the bottom of the crucible was an additional evidence for the lack of particles on the bottom. A reliable proof of homogeneity was the transparency of the obtained solution which allowed seeing the bottom through the solution under special irradiation. By step-wise lowering of the temperature (in most cases by 10 1C every 30 min), the temperature of primary crystallization was determined by the appearance of nuclei on the immersed platinum probe. The appearance of the first crystals on the platinum probe immersed in the solution was assured by the temperature difference between the solution layer at the hot bottom and walls of the crucible and the colder center on the solution surface. The temperature of the solution with the immersed platinum probe and the first nuclei was raised by a step of 5 1C every 30 min up to the temperature when the nuclei started to dissolve. Then the temperature was reduced back using the same step until a growth of the nuclei was observed again. In this way the temperature of saturation (liquidus temperature) was determined with an accuracy of 7 5 1C. It was established that the metastable zone in this system comprises about 10–15 1C, i.e. the real temperature of the first crystallization is 10–15 1C above that when the first appearance of nuclei on the pure platinum probe was observed. The latter were separated mechanically, cleaned in hot water, dried and submitted for X-ray phase analysis. Powder XRD data were recorded in the range from 10 to 801 (2θ) with a constant step of 0.021 and 1 s/step counting time at room temperature on a Shimadzu XRD-7000 powder diffractometer using filtered Cu Kα radiation.
Fig. 1. Temperature and concentration regions of crystallization in the system Na2O B2O3–CaO–GeO2. Some calcium germinate compounds and its melting temperatures are shown on the bottom axes.
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To find out the concentration limits of crystallization of the different phases for each solvent, 20–40 separate compositions were treated in the described way. In the cases of unknown phases, X-ray diffraction data were collected and presented.
3. Results and discussion 3.1. Alkali borate fluxes Fig. 1 shows the temperature and concentration regions of crystallization of the phases obtained in the system Na2O B2O3 (solvent)–CaO – GeO2. Here and in the following figures the crystallization regions are depicted by solid lines; in each region the isotherms of crystallization are shown. The weight percentages of the CaGeO2 phases on the bottom axes are indicated. No studies of the systems outside the dotted lines were performed owing to the lack of homogeneous solutions at 1200 1C, strong evaporation or rather high viscosity. An extremely high dissolving capacity of the examined solvent toward CaO and GeO2 was observed. Up to about 60 wt% of (CaOþGeO2) there was no crystallization of the solutions (instead of which glass formation occurred) or primary crystallization of some of the sodium borates (NaBO2, Na2B4O7) was observed. Above 60 wt% of (CaOþGeO2) in the system, CaGeO3 (ICDD 86-1875) crystallized in the zone richer in GeO2; Ca5Ge3O11 (ICDD 52-0158) crystallized in the zone with a small excess of GeO2; Ca3B2O6 (ICDD 26-0347) and
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Ca2GeО4 (ICDD 26-0304) crystallized in the zone of almost equal percentages of CaO and GeO2 or of small excess of CaO. The regions of germanate phases cover up to 88 wt% of dissolved (CaOþGeO2) where the crystallization temperature reach to 1150 1C. The crystallization temperature strongly depends on the CaO/GeO2 ratio. Upon increasing the CaO content, the crystallization temperature of the germanate phases sharply increases, while the increase in the GeO2 content has only a slight effect. Thus, CaGeO3, which is rich in GeO2, can be grown in the temperature range from 950 to 1050 1C; Ca5Ge3O11 can be grown in the temperature range from 1000 to 1100 1C, while Ca2GeO4, which is the compound richest in CaO, crystallizes above 1100 1C. Actually the system solvent-CaO–GeO2 repeats the behavior of the system CaO–GeO2, where moving from GeO2 to CaO several germanate compounds crystallized and the crystallization temperature sharply rises especially after 34.9 wt% CaO (CaGeO3 compound) (see melting temperature of the calcium germinates on the Fig. 1). It should be noted that owing to the rather high content of solute (CaOþ GeO2), the localization of regions of the three crystallizing germanates is not influenced by the composition of the solvent—they are positioned at CaO/GeO2 ratios almost corresponding to the ranges in the binary system (CaO–GeO2) [18]. Fig. 2 presents the results of the studies of the system Na2O 1.5B2O3 (solvent)–CaО–GeO2. The same regularities are observed as those for the system Na2O B2O3 (solvent)–CaO–GeO2 as regards the dissolving capacity, the type of crystallizing phases and their relative positions. The essential difference between them is due to the higher content of B2O3 in the latter solvent. As a result of this, the range of
Fig. 2. Temperature and concentration regions of crystallization in the system Na2O 1.5B2O3–CaO–GeO2.
Fig. 3. Temperature and concentration regions of crystallization in the system Na2O 2.0B2O3–CaO–GeO2.
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Table 1 X-ray powder diffraction data for the N1 phase. d (Å)
I
5.13937 4.83489 3.63268 3.55058 3.51947 2.90247 2.70899 2.69441 2.5665 2.41493 2.29522 2.2662 2.18476 2.16302 2.07322 2.01075 1.93723 1.7587 1.70991 1.69803 1.6631 1.65369 1.62253 1.61189 1.53664 1.47079 1.4415 1.42244 1.36447 1.36275 1.3463 1.31716 1.29762 1.2824 1.24415 1.22558 1.20899 1.2037
7 6 8 4 73 100 6 94 15 6 40 28 6 28 5 6 34 26 4 28 53 9 22 27 9 11 6 5 13 5 10 5 9 5 3 4 11 5
Fig. 4. Photograph of spontaneously nucleated crystals–unknown phase N1.
Ca3B2O6 crystallization is significantly broadened and even a crystallization range of Ca2B2O5 (ICDD 79-1516) is registered. These borate phases push the germanate ones to still higher concentrations of CaO and GeO2. Thus, for example, in the latter solvent Ca2GeO4 crystallizes only above 75 wt% of (CaOþGeO2) (above 70 wt % in the solvent Na2O B2O3). Rather different is the behavior of the ternary system Na2O 2B2O3 (solvent)–CaО–GeO2 (Fig. 3). Here the crystallization region of Ca2B2O5 is considerably broader and Ca3B2O6 is missing. Instead, in this system, a crystallization region of a novel unknown phase denoted as N1, was registered. The N1 phase is situated in the region rich in GeO2.
The diffractogram of this new phase is shown in Table 1 and its photograph is presented on Fig. 4. Data about the chemical composition and structure of this novel phase will be reported separately. The trend of the dissolving capacity to increase with the increase in the B2O3 content continues, whereby the germanate phases crystallize from solutions still richer in CaO and GeO2. For example, Ca2GeO4 crystallizes from the Na2O 2B2O3 solvent at a concentration of (CaOþ GeO2) above 82%. The comparison between the three systems discussed above reveals that upon increasing the B2O3 content in the solvent, the crystallization fields of the germanate phases are slightly shifted toward solutions richer in GeO2. Fig. 5 presents the system Li2O B2O3 (solvent)–CaO–GeO2. The comparison of the crystallization ranges observed in this system with those in the corresponding system with Na2O B2O3 as a solvent shows the following: (i) crystallizing phases of equal chemical composition and position in the system are obtained; (ii) crystallization takes place in almost the same temperature ranges; (iii) Li2O B2O3 displays a slightly higher dissolution capacity; and (iv) the germanate phases are shifted to the CaO-rich region in the Li2O B2O3 solvent. 3.2. Alkali molybdate fluxes Along with the alkali-borate systems, alkali molybdates were examined as solvents—Na2O 1.5MoO3; Na2O 2MoO3 and Li2O MoO3. Fig. 6 presents the system Na2O 1.5MoO3 (solvent)–CaO–GeO2. The alkali-molybdate solvents display a different behavior than the alkaliborate ones. First of all, their dissolving capacity for CaO and GeO2 is considerably lower. The phases crystallizing up to 1150 1C in alkalimolybdate solvents are at concentrations of the solutes (CaOþ GeO2) up to 40%, while the dissolving capacity of the borate solvents reached up to 95%. Two phases crystallize in the concentration range 10–40 wt% of the pseudo-ternary system examined—Na2CaGe6O14 (ICDD 48-0298) in the GeO2-rich region; and CaMoO4 (ICDD 77-2238) in the CaO-rich region. The germanate phases crystallizing from the borate solvents are missing in this case. The comparison between alkali-borate and alkali-molybdate solvents shows only one common feature. In both cases the orientation of the crystallization isotherms is relatively the same. The isotherms correspond to an almost constant CaO concentration, i.e., the crystallization temperature strongly depends on the concentration of CaO and considerably less on that of GeO2. For instance, it can be seen on Fig. 6 that the crystallization temperature is constant (1050 1C) for constant CaO concentration (10 wt %) and different GeO2 concentrations—5 wt% (where CaMoO4 crystallizes), and 15 wt % (where Na2CaGe6O14 crystallizes). In Fig. 7 the system Na2O 2MoO3 (solvent)–CaO–GeO2 is presented. With regard to Na2O 1.5MoO3, this solvent has a lower dissolving capacity—up to 28 wt% of dissolved (CaOþGeO2) at a temperature of crystallization of 1150 1C. In the whole concentration range from 4 to 28 wt% of (CaOþ GeO2), only CaMoO4 crystallizes. Single tests revealed that a further increase in MoO3 concentration in the solvent lowered the dissolving capacity, which is opposite to B2O3. The same behavior was displayed by solvents with a high content of Na2O (e.g., Na2O·MoO3). In these cases also significant evaporation was registered. Fig. 8 presents the pseudo-ternary system Li2O MoO3 (solvent)– CaO–GeO2. Between 8 and 40 wt% of dissolved solids (CaOþGeO2) four phases crystallize in the system: Li2GeO3 (ICDD 83-1518) and a novel unknown phase N2 in the GeO2-rich region; Li2CaGeO4 (ICDD 72-1730) and Ca2GeO4 in the region of nearly equal weight concentrations of CaO and GeO2. As expected, the lithium-containing phases crystallize from solutions richer in the solvent (Li2O MoO3). The diffractogram of the novel N2 phase is shown in Table 2 and its photograph is presented on Fig. 9. Data about the chemical composition and structure of this novel phase will be reported separately.
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Fig. 5. Temperature and concentration regions of crystallization in the system Li2O B2O3–CaO–GeO2.
Fig. 6. Temperature and concentration regions of crystallization in the system Na2O 1.5MoO3–CaO–GeO2.
It is worth noting that the structure of Li2CaGeO4 is built up from GeO4 tetrahedra, so Li2CaGeO4 is a potential matrix to be doped with Cr4 þ as a laser-active ion. Special attention should be paid to the broad concentration region of crystallization of Ca2GeO4 (from 25 to above 40 wt% of dissolved solids). Moreover, Ca2GeO4 crystallizes from these solutions at about 950 1C (the minimum crystallization temperature of Ca2GeO4 from borate solutions is about 1100 1C). The diffractograms of the crystals grown at several points of this region reveal that the crystallized phase has a structure of Ca2GeO4, but with a certain reduction of the parameters of the crystal lattice. Most probably, in this case solid solutions of (CaLi)2GeO4 are formed instead of a pure Ca2GeO4 phase. These solid solutions would be of particular interest as matrices to be doped with Cr4 þ , as the different lithium content would alter the absorption and the emission spectra due to the specific and variable local environment of the CrO4 tetrahedra. A detailed study of this region will be made in order to throw light on the existence of (CaLi)2GeO4 solid solutions. Of significance for crystal growth by the flux method is some additional characteristics of the solutions, such as the extent of
evaporation of the solution components and the viscosity of the solution. The control of the weight of the solutions prior to and after each run revealed that the weight losses for all examined solutions did not exceed 0.4 wt% per 24 h. Higher evaporation is observed for the molybdate solutions at higher temperatures. Single tests revealed that the dynamic viscosity of the solutions is within the range of 10–30 cP for the molybdate solutions and 50–150 cP for the borate solutions, so the viscosity allows successful crystal growth. Upon increasing the amount of B2O3 in the solvent and decreasing the temperature, the viscosity tends to higher values.
4. Conclusions The results of the performed study give a basis for producing single crystals by the flux method from a series of germanate compounds with various application areas. Cr4þ -doped single crystals of Ca2GeO4, which are among the best prospective materials for solid state lasers
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Fig. 7. Temperature and concentration regions of crystallization in the system Na2O 2MoO3.
Fig. 8. Temperature and concentration regions of crystallization in the system Li2O MoO3.
emitting in the 1.1–1.6 μm range, can be successfully grown from a Na2O B2O3 solvent at a recommended initial composition of 24 wt% solvent, 42 wt% CaO and 34 wt% GeO2 (composition 1, Fig. 1), as well as from a Li2O B2O3 solvent at a recommended initial composition of 17 wt% solvent, 46 wt% CaO and 37 wt% GeO2 (composition 2, Fig. 5). The calculation shows that the precipitation is 0.035 g per gram solution for one degree, so 17.5 g Ca2GeO4 single crystal could be
obtained from 100 g solution after cooling from 1150 to 1100 1C. Although the solutions contain high amounts of CaO and GeO2, there is no noticeable evaporation and the viscosity is of the order of 60 cP. The initial crystallization temperature that could be used is about 1150 1C which is considerably lower than that with the solvent CaF2 used so far (above 1350 1C) [16]. The problems encountered upon using CaCl2 as a solvent are also eliminated (evaporation,
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Table 2 X-ray powder diffraction data for the N2 phase. d (Å)
I
d (Å)
I
5.59621 4.67438 4.24187 4.17924 3.77313 3.74983 3.65734 3.44133 3.35972 3.2088 3.18853 2.97984 2.82855 2.80188 2.77139 2.70055 2.68686 2.67306 2.63616 2.61505 2.60054 2.52589 2.40992 2.34076 2.26425 2.23855 2.22673 2.16937 2.15632 2.09619 2.09066 2.03057 1.9911 1.98501 1.9613 1.92193 1.89311 1.88897 1.8687 1.84296 1.82925 1.82473
7 44 83 11 11 19 77 48 48 3 23 4 21 25 52 4 7 59 4 100 3 73 18 18 44 70 12 4 6 13 12 21 17 20 13 9 3 7 7 4 21 12
1.72576 1.72119 1.71571 1.69861 1.69267 1.68215 1.63354 1.62518 1.60462 1.59459 1.57105 1.55954 1.54025 1.53005 1.52321 1.50569 1.48671 1.47062 1.43953 1.42466 1.42073 1.41531 1.4019 1.39445 1.3721 1.35428 1.34667 1.33454 1.33118 1.32509 1.31918 1.309 1.30524 1.29898 1.29489 1.27012 1.26316 1.25695 1.24499 1.22066 1.2131 1.20272
4 13 8 5 3 50 5 9 7 7 27 58 40 16 24 9 5 6 18 5 10 33 20 17 8 3 20 7 9 8 10 5 5 5 5 11 7 11 19 8 3 8
Fig. 9. Photograph of spontaneously nucleated crystals–unknown phase N2.
hygroscopicity) [15,16]. These novel solvents will be used for growing single crystals with different chromium content and studying the laser characteristics of Cr4þ :Ca2GeO4 laser active media. The same solvents can be used for growing single crystals of Ca5Ge3O11 (another potential laser matrix) and single crystals of CaGeO3. Suitable initial solutions for the growth of Ca5Ge3O11 with high concentrations of CaO and GeO2 and appropriate physicochemical properties are the compositions denoted with 3 in Fig. 1and 4 in Fig. 5, with crystallization temperatures of about 1050 1C and 1100 1C, respectively. For growing CaGeO3 single
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crystals, the initial compositions denoted with 5 in Fig. 1 and 6 in Fig. 5 with crystallization temperatures of about 1030 1C and 1050 1C, respectively, can be used. The recommended conditions for growing lithium germanates (Li2GeO3 and Li2CaGeO4) from a Li2O MoO3 solvent are also given (Fig. 8). The crystallization regions of the two phases are rather broad and the crystallization temperatures are in the range of 950–1050 1C. Exemplary initial solutions could be those denoted by 7 and 8 in Fig. 8 (for growing single crystals of Li2CaGeO4 and Ca2GeO4, respectively). Of interest is also the possibility of growing single crystals of Na2CaGe6O14 (9, Fig. 6). Elucidation of the structure, microstructure and optical properties of these germanates could throw light on their potential application. Special attention will be paid to the possibility of growing single crystals from (CaLi)2GeO4 solid solutions from the system Li2O MoO3–CaO–GeO2 (Fig. 8). The preserved olivine structure of Ca2GeO4 with partial substitution of Ca for Li is a potentiality for alteration of the optical properties of Ca2GeO4. The structures and potential applications of the two novel compounds will also be investigated.
Acknowledgments The research is supported by a grant (agreement of August 27, 2013 no. 02.В.49.21.0003 between the Ministry of Education and Science of the Russian Federation and N.I. Lobachevsky State University of Nizhni Novgorod).
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[15] F. Nishi, Y. Takéuchi, Two-layer structure of tricalcium germanate, Ca3 x[GeO4] (O1 2x,F2x) with xE 0.275, Acta Crystallogr. C 40 (Part 5) (1984) 730–733. http: //dx.doi.org/10.1107/S0108270184005515. [16] A.B Bykov, V. Petricevic, J. Steiner, Di Yao, L.L. Isaacs, M.R. Kokta, R.R. Alfano, Flux growth and characterization of Cr4 þ : Ca2GeO4 crystals as a new near infrared tunable laser material, J Cryst. Growth 211 (1–4) (2000) 295–301. http://dx.doi.org/10.1016/S0022-0248(99)00770-8.
[17] V.V Dozmarov, L.D Iskhakova, V.N Protopopov, E.M Dianov, Growing Cr4 þ : Ca2GeO4 large crystals by the solution melt method, in: F. Gerard (Ed.), Advances in Condensed Matter and Materials Research, 3, Nova Science Publishers Inc, New York, 2002, pp. 153–158, ISBN 13: 9781590331477. [18] A.K. Shirvinskaya, R.G. Grebenshchikov, N.A. Toropov, Phase transitions in Ca2GeO4, Izv. Akad. Nauk SSSR, Neorg. Mater. 2 (1966) 332–336.
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Novel solvents for the single crystal growth of germanate phases by the flux method V.A. Ivanov a, M.O. Marychev a,n, P.V. Andreev a, I. Koseva b, P. Tzvetkov b, V. Nikolov a a b
N.I. Lobachevsky State University of Nizhni Novgorod, Nizhni Novgorod 603950, Russia Bulgarian Academy of Science, Institute of General and Inorganic Chemistry, BU-1113 Sofia, Bulgaria
art ic l e i nf o
a b s t r a c t
Article history: Received 23 March 2015 Received in revised form 27 April 2015 Accepted 29 April 2015 Communicated by: V. Fratello Available online 9 May 2015
A series of alkali-borate (Na2O B2O3, Na2O 1.5B2O3, Na2O 2B2O3 and Li2O B2O3), and a series of alkalimolybdate (Na2O 1.5MoO3, Na2O 2MoO3 and Li2O MoO3) solvents were studied with a view to find out the suitable conditions for growing single crystals from germanate phases by the flux method. The ternary systems solvent-CaO–GeO2 were investigated and crystallization temperature and crystalized phase were determined. As a main result the concentration and temperature regions of crystallization of calcium germanates (Ca5Ge3O11, CaGeO3) and of some alkali germanates (Li2GeO3, Li2CaGeO4, Na2CaGe6O14) were experimentally determined for a first time. The conditions found for Ca2GeO4 growth are significantly more favorable than those known so far. Additionally during this study, two novel unknown phases were obtained. X-ray diffractograms of the novel phases are presented. The obtained conditions are a basis for successful single crystal growth by the flux method for all mentioned germanate compounds. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Phase diagrams A1. Solvents A1. X-ray diffraction A2. Growth from high temperature solutions A2. Single crystal growth B1. Oxides
1. Introduction Solid-state lasers emitting in the spectral region of 1.1–1.6 μm find increasing applications in medicine, ecology, telecommunications, instrument-building, etc. [1–3]. Several single crystals doped with ions of the transition elements emit in this region. Of major importance among them are single crystals doped with Cr4þ . The laser media most often used so far are Cr4þ doped Mg2SiO4 (forsterite) and Y3Al5O12 (YAG) [4,5]. These media have, however, several drawbacks, e.g., low quantum yield due to non-emitting transitions; presence of Cr3 þ in the matrix and limited doping capacity with Cr4þ . Last but not least are the problems of crystal growth related to the high melting temperatures of these two compounds [6,7]. Recently, a number of compounds have been studied as potential substitutes for Mg2SiO4 and Y3Al5O12. Very few of these studies have, however, yielded positive results. A major group of compounds with potential application are the Cr4 þ doped compounds with olivine structure. Unfortunately, most of these compounds display drawbacks such as incongruent melting, phase transitions, and non-radiative emission. For example, single crystal growth of Mg2GeO4 is accompanied by significant evaporation of GeO2 [8]; CaMgSiO4 melts incongruently [9]; Ca2SiO4 undergoes several polymorphous transitions [10]. n Correspondence to: Faculty of Physics, N.I. Lobachevsky State University of Nizhni Novgorod, Gagarin Avenue, 23, build. 3, off. 403, Nizhni Novgorod 603950, Russia. Tel.: þ 7 903 604 14 12. E-mail address: [email protected] (M.O. Marychev).
http://dx.doi.org/10.1016/j.jcrysgro.2015.04.042 0022-0248/& 2015 Elsevier B.V. All rights reserved.
The only material with olivine structure (besides forsterite) that has provided single crystals of satisfactory size and quality and has demonstrated laser emission of high efficiency is Cr4 þ :Ca2GeO4 [11,12]. As a representative of the olivines, this compound also undergoes a phase transition, but at a higher temperature (1450 1C). The low-temperature modification with olivine structure is very suitable to be doped with Cr4 þ ; germanium is situated in the required tetrahedral environment and Cr4 þ and Ge4 þ are of the same valence state and have close ionic radii (0.041 и 0.039 nm, respectively). Moreover, the large ionic radius of Ca2þ does not allow its substitution by Cr3þ and Cr2þ , which are undesirable for this application [13]. Attempts have been made to grow single crystals of Cr4þ :Ca2GeO4 by the flux method at temperatures lower than 1450 1C (the phase transition temperature). The reported studies show that the use of CaCl2 as a solvent is not appropriate due to the intense evaporation of CaCl2 and the formation of hygroscopic phases that enter in the crystal and destroying it [14,15]. When CaF2 is used as a solvent, crystal growth should be carried out at a temperature above 1350 1C, where solvent evaporation takes place; moreover, the process should take place in an iridium crucible in a nitrogen atmosphere [16]. The use of a lithium-molybdate solvent is also reported, but no details on solution chemical composition are given [17]. The main aim of the present work was to find out a novel, more appropriate solvent for the growth of single crystals of Ca2GeO4 by studying a series of alkali-borate and alkali-molybdate solvents. During the studies, appropriate conditions for the growth of other germanates with potential applications, such as Ca5Ge3O11, CaGeO3,
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Li2GeO3, Li2CaGeO4, and Na2CaGe6O14, were also found. Additionally, as a result of the studies, two novel phases, unknown so far, were discovered.
2. Experimental First a group of alkali-borate compounds (Na2O B2O3, Na2O 1.5B2O3, Na2O 2B2O3 and Li2O B2O3) were examined as solvents. It is known that these solvents are characterized by relatively low melting temperatures and high dissolving capacity of a number of compounds of different compositions. The number of solvents was limited to the compositions shown above, because both the viscosity of solvents with a higher B2O3 content and the volatility of solvents with a higher alkali oxide content increased. The second group of solvents included alkali-molybdate compounds (Na2O 1.5MoO3, Na2O 2MoO3 and Li2O MoO3). These solvents are also characterized by low melting temperatures, but their dissolving capacity is lower than that of the alkali-borate solvents. On the other hand, the viscosity of the solutions based on the molybdate solvents is significantly lower, which considerably facilitates the mass transfer during crystal growth. As a result, irrespective of the lower concentrations of dissolved solids in this case, the crystals grow with a considerably higher rate and display a better quality. The compositions of the alkali-molybdate solvents were also limited, as shown above, because of the sharp decrease in solubility at higher MoO3 contents and the increased evaporation at higher alkali oxide contents (established in the preliminary experiments). The crystallization temperature was limited to 1150 1C, i.e. a temperature of solution homogenization was about 1200 1C. The evaporation of the solutions at a crystallization temperature above 1150 1C during the prolonged flux process would lead to poorly controlled growth conditions and low reproducibility of the results. The concentration borders are shown on the figures. The main purpose of the study was to find out the temperature and concentration regions of crystallization of lithium-, sodiumand calcium-germanate phases with potential application as solidstate laser media (matrices). Germanates containing Mg and Zn, as well as Al, Ga, Sc and In will be the subject of future studies. The ternary systems solvent-CaO–GeO2 were subjected to spontaneous crystallization as homogeneous solutions upon lowering their temperature down to the crystallization temperature (saturation temperature). The following starting reagents were used: Li2CO3
(99.99%), Na2CO3 (99.6%), H3BO3 (99.9%), MoO3 (99.9%), CaCO3 (99.5%) and GeO2 (99.999%). The reagents in a total amount of 20–25 g were weighed with a precision of7 0.01 g. They were mechanically mixed and were transferred to a platinum crucible of 3 cm diameter and 3 cm height. Depending on the composition and the temperature of the final solution, the depth of the solution was about 1 cm. A vertical Kanthal resistance furnace capable of temperatures up to 1250 1C with programmable temperature control and precision of 0.1 1С (controller-programmer type EUROTHERM 2704) was used. After decomposition of the carbonates and the boric acid, the solution was homogenized for 1–3 h at a temperature exceeding the assumed crystallization temperature by about 50 1C. An indication that a homogeneous solution was obtained was the solidification of the solution as a transparent glassy mass without any nondissolved particles to be seen when the platinum probe was removed and observed under microscope ( 100). The “sticking” of the platinum probe to the bottom of the crucible was an additional evidence for the lack of particles on the bottom. A reliable proof of homogeneity was the transparency of the obtained solution which allowed seeing the bottom through the solution under special irradiation. By step-wise lowering of the temperature (in most cases by 10 1C every 30 min), the temperature of primary crystallization was determined by the appearance of nuclei on the immersed platinum probe. The appearance of the first crystals on the platinum probe immersed in the solution was assured by the temperature difference between the solution layer at the hot bottom and walls of the crucible and the colder center on the solution surface. The temperature of the solution with the immersed platinum probe and the first nuclei was raised by a step of 5 1C every 30 min up to the temperature when the nuclei started to dissolve. Then the temperature was reduced back using the same step until a growth of the nuclei was observed again. In this way the temperature of saturation (liquidus temperature) was determined with an accuracy of 7 5 1C. It was established that the metastable zone in this system comprises about 10–15 1C, i.e. the real temperature of the first crystallization is 10–15 1C above that when the first appearance of nuclei on the pure platinum probe was observed. The latter were separated mechanically, cleaned in hot water, dried and submitted for X-ray phase analysis. Powder XRD data were recorded in the range from 10 to 801 (2θ) with a constant step of 0.021 and 1 s/step counting time at room temperature on a Shimadzu XRD-7000 powder diffractometer using filtered Cu Kα radiation.
Fig. 1. Temperature and concentration regions of crystallization in the system Na2O B2O3–CaO–GeO2. Some calcium germinate compounds and its melting temperatures are shown on the bottom axes.
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To find out the concentration limits of crystallization of the different phases for each solvent, 20–40 separate compositions were treated in the described way. In the cases of unknown phases, X-ray diffraction data were collected and presented.
3. Results and discussion 3.1. Alkali borate fluxes Fig. 1 shows the temperature and concentration regions of crystallization of the phases obtained in the system Na2O B2O3 (solvent)–CaO – GeO2. Here and in the following figures the crystallization regions are depicted by solid lines; in each region the isotherms of crystallization are shown. The weight percentages of the CaGeO2 phases on the bottom axes are indicated. No studies of the systems outside the dotted lines were performed owing to the lack of homogeneous solutions at 1200 1C, strong evaporation or rather high viscosity. An extremely high dissolving capacity of the examined solvent toward CaO and GeO2 was observed. Up to about 60 wt% of (CaOþGeO2) there was no crystallization of the solutions (instead of which glass formation occurred) or primary crystallization of some of the sodium borates (NaBO2, Na2B4O7) was observed. Above 60 wt% of (CaOþGeO2) in the system, CaGeO3 (ICDD 86-1875) crystallized in the zone richer in GeO2; Ca5Ge3O11 (ICDD 52-0158) crystallized in the zone with a small excess of GeO2; Ca3B2O6 (ICDD 26-0347) and
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Ca2GeО4 (ICDD 26-0304) crystallized in the zone of almost equal percentages of CaO and GeO2 or of small excess of CaO. The regions of germanate phases cover up to 88 wt% of dissolved (CaOþGeO2) where the crystallization temperature reach to 1150 1C. The crystallization temperature strongly depends on the CaO/GeO2 ratio. Upon increasing the CaO content, the crystallization temperature of the germanate phases sharply increases, while the increase in the GeO2 content has only a slight effect. Thus, CaGeO3, which is rich in GeO2, can be grown in the temperature range from 950 to 1050 1C; Ca5Ge3O11 can be grown in the temperature range from 1000 to 1100 1C, while Ca2GeO4, which is the compound richest in CaO, crystallizes above 1100 1C. Actually the system solvent-CaO–GeO2 repeats the behavior of the system CaO–GeO2, where moving from GeO2 to CaO several germanate compounds crystallized and the crystallization temperature sharply rises especially after 34.9 wt% CaO (CaGeO3 compound) (see melting temperature of the calcium germinates on the Fig. 1). It should be noted that owing to the rather high content of solute (CaOþ GeO2), the localization of regions of the three crystallizing germanates is not influenced by the composition of the solvent—they are positioned at CaO/GeO2 ratios almost corresponding to the ranges in the binary system (CaO–GeO2) [18]. Fig. 2 presents the results of the studies of the system Na2O 1.5B2O3 (solvent)–CaО–GeO2. The same regularities are observed as those for the system Na2O B2O3 (solvent)–CaO–GeO2 as regards the dissolving capacity, the type of crystallizing phases and their relative positions. The essential difference between them is due to the higher content of B2O3 in the latter solvent. As a result of this, the range of
Fig. 2. Temperature and concentration regions of crystallization in the system Na2O 1.5B2O3–CaO–GeO2.
Fig. 3. Temperature and concentration regions of crystallization in the system Na2O 2.0B2O3–CaO–GeO2.
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Table 1 X-ray powder diffraction data for the N1 phase. d (Å)
I
5.13937 4.83489 3.63268 3.55058 3.51947 2.90247 2.70899 2.69441 2.5665 2.41493 2.29522 2.2662 2.18476 2.16302 2.07322 2.01075 1.93723 1.7587 1.70991 1.69803 1.6631 1.65369 1.62253 1.61189 1.53664 1.47079 1.4415 1.42244 1.36447 1.36275 1.3463 1.31716 1.29762 1.2824 1.24415 1.22558 1.20899 1.2037
7 6 8 4 73 100 6 94 15 6 40 28 6 28 5 6 34 26 4 28 53 9 22 27 9 11 6 5 13 5 10 5 9 5 3 4 11 5
Fig. 4. Photograph of spontaneously nucleated crystals–unknown phase N1.
Ca3B2O6 crystallization is significantly broadened and even a crystallization range of Ca2B2O5 (ICDD 79-1516) is registered. These borate phases push the germanate ones to still higher concentrations of CaO and GeO2. Thus, for example, in the latter solvent Ca2GeO4 crystallizes only above 75 wt% of (CaOþGeO2) (above 70 wt % in the solvent Na2O B2O3). Rather different is the behavior of the ternary system Na2O 2B2O3 (solvent)–CaО–GeO2 (Fig. 3). Here the crystallization region of Ca2B2O5 is considerably broader and Ca3B2O6 is missing. Instead, in this system, a crystallization region of a novel unknown phase denoted as N1, was registered. The N1 phase is situated in the region rich in GeO2.
The diffractogram of this new phase is shown in Table 1 and its photograph is presented on Fig. 4. Data about the chemical composition and structure of this novel phase will be reported separately. The trend of the dissolving capacity to increase with the increase in the B2O3 content continues, whereby the germanate phases crystallize from solutions still richer in CaO and GeO2. For example, Ca2GeO4 crystallizes from the Na2O 2B2O3 solvent at a concentration of (CaOþ GeO2) above 82%. The comparison between the three systems discussed above reveals that upon increasing the B2O3 content in the solvent, the crystallization fields of the germanate phases are slightly shifted toward solutions richer in GeO2. Fig. 5 presents the system Li2O B2O3 (solvent)–CaO–GeO2. The comparison of the crystallization ranges observed in this system with those in the corresponding system with Na2O B2O3 as a solvent shows the following: (i) crystallizing phases of equal chemical composition and position in the system are obtained; (ii) crystallization takes place in almost the same temperature ranges; (iii) Li2O B2O3 displays a slightly higher dissolution capacity; and (iv) the germanate phases are shifted to the CaO-rich region in the Li2O B2O3 solvent. 3.2. Alkali molybdate fluxes Along with the alkali-borate systems, alkali molybdates were examined as solvents—Na2O 1.5MoO3; Na2O 2MoO3 and Li2O MoO3. Fig. 6 presents the system Na2O 1.5MoO3 (solvent)–CaO–GeO2. The alkali-molybdate solvents display a different behavior than the alkaliborate ones. First of all, their dissolving capacity for CaO and GeO2 is considerably lower. The phases crystallizing up to 1150 1C in alkalimolybdate solvents are at concentrations of the solutes (CaOþ GeO2) up to 40%, while the dissolving capacity of the borate solvents reached up to 95%. Two phases crystallize in the concentration range 10–40 wt% of the pseudo-ternary system examined—Na2CaGe6O14 (ICDD 48-0298) in the GeO2-rich region; and CaMoO4 (ICDD 77-2238) in the CaO-rich region. The germanate phases crystallizing from the borate solvents are missing in this case. The comparison between alkali-borate and alkali-molybdate solvents shows only one common feature. In both cases the orientation of the crystallization isotherms is relatively the same. The isotherms correspond to an almost constant CaO concentration, i.e., the crystallization temperature strongly depends on the concentration of CaO and considerably less on that of GeO2. For instance, it can be seen on Fig. 6 that the crystallization temperature is constant (1050 1C) for constant CaO concentration (10 wt %) and different GeO2 concentrations—5 wt% (where CaMoO4 crystallizes), and 15 wt % (where Na2CaGe6O14 crystallizes). In Fig. 7 the system Na2O 2MoO3 (solvent)–CaO–GeO2 is presented. With regard to Na2O 1.5MoO3, this solvent has a lower dissolving capacity—up to 28 wt% of dissolved (CaOþGeO2) at a temperature of crystallization of 1150 1C. In the whole concentration range from 4 to 28 wt% of (CaOþ GeO2), only CaMoO4 crystallizes. Single tests revealed that a further increase in MoO3 concentration in the solvent lowered the dissolving capacity, which is opposite to B2O3. The same behavior was displayed by solvents with a high content of Na2O (e.g., Na2O·MoO3). In these cases also significant evaporation was registered. Fig. 8 presents the pseudo-ternary system Li2O MoO3 (solvent)– CaO–GeO2. Between 8 and 40 wt% of dissolved solids (CaOþGeO2) four phases crystallize in the system: Li2GeO3 (ICDD 83-1518) and a novel unknown phase N2 in the GeO2-rich region; Li2CaGeO4 (ICDD 72-1730) and Ca2GeO4 in the region of nearly equal weight concentrations of CaO and GeO2. As expected, the lithium-containing phases crystallize from solutions richer in the solvent (Li2O MoO3). The diffractogram of the novel N2 phase is shown in Table 2 and its photograph is presented on Fig. 9. Data about the chemical composition and structure of this novel phase will be reported separately.
V.A. Ivanov et al. / Journal of Crystal Growth 426 (2015) 25–32
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Fig. 5. Temperature and concentration regions of crystallization in the system Li2O B2O3–CaO–GeO2.
Fig. 6. Temperature and concentration regions of crystallization in the system Na2O 1.5MoO3–CaO–GeO2.
It is worth noting that the structure of Li2CaGeO4 is built up from GeO4 tetrahedra, so Li2CaGeO4 is a potential matrix to be doped with Cr4 þ as a laser-active ion. Special attention should be paid to the broad concentration region of crystallization of Ca2GeO4 (from 25 to above 40 wt% of dissolved solids). Moreover, Ca2GeO4 crystallizes from these solutions at about 950 1C (the minimum crystallization temperature of Ca2GeO4 from borate solutions is about 1100 1C). The diffractograms of the crystals grown at several points of this region reveal that the crystallized phase has a structure of Ca2GeO4, but with a certain reduction of the parameters of the crystal lattice. Most probably, in this case solid solutions of (CaLi)2GeO4 are formed instead of a pure Ca2GeO4 phase. These solid solutions would be of particular interest as matrices to be doped with Cr4 þ , as the different lithium content would alter the absorption and the emission spectra due to the specific and variable local environment of the CrO4 tetrahedra. A detailed study of this region will be made in order to throw light on the existence of (CaLi)2GeO4 solid solutions. Of significance for crystal growth by the flux method is some additional characteristics of the solutions, such as the extent of
evaporation of the solution components and the viscosity of the solution. The control of the weight of the solutions prior to and after each run revealed that the weight losses for all examined solutions did not exceed 0.4 wt% per 24 h. Higher evaporation is observed for the molybdate solutions at higher temperatures. Single tests revealed that the dynamic viscosity of the solutions is within the range of 10–30 cP for the molybdate solutions and 50–150 cP for the borate solutions, so the viscosity allows successful crystal growth. Upon increasing the amount of B2O3 in the solvent and decreasing the temperature, the viscosity tends to higher values.
4. Conclusions The results of the performed study give a basis for producing single crystals by the flux method from a series of germanate compounds with various application areas. Cr4þ -doped single crystals of Ca2GeO4, which are among the best prospective materials for solid state lasers
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Fig. 7. Temperature and concentration regions of crystallization in the system Na2O 2MoO3.
Fig. 8. Temperature and concentration regions of crystallization in the system Li2O MoO3.
emitting in the 1.1–1.6 μm range, can be successfully grown from a Na2O B2O3 solvent at a recommended initial composition of 24 wt% solvent, 42 wt% CaO and 34 wt% GeO2 (composition 1, Fig. 1), as well as from a Li2O B2O3 solvent at a recommended initial composition of 17 wt% solvent, 46 wt% CaO and 37 wt% GeO2 (composition 2, Fig. 5). The calculation shows that the precipitation is 0.035 g per gram solution for one degree, so 17.5 g Ca2GeO4 single crystal could be
obtained from 100 g solution after cooling from 1150 to 1100 1C. Although the solutions contain high amounts of CaO and GeO2, there is no noticeable evaporation and the viscosity is of the order of 60 cP. The initial crystallization temperature that could be used is about 1150 1C which is considerably lower than that with the solvent CaF2 used so far (above 1350 1C) [16]. The problems encountered upon using CaCl2 as a solvent are also eliminated (evaporation,
V.A. Ivanov et al. / Journal of Crystal Growth 426 (2015) 25–32
Table 2 X-ray powder diffraction data for the N2 phase. d (Å)
I
d (Å)
I
5.59621 4.67438 4.24187 4.17924 3.77313 3.74983 3.65734 3.44133 3.35972 3.2088 3.18853 2.97984 2.82855 2.80188 2.77139 2.70055 2.68686 2.67306 2.63616 2.61505 2.60054 2.52589 2.40992 2.34076 2.26425 2.23855 2.22673 2.16937 2.15632 2.09619 2.09066 2.03057 1.9911 1.98501 1.9613 1.92193 1.89311 1.88897 1.8687 1.84296 1.82925 1.82473
7 44 83 11 11 19 77 48 48 3 23 4 21 25 52 4 7 59 4 100 3 73 18 18 44 70 12 4 6 13 12 21 17 20 13 9 3 7 7 4 21 12
1.72576 1.72119 1.71571 1.69861 1.69267 1.68215 1.63354 1.62518 1.60462 1.59459 1.57105 1.55954 1.54025 1.53005 1.52321 1.50569 1.48671 1.47062 1.43953 1.42466 1.42073 1.41531 1.4019 1.39445 1.3721 1.35428 1.34667 1.33454 1.33118 1.32509 1.31918 1.309 1.30524 1.29898 1.29489 1.27012 1.26316 1.25695 1.24499 1.22066 1.2131 1.20272
4 13 8 5 3 50 5 9 7 7 27 58 40 16 24 9 5 6 18 5 10 33 20 17 8 3 20 7 9 8 10 5 5 5 5 11 7 11 19 8 3 8
Fig. 9. Photograph of spontaneously nucleated crystals–unknown phase N2.
hygroscopicity) [15,16]. These novel solvents will be used for growing single crystals with different chromium content and studying the laser characteristics of Cr4þ :Ca2GeO4 laser active media. The same solvents can be used for growing single crystals of Ca5Ge3O11 (another potential laser matrix) and single crystals of CaGeO3. Suitable initial solutions for the growth of Ca5Ge3O11 with high concentrations of CaO and GeO2 and appropriate physicochemical properties are the compositions denoted with 3 in Fig. 1and 4 in Fig. 5, with crystallization temperatures of about 1050 1C and 1100 1C, respectively. For growing CaGeO3 single
31
crystals, the initial compositions denoted with 5 in Fig. 1 and 6 in Fig. 5 with crystallization temperatures of about 1030 1C and 1050 1C, respectively, can be used. The recommended conditions for growing lithium germanates (Li2GeO3 and Li2CaGeO4) from a Li2O MoO3 solvent are also given (Fig. 8). The crystallization regions of the two phases are rather broad and the crystallization temperatures are in the range of 950–1050 1C. Exemplary initial solutions could be those denoted by 7 and 8 in Fig. 8 (for growing single crystals of Li2CaGeO4 and Ca2GeO4, respectively). Of interest is also the possibility of growing single crystals of Na2CaGe6O14 (9, Fig. 6). Elucidation of the structure, microstructure and optical properties of these germanates could throw light on their potential application. Special attention will be paid to the possibility of growing single crystals from (CaLi)2GeO4 solid solutions from the system Li2O MoO3–CaO–GeO2 (Fig. 8). The preserved olivine structure of Ca2GeO4 with partial substitution of Ca for Li is a potentiality for alteration of the optical properties of Ca2GeO4. The structures and potential applications of the two novel compounds will also be investigated.
Acknowledgments The research is supported by a grant (agreement of August 27, 2013 no. 02.В.49.21.0003 between the Ministry of Education and Science of the Russian Federation and N.I. Lobachevsky State University of Nizhni Novgorod).
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