High-pressure synthesis and crystal structure of α-Y₂B₄O₉

2.1 Crystal structure

The yttrium borate α-Y₂B₄O₉ crystallizes in the monoclinic space group C2/c (no. 15) with the lattice parameters a=25.084(2), b=4.3913(2), c=24.726(2) Å, and β=99.97(1)° (Table 1). The unit cell (V=2682.4(2) ų) contains Z=20 formula units. The asymmetric unit comprises 38 atoms (5×Y, 10×B, and 23×O), all located on Wyckoff positions 8f except for O6, which resides on Wyckoff site 4e.

α-Y₂B₄O₉ exhibits a crystal structure isotypic to that of α-Ln₂B₄O₉ (Ln=Sm–Ho) [5], [6]. The fundamental building block (FBB; see Fig. 1) consists of two edge-sharing tetrahedra in the center, which are surrounded by corner- sharing tetrahedra. Together with the edge-sharing tetrahedra, the corner-linked tetrahedra form five- and six-membered rings. Furthermore, they form three-membered rings and are linked to single tetrahedra, which are not part of any rings within the FBB. Within the crystallographic ac plane, the FBBs are interconnected by these single tetrahedra and, additionally, by one tetrahedron of each five-membered ring. The linkage of the FBBs in all three dimensions causes the formation of further rings of various sizes (Fig. 2). An analysis of the cycle class sequence is given in [5].

Fig. 1:

Fundamental building block (FBB) of α-Y₂B₄O₉. Edge-sharing tetrahedra are drawn in red, corner-linked tetrahedra in dark blue, and tetrahedra interconnecting the FBBs in light blue.

Fig. 2:

Crystal structure of α-Y₂B₄O₉ viewed along [01̅0]. The color code of the tetrahedra is identical to that of Fig. 1.

The B–O distances and O–B–O angles vary between 1.382(4) and 1.576(4) Å and 94.0(2)–119.5(3)°, respectively (Tables 2 and 3 ). In accordance with α-Ln₂B₄O₉ (Ln=Sm–Ho), the B8 tetrahedron (i.e. the [BO₄] tetrahedron with B8 as central atom) shows the strongest distortion. The average B–O bond lengths and O–B–O angles lie in the range of 1.462–1.496 Å and 109.2–109.4° and are thus in line with the values reported by Zobetz (1.476(35) Å/ 109.44(2.78)°) [7].

The borate framework features channels which host the yttrium atoms (Fig. 2). The five different yttrium sites show coordination numbers (CN) between 8 and 11 with Y–O bond lengths in the range of 2.203(2)–3.095(2) Å (Table 4). In comparison to α-Ln₂B₄O₉ (Ln=Sm–Ho), these values come closest to the corresponding Ho–O distances (2.212(7)–3.084(7) Å) because of the nearly identical ionic radii of Y³⁺ (r(Y³⁺)=1.215 Å for CN 9) and Ho³⁺ (r(Ho³⁺)=1.212 Å for CN 9) [8].

Bond valence sums (BVS) were calculated according to the bond-length/bond-strength (BLBS) concept (Table 5) [9], [10]. The calculated values correspond to the expected formal ionic charges within the limits of this concept. The slightly decreased BVS of Y5 (2.62) and O21 (−1.64) were also observed for the corresponding sites in α-Ln₂B₄O₉ (Ln=Eu, Gd, Tb) [5].

Table 6 shows a comparison of the lattice parameters and cell volumes of α-Y₂B₄O₉ and α-Ln₂B₄O₉ (Ln=Sm–Ho). The positional parameters are listed in Table 7.

Further details on the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +497247-808-666; e-mail: crysdata@fiz-karlsruhe.de) on quoting the deposition number CSD-433610 for α-Y₂B₄O₉.

2.2 X-ray powder diffraction

A Rietveld refinement was carried out between 2.0 and 32.0° in 2θ using the structural model obtained through single-crystal diffraction. Apart from an amorphous phase, the sample consisted only of α-Y₂B₄O₉. More details are given in Table 1. However, the obtained results can only be considered as approximate values because of a high roughness of the measured curve, which is probably due to a moderate crystallinity of the sample and the fluorescence of yttrium caused by molybdenum radiation.

2.3 Vibrational spectroscopy

The single-crystal IR and Raman spectra are displayed in Fig. 3. Both spectra exhibit some unexpected but only minor peaks at wavenumbers >1400 cm⁻¹. The origin of these peaks is uncertain, but we assume they can be ascribed to surface impurities or inclusions in the crystals.

Fig. 3:

Single-crystal IR (red) and Raman (black) spectrum of α-Y₂B₄O₉.

DFT calculations for (NH₄)YB₈O₁₄ [4] showed that stretching vibrations of the [BO₄] tetrahedra give rise to IR and Raman modes in the wavenumber range ~1300–850 cm⁻¹, while the corresponding bending vibrations can be observed between ~1100 and 300 cm⁻¹. Complex B–O–Y vibrations occur from ~800 through to 700 cm⁻¹, whereas vibrational modes in the wavenumber ranges ~500–250 cm⁻¹ and ~400–100 cm⁻¹ are associated with Y–O stretching and bending modes, respectively.

4.1 Synthesis

A 1:4 molar mixture of Y₂O₃ (Sigma-Aldrich, USA, 99.99%) and H₃BO₃ (Carl Roth, Germany, ≥99.8%) was weighed and ground in an agate mortar under ambient conditions. The mixture was then filled into a crucible, closed with a lid (both made of α-BN; Henze Boron Nitride Products AG, Germany), and placed in an 18/11 assembly. A Walker-type multi-anvil module and a 1000 t press (both Max Voggenreiter GmbH, Germany) were employed to compress the sample. More details regarding the experimental setup can be found in the literature [11], [12], [13].

The sample was compressed to ~12.3 GPa within 334 min and then heated to ~1020°C over 10 min. This temperature was maintained for 30 min before the sample was cooled down to 200°C in 10 min and subsequently quenched to room temperature by switching off the heating. Afterwards, the pressure was released within 1000 min. The colorless reaction product was mechanically separated from the BN crucible.

4.2 Single-crystal structure analysis

The single-crystal intensity data were acquired using a Bruker D8 Quest Kappa diffractometer equipped with a Photon 100 detector. Monochromatized MoKα radiation (λ=0.7107 Å) was generated by an Incoatec microfocus X-ray tube (power settings: 50 kV and 1 mA) and a multilayer optic. Sadabs [14] was employed to correct the data set for absorption effects. The structure was solved using Shelxt (version 2014/4) [15] and further refined with Shelxl (version 2016/6) [16] as implemented in the WinGX suite (version 2013/3) [17]. Atom labels were chosen according to α-Gd₂B₄O₉ [5] to allow for an easier comparison of the structures.

4.3 X-ray powder diffraction

X-ray powder diffraction was carried out on a STOE Stadi P powder diffractometer in transmission geometry. The flat sample was irradiated with Ge(111)-monochromatized MoK_α1 radiation (λ=0.7093 Å). The diffracted radiation was detected by means of a Dectris Mythen 1 K detector. Due to the strong fluorescence of yttrium by molybdenum radiation, a horizontal background line was subtracted from the original diffractogram before the Rietveld refinement was accomplished using Topas 4.2 [18].

4.4 Vibrational spectroscopy

The single-crystal Raman measurement was carried out on a Horiba Jobin Yvon Labram-HR-800 spectrometer. A 532 nm frequency-doubled Nd:YAG laser and a 50× objective lens were used to excite an area of approximately 5 μm in diameter. The scattered light was dispersed by an optical grating with 1800 lines mm⁻¹ and collected by a 1024×256 Andor CCD detector in the range of 100–3700 cm⁻¹. Two spectra with an acquisition time of 100 s each were averaged employing Labspec [19] (version 5.93.20). The background was approximated via a fifth order polynomial line segment function and subtracted from the original spectrum.

A Bruker Vertex 70 FT-IR spectrometer (spectral resolution 4 cm⁻¹) equipped with a Globar mid-IR source and a KBr beam splitter was employed to acquire the absorption spectrum of a single crystal in the range of 600–4000 cm⁻¹. The spectrometer is coupled to a Bruker Hyperion 3000 microscope, which is equipped with a mercury cadmium telluride (MCT) detector. For the measurement in transmittance mode, the sample was placed on a BaF₂ window. One-hundred sixty scans of the sample were recorded using a 15× IR objective. Data processing was accomplished employing Opus [20].