The UV-phosphor strontium fluorooxoborate Sr[B₅O₇F₃]:Eu

2.1 Crystal structure

Sr[B₅O₇F₃] crystallises in the same structure type as its calcium counterpart, which was published as Ca₂[B₁₀O₁₄F₆] with a doubled sum formula but half the formula units per unit cell. The title compound adopts the non-centrosymmetric space group Ccm2₁. The fundamental building block (FBB) is shown in Fig. 1 and consists of two trigonal planar BO₃ units and three BO₃F tetrahedra; two tetrahedra and one BO₃ triangle form a six-membered ring. Two of those rings are condensed via a common edge and thus the FBB can be described according to Burns et al. [9] by the descriptor 2∆3□:[Φ]<∆2□>|<∆2□>|.

Fig. 1:

FBB of Sr[B₅O₇F₃], featuring the slightly bent bicyclic structure. Boron atoms in green, oxygen atoms in red, fluorine atoms in yellow.

The FBBs are connected via common O31^[2] oxygen atoms bridging two adjacent boron atoms to form layers in the (100) plane. Thus, four FBBs form an 18-membered ring, in which the strontium atoms are situated. All fluorine atoms point out of the planes (Fig. 2). The atoms O12, B2 and F2 lie on a mirror plane together with the shared edge. Table 1 summarizes selected interatomic distances and angles.

Fig. 2:

Layers of Sr[B₅O₇F₃] viewed along the [010] direction (upper picture) and the [001] direction (lower picture). Strontium atoms in grey, boron atoms in green, oxygen atoms in red, fluorine atoms in yellow.

An unusual feature of the structure is the oxygen atom O12^[3] connecting three adjacent BO₃F tetrahedra within the FBB. The distances between O12^[3] and the neighbouring boron atoms B1 and B2 are 157.4(17) and 142.6(17) pm, respectively, and thus considerably larger than usual B_□–O^[2] bonds (see caption to Table 1 for definition of B_□ and B∆) which are usually in the range from 145 to 147 pm, as observed in Sn[B₂O₃F₂] and Ba[B₄O₆F₂] [5], [6]. Such rarely found three-fold-coordinated oxygen atoms O^[3] were reported before in borates like Ni₃B₁₈O₂₈(OH)₄·H₂O or LiBa₃(OH)[B₉O₁₆][B(OH)₄] [10], [11], and in alumosilicates like Be₃Al₂Si₆O₁₈ or SiAl₂O₅ [12], [13], [14]. The average B–O bond lengths in the title compound of 137 pm (B∆–O) and 148 pm (B_□–O) agree well with the sum of the ionic radii [15]. Yet the variance of the bond lengths is slightly larger, compared with other, similar compounds. This variance, as well as the elongated bonds with the O12 atom are also reflected in the slightly larger but still uncritical deviations of the tetrahedral units from a regular tetrahedron, calculated according to Balic-Žunic and Makovicky [16], [17] to be 0.68% (B1) and 0.27% (B2).

The strontium cation is situated within the layers of the anionic substructure, in a ring built up by four B₅O₇F₃²⁻ FBBs. Its nine-fold coordination consists of six oxygen atoms of the aforementioned ring and by three fluorine atoms of the layers above and below. The coordination polyhedron can be described as a strongly distorted triangular cupola (Johnson solid J₃). The Sr–O distances range from 258 to 262 pm, which is slightly shorter than the sum of their ionic radii (267 pm) [15]. The Sr–F distances are quite different, as the different fluorine anions F1 and F2 coordinate the strontium cation from different layers. Both distances (239 pm for Sr–F1, 256 pm for Sr–F2) are shorter than the sum of their ionic radii (260 pm).

2.2 Infrared Spectroscopy

The IR spectrum of Sr[B₅O₇F₃] was recorded in the range from 4000 to 400 cm⁻¹ (Fig. 3). The inset shows the whole measured area with only very weak bands caused by traces of adhesive moisture beyond 3000 cm⁻¹. The assignments listed in Table 2 are based on the infrared spectrum of Ba[B₄O₆F₂] and confirmed by DFT calculations, as well as on other reference data [18], [19]. As expected, considering the comparable structural units (BO₃ and BO₃F), the spectra are similar. The bands around 1500–1275 cm⁻¹ and the band near 960 cm⁻¹ are caused by the asymmetric and symmetric stretching modes of the trigonal BO₃ units. According to our calculations on comparable infrared spectra [5], [6] the asymmetric and symmetric stretching modes of the B–O bonds of tetrahedral BO₃F units are found in the regimes 1100–980 and 960–850 cm⁻¹, respectively. Characteristic stretching modes of B–F bonds in fluorooxoborates are found in the regimes 1000–920 and 860–730 cm⁻¹, deformation modes of BO₃F tetrahedra typically lie around 860 cm⁻¹ for the similar Ba[B₄O₆F₂] [6]. Based on these regimes we tentatively assigned the experimentally observed modes of the title compound according to Table 2. Aside from a slight shift in wavenumbers for most bands, the most obvious difference is an additional band around 1220 cm⁻¹ in the spectrum of Sr[B₅O₇F₃]. This was also reported by Mutailipu et al. [7], but was not explained further. We suspect that this band is caused by B–O stretching vibrations including the three-fold-coordinated O12^[3] as it does not appear in Ba[B₄O₆F₂]. A B–O^[3] bond is presumably weaker than a B–O^[2] bond causing a shift to smaller wavenumbers from the standard regime. The presence of an infrared band in this range in other borates featuring three-fold-coordinated oxygen [10], [20], [21], [22] supports our tentative assignment. A respective asymmetric mode is to be expected at lower frequencies, maybe the sharp vibration around 720 cm⁻¹ might fit – but this is rather speculative as the spectrum is very busy in this regime.

Fig. 3:

IR spectrum of Sr[B₅O₇F₃]; the inset shows the complete region up to 4000 cm⁻¹.

2.3 Fluorescence spectroscopy on Sr[B₅O₇F₃]:Eu²⁺

The fluorescence spectrum of Sr[B₅O₇F₃]:Eu²⁺ (Fig. 4) features an emission with a single 5d-4f transition peaking at 372 nm with a full width at half maximum (FWHM) of 21 nm. The excitation spectrum comprises numerous peaks assigned to 4f-5d transitions. Below 370 nm, 4f-4f transitions of Eu²⁺ can be excited as well [23], [24], but they are hard to identify since the resolution of the spectrum is limited by the employed deuterium lamp. The width of all excitation peaks remains equal, even at lower temperatures, as shown in Fig. 5.

Fig. 4:

Photoluminescence spectrum of Sr[B₅O₇F₃]:Eu²⁺; emission spectrum (violet line) measured at λ_exc=239 nm, excitation spectrum (black line) measured at λ_em=372 nm.

Fig. 5:

Photoluminescence spectrum of Sr[B₅O₇F₃]:Eu²⁺; emission and excitation at liquid nitrogen temperature (blue line) and room temperature (grey line); the spectra were measured with λ_exc=239 nm and λ_em=372 nm, respectively.

Compared to the fluorescence spectrum of Ba[B₄O₆F₂]:Eu²⁺, the emission spectrum of Sr[B₅O₇F₃]:Eu²⁺ shows no sharp peak which could be attributed to a 4f-4f transition, but the emission peak of the 5d-4f transition shows the same FWHM; this is reasonable as both host structures are quite similar. Both structures comprise layered polymeric fluorooxoborate anions; the cations situated in the voids of these layers are weakly coordinated by oxygen and fluorine atoms with large interatomic distances due to rather large coordination numbers (Sr[B₅O₇F₃]: 9, Ba[B₄O₆F₂]: 13). Since these distances are significantly shorter in the title compound compared to Ba[B₄O₆F₂], it is not surprising that the emission wavelength is slightly red-shifted from 366 to 372 nm and that the 4f-4f transition cannot be observed in Sr[B₅O₇F₃]:Eu²⁺ due to efficient relaxation from the ⁶P (4f) to the 5d states. Nevertheless, the ligand field splitting and nephelauxetic effect of the 5d states in Eu²⁺ is still small enough that the emission of the d-f transition lies in the UV regime of the electromagnetic spectrum.

4.1 Synthesis of Sr(BF₄)₂

The synthesis of the starting material Sr(BF₄)₂ was achieved according to the following equation:

Three grams of SrCO₃ (20 mmol, Riedel-de Häen, 96% purity) were suspensed in 5 mL of demineralised water inside of a PVC container. 4.8 mL of HBF₄ (38 mmol, Alfa Aesar, 50% solution). The excess of SrCO₃ ensured the complete reaction of the tetrafluoroboric acid. The suspension was then filtered and the filtrate was slowly dried in vacuo, yielding phase-pure Sr(BF₄)₂.

4.2 Synthesis of Sr[B₅O₇F₃] and Sr[B₅O₇F₃]:Eu²⁺

The synthesis of Sr[B₅O₇F₃] was achieved according to the following equation:

The pre-dried educts Sr(BF₄)₂ (255.0 mg, 0.9761 mmol) and B₂O₃ (160.0 mg, 2.298 mmol, Alfa Aesar, 99.999% purity) were weighed and ground in an agate mortar in air and transferred into a BN crucible. This mixture was dried at T=105°C overnight in a compartment drier to get rid of any remaining moisture. The powder was then compressed inside the crucible which was then screwed and sealed in a silica ampoule under argon atmosphere. The sample was heated to 500°C with a rate of 50 K h⁻¹, held at that temperature for 500 h and then cooled to room temperature with a rate of 5 K h⁻¹. 263.1 mg (based on Sr[B₅O₇F₃]: 0.8469 mmol, 86.8% yield) of a slightly greyish, air and moisture insensitive powder were isolated.

For the synthesis of the doped sample, 1 mol% of the Sr(BF₄)₂ was replaced by EuF₃ (ChemPur, 99.9% purity). The mixture of dry educts was ground and pressed into a pellet inside of a glovebox and then filled into a silver crucible and sealed in a silica glass ampule under vacuum (2.5×10⁻³ mbar). The sample was heated to 500°C with a rate of 50 K h⁻¹, held at that temperature for 50 h and then cooled to 400°C in a first step and then to room temperature with the rates of 1 K h⁻¹ and 40 K h⁻¹, respectively. The product was used for fluorescence spectroscopy without any further purification.

4.3 Spectroscopic methods

IR spectra were recorded on a Bruker EQUINOX 55 FT-IR spectrometer equipped with a Platinum ATR unit in the range 4000–400 cm⁻¹ with a resolution of 4 cm⁻¹ and 32 scans. Fluorescence excitation and emission spectra were recorded on a Horiba Fluoromax-4 spectrometer, scanning a range from 200 to 800 nm.

4.4 X-ray structure determination

The structure of Sr[B₅O₇F₃] was solved from powder X-ray diffraction data. These data were collected on a Bruker D8 Advance capillary diffractometer in the range of 5.000≤2θ≤119.966°, with steps of 0.02° and an irradiation of 36 s per step. The structure was solved by Direct Methods using the program Expo2014 [33], further refinement was conducted with the program package Fullprof [34] with the graphical interface WinPLOTR [35]. During the structure refinement, all atom positions were refined without any restraints. After some refinement cycles, the z position of the strontium atom was fixed to fix the structure along [001]. The XRD pattern including the refinement is portrayed in Fig. 6, the results of the structure refinement are listed in Tables 3 and 4.

Fig. 6:

Observed (red dots) and calculated (black line) powder diffraction pattern of Sr[B₅O₇F₃], as well as the difference plot (blue line); the green vertical lines correspond to the possible Bragg-peak positions of Sr[B₅O₇F₃]; the only peak of a possible side phase (SrF₂) is labelled with a blue arrow.

Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-555; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-informationsdienste.de/en/DB/icsd/depot_anforderung.html) on quoting the deposition number CSD-1962709.