Flux Growth and Crystal Structure Refinement of Calcite Type Borate Gabo 3

A single crystal of gallium borate, GaBO3, 4 × 4 × 0.2 mm 3 in size has been grown by spontaneous crystallization with a molten flux based on a Bi2O3-3B2O3 solvent. From single crystal X-ray diffraction measurement, GaBO3 was found to crystallize in the trigonal calcite type, space group R-3c, with cell dimensions a = 4.56590(10) and c = 14.1764(4) Å, Z = 6. Layers of distorted [GaO6] octahedra are interleaved by layers of triangular planar [BO3] unites. The transmission spectrum on a single crystal indicated that the band gap of GaBO3 is 3.62 eV.


Introduction
Calcium carbonate, CaCO3, exists in three different polymorphs: The calcite, vaterite, and aragonite structures [1].Triangle planar BO3 3− groups can replace CO3 2− to form metal orthoborates with the nominal formula A III BO3 which have been determined to be isostructural with different forms of CaCO3.Usually, small cations of A 3+ lead to the crystallization of A III BO3 in a calcite type structure which belongs to the trigonal space group R-3c.The A 3+ caions occupy octahedral positions and can be substituted by Al 3+ , In 3+ , Fe 3+ and Sc 3+ [2][3][4][5][6].These compounds have attracted attention because of their OPEN ACCESS potential applications as photoluminescence materials, laser media, scintillating materials and magnetic materials [7,8].
Gallium borate, GaBO3, has been well studied with respect to luminescence properties [9][10][11], thermal behavior [12,13] and thermochemistry [14,15] in phase equilibria in the Ga2O3-B2O3 system.Gallium borate melts incongruently, decomposes to β-Ga2O3 above 918 °C, and crystallizes in the calcite-type structure with the unit cell a = 4.568 Å and c = 14.182Å by powder X-ray diffraction [16].Single crystal of calcite-type borates usually are grown by the hydrothermal method [4], high pressure solid state reaction [16] or in a flux.The B2O3 [13], Li2O-B2O3 [5,6], B2O3-PbO-PbF2 [17] systems have been proved to be suitable flux for growing calcite-type borates.Recently, Vitzthum et al. have also reported the structure of GaBO3 determined from crystals synthesized under high pressure [18].We report a flux-based crystal growth procedure under ambient pressures to grow crystals up to 4 × 4 × 0.2 mm 3 in size.In this contribution, the flux-based crystal growth procedure, crystal structure, and optical properties of GaBO3 are reported.

Crystal Growth
According to a previous study [12] of the thermochemistry of GaBO3 and phase equilibria in the Ga2O3-B2O3 system, the compound GaBO3 melts incongruently and decomposes to β-Ga2O3 above 918 °C.Thus, a flux must be used to grow single crystals.Here, transparent and light yellowish GaBO3 crystals have been grown by spontaneous crystallization in a molten flux based on the Bi2O3-3B2O3 solvent, which has a low melting temperature (708 °C) [19] and good solubility for the calcite type borates [2].There are two key experimental considerations in order to obtain phase pure GaBO3: (1) The ratio of GaBO3 and Bi2O3-3B2O3; and (2) Annealing procedure.Without enough flux or annealing procedure, single crystals of β-Ga2O3 will be obtained as a secondary phase.Simultaneously, too much flux will lead to the formation of a glass because of the high viscosity of B2O3.As shown in Figure 1, the GaBO3 crystals have the typical morphology of the calcite type showing forms of well-formed hexagonal plates elongated along the c-axis, with sizes up to 4 × 4 × 0.2 mm 3 .They are chemically stable with respect to hot water and strong acid.

Structural Analysis
GaBO3 is isostructural with the mineral calcite CaCO3 and other calcite type borates, such as AlBO3, InBO3, FeBO3 and ScBO3.The cell dimensions of GaBO3 (a = 4.56590 (10) and c = 14.1764(4)Å) are larger than AlBO3 (a = 4.4638(3) and c = 13.745(1)Å) [4] and smaller than InBO3 (a = 4.8217( 8) and c = 15.438(1)Å) [6].The structure of GaBO3 is illustrated in Figure 2. GaBO3 crystallizes in the centrosymmetric trigonal space group R-3c.In the asymmetric unit of GaBO3, Ga, B, and O occupy only one crystallographically unique positions, respectively.The bond length and bond angles are listed in Table 1.It is worthwhile to note that our crystal structure (cell constants, bond distances and angles) agrees very closely with those reported by Vitzthum et al. [18].As shown in Figure 4, the powder X-ray diffraction patterns of GaBO3, as-grown crystals and the theoretical simulations from single crystal structures match each other very well.The differences in peak intensity for the same crystallographic index between the two patterns are believed to be caused by the preferred orientation of the powder samples.The powder patterns also confirmed the presence of small amount of β-Ga2O3 as a secondary phase.The result of ICP elemental analysis of GaBO3 is calculated on the basis of one B atom and three O atoms in each formula unit.The result of Ga0.94BO3 is consistent with the compositions determined from the single crystal X-ray analysis.

Optical Properties
To confirm the band gap of GaBO3, transmittance (T) and reflectance (R) spectra were recorded from single crystals.From T and R, the absorption coefficient (α) can be determined as a function of incident photon energy.As shown in Figure 5, a linear fit to α 2 versus energy gives a band gap 3.62 eV for GaBO3 crystal.

Growth
Single crystals of GaBO3 were grown from a high temperature solution by using Bi2O3-3B2O3 as a flux.This solution was prepared in a platinum crucible with an Al2O3 lid by melting a reagent-grade mixture of Bi2O3 (99.999%,Alfa-Aesar, Ward Hill, MA, USA), Ga2O3 (99.99%,Alfa-Aesar) and B2O3 (99.999%,Alfa-Aesar) in a molar ratio of Ga2O3:Bi2O3:B2O3 = 3:4:15.The mixture was heated in a programmable temperature electric furnace at 1000 °C, and held for one day until the melt became transparent and clear.The homogenized melt solution was then cooled rapidly (50 °C/h) to the initial temperature of crystallization 850 °C, and then cooled slowly to the final crystallization temperature 700 °C at the rate of 3 °C/h.After annealing for three days at 700 °C, the melt was allowed to cool to room temperature by turning off the power of the furnace.The flux attached to the crystal was readily dissolved in nitric acid and hot water.

Elemental Analysis
Elemental analysis of the crystals was performed using a Jobin Yvon Ultima2 inductively coupled plasma optical emission spectrometer (ICP-OES) with Sepex Certiprep standards.The crystal samples were dissolved in a mixture of nitric acid (2 mL), phosphoric acid (3 mL) and hydrochloric acid (5 mL) by microwave digestion at 220 °C for 2 h.

X-ray Measurements
X-ray diffraction patterns of polycrystalline materials were obtained on a Rigaku Ulitma powder X-ray diffractometer (Rigaku Americas, Woodlands, TX, USA) by using Cu Kα radiation (λ = 1.540598Å) at room temperature in the angular range of 2θ = 5°-65° with a scan step width of 0.05° and a dwell step of 2 s.
Single crystal X-ray diffraction data were collected at 100 K on a Bruker Kappa APEX II CCD diffractometer (Bruker AXS Inc., Madison, WI, USA) with monochromatic Mo Kα radiation (λ = 0.71073 Å).A transparent crystal block was mounted on a glass fiber with epoxy for structure determination.The data were integrated using the SAINT program (Bruker AXS Inc., Madison, WI, USA).Absorption corrections based on the Multi-scan technique were applied with SADABS (Bruker AXS Inc., Madison, WI, USA).The structure was solved by direct methods using SHELXS-97 [20] and then refined by full-matrix least-squares refinement on F 2 with SHELXL-97 [20] found in the software suite WinGX v2013.3[21].The structure was verified using ADDSYM algorithm from the program PLATON [22], and no higher symmetries were found.Relevant crystallographic data and details of the experimental conditions are summarized in Table 2. Atomic coordinates and isotropic displacement coefficients are listed in Table 3.The Cif document could be found in supplementary information.

Optical Measurements
The optical transmission and reflection spectra of single crystal samples were measured with a PerkinElmer Lambda 1050 UV/Vis/NIR spectrometer over the range of 180-860 nm with a photomultiplier tube (PMT).In the transmission configuration, the transmittance T, reflectivity R, and absorption coefficient α are related by the expression where d is the thickness of the sample (d = 0.2 mm).The reflectivity was obtained by normalizing the reflectance of the sample to that of a silicon reference, which has a reflectivity of 0.3 at the wavelength regime > 1300 nm.The band gap energy is determined by plotting the square of absorption coefficient, α 2 , versus photon energy.Extrapolating the linear part of the curve to zero and finding the point of interception with the E axis gives the corresponding band gap energy.

Conclusions
Transparent and yellowish calcite type borate GaBO3 crystals have been grown using Bi2O3-B2O3 as a flux by spontaneous crystallization for the first time.The crystal structure of GaBO3 is refined by single crystal X-ray diffraction.It crystallizes in space group R-3c with cell dimensions a = 4.56590 (10) and c = 14.1764(4)Å.The transmission spectrum results indicated that band gap of GaBO3 compound is 3.62 eV.
The B atoms are coordinated to three O atoms to form planar [BO3] triangles with B-O bond lengths 1.3789(5) Å and O-B-O bond angles 120°.The Ga atoms are bound to six O atoms to form distorted [GaO6] octahedra with Ga-O bond lengths 1.9875(2) Å and O-Ga-O bond angles from 88.281(8)° to 180.00(3)°.Each O atom is threefold coordinated with two Ga atoms and one B atom.The compound adopts the classical calcite structure type with Ga-centered distorted octahedra connected by sharing vertices with the isolated [BO3] triangles that extend parallel to the ab plane.The [BO3] borate groups are distributed in layers so that the [BO3] triangles present reversed orientations in alternating layers, while each [GaO6] octahedron share only vertices with other six [GaO6] octahedra (Figure 3), three from the upper layer and three from the lower layer, resulting in the final 3D framework.

Figure 3 .
Figure 3.The connection of the polyhedra in the structure of GaBO3 shown along the c-axis.

Figure 4 .
Figure 4. X-ray powder diffraction patterns of (a) simulation results, and (b) crystal sample.

Figure 5 .
Figure 5. Square of absorption coefficient vs. photon energy of a GaBO3 sample.The optical band gap of the sample was estimated to be 3.62 eV.

Table 2 .
Crystal data and structure refinement for GaBO3.

Table 3 .
Atomic positions and isotropic displacement factors for GaBO3.