High-pressure synthesis and characterization of the first cerium fluoride borate CeB2O4F.

CeB2O4F is the first cerium fluoride borate, which is exclusively built up of one-dimensional, infinite chains of condensed trigonal-planar [BO3]3- groups. This new cerium fluoride borate was synthesized under high-pressure/high-temperature conditions of 0.9 GPa and 1450 °C in a Walker-type multianvil apparatus. The compound crystallizes in the orthorhombic space group Pbca (No. 61) with eight formula units and the lattice parameters a=821.63(5), b=1257.50(9), c=726.71(6) pm, V=750.84(9) Å3, R1=0.0698, and wR2=0.0682 (all data). The structure exhibits a 9+1 coordinated cerium ion, one three-fold coordinated fluoride ion and a one-dimensional chain of [BO3]3- groups. Furthermore, IR spectroscopy, Electron Micro Probe Analysis and temperature-dependent X-ray powder diffraction measurements were performed.

Introduction

In the past, our research using the multianvil high-pressure technique provided us with a variety of interesting results in the chemistry of borates [1]. In the last two years, we extended our interests into the filed of rare-earth fluoride borates. The first known compounds in the system RE-B–O–F were the rare-earth fluoride borates RE3(BO3)2F3 (RE=Sm, Eu, Gd) [2,3] and Gd2(BO3)F3 [4]. These structurally related compounds were synthesized by heating of stoichiometric mixtures of rare-earth sesquioxides, rare-earth fluorides and boron sesquioxide under ambient pressure conditions. Kazmierczak et al. succeeded in the synthesis of the first divalent rare-earth fluoride borate Eu5(BO3)3F [5] with an apatite like structure, in which the rare-earth cation substitutes Ca2+ and the [BO3]3− anions replace the phosphate tetrahedra. A closer look at the periodic table shows rare-earth fluoride borates for the light lanthanoids (lanthanum to samarium) like La4B4O11F2 [6], Pr4B3O10F [7] or Sm3(BO3)2F3 [2]. However, all attempts to synthesize a cerium fluoride borate failed. Now, the combination of relatively mild pressure conditions with extremely high temperatures led to the first cerium fluoride borate CeB2O4F. Beside Ce4B14O27 [8], δ-Ce(BO2)3 [9] and γ-Ce(BO2)3 [10], CeB2O4F is the fourth cerium-containing borate synthesized at high-pressure conditions. Due to the mild pressure conditions, the two different boron atoms in CeB2O4F are still coordinated by three oxygen atoms. These [BO3]3− groups form one-dimensional chains. We report the synthesis, structural details and properties of the new high-pressure cerium fluoride borate CeB2O4F.

Experimental section

Synthesis

The cerium fluoride borate CeB2O4F was synthesized from a non-stoichiometric mixture of 79.2 mg CeO2 (Auer-Remy, Hamburg, Germany, 99.9%), 80.1 mg B2O3 (Strem Chemicals, Newburyport, USA, 99.9+%) and 90.7 mg CeF3 (Strem Chemicals, Newburyport, USA 99.9+%) according to the idealized Eq. (1).

The starting materials were finely ground and filled into a boron nitride crucible (Henze BNP GmbH, HeBoSint® S100, Kempten, Germany). The boron nitride crucible was placed into an 18/11-assembly and compressed by eight tungsten carbide cubes (TSM-10, Ceratizit, Reutte, Austria). To apply the pressure, a 1000 t multianvil press with a Walker-type module (both devices from the company Voggenreiter, Mainleus, Germany) was used. A detailed description of the assembly preparation can be found in Refs. [11-15]. In detail, the 18/11 assembly was compressed up to 0.9 GPa in 45 min and heated to 1450 °C (cylindrical graphite furnace) in the following 15 min, kept there for 5 min and cooled down to 450 °C in 25 min at constant pressure. After natural cooling down to room temperature by switching off the heating, a decompression period of 2.5 h was required. The recovered MgO-octahedron (pressure transmitting medium, Ceramic Substrates & Components Ltd., Newport, Isle of Wight, UK) was broken apart and the sample carefully separated from the surrounding graphite and boron nitride crucible. The new compound CeB2O4F was gained in the form of colorless, air- and water-resistant crystals. The reproducibility of this synthesis is difficult, because small variations of pressure or temperature led to an amorphous substance or in the majority of experiments, no reaction was observed. Interestingly, the addition of elementary phosphorus to the reaction mixture led to the formation of CePO4, which obviously favours the crystallization of CeB2O4F. Only the syntheses with elementary phosphorous yielded to single crystals with sufficient quality for a single-crystal X-ray diffraction study.

Crystal structure analysis

The powder diffraction pattern of CeB2O4F was obtained in transmission geometry from flat samples of the reaction product, using a STOE STADI P powder diffractometer with MoK radiation (Ge monochromator, λ=70.93 pm). Fig. 1 shows the diffraction pattern of CeB2O4F (top), as well as a reflection of compressed hexagonal boron nitride from the crucible, marked with a line. The experimental powder pattern tallies well with the theoretical pattern simulated from single-crystal data (bottom). Indexing the reflections of CeB2O4F, we received the lattice parameters a=820.4(5), b=1257.7(4) and c=726.6(3)  pm and a unit-cell volume of 749.7(4) Å3. This confirms the lattice parameters, obtained from the single-crystal X-ray diffraction study (Table 1).

Fig. 1

Top: Experimental powder pattern of CeB2O4F (space group: Pbca); one reflection of compressed hexagonal BN is indicated with a green line. Bottom: Theoretical powder pattern of CeB2O4F based on single-crystal diffraction data. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1

Crystal data and structure refinement of CeB2O4F (space group: Pbca) (standard deviations in parentheses).

Empirical formula	CeB2O4F
Molar mass, g mol−1	244.74
Crystal system	Orthorhombic
Space group	Pbca (No. 61)
Lattice parameters from powder data
Powder diffractometer	STOE Stadi P
Radiation	MoKα1 (λ=70.93 pm)
a, pm	820.4(5)
b, pm	1257.7(4)
c, pm	726.6(3)
V, Å3	749.7(4)
Single crystal data
Single crystal diffractometer	Enraf-Nonius Kappa CCD
Radiation	MoKα (λ=71.073 pm) (graded multilayer X-ray optics)
a, pm	821.63(5)
b, pm	1257.50(9)
c, pm	726.71(6)
V, Å3	750.84(9)
Formula units per cell	8
Calculated density, g cm−3	4.33
Crystal size, mm3	0.0083×0.0045×0.0003
Temperature, K	293(2)
Absorption coefficient, mm−1	12.03
F(0 0 0)	872
θ range, °	3.2–30.0
Range in hkl	±11, −15/+17, ±10
Total no. of reflections	5579
Independent reflections	1093 (Rint=0.0755)
Reflections with I≥2σ(I)	758 (Rσ=0.0451)
Data/parameters	1093/74
Absorption correction	Multi-scan (Scalepack [16])
Goodness-of-fit on Fi2	1.152
Final R indices [I≥2σ(I)]	R1=0.0395
	wR2=0.0624
R indices (all data)	R1=0.0698
	wR2=0.0682
Largest diff. peak and hole, e·Å−3	1.80/−0.95

Small single crystals of the new cerium fluoride borate CeB2O4F could be isolated by mechanical fragmentation. The single crystal intensity data of CeB2O4F were collected at room temperature using a Nonius Kappa-CCD diffractometer with graphite-monochromatized MoKα radiation (λ=71.073 pm). A semiempirical absorption correction based on equivalent and redundant intensities (Scalepack [16]), was applied to the intensity data. All relevant details of the data collection and evaluation are listed in Table 1.

The structure solution and parameter refinement (full-matrix least-squares against F2) were performed using the Shelx-97 software suite [17,18] with anisotropic atomic displacement parameters for all atoms. According to the systematic extinctions, the orthorhombic space group Pbca was derived. The relatively high R-indices of R1=0.0698 and wR2=0.0682 (all data) result from the large number of low intensity reflections of the type hkl for h≠2n. The final difference Fourier syntheses did not reveal any significant residual peaks in all refinements. The positional parameters of the refinements, anisotropic displacement parameters, interatomic distances and interatomic angles are listed in Tables 2–5. Additional details of the crystal structure investigations may be obtained from the Fachinformationszentrum Karlsruhe (crysdata@fiz-karlsruhe.de, http://www.fiz-informationsdienste.de/en/DB/icsd/depotanforderung.html), D-76344 Eggenstein-Leopoldshafen (Germany) on quoting the Registry No. CSD-425861.

IR spectroscopy

The FTIR-ATR (Attenuated Total Reflection) spectra of powders were measured with a Bruker Alpha-P spectrometer with a diamond ATR-crystal (2×2 mm), equipped with a DTGS detector in the spectral range of 400–4000 cm−1 (spectral resolution 4 cm−1). 24 scans of the sample were acquired. A correction for atmospheric influences using the Opus 7.0 software was performed.

Electron micro probe analysis

The presence of fluorine in CeB2O4F was determined by Electron Micro Probe Analysis (EPMA) by means of wavelength-dispersive X-ray spectroscopy (WDX) using a JEOL JXA 8100 Superprobe under an acceleration voltage of 15 kV. The analyzed spot size was approximately 40 µm×40 µm. The spectrum was acquired with an appropriate crystal (LDE1) in the range of FKα1 maximum (spectrometer conditions: step size=40 µm, dwell time=1500 ms). A fluorine-free cerium doped glass was used as standard (REE3 [19]). The sample was prepared by placing a single crystal on carbon pads and subsequently coating them with a thin conductive carbon film.

Temperature programmed X-ray powder diffraction experiments

Temperature-programmed X-ray powder diffraction experiments were done on a STOE STADI P powder diffractometer [MoKα1 radiation (λ=70.93 pm)] with a computer controlled STOE furnace: The sample was enclosed in a silica capillary and heated from room temperature to 1100 °C in 100 °C steps. The heating rate was set to 40 °C/min. Afterwards, the sample was cooled down to room temperature in 100 °C steps (cooling rate: 50 °C/min). After each heating step, a diffraction pattern was recorded over the angular range 2°≤2θ≤45°.

Results and discussion

Crystal structure of CeB2O4F

CeB2O4F crystallizes in the orthorhombic space group Pbca (no. 61) with 8 formula units per unit cell. The structure is composed of linked [BO3]3− groups, 9+1 coordinated cerium cations and threefold coordinated fluoride anions. Fig. 2 gives a view of the crystal structure of CeB2O4F showing infinite chains of linked BO3 groups along a. This structural motif consists of two crystallographically different BO3 groups (Δ), which can be descripted with the fundamental building block 1Δ:Δ (after Burns et al. [20]). In the chemistry of borates there are several compounds known which are built up by infinite chains of corner-linked BO4 tetrahedra (fundamental building block 1Δ:Δ), e.g. the mineral vimsite [20]. In contrast, only a few compounds with this rare strucutral motif of infinite chains of trigonal BO3 groups are known. Chains of BO3 triangles were first discovered by Zachariasen and Ziegler in 1932 in the calcium metaborate CaB2O4 [21]. This structure was again refined by Marezio et al. [22]. Furthermore, Höhne et al. reported the crystal structure of α-LiBO2 in 1963; a second compound with this fundamental building block [23]. Also Zachariasen reported the crystal structure of α-LiBO2 [24] and Kirfel et al. redetermined its structure [25]. This structural motif is also present in the alkali metal borate SrB2O4 [26]. To the best of our knowledge, the fundamental building block 1Δ:Δ was never observed in the chemistry of fluorido- and fluoride-borates up to now. A closer view on the ac-plane (Fig. 3) shows the wave-like modulation of the borate chains with the formal wavelength of λ=a in the structure of CeB2O4F. Fig. 4 shows the chain of linked BO3 groups in CeB2O4F. The linkage of the individual BO3 units occurs by the oxygen atoms O2 and O3, whereas the oxygen atoms O1 and O4 represent the terminal positions of the chain. The boron-oxygen distances inside the BO3 units are between 132.4(9) and 142.0(8) pm with a mean value of 137.1 pm for B1 and between 130.9(8) and 140.6(8) pm with a mean value of 136.9 pm for the B2 atom. These values fit very well with the usual boron-oxygen distance of 137 pm inside trigonal-planar [BO3]3− groups [27-29]. As expected, the boron-oxygen distances of the linking oxygen atoms (O2 and O3) are considerably larger than the B–O bond-length of the terminal oxygen atoms (O1 and O4) (see Table 4). As expected for trigonal-planar [BO3]3− groups, the mean O–B–O angles are 120.0° and 119.9° for B1 and B2, respectively. Fig. 5 displays the coordination sphere of the cerium cation in the structure of CeB2O4F, which is coordinated by seven oxygen and three fluorine atoms. The interatomic Ce–O/F distances are between 236.0(4) and 302.6(5) pm with a mean value of 259.3 pm. The largest Ce–O distance is about 26.4 pm longer than the second largest (276.2(5) pm). Therefore, the coordination sphere is described as a 9+1 coordination. The fluoride ion in CeB2O4F is coordinated by three cerium ions (Fig. 6). The bond lengths range between 236.0(4) and 261.9(4) pm with a mean value of 252.7 pm. In comparison to other rare-earth fluoride borates, the mean value of 252.7 is quite large, e.g., the largest mean F–RE distance of threefold coordinated fluoride ions in La4B4O11F2 [6] is 250.7 pm and between 240.8 pm (F4 in Yb5(BO3)2F9) and 244.8 pm (F4 in Dy5(BO3)2F9) in RE5(BO3)2F9 [30-33]. The fluoride anion also shows a significant displacement of the plane, which is spanned by the three cerium cations. The mean value of the Ce–F–Ce angle in CeB2O4F is 112.6°. The other known rare-earth fluoride borates, which contain FRE3 units (RE5(BO3)2F9 (RE=Dy–Yb) and La4B4O11F2) show nearly perfect trigonal planar FRE3 units with RE–F–RE angles around 120°. Another interesting structural attribute are the Ce–F layers in the ac-plane. Each unit cell contains 2 cerium-fluorine layers at b/2 and b. The layers are built up by two types of connected rings, which contain four and eight atoms, respectively. The cerium-fluorine plane at b/2 is shown in Fig. 7. While the “Ce2F2“ ring is planar, the “Ce4F4” ring has a chair-like configuration.

Fig. 2

Crystal structure of the new rare-earth fluoride borate CeB2O4F (space group: Pbca), showing chains of linked BO3 groups along a. (Ce3+: large spheres; F−: small spheres).

Fig. 3

Wave-like modulation of the boron–oxygen chains built up from BO3 groups in the crystal structure of CeB2O4F, showing a formal wavelength of λ=a. (Ce3+: large spheres; F−: small spheres).

Fig. 4

Chain of linked [BO3]3− groups in CeB2O4F (space group: Pbca).

Table 4

Interatomic distances (pm) in CeB2O4F (space group: Pbca), calculated with the single-crystal lattice parameters.

Ce1–F1a	236.0(4)	B1–O1	132.4(9)	F1–Ce1a	236.0(4)
Ce1–O4a	241.0(4)	B1–O3	136.9(9)	F1–Ce1b	260.2(4)
Ce1–O1a	245.9(5)	B1–O2	142.0(8)	F1–Ce1c	261.9(4)
Ce1–O4b	249.6(4)		Ø=137.1		Ø=252.7
Ce1–O1b	255.7(5)
Ce1–F1b	260.2(4)	B2–O4	130.9(8)
Ce1–F1c	261.9(4)	B2–O3	139.1(8)
Ce1–O2	263.5(5)	B2–O2	140.6(8)
Ce1–O3	276.2(5)		Ø=136.9
Ce1–O1c	302.6(5)
	Ø=259.3

Fig. 5

Coordination sphere of the Ce3+ ion in CeB2O4F.

Fig. 6

The coordination sphere of F− ion in CeB2O4F (space group: Pbca) showing a displacement of the fluorine atom to the plan spanned by the three cerium cations.

Fig. 7

Ce–F layers in the ac-plane build up by linked “Ce2F2” and “Ce4F4” rings; large spheres: Ce3+ cations, small spheres: F− anions.

Additionally, the bond valence sums for all atoms of CeB2O4F were calculated, using the bond-length/bond-strength (ΣV) [34,35] and the CHARDI concept (charge distribution in solids, ΣQ) [36]. The results of these calculations are listed in Table 6. All calculated values corresponded well with the expected values of the formal ionic charges.

Table 6

Charge distribution in CeB2O4F (space group: Pbca), calculated with the bond-length/ bond-strength (∑V) and the Chardi (∑Q) concept.

	Ce1	B1	B2	O1	O2	O3	O4	F1
∑V	+3.06	+3.02	+3.04	−2.00	−2.06	−2.15	−2.07	−0.84
∑Q	+2.97	+3.05	+2.98	−1.97	−1.86	−2.06	−2.08	−1.03

The MAPLE-values (madelung part of lattice energy) [37-39] were calculated in order to compare the results with the MAPLE-values, received from the summation of the binary components A-type Ce2O3 [40], CeF3 [41] and the high-pressure modification B2O3-II [42]. The value of 28614 kJ mol−1 was obtained in comparison to 28457 kJ mol−1 (deviation=0.6%), starting from the binary oxides [1/3·Ce2O3 (14150 kJ mol−1)+1/3·CeF3 (5408 kJ mol−1)+1·B2O3-II (21938 kJ mol−1)].

FTIR spectroscopy

The spectrum of the FTIR-ATR measurement of CeB2O4F is displayed in Fig. 8. The assignments of the vibrational modes are based on a comparison with the experimental data of borates, containing trigonal [BO3]3− groups [31-34,43-45]. Absorption bands at 1200–1450 cm−1, between 600 and 800 cm−1 and below 500 cm−1 are expected for borates containing trigonal [BO3]3− groups. In the FTIR spectrum of the new cerium fluoride borate, the expected [BO3]3− modes are detected between 1150 and 1450 cm−1 and between 600 and 800 cm−1. Furthermore, no OH or water bands could be detected in the range of 3000–3600 cm−1.

Fig. 8

Powder-FT-IR reflectance spectrum of CeB2O4F (space group: Pbca) in the range of 400–1800 cm−1.

Elemental analyses

The differentiation between fluoride ions, hydroxyl groups or oxygen anions is not possible by X-ray single crystal diffraction measurements. The presence of OH-groups was eliminated by IR-spectroscopy. WDX-spectroscopy on a single crystal of CeB2O4F was performed to certify the presence of fluorine in the crystal structure. The WDX-spectrum in Fig. 9 shows the presence of fluorine in the sample. Therefore, a tetravalent cerium oxide borate can be excluded.

Fig. 9

WDX-spectra of CeB2O4F (black) and of a fluoride-free, cerium standard (red, dashed). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Thermal behavior of CeB2O4F

Considering the high-pressure/high-temperature conditions during the synthesis, the assumed metastable character of CeB2O4F was investigated. Fig. 10 illustrates the temperature programmed X-ray powder diffraction patterns of CeB2O4F, showing a decomposition of the high-pressure phase into α-CeB3O6 and CeF3 at a temperature of 800 °C. At 900 °C α-CeB3O6, CeO2 and CeF3 can be identified. Temperatures above 1000 °C led to CeO2 and an amorphous product. Despite of the metastable character of CeB2O4F, it is stable up to 700 °C.

Fig. 10

Temperature-programmed X-ray powder patterns, showing the decomposition of CeB2O4F.

Conclusions

In this article, we described the synthesis of CeB2O4F under high-pressure/high-temperature conditions. This compound is the first known cerium fluoride borate and also the first compound in the class of fluoride borates which is composed of infinite one-dimensional chains of BO3 units. In addition, these chains represent a new fundamental building block in the chemistry of borates. To investigate the stability field of this structure type, additional experiments will be performed with the neighbouring rare-earth cations La3+ and Pr3+.

Table 2

Atomic coordinates and equivalent isotropic displacement parameters Ueq (Å2) of CeB2O4F (space group: Pbca). All atoms are positioned on the Wyckoff-site 8c. Ueq is defined as one third of the trace of the orthogonalized Uij tensor (standard deviations in parentheses).

Atom	x	y	Z	Ueq
Ce1	0.11249(3)	0.47616(3)	0.25261(6)	0.0063(2)
F1	0.1347(4)	0.0494(4)	0.4319(6)	0.0123(9)
B1	0.1571(8)	0.1722(6)	0.154(2)	0.006(2)
B2	0.4091(7)	0.2725(6)	0.238(2)	0.011(2)
O1	0.1003(5)	0.0854(4)	0.0734(7)	0.009(2)
O2	0.3266(5)	0.1786(4)	0.1900(7)	0.010(2)
O3	0.0700(5)	0.2599(4)	0.2068(7)	0.015(2)
O4	0.3561(5)	0.3704(4)	0.2265(7)	0.012(2)

Table 3

Anisotropic displacement parameters (Uij/Å2) for CeB2O4F (space group: Pbca).

Atom	U11	U22	U33	U12	U13	U23
Ce1	0.0058(2)	0.0078(2)	0.0054(2)	0.0007(2)	−0.0001(2)	0.0002(2)
F1	0.011(2)	0.017(2)	0.009(2)	0.002(2)	−0.003(2)	0.000(2)
B1	0.007(3)	0.006(3)	0.005(3)	−0.001(3)	0.004(3)	0.000(3)
B2	0.005(2)	0.010(3)	0.017(4)	0.000(2)	0.007(4)	−0.005(4)
O1	0.010(2)	0.007(3)	0.010(2)	−0.003(2)	0.001(1)	−0.001(2)
O2	0.003(2)	0.006(2)	0.020(2)	−0.001(2)	−0.001(2)	0.000(2)
O3	0.004(2)	0.010(2)	0.029(3)	0.000(2)	0.006(2)	−0.003(2)
O4	0.008(2)	0.012(2)	0.015(3)	0.000(2)	−0.002(2)	0.001(2)

Table 5

Interatomic angles (deg) in CeB2O4F (space group: Pbca), calculated with the single-crystal lattice parameters.

O1–B1–O3	127.1(6)	O4–B2–O3	116.3(6)	Ce1a–F1–Ce1b	113.6(2)
O1–B1–O2	118.3(6)	O4–B2–O2	127.8(6)	Ce1a–F1–Ce1c	120.3(2)
O3–B1–O2	114.5(6)	O3–B1–O2	115.7(6)	Ce1b–F1–Ce1c	103.8(2)
	Ø=120.0		Ø=119.9		Ø=112.6