Gd₄(BO₂)O₅F – a gadolinium borate fluoride oxide comprising a linear BO₂ moiety

2.1 Crystal structure determination

The diffraction pattern suggested a hexagonal unit cell (a = 381, c = 1573 pm) with reflection conditions pointing towards space group P6₃/mmc (no. 194). Since the structure model solved and refined in this space group comprised exactly semi-occupied boron positions and further disorder, a careful inspection of the diffraction images was performed which revealed very weak superstructure reflections. Taking into account these additional reflections a primitive orthorhombic unit cell with an approximate ratio b/a ≈ 1.733 ≈ 3 was indexed (a = 381.42(7), b = 660.88(11), c = 1574.6(4) pm). This special b/a ratio yielded by threefold partially merohedrally twinning a drilling dissembling the above mentioned hexagonal unit cell; the reflections breaking the C centring of the orthohexagonal unit cell are the aforementioned weak superstructure reflections. The final twin refinement was performed in space group Pmmn (no. 59, a = 1574.6(4) pm, b = 381.42(7) pm, c = 660.88(11) pm) which delivered the most stable and reliable structure refinement after a careful check for the presence or absence of an inversion centre by refining and analysing the structure model in respective space groups like e.g. Pmn2₁. Relevant data of the crystal structure determination are listed in Table 1, selected interatomic distances and bond angles are given in Table 2. The assignment of oxygen and fluorine was decided in combination with MAPLE calculations, interatomic distances and considerations based on Pauling’s rules (see below).

2.2 Crystal structure

Nickel arsenide crystallises in space group P6₃/mmc (no. 194, ratio c/a ≈ 1.39) [18]. The initially found hexagonal unit cell features a very similar ratio c/3a ≈ 1.376 suggesting a close relationship of the title compound’s crystal structure with that of nickel arsenide. By this relationship the crystal structure of Gd₄(BO₂)O₅F and also the observed twinning as a drilling may be understood and followed using the group-subgroup scheme in the Bärnighausen formalism [19–22] shown in Fig. 1. It starts with the structural data of NiAs including subsequently occupied tetrahedral and trigonal prismatic voids on Wyckoff positions 4f and 2b. Tripling along [001] yields already the dimensions of the previously mentioned hexagonal unit cell. A suited example adopting a similar structure is InGaZn₂O₅ [23] showing linearly coordinated gallium atoms between ZnO layers, illustrated by the notation In[ZnO]₂[GaO₂]O. Compared with the title compound, i.e. [GdO]₄[BO₂]OF in this notation, the site of Ga(1) adopts that of the boron atoms, but in contrast to the title compound this site is fully occupied in InGaZn₂O₅ in P6₃/mmc. After transformation to the orthohexagonal cell in Cmcm (no. 63) three different orientations of the resulting structure are possible, presumably yielding the twinning during crystal growth. By loosing the C centring the final space group Pmmn (no. 59) is reached where the respective Ga(1) site of InGaZn₂O₅ splits and a fully occupied site B(1) is achieved.

Fig. 1:

Group–subgroup scheme in the Bärnighausen formalism [19–22] showing the symmetry relations between NiAs, InGaZn₂O₅ and Gd₄(BO₂)O₅F (at the last stage of the scheme also the calculated coordinates derived from the NiAs type are shown).

The crystal structure of Gd₄(BO₂)O₅F is composed of dense layers 2∞[OGd4/4] formed by edge-sharing [OGd₄] tetrahedra terminated by fluorine and further oxygen atoms. Between these oxygen atoms the boron atoms are part of linear BO₂⁻ anions (Fig. 2) located in trigonal prismatic voids (Fig. 3a). The bond length B–O was refined to 121.3 pm and the angle O–B–O to a value of 180(4)° in accordance with the expectation for a moiety isolectronic with CO₂ (115 pm, 180° [24]) and NO₂⁺ (108–114 pm, avg. 111 pm, 175–180° [25–27]); the sum of ionic radii cannot be calculated due to the lack of an ionic radius for linearly coordinated B³⁺. Calvo and Faggiani found a bond length of 125(2) pm in (Ca,Sr)_9+yNa_x(BO₂)_x+2y(PO₄)₆ (x+ 2y< 1) [17]. The coordination polyhedron around both gadolinium atoms shown in Fig. 3b can be described as a tricapped tetrahedron comprising coordination distances between 233 and 256 pm (Gd1) and between 227 and 248 pm (Gd2). These distances are also in agreement with the sum of the ionic radii of 238 pm [28]. Selected interatomic distances and bond angles are listed in Table 2.

Fig. 2:

Perspective view of the crystal structure of Gd₄(BO₂)O₅F approx. along [010] illustrating the layer of OGd₄ tetrahedra (shown as closed tetrahedra); Gd atoms grey, boron brown, fluorine green and oxygen red.

Fig. 3:

Local surroundings of the linear BO₂⁻ moiety and the metal ions. (a) The BO₂⁻ anion in its trigonal prismatic void; (b) coordination spheres of the gadolinium atoms (site symmetry .m. for Gd1 and Gd2) viewed along [001]; same colour code as in Fig. 1.

2.3 Electrostatic calculations

The electrostatic consistency of the structure model was proved by calculations based on the MAPLE concept (Madelung Part of Lattice Energy) [29–31]. A structure model is electrostatically consistent if the sum of MAPLE values of chemically similar compounds like Gd₂(BO₃)F₃ [13], B₂O₃ [32] and C-type Gd₂O₃ [33] deviates from the MAPLE value of the compound of interest, i.e. Gd₄(BO₂)O₅F, by less than approx. 1 %. According to our calculations, the structure model of Gd₄(BO₂)O₅F thus shows electrostatic consistency as presented in Table 3.

Based on X-ray single-crystal data an unequivocal assignment of oxygen and fluorine atoms to respective sites is difficult; therefore all reasonable models of O/F ordering on the different sites have been investigated by these calculations which have proven to give reliable assignments for the assignment of oxygen and nitrogen to their respective sites in oxonitridosilicates [34, 35]. Indeed, the calculations for Gd₄(BO₂)O₅F gave significantly different Madelung parts of lattice energy as well as striking differences of partial Madelung factors leading to the final assignment of fluorine solely to site F3 which accordingly is occupied 1:1 by oxygen and fluorine. This site is also the most probable one with regard to Pauling’s rules as it provides the smallest possible coordination number the fluorine atoms may experience in Gd₄(BO₂)O₅F.

2.4 Raman spectroscopy

Figure 4 shows the relevant region of the Raman spectrum recorded on a single crystal of Gd₄(BO₂)O₅F. It comprises a single intense peak at 1364 cm⁻¹. The only Raman-active vibrational mode in linear three-atomic molecules Y–X–Y (atom types X and Y) is the symmetric stretching mode. In cationic NO₂⁺ this vibration was recorded at 1396 cm⁻¹ [36], in neutral CO₂ at 1388 cm⁻¹ [36]; for the anionic species BN₂³⁻ and BC₂⁵⁻ this vibration was excited in the range 1028 and 1107 cm⁻¹ [37] and at 1041 cm⁻¹ [16], respectively. Considering the obvious trend that with decreasing atomic mass of Y and increasing negative charge the excitation energies also decrease, the Raman peak at 1364 cm⁻¹ is in line with observations and theoretical considerations on 16-electron molecular moieties [38]. Moreover, very weak peaks were observed at 805 and 2083 cm⁻¹, which presumably are due to the respective IR active δ and ν_as modes.

Fig. 4:

Relevant region of a Raman spectrum recorded of a single-crystal of Gd₄(BO₂)O₅F showing the Raman-active vibration.

3.1 Synthesis of Gd₄(BO₂)O₅F

Under argon, a homogenised mixture of Gd₂O₃ (236 mg, 0.651 mmol, ChemPur, 99.99 %), B₂O₃ (22 mg, 0.32 mmol, ChemPur, 99.9 %) and NaF (15 mg, 0.36 mmol, ChemPur, 99.9 %) was transferred into a boron nitride crucible which was then enclosed in a sealed tantalum ampoule; the reaction was performed under a constant argon flow by applying the following temperature program: the reaction mixture was heated to 800 °C at a rate of 480 °C/h, subsequently heated further to 1500 °C at a rate of 120 °C h⁻¹, and this temperature was held for 72 h. After cooling to room temperature at a rate of 30 °C h⁻¹, some colourless crystals of Gd₄(BO₂)O₅F were obtained as a side product; by means of energy dispersive X-ray spectroscopy (EDX) Gd was detected as the only metal present in the single crystals.

3.2 X-ray structure determination

X-Ray diffraction data were collected from two carefully selected single crystals enclosed in a Lindemann tube on a Stoe IPDS 2 diffractometer using MoK_α radiation and – in the case of the HKLF4 integration – corrected numerically for absorption [39]. The crystal structure of the title compound was solved by direct methods and subsequently refined in space group Pmmn (no. 59, a = 1574.6(4), b = 381.42(7), c = 660.88(11) pm) using the Shelxtl program package [40] based on HKLF5 and HKLF4 data.

The relevant crystallographic data and further details of the X-ray data collection are summarised in Table 1. In Table 2 selected interatomic distances and bondangles are listed. Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-informationsdienste.de/en/DB/icsd/depot_anforderung.html) on quoting the deposition number CSD-429779.

3.3 Raman spectroscopy

The Raman spectrum was recorded using a spectrometer comprising a double monochromator and different excitation lasers (krypton, argon and Ti:sapphire, 90° and 180° geometry), providing excitations in the range 330–830 nm, equipped with a cryostat (15–330 K) and a heating stage for high-temperature measurements up to 1000 K.