Gallium compounds have several applications in industry, for example in LEDs , photovoltaic materials , or lasers . Recently, there also have been reports about the photocatalytic activity of gallium borates and their ability to split water . Among the wide field of gallium compounds, our working group currently focuses on two systems: rare-earth orthogallates and gallium borates.
Basically, the field of rare-earth orthogallates is well explored. As early as 1967, Marezio et al.  released a list of several rare-earth orthogallates (RE = La–Lu except Ce and Pm). The structural investigations for this list were based on powder diffraction data, and all compounds were found to be orthorhombic. Thirty years later, the so far missing CeGaO3 was described by Shishido et al.  to crystallize tetragonally. In 2009, our group succeeded in the synthesis of HoGaO3 single crystals . During further investigations, we recently obtained single crystals of DyGaO3 via a high-pressure/high-temperature synthesis starting from C-Dy2O3 and β-Ga2O3. Due to these results, we could confirm the lattice parameters and structural data of the literature for DyGaO3.
In the field of gallium borates, there are only three compositions known so far: trigonal GaBO3 , monoclinic Ga4B2O9 , and trigonal HGa3B6O12(OH)4 . All three structures were refined from powder diffraction data. In 2014, Wenliang et al. found single crystals of HGa3B6O12(OH)4 and identified the phase as rhombohedral B22Ga9H21O57 . In this work, we managed to synthesize samples of GaBO3 with a degree of crystallinity that allowed us to perform a single-crystal structure determination for the first time. Hence, we were able to confirm the lattice parameters given by Bither and Young .
In the following, the syntheses and structures of the dysprosium orthogallate DyGaO3 and the gallium borate GaBO3 are described in detail. Additionally, the results of Raman-spectroscopic studies of DyGaO3 are presented.
2 Experimental section
2.1 Synthesis of DyGaO3
Our intention was to obtain single crystals of DyGaO3 with a size suitable for X-ray diffraction. Therefore, we mixed and grounded a mixture of the stoichiometric ratio 1:1 of C-Dy2O3 and β-Ga2O3. The fine white powders of cubic Dy2O3 (Strem Chemicals, Kehl, Germany, 99.9 %) and β-Ga2O3 (Strem Chemicals, Kehl, Germany, 99.998 %) were filled into a crucible and closed with a lid of α-BN (Henze Boron Nitride Products AG, Kempten, Germany).
The compounds were compressed and heated by a multi-anvil device based on a Walker-type module (from the company Voggenreiter, Mainleus, Germany) consisting of an 18/11-assembly, which was surrounded by eight tungsten carbide cubes (Ceratizit Austria GmbH, Reutte, Austria). A detailed description can be found in Refs. [11–13]. For the synthesis of DyGaO3, a pressure of 8.5 GPa was necessary and adjusted within 4 h. Holding the pressure, the sample was heated up to 1350 °C in 15 min. The temperature was maintained for 18 min before the heating was down-regulated to 650 °C in a period of 25 min. After that, the heater was turned off so the sample could cool down to room temperature, and simultaneously, the decompression process started, which lasted 11 h 30 min.
After the synthesis, we tried to separate the product from the boron nitride crucible and obtained a grey sparkling, hard, crystalline substance, which was insensitive to air and light.
In experiments with lower temperatures and less pressure, leftovers of the reactant Dy2O3 remained.
2.2 Synthesis of GaBO3
The synthesis of GaBO3 single crystals was accomplished with the starting materials Ga(NO3)3·8H2O (Strem Chemicals, Kehl, Germany, 99.99 %) and H3BO3 (99.5 %, Carl Roth, Karlsruhe, Germany), which were mixed in a stoichiometric ratio of 2:1 and grounded in an agate mortar. This blend was transferred into a Pt sagger and afterwards encased with a crucible and lid of α-BN. The experimental set-up was the same as described for the synthesis of DyGaO3. The product GaBO3 was formed under extreme conditions of 8 GPa and 700 °C, which were accomplished in about 6 h. The temperature of 700 °C was held for 7 min, and afterwards, a lower temperature of 300 °C was applied in 10 min so that the crystals had time to crystallize well. After that, the heating was turned off, and consequently, the reaction mixture was quenched to room temperature. Then, the 13-h process of decompression started.
After the synthesis, the Pt capsule was freed from its surroundings and then cut open with a scalpel. At first, the product appeared jelly-like, but after a few minutes, it dried and some small, colorless, rhombic crystals appeared.
We also obtained GaBO3 during other syntheses with varied molar ratios or temperature/pressure conditions, but only the way above described led to single crystals of a measurable size.
2.3 Crystal structure analyses
The products of the reactions C-Dy2O3:β-Ga2O3 = 1:1 and Ga(NO3)3·8H2O:H3BO3 = 2:1 were analyzed via X-ray powder diffraction on a Stoe Stadi P powder diffractometer (STOE & Cie GmbH, Darmstadt, Germany) that was equipped with Ge(111)-monochromatized MoKα1 radiation (λ = 70.93 pm) working in transmission geometry.
In Fig. 1, the experimental powder diffraction pattern of DyGaO3 is compared to the theoretical pattern obtained from the single-crystal data, and obviously, it matches well. Beside the main phase DyGaO3, the experimental diffractogram exhibits two additional reflections at 2θ = 12.7 and 16.4° of a yet unidentified byproduct. A total of 73 reflections of the powder pattern were indexed and refined , leading to lattice parameters that fit quite well with those received from the single-crystal data (Table 1). The powder was examined with a polarization contrast microscope and suitable single crystals were chosen and isolated mechanically. For the collection of the single-crystal data, a Stoe IPDS-I diffractometer with MoKα1 radiation (λ = 71.073 pm) that worked at room temperature was used. Furthermore, a numerical absorption correction  was applied to the intensity data. According to the systematic reflection conditions, the space groups Pna21 (no. 33) and Pnma (no. 62) were derived. Due to the structure solution and the parameter refinement with anisotropic displacement parameters for all atoms using the SHELXS/L-97 software suite [17, 18], the space group Pnma was found to be correct. All relevant details of the data collection of orthorhombic DyGaO3 can be found in Table 1. In Tables 2–5, the positional parameters, anisotropic displacement parameters, interatomic distances, and interatomic angles of DyGaO3 are listed.
In Fig. 2, the experimental powder diffraction pattern of the product from the reaction of Ga(NO3)3·8H2O with H3BO3 is compared with the theoretical pattern of GaBO3 simulated from the single-crystal data. Except for some minor reflections in the experimental powder diffraction pattern at 12.23, 12.58, 16.15, 16.72, and 37.91° in 2θ, which may result from an unidentified byproduct, these two patterns agree very well. A total of 21 reflections could be indexed and refined, and the lattice parameters a = 457.00(4), c = 1418.6(2) pm, and V = 0.25657(3) nm3 were obtained from the powder data. As described above for DyGaO3, single crystals of GaBO3 were collected and examined by X-ray diffraction. The systematic reflection conditions led to the space groups R3c and R3̅c. The structure solution and the parameter refinement with anisotropic displacement parameters for all atoms (full-matrix least-squares against F2), both operated with SHELXS/L-97 [17, 18], confirmed the space group R3̅c. Table 1 provides all relevant details of the data collection of GaBO3. The positional parameters, anisotropic displacement parameters, interatomic distances, and interatomic angles of GaBO3 can be found in Tables 7–10.
Further details on the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: , http://www.fiz-karlsruhe.de/request_for_deposited_data. html) on quoting the deposition numbers CSD-429061 for DyGaO3 and CSD-429062 for GaBO3.
2.4 Vibrational spectroscopy
Confocal Raman spectra of DyGaO3 single crystals in the range of 100–4000 cm–1 were recorded with a Horiba Jobin Yvon Labram-HR 800 Raman microspectrometer. The samples were excited using the 532 nm emission line of a frequency-doubled 100 mW Nd:YAG laser and the 633 nm emission line of a 17-mW helium-neon laser under an Olympus 100× objective lens with a numerical aperture of 0.9. The size of the laser spot on the surface was approximately 1 μm in diameter. The scattered light was dispersed by an optical grating with 1800 lines mm–1 and collected by a 1024 × 256 open-electrode CCD detector. The spectral resolution, determined by measuring the Rayleigh line, was about 2 cm–1. The spectra were recorded unpolarized at ambient conditions. The accuracy of the Raman line shifts, calibrated by measuring a silicon standard, was in the order of 0.5 cm–1. Background and Raman bands were fitted by the built-in spectrometer software Labspec 5  to first- or second-order polynomial and convoluted Gaussian-Lorentzian functions, respectively. The calculation of the number and symmetry of the vibrational modes was carried out by using the crystallographic information file and the SAM program [20–23].
3 Results and discussion
3.1 Crystal structure of DyGaO3
Like other rare-earth orthogallates (RE = La, Pr, Nd, Sm–Lu ), DyGaO3 is isostructural with the orthoferrites (GdFeO3 type ). Their orthorhombic perowskite-like structure was already described by Geller et al. [24, 25], Marezio et al. , and Will and Eberspächer . In 1967, Marezio et al.  first obtained the compound DyGaO3 and published the lattice parameters derived from powder data. Geller et al.  followed 7 years later with a publication of their own lattice parameters of DyGaO3, also derived from powder data. Our lattice parameters of DyGaO3 that we obtained from powder data (a = 552.88(2), b = 755.15(3), c = 528.07(2) pm) and single-crystal data (a = 552.6(2), b = 754.5(2), c = 527.7(2) pm) correspond well with those from Marezio et al. and Geller et al. A comparison of these lattice parameters can be found in Table 6.
DyGaO3 crystallizes in the orthorhombic space group Pnma. Like in cubic perovskite structures, the oxygen and rare-earth ions form a face-centered structure, in which the Ga3+ ions occupy every fourth octahedrally coordinated position. It is a distorted network of corner sharing octahedra with Ga3+ in the center accommodating eightfold coordinated cations Dy3+. A representative view of the structure is given in Fig. 3. Unlike cubic perovskites, the GaO6 octahedra are distorted along the c axis and tilted against each other in an average Ga–O–Ga angle of 146.6° instead of 180°. Additional interatomic angles can be found in Table 5. The lengths of the bonds between Ga and O range from 197.0(2) to 201.0(4) pm, with an average length of 198.6 pm (Table 4). Compared to other published values of octahedrally coordinated Ga atoms [7, 28], they assort well. The Dy–O bond lengths are in the ranges from 225.4(7) to 230.5(7) pm for the first four oxygen atoms and from 250.7(5) to 264.6(5) for the second four. The average length of 242.6 pm is in good accordance to values found in the literature [29, 30].
3.2 Vibrational Spectroscopy of DyGaO3
From the selection rules of factor group D2h (space group Pnma), a total number of 57 optical modes are predicted for orthorhombic DyGaO3 with the following irreducible representations: Γopt = 7Ag + 8Au + 5B1g + 9B1u + 7B2g + 7B2u + 5B3g + 9B3u. These calculations for the orthorhombic structure show that 24 modes (7Ag + 5B1g + 7B2g + 5B3g) are Raman-active while 25 modes are IR-active (9 B1u + 7B2u + 9B3u). In addition, three modes are acoustic (B1u + B2u + B3u) and eight modes are inactive (8 Au). As a result, only nondegenerated modes are expected. The Raman spectrum of DyGaO3 (Fig. 4), excited with the 532 nm emission line of a frequency-doubled Nd:YAG laser, exhibits strong bands at 504, 490, 411, 265, and 113 cm–1, medium modes at 394, 345, 327, 301, and 159 cm–1, and weak bands at 372, 143, and 110 cm–1. The intensities of the Raman bands depend on the orientation of the crystals. In contrast to the Raman spectrum of NdGaO3 , the high-wavenumber A1g mode (469 cm–1) is shifted to higher wavenumbers (504 cm–1), which can be attributed to the lattice contraction caused by the smaller Dy3+ ions. According to Iliev et al. , the vibrational modes of orthorhombic perovskites can be assigned to out-of-phase and in-phase stretching and bending vibrations, involving mixed vibrations of Dy (in our case) and O atoms (along the b axis or in the ac planes), as well as rotation of the GaO6 octahedra. The Ga atoms generate no Raman-active vibrations due to the local symmetry.
3.3 Crystal structure of GaBO3
In the ternary system Ga–B–O, two compositions are known so far: trigonal GaBO3 and monoclinic Ga4B2O9 . For GaBO3, which was first described in 1973 , as well as for Ga4B2O9 (discovered in 2010 ), only powder data can be found in the literature. In 1973, Bither and Young  published the lattice parameters of GaBO3 (a = 456.8, c = 1418.2 pm, V = 0.2563 nm3), which crystallizes in the calcite structure type. The lattice parameters obtained from our single crystals (a = 457.10(6), c = 1419.2(3) pm, V = 0.25681(7) nm3) correspond to these data. Trigonal GaBO3 crystallizes with six formula units per cell, isostructural to CaCO3 in the calcite-structure type (space group R3̅c) (Fig. 5).
In the distorted GaO6 octahedra, the O–Ga–O angles between the axial O atoms are exactly 180°, whereas the angles between the equatorial O atoms are distorted (91.7 and 88.3°), as in DyGaO3. A complete list of angles in GaBO3 can be found in Table 10. The Ga–O bond lengths are nearly the same as in DyGaO3 (199.02(7) pm in GaBO3 and 198.6 pm in DyGaO3). The BO3 groups are perfectly planar with O–B–O angles of 120°, and the B–O bond lengths of 138.0(2) pm also match well with values from the literature for planar BO3 groups . Via corner sharing, the GaO6 octahedra form layers, which are further connected through the planar BO3 groups that share each O atom with one different octahedron (Fig. 6). Examined under the symmetry of a hexagonal prism, a structure of alternating Ga3+ and BO33– layers can be recognized (Fig. 7).
This work is dedicated to the successful high-pressure/high-temperature syntheses of orthorhombic DyGaO3 and trigonal GaBO3, which were obtained under the conditions of 8.5 GPa/1350 °C and 8 GPa/700 °C, respectively. Both compounds have already been known in literature; however, we were the first to obtain single crystals leading to data with a higher accuracy. Raman-spectroscopic examinations were performed on DyGaO3.
Special thanks go to Dr. Gunter Heymann and Dr. Klaus Wurst (both from the University of Innsbruck) for the recording of the single-crystal data sets.
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Published Online: 2015-03-21
Published in Print: 2015-04-01