CRYSTAL GROWTH & DESIGN
Growth of ErAl3(BO3)4 Single Crystals from a K2Mo3O10 Flux
2006 VOL. 6, NO. 8 1766-1768
Katsuya Teshima,* Yuichiro Kikuchi,† Takaomi Suzuki, and Shuji Oishi* Department of EnVironmental Science and Technology, Faculty of Engineering, Shinshu UniVersity, 4-17-1 Wakasato, Nagano 380-8553, Japan ReceiVed NoVember 28, 2005; ReVised Manuscript ReceiVed June 9, 2006
ABSTRACT: ErAl3(BO3)4 crystals were successfully grown by the slow-cooling method in a K2Mo3O10 flux. The formation of ErAl3(BO3)4 crystals was greatly dependent on the solute concentration of starting mixtures, and mixtures containing a solute of 32-41 mol % produced ErAl3(BO3)4 crystals. The obtained ErAl3(BO3)4 crystals had sizes up to 2.4 × 1.4 × 1.2 mm and a transparent pearl pink color. Their form was a hexagonal {112h0} prism with pyramidal {101h1} end faces. Introduction Rare-earth aluminum borate crystals (RAl3(BO3)4; R ) Y, La-Lu) have attracted much attention, due to their potential applications in various industrial fields because of their excellent optical properties.1-20 These RAl3(BO3)4 crystals are of great interest for their luminescence and very low concentration quenching. Nowadays, there is a persistent demand for optical devices, such as self-frequency-doubling lasers and laser diode pumps.15,16,18,19 Among these RAl3(BO3)4 crystals, YAl3(BO3)4 is well-known to be a nonlinear crystal and is an excellent host for rare-earth ions such as Nd3+, Yb3+, and Er3+. Crystals of YAl3(BO3)4 doped with rare-earth elements and Nd-stoichiometric NdAl3(BO3)4 have already been established as laser materials.3,5,19 The NdAl3(BO3)4 laser material has very low fluorescence quenching, despite a high Nd concentration. Furthermore, crystals of YAl3(BO3)4 doped with Er have been demonstrated to be potential self-frequency-doubling laser materials. Er3+ ions are promising candidates as dopants for infrared-pumped visible luminescence and laser emission.20 In the same way, ErAl3(BO3)4 should be a promising efficient Er laser material. Some attempts have been made to synthesize RAl3(BO3)4 crystals from high-temperature solutions using the binary or pseudo-binary systems K2SO4-3MoO3,1,3 BaO-B2O3,5 BaCO3B2O3,5 Bi2O3-B2O3,12 K2Mo3O10-B2O316 and K2CO3-3MoO310 and the pseudo-ternary system K2Mo3O10-Nd2O3-B2O38 as fluxes. In addition, RAl3(BO3)4 crystals were grown by liquidphase epitaxy,6 top-seeded solution growth.16,17,20 Among these techniques, flux growth is very convenient and can produce ErAl3(BO3)4 crystals. YAl3(BO3)4 doped with rare-earth ions and NdAl3(BO3)4 crystals have been frequently studied over the last few decades. There have been only a few reports on the crystal growth of Er-stoichiometric ErAl3(BO3)4 and its properties; however, detailed results such as the optimal growth conditions and crystal properties have not been described.1,2,13 In this paper, our main aim is to find out the reproducible optimum growth conditions of ErAl3(BO3)4 crystals by slow cooling from a K2Mo3O10 flux. In addition, the morphology of the resulting crystals was also examined. Experimental Section Single crystals of ErAl3(BO3)4 were grown by a flux slow-cooling method. A stoichiometric mixture of reagent-grade Er2O3 (Wako Pure * To whom correspondence should be addressed. E-mail: teshima@ gipwc.shinshu-u.ac.jp (K.T.);
[email protected] (S.O.). † Present address: TS Communications Ltd., 5-12-10 Higashi Ohi, Shinagawa 140-0011, Japan
Table 1. Typical Growth Conditions of the ErAl3(BO3)4 Crystals run no.
solute concn (mol %)
1 2 3 4 5 6 7 8 9 10 11
5 10 15 20 25 32 33 35 37 39 41
amt of solute/g Er2O3 Al2O3 B2O3
amt of flux/g K2CO3 MoO3
0.456 0.916 1.380 1.848 2.319 2.986 3.082 3.274 3.467 3.661 3.855
6.265 5.960 5.652 5.341 5.028 4.588 4.522 4.394 4.266 4.137 4.008
0.365 0.733 1.104 1.478 1.855 2.388 2.465 2.618 2.773 2.927 3.083
0.332 0.667 1.005 1.345 1.689 2.174 2.244 2.384 2.524 2.665 2.806
19.576 18.621 17.659 16.689 15.710 14.327 14.128 13.729 13.239 12.927 12.524
Chemical Industries, Ltd.), Al2O3 (Wako Pure Chemical Industries, Ltd.), and B2O3 (Wako Pure Chemical Industries, Ltd.) was used as a solute, and reagent-grade K2CO3 (Wako Pure Chemical Industries, Ltd.) and MoO3 (Allied Materials, Co., Ltd.) powders were chosen as the flux. The typical growth conditions are given in Table 1. The concentration of the solute was varied from 5 to 41 mol % (1.153 to 9.744 g) of the flux. The flux composition was fixed at K2Mo3O10. The solute and flux powder were weighed out, mixed together, and put into platinum crucibles with a diameter of 36 mm and a height of 40 mm. The weights of these mixtures were kept at approximately 25 g for all growth runs. The lids were loosely fitted, and the crucibles were inserted into an electric furnace with silicon carbide heating elements. The crucibles were heated in air from room temperature to the holding temperature (1100 °C) at 45 °C h-1. They were held at this temperature for 10 h and then cooled to 450 °C at a rate of 5 °C h-1. When the cooling program was completed, the crucibles were cooled to room temperature. The crystalline products were then taken out by dissolving the flux in warm water. The grown crystals were observed by the use of an optical microscope and a scanning electron microscope (SEM, Hitachi, S-4100) and were further investigated by means of X-ray diffraction (XRD, Shimadzu, XRD-6000) and an energy-dispersive X-ray spectrometer (EDS, Horiba, EMAX). The length (L), width (W), and thickness (T) of the crystals were measured. The average length (Lav) and width (Wav) of the first 10 largest crystals were calculated for each growth run. The morphology was investigated by the use of XRD and interfacial angle data. The lattice parameters were obtained on the basis of the powder XRD. The densities of the grown crystals were determined pycnometrically.
Results and Discussion Flux Growth of ErAl3(BO3)4 Crystals. Well-formed prismatic ErAl3(BO3)4 crystals of sizes up to L ) 2.4 mm, W ) 1.4 mm, and T ) 1.2 mm were grown from K2Mo3O10 flux in runs 6-11 (solute concentration: 32-41 mol %). No growth of ErAl3(BO3)4 crystals was observed in runs 1-5. Typical
10.1021/cg050631a CCC: $33.50 © 2006 American Chemical Society Published on Web 07/11/2006
Growth of ErAl3(BO3)4 Single Crystals
Crystal Growth & Design, Vol. 6, No. 8, 2006 1767
Figure 1. Optical micrograph showing ErAl3(BO3)4 crystals grown from K2Mo3O10 flux.
Figure 3. Optical micrograph showing KErMo2O8 byproduct crystals.
Figure 2. Relationship between solute concentration and grown crystal sizes (Lav and Wav).
ErAl3(BO3)4 crystals are shown in Figure 1. The grown crystals were colored transparent pearl pink. Figure 2 shows the variation in the average length (Lav) and width (Wav) of the ErAl3(BO3)4 crystals with the solute concentration. Large crystals with Lav ) 2.0 mm and Wav ) 1.1 mm were grown from a mixture containing 37 mol % solute (run 9). The Lav and Wav values decreased on either increasing or decreasing the solute concentration. Mixtures containing a solute of 33-41 mol % produced crystals with Lav values of 1.1-2.0 mm and Wav values of 0.4-1.1 mm. The aspect ratios (Lav/Wav) of the crystals were in the region of 1.6-2.7. A few aggregates up to 5 mm in size were grown from the mixture containing 32 mol % solute (run 6). They consisted of a large number of small ErAl3(BO3)4 crystals, which individually were 10-500 µm (Lav ) 410 and Wav ) 170 µm). Byproduct crystals (KErMo2O8) formed in all growth runs. Mixtures containing a solute of 5-25 mol % produced only KErMo2O8 crystals. When the solute concentration was 33 mol %, the product ratio by weight ErAl3(BO3)4:KErMo2O8 was about 3:1. The product ratio of KErMo2O8 crystals gradually decreased with an increase in the solute concentration. At a solute concentration of 41 mol %, less than 10% of KErMo2O8 crystals were present. Furthermore, only a few MoO3 crystals, which were transparent yellow, were also grown in some runs. Typical platelike crystals of KErMo2O8 are shown in Figure 3. The obtained KErMo2O8 crystals were up to 11.2 × 5.0 × 0.2 mm in size and were transparent and light pink. Their form was a tetragonal thin plate.
Figure 4. X-ray diffraction patterns (Cu KR) of ErAl3(BO3)4 crystals: (a) prismatic crystals whose well-developed faces were laid in parallel with the holder plate; (b) pulverized crystallites; (c) ErAl3(BO3)4 JCPDS data.21
Characteristics of the ErAl3(BO3)4 Crystals. Prismatic ErAl3(BO3)4 crystals were obtained up to 2.4 × 1.4 × 1.2 mm, which were transparent and pearl pink. The grown crystals were investigated by XRD in order to identify the transparent pearl pink crystals and to determine the Miller indices of the crystal faces. Figure 4 shows XRD profiles of data for the ErAl3(BO3)4 prismatic crystals and pulverized crystallites, along with JCPDS data.21 The pulverized crystallite pattern (Figure 4b) was the same as that of JCPDS data21 (Figure 4c). In addition, the EDS data showed that erbium and aluminum atoms were homogeneously distributed in the crystals, and potassium and molybdenum atoms from the flux were not detected. As a result, the grown crystals were obviously identified as ErAl3(BO3)4 by their XRD patterns and EDS data. Figure 5 shows a typical example of the grown ErAl3(BO3)4 crystals and a schematic drawing of the crystal habit. The prismatic ErAl3(BO3)4 crystals had a form of hexagonal prism with pyramidal end faces. The surfaces of these crystals were
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{101h1} faces. They exhibited a pearl pink color and were up to 2.4 × 1.4 × 1.2 mm in size. The ErAl3(BO3)4 generation and their sizes depended considerably on the solute concentration of starting mixtures. When the solute concentrations of starting mixtures were 32-41 mol %, the ErAl3(BO3)4 crystals, which were objective ones in this study, were synthesized from the high-temperature solutions. Large crystals with 2.0 mm (Lav) and 1.1 mm (Wav) were grown from the mixture containing 37 mol % solute. In addition, KErMo2O8 crystals as a byproduct formed in all growth runs with mixtures containing 5-41 mol % solute. References Figure 5. Typical ErAl3(BO3)4 crystal and drawing of crystal habit bounded by the {112h0} and {101h1} faces.
very flat (Figure 5a). The XRD pattern of orientated prismatic crystals showed that the diffraction intensities of the (112h0), (224h0), and (336h0) planes in the hexagonal setting were predominant (Figure 4a). The indices of prismatic faces were {112h0}. The interfacial angles between prismatic-pyramidal faces, prismatic-prismatic faces, and pyramidal-pyramidal faces were respectively 54 ( 1, 60 ( 1, and 71 ( 1°. These values were in good agreement with the calculated interfacial angle of 54.6° between the (112h0) and (101h1) faces, 60.0° between the (112h0) and (21h1h0) faces, and 70.9° between the (101h1) and (011h1) faces, respectively. On the basis of the XRD data and interfacial angle measurements, the crystals were found to be bounded by the {112h0} and {101h1} faces (Figure 5b). On the basis of the powder XRD data, the lattice parameters were determined as a ) 0.929(1) nm and c ) 0.723(2) nm. These values agree approximately with those (a ) 0.928 nm and c ) 0.724 nm) from the literature.22 The crystal density determined pycnometrically, 4.47(3) g cm-3, agrees with the literature value, 4.46 g cm-3, and the calculated density using the lattice parameters, 4.459 g cm-3. Conclusions Transparent ErAl3(BO3)4 crystals were readily grown by the slow cooling of a K2Mo3O10 flux. The ErAl3(BO3)4 crystals formed as a hexagonal prism with pyramidal end faces. The prismatic crystals were bounded by well-developed {112h0} and
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