Synthesis and crystal structure of the rare earth borogermanate EuGeBO₅

2.1 Synthesis and analyses

All starting materials were used as-received without further purification. A powdery sample of the title compound was obtained by a high-temperature solid-state reaction with KI as a flux. The employment of KI can help reduce reactive temperature and make the reaction happened more uniformly because of its low melting point and reactive inertness in such system. The starting materials were H₃BO₃ (99.9%, Aladdin, China), GeO₂ (99.9%, Aladdin, China), and EuO (99.9%, Aladdin, China). The sample had a total mass of 500 mg with an addition of 400 mg KI (99%, Aladdin, China), and the molar ratios of Eu:Ge:B were 1:1:1. The mixture of starting materials was ground into a fine powder in an agate mortar and pressed into a pellet, followed by loading into a corundum crucible. The sample was placed into a muffle furnace, slowly heated to 1223 K in 24 h, and maintained for 7 days, finally cooled down to 573 K at a speed of 5 K h⁻¹ and powered off. The colorless single crystals of the title compound were stable in moisture and air. They were washed using ethanol and water under ultrasound irradiation, and the purity was confirmed by powder X-ray diffraction (PXRD). The PXRD pattern was collected with a Bruker D8 advance diffractometer (Germany) at 40 kV and 100 mA with CuKα radiation (λ=1.5406 Å) with a scan speed of 5° min⁻¹ at room temperature. The simulated pattern was produced from single-crystal data with the program Mercury (v2.3) provided by the Cambridge Crystallographic Data Center. The PXRD pattern (Fig. 1) corresponds well with the simulated one, indicating a single phase sample. A semiquantitative microscopic elemental analysis on the as-prepared single crystals was performed on a field-emission scanning electron microscope (Zeiss Supra 55, Germany) equipped with an energy-dispersive X-ray spectroscope (Bruker Quantax, Germany), which confirmed the presence of Eu, Ge, and O with the approximate ratios of 1:1:5, and no other elements were detected (boron could not be detected using energy-dispersive X-ray spectroscope because it is too light).

Fig. 1:

PXRD pattern of EuGeBO₅. The red and black patterns are the experimental and simulated ones, respectively.

2.2 Structure determination

The intensity data set of EuGeBO₅ was collected on Bruker D8 QUEST X-ray diffractometer with graphite-monochromatized MoKα radiation (λ=0.71073 Å). Its structure was solved by direct methods and refined by full-matrix least-squares techniques on F² with anisotropic displacement parameters for all atoms. All the calculations were performed with the Shelxtl-97 crystallographic software package [17]. The final refinement included a secondary extinction correction. It has to be mentioned that atoms B1, O4, and O5 were nonpositive definite in the first refinement, so the command ISOR was applied to restrict them altogether with 18 restraints. The crystallographic data, atomic coordinates, equivalent isotropic displacement parameters, and bond lengths are listed in Tables 1–3, respectively.

Further details of the crystal structure investigation may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-mail: crysdata@fiz-karlsruhe.de) on quoting the deposition number CSD-430764.

3.1 Synthesis

The title compound was synthesized in a corundum crucible, not in a platinum crucible as used for the synthesis of many other REMM′O₅ compounds [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. First, a powdered sample of EuGeBO₅ was obtained through the reaction EuO+2GeO₂+2H₃BO₃ targeting to synthesize EuGe₂B₂O₈. No single crystal could be picked up in this synthesis to collect the single-crystal data. Fortunately, the main product of the reaction was the monoclinic phase EuGeBO₅ as the PXRD pattern is very similar to that of NdGeBO₅. Until now, only powder data for EuGeBO₅ had been available [16]. We made efforts to prepare single crystals of EuGeBO₅ to obtain its structural data in more detail. As described in the Experimental section, the single crystals of EuGeBO₅ were finally obtained using KI as a flux and reducing the reactive temperature to 1223 K, where the flux KI could be washed away easily with water. Here, it has to be mentioned that the oxidation state change of Eu from divalent to trivalent was probably induced by oxygen in the air as the reactive system was exposed to air. Our synthetic method is different from that used for the syntheses of the other REMM′O₅ compounds. The method should also be advantageous for the synthesis of other REMM′O₅ compounds with the monoclinic structure type.

3.2 Crystal structure

EuGeBO₅ crystallizes in the monoclinic space group P2₁/c, isotypic with the monoclinic phase of NdGeBO₅ [18]. Rulmont and Tarte reported its unit cell parameters by indexing the PXRD data and refinement using a least-squares program. They reported the values a=9.9960(10), b=7.5260(14), c=4.8874(9) Å, and β=91.65(1)° [16]. By comparison, our parameters obtained from single-crystal diffraction are a=4.8860(5), b=7.5229(8), c=9.9587(10) Å, and β=91.709(3)°. The reinvestigation of monoclinic EuGeBO₅ using single-crystal data resulted in a more precise structure determination.

There are one Eu, one Ge, one B, and five O atoms in the asymmetric unit of the EuGeBO₅ structure. Ge and B atoms are coordinated with four O atoms to form GeO₄ and BO₄ tetrahedra, respectively, and each Eu atom has eight neighboring O atoms (Fig. 2). All three BO₄, GeO₄, and EuO₈ units share corners with each other.

Fig. 2:

Coordination geometry of EuGeBO₅.

The structure of EuGeBO₅ can be considered as a pseudo-layer structure, where the layers are constructed from BO₄ and GeO₄ tetrahedra, and Eu ions occupy the cavities between the layers (Fig. 3). From Fig. 4, it can be observed that the layers extend parallel to the ab plane. There are no GeO₄ tetrahedra or BO₄ tetrahedra interconnected with themselves, but GeO₄ and BO₄ tetrahedra alternate to form a layer that can be regarded as polyanionic group (GeBO₅)³⁻.

Fig. 3:

Crystal structure of EuGeBO₅. For the sake of clarity, Eu–O bonds were omitted. Dark and light blue polyhedra represent (BO₃)³⁻ and (GeO₄)⁴⁻ units, respectively.

Fig. 4:

(GeBO₅)³⁻ polyanion layer in the ab plane constructed by (BO₄)⁵⁻ and (GeO₄)⁴⁻ polyhedra. Dark and light blue polyhedra represent (BO₄)⁵⁻ and (GeO₄)⁴⁻ units, respectively.

The Ge–O and B–O distances in EuGeBO₅ are in the range of 1.694(6)–1.753(5) and 1.409(10) to 1.599(10) Å, respectively, which are consistent with the distances of Ge–O bonds found in SrGe₂B₂O₈ [19] and the B–O bonds found in Cs₂B₄SiO₉ [20]. The Eu–O bond lengths of 2.313(6) to 2.534(6) Å are in agreement with those in Eu₂GeB₂O₈ [7].

Although the crystal structure of EuGeBO₅ as we described above is relatively simple, the REMM′O₅ (M=Si, Ge; M′=B, Al) series demonstrates various structure types with different types of (MM′O₅)³⁻ polyanions (Fig. 5). It is interesting to summarize and compare these polyanion structures. Except for the monoclinic phase, there are four other types of structures for the REMM′O₅ series reported, namely, the orthorhombic Pbam and P2₁2₁2₁ phases and the trigonal P3₁ and P3₁21 phases. Among them, the space groups P2₁2₁2₁, P3₁, and P3₁21 are suitable for homochiral structures, which make these REMM′O₅ compounds potential second-order nonlinear optical materials.

Fig. 5:

(MM′O₅)³⁻ (M=Si, Ge; M′=B, Al) polyanions in the structures of REMM′O₅ compounds. M, M′, and O atoms are represented as turquoise, blue, and yellow spheres, respectively.

The (MM′O₅)³⁻ polyanions in PrGeBO₅ (space group P3₁21) [5], LaGeBO₅ (space group P3₁) [21], LaSiAlO₅ (space group P2₁2₁2₁) [22], and YGeAlO₅ (space group Pbam) [23] are totally different. In PrGeBO₅, BO₄ tetrahedra are isolated and GeO₄ tetrahedra share edges to form a chain along the c direction. In LaGeBO₅, the polyanion is constructed from BO₄ tetrahedra and a chain is formed with GeO₄ tetrahedra through corner sharing. In LaSiAlO₅, SiO₄ tetrahedra are isolated and AlO₄ tetrahedra share corners to construct chains along the a direction. Differently, Ge and Al in YGeAlO₅ are five- and six-coordinated with O atoms to form tetragonal pyramids and octahedra, respectively. AlO₆ octahedra share edges to form a chain along the c direction, and two GeO₅ pyramids always connect to form dimers to link neighboring Al atoms through corner sharing in the ab plane. In conclusion, the (MM′O₅)³⁻ polyanions have different dimensionalities in the structures of LaGeBO₅ (space group P3₁), EuGeBO₅ (space group P2₁/a), and YGeAlO₅ (space group Pbam), respectively. In PrGeBO₅ (space group P3₁21) and LaSiAlO₅ (space group P2₁2₁2₁), the BO₄ and SiO₄ tetrahedra are isolated.