Powder neutron diffraction of Tl₂BeF₄ at six temperatures from room temperature to 1.5 K

The structure of thallium fluoro­beryllate, Tl₂BeF₄, has been analysed by the Rietveld method on neutron diffraction patterns collected at 1.5, 50, 100, 150, 200 and 300 K, with the aim of detecting low-temperature instabilities. Atomic parameters based on the isomorphic β-K₂SO₄ crystal in the paraelectric phase were used as the starting model at room temperature; no evidence for any phase transition has been detected at lower temperature. The structure was determined in the ortho­rhom­bic space group Pnma. All the atoms (except one F atom) occupy sites with m symmetry. We have compared the structure with those of other compounds of the β-K₂SO₄ family, at room temperature, in order to gain insight into their observed instabilities. The irregular coordination of the cations may indicate stereochemical activity of the Tl^I lone pair but does not indicate a possible structural instability.

S1. Comment

We are interested in the large family of ferroelectric compounds having the general formula A₂BX₄, with the β-K₂SO₄ structure in the paraelectric phase (space group Pnma, Z = 4). The most widely studied compounds are K₂SeO₄, (NH₄)₂SO₄ and (NH₄)₂BeF₄; they exhibit interesting properties, related to a low-temperature phase transition, which are strikingly different. For instance, the polar axis of (NH₄)₂BeF₄ is the b axis, while that of (NH₄)₂SO₄ (Hoshino et al., 1958) and K₂SeO₄ is the c axis (Yamada et al., 1984). (NH₄)₂SO₄ has the ferroelectric phase transition at T_c = 223.5 K (Schlemper & Hamilton, 1966), while an incommensurate phase is observed in (NH₄)₂BeF₄ between T_i = 182.9 K and T_c = 177.2 K (Srivastava et al., 1999), and in K₂SeO₄ between T_i = 192.5 K and T_c = 93 K (Aiki & Hukuda, 1969). A review by Fabry & Pérez-Mato (1994) reported that the instability of a large number of A₂BX₄ compounds is related to the behaviour of the 11-coordinate cation, rather than to the nine-coordinate cation, and especially to the bond strength of the shorter cation–anion contact parallel to the pseudo-hexagonal a axis. A detailed and comparative study of this type of compound is necessary. Alkali metal fluoroberyllates have been exceptional cases of A₂BX₄-type compounds in the past, but we have recently studied them by neutron powder diffraction with refinement in Pnma at room temperature and 1.5 K (da Silva et al., 2005). Moreover, thallium oxysalts, such as Tl₂SO₄, Tl₂CrO₄ and Tl₂SeO₄, have also been studied; the last of these undergoes a phase transition at 72 K (Friese et al., 2004). The different behaviour of thallium compounds with respect to other β-K₂SO₄ compounds is a result of the stereoactivity of the 6s² lone pair in the Tl^I cation (Fabry & Breczewsky, 1993).

The aim of the present work is to compare the structure of Tl₂BeF₄ with the rest of this class of compounds to assess the role of the cations in the structural instability. The correct crystal structure of Tl₂BeF₄ has not been reported previously [only the cell parameters, by Arend et al. (1980)] and a structural phase transition has never been observed. We have measured the neutron powder diffraction patterns of Tl₂BeF₄ at room temperature and lower temperatures down to 1.5 K and we have refined the crystal structure using the Rietveld method in space group Pnma for all temperatures. The fitted diffraction profile at room temperature is shown in Fig. 1 and the variation of the unit cell volume with temperature is shown in Fig. 2. From DSC analysis and from Fig. 2, no thermal anomaly is observed. The structure of Tl₂BeF₄ consists of isolated BeF₄²⁺ tetrahedra, with Tl⁺ ions distributed between them. The cations are placed in two different cavities — one of them, within a slightly distorted bipyramidal hole, remains 11-coordinate, while the other cation, within the distorted octahedral hole, is surrounded by nine F atoms. As in other fluoroberyllates (da Silva et al., 2005) and in thallium selenate (Friese et al., 2004), the average Be—F bond length of the fluoroberyllate anion increases at low temperature. At variance with other compounds showing structural instability (González-Silgo et al., 1997; Solans et al., 1998), (1) we do not observe any significant rotation of the tetrahedra around the b axis, so that the environment of the cations is not changed with temperature, and (2) the bond valence sum (Brown, 1992) at both the Tl1 and the Tl2 sites is close to 1 v.u. (valence units) for the entire temperature range. Otherwise, the five F atoms nearest to Tl1 are at distances less than 3.1 Å and lie on the same side of this cation (see Fig. 3a), thus indicating the stereoactivity of the Tl1 lone pair, as in other thallium compounds (Fabry & Breczewsky, 1993); this situation is maintained at lower temperatures. With respect to the Tl2 environment, the stereoactivity of the lone pair is less clear, although a tendency is observed at lower temperature; at 1.5 K, the three F atoms nearest this cation are at distances that are 0.1 Å shorter than the other Tl2—F contacts and lie approximately on the same side of Tl (Fig. 3b).

S2. Experimental

Tl₂BeF₄ was obtained via the reaction BeF₂(s) + H₂F₂(aq) + Tl₂CO₃(s) → Tl₂BeF₄ + CO₂ + H₂O. BeF₂, H₂F₂ (48%) and Tl₂CO₃ were analytical grade. After completion of the reaction at pH 7, the sample was slowly evaporated at room temperature. After a few days, a polycrystalline powder was obtained. The sample was not evaporated to dryness in order to avoid possible contamination. The polycrystalline powder was filtered and dried. Thermal analyses were carried out on a Perkin–Elmer differential scanning calorimeter Pyris I-DSC. The sample was measured in the temperature range 98–293 K, with a warming rate of 10 K min⁻¹ and a total scale sensitivity of 0.1 mW. The sample mass was 27.76 mg. This analysis showed no thermal anomaly. The high-resolution neutron powder diffractometer 3 T2 at the Orphée reactor of Laboratoire Léon Brillouin (Saclay, France) was used for data collection with high direct space resolution (λ = 1.2251 Å, Q_max = 9.2 Å⁻¹). A helium cryostat was used in order to keep the sample at the correct temperature during the diffraction experiments. We present the data collected at 1.5, 50, 100, 150, 200 and 300 K.

S3. Refinement

Rietveld refinement of the six powder diffraction patterns was carried out with the program FULLPROF (Rodríguez-Carvajal, 2005) and the visualization of the results was performed using the program WINPLOTR (Roisnel & Rodríguez-Carvajal, 2005). The experimental profiles were all modelled using a pseudo-Voigt profile shape function, with five adjustable parameters (U, V, W, η and X); initial values were obtained from the instrumental resolution parameters. After preliminary refinements to establish the scale factor, zero point displacement, cell parameters and profile parameters, several cycles were performed to refine the atomic positions and isotropic displacement parameters. The starting values for the atomic positions were those of the β-K₂SO₄ structure, in space group Pnma. Space group Pn2₁a was also examined, without any improvement. Owing to the poor values obtained for the agreement factors and the modulated residuals that remained after the refinements, it was necessary to apply a Fourier filtering treatment, in order to model the background and to improve the results. This can be justified because from the modulated residues it has been possible to calculate the radial distribution function of other fluoroberyllates, because of the coexistence of an amorphous phase with the crystalline material (da Silva et al., 2005).

Computing details

For all compounds, cell refinement: FULLPROF (Rodríguez-Carvajal, 2005); program(s) used to refine structure: FULLPROF; molecular graphics: Diamond (Brandenburg & Berndt, 1999); software used to prepare material for publication: FULLPROF.

Figures

	Fig. 1. Plot of the Rietveld refinement result for Tl2BeF4 at room temperature, showing the calculated (line), observed (circle) and difference (lower) profiles.
	Fig. 2. Variation of the unit-cell volume with temperature for Tl2BeF4, showing linear behaviour without anomalies.
	Fig. 3. Arrangement of the nearest-neighbour BeF4 tetrahedra to (a) nine- and (b) 11-coordinated Tl^+ cations, showing the shortest (thick solid line) and longest (thin dashed line) bonds. Intermediate bonds are drawn with a thin solid line. [Symmetry codes: (i) x, y, z; (ii) 1 − x, 1/2 + y, 1 − z; (iii) 1/2 + x, 1/2 − y, 1/2 − z; (iv) 1/2 − x, 1 − y, 1/2 + z; (v) −x, 1/2 + y, 1 − z; (vi) 1 − x, 1 − y, 1 − z; (vii) 1/2 − x, 1/2 + y, 1/2 + z; (viii) x, 1/2 − y, z; (ix) x, 1 + y, z; (x) 1/2 − x, 1 − y, −1/2 + z; (xi) 1/2 + x, 3/2 − y, 1/2 − z; (xii) 1/2 + x, 1/2 − y, 1/2 − z; (xiii) 1/2 + x, 1 + y, 1/2 − z.]

(I_300K) Thallium Fluoroberyllate

Crystal data

Tl2BeF4	V = 476.39 (2) Å^3
Mr = 493.77	Z = 4
Orthorhombic, Pnma	Dx = 6.884 Mg m^−3
Hall symbol: -P 2ac 2n	Neutron radiation, λ = 1.2251 Å
a = 7.7238 (2) Å	T = 300 K
b = 5.90226 (17) Å	white
c = 10.4499 (3) Å	cylinder, 50 × 10 mm

Data collection

3T2 Line diffractometer	Data collection mode: transmission
Radiation source: nuclear reactor	Scan method: step
Ge 335 monochromator	2θmin = 6.30°, 2θmax = 125.45°, 2θstep = 0.05°
Specimen mounting: vanadium can

Refinement

Refinement on Inet	Excluded region(s): 0° to 10°; no reflection region with a complex background shape
Least-squares matrix: full with fixed elements per cycle	Profile function: pseudo-Voigt
Rp = 0.036	35 parameters
Rwp = 0.048	0 restraints
Rexp = 0.076	Weighting scheme based on measured s.u.'s
RBragg = 0.075	(Δ/σ)max = 0.01
R(F) = 0.055	Background function: Fourier filter
χ^2 = 0.395	Preferred orientation correction: Not used
2384 data points

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Tl1	0.1679 (6)	0.25000	0.0901 (5)	0.0299 (13)*
Tl2	−0.0078 (6)	0.25000	0.6943 (4)	0.0282 (13)*
Be1	0.2343 (6)	0.25000	0.4195 (5)	0.0211 (12)*
F1	0.3021 (7)	0.0370 (9)	0.3517 (5)	0.0401 (16)*
F2	0.0355 (10)	0.25000	0.4167 (9)	0.045 (2)*
F3	0.2997 (11)	0.25000	0.5575 (8)	0.041 (2)*

Geometric parameters (Å, º)

Be1—F1	1.535 (6)	Tl1—F3^ii	2.9812 (14)
Be1—F2	1.536 (9)	Tl1—F3^vii	2.9812 (14)
Be1—F3	1.528 (10)	Tl2—F1^viii	2.875 (7)
Tl1—F1	3.182 (7)	Tl2—F1^ix	2.846 (6)
Tl1—F1^i	3.152 (7)	Tl2—F1^x	2.875 (7)
Tl1—F1^ii	3.022 (7)	Tl2—F1^xi	2.846 (6)
Tl1—F1^iii	3.022 (7)	Tl2—F2	2.920 (10)
Tl1—F1^iv	3.182 (7)	Tl2—F2^xii	3.178 (4)
Tl1—F1^v	3.152 (7)	Tl2—F2^viii	3.178 (4)
Tl1—F2	3.563 (11)	Tl2—F3	2.772 (10)
Tl1—F2^vi	2.840 (9)	Tl2—F3^xiii	2.990 (9)
Tl1—F3^i	3.235 (10)
F1—Be1—F1^iv	110.0 (3)	F1—Be1—F3	108.8 (2)
F1—Be1—F2	109.4 (2)	F2—Be1—F3	110.4 (5)

Symmetry codes: (i) x−1/2, −y+1/2, −z+1/2; (ii) −x+1/2, −y, z−1/2; (iii) −x+1/2, y+1/2, z−1/2; (iv) x, −y+1/2, z; (v) x−1/2, y, −z+1/2; (vi) x+1/2, −y+1/2, −z+1/2; (vii) −x+1/2, −y+1, z−1/2; (viii) −x, y+1/2, −z+1; (ix) −x+1/2, −y, z+1/2; (x) −x, −y, −z+1; (xi) −x+1/2, y+1/2, z+1/2; (xii) −x, y−1/2, −z+1; (xiii) x−1/2, −y+1/2, −z+3/2.

(I_200K) Thallium Fluoroberyllate

Crystal data

Tl2BeF4	V = 470.03 (2) Å^3
Mr = 493.77	Z = 4
Orthorhombic, Pnma	Dx = 6.977 Mg m^−3
Hall symbol: -P 2ac 2n	Neutron radiation, λ = 1.2251 Å
a = 7.69999 (15) Å	T = 200 K
b = 5.86734 (11) Å	white
c = 10.4039 (2) Å	cylinder, 50 × 10 mm

Data collection

3T2 Line diffractometer	Data collection mode: transmission
Radiation source: nuclear reactor	Scan method: step
Ge 335 monochromator	2θmin = 6.30°, 2θmax = 125.45°, 2θstep = 0.05°
Specimen mounting: vanadium can

Refinement

Rp = 0.042	Profile function: pseudo-Voigt
Rwp = 0.055	35 parameters
Rexp = 0.076	0 restraints
RBragg = 0.072	Weighting scheme based on measured s.u.'s
R(F) = 0.047	(Δ/σ)max = 0.01
χ^2 = 0.526	Background function: Fourier filter
2384 data points	Preferred orientation correction: Not used
Excluded region(s): 0° to 10°; no reflection region with a complex background shape

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Tl1	0.1667 (4)	0.25000	0.0908 (4)	0.0216 (10)*
Tl2	−0.0083 (4)	0.25000	0.6944 (3)	0.0160 (9)*
Be1	0.2352 (4)	0.25000	0.4199 (4)	0.0132 (9)*
F1	0.3044 (5)	0.0359 (7)	0.3526 (4)	0.0278 (11)*
F2	0.0353 (7)	0.25000	0.4154 (7)	0.0301 (14)*
F3	0.3013 (8)	0.25000	0.5597 (6)	0.0277 (16)*

Geometric parameters (Å, º)

Be1—F1	1.534 (5)	Tl1—F3^ii	2.9617 (10)
Be1—F2	1.540 (6)	Tl1—F3^vii	2.9617 (10)
Be1—F3	1.541 (7)	Tl2—F1^viii	2.873 (5)
Tl1—F1	3.181 (6)	Tl2—F1^ix	2.826 (5)
Tl1—F1^i	3.116 (5)	Tl2—F1^x	2.873 (5)
Tl1—F1^ii	3.001 (5)	Tl2—F1^xi	2.826 (5)
Tl1—F1^iii	3.001 (5)	Tl2—F2	2.922 (8)
Tl1—F1^iv	3.181 (6)	Tl2—F2^xii	3.155 (3)
Tl1—F1^v	3.116 (5)	Tl2—F2^viii	3.155 (3)
Tl1—F2	3.525 (8)	Tl2—F3	2.765 (7)
Tl1—F2^vi	2.839 (6)	Tl2—F3^xiii	2.949 (7)
Tl1—F3^i	3.220 (7)
F1—Be1—F1^iv	110.0 (2)	F1—Be1—F3	108.43 (15)
F1—Be1—F2	109.47 (14)	F2—Be1—F3	111.0 (4)

Symmetry codes: (i) x−1/2, −y+1/2, −z+1/2; (ii) −x+1/2, −y, z−1/2; (iii) −x+1/2, y+1/2, z−1/2; (iv) x, −y+1/2, z; (v) x−1/2, y, −z+1/2; (vi) x+1/2, −y+1/2, −z+1/2; (vii) −x+1/2, −y+1, z−1/2; (viii) −x, y+1/2, −z+1; (ix) −x+1/2, −y, z+1/2; (x) −x, −y, −z+1; (xi) −x+1/2, y+1/2, z+1/2; (xii) −x, y−1/2, −z+1; (xiii) x−1/2, −y+1/2, −z+3/2.

(I_150K) Thallium Fluoroberyllate

Crystal data

Tl2BeF4	V = 466.82 (1) Å^3
Mr = 493.77	Z = 4
Orthorhombic, Pnma	Dx = 7.025 Mg m^−3
Hall symbol: -P 2ac 2n	Neutron radiation, λ = 1.2251 Å
a = 7.69020 (13) Å	T = 150 K
b = 5.84889 (9) Å	white
c = 10.37848 (17) Å	cylinder, 50 × 10 mm

Data collection

3T2 Line diffractometer	Data collection mode: transmission
Radiation source: nuclear reactor	Scan method: step
Ge 335 monochromator	2θmin = 6.30°, 2θmax = 125.45°, 2θstep = 0.05°
Specimen mounting: vanadium can

Refinement

Rp = 0.047	Profile function: pseudo-Voigt
Rwp = 0.063	35 parameters
Rexp = 0.076	0 restraints
RBragg = 0.075	Weighting scheme based on measured s.u.'s
R(F) = 0.047	(Δ/σ)max = 0.01
χ^2 = 0.682	Background function: Fourier filter
2384 data points	Preferred orientation correction: Not used
Excluded region(s): 0° to 10°; no reflection region with a complex background shape

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Tl1	0.1664 (4)	0.25000	0.0909 (3)	0.0170 (8)*
Tl2	−0.0098 (4)	0.25000	0.6941 (3)	0.0119 (7)*
Be1	0.2361 (4)	0.25000	0.4200 (3)	0.0117 (7)*
F1	0.3062 (5)	0.0356 (6)	0.3527 (4)	0.0218 (9)*
F2	0.0355 (6)	0.25000	0.4158 (5)	0.0237 (11)*
F3	0.3022 (6)	0.25000	0.5606 (5)	0.0181 (12)*

Geometric parameters (Å, º)

Be1—F1	1.533 (4)	Tl1—F3^ii	2.9512 (8)
Be1—F2	1.543 (6)	Tl1—F3^vii	2.9512 (8)
Be1—F3	1.545 (6)	Tl2—F1^viii	2.867 (5)
Tl1—F1	3.180 (5)	Tl2—F1^ix	2.820 (5)
Tl1—F1^i	3.096 (5)	Tl2—F1^x	2.867 (5)
Tl1—F1^ii	2.991 (5)	Tl2—F1^xi	2.820 (5)
Tl1—F1^iii	2.991 (5)	Tl2—F2	2.909 (6)
Tl1—F1^iv	3.180 (5)	Tl2—F2^xii	3.145 (2)
Tl1—F1^v	3.096 (5)	Tl2—F2^viii	3.145 (2)
Tl1—F2	3.519 (6)	Tl2—F3	2.771 (6)
Tl1—F2^vi	2.839 (6)	Tl2—F3^xiii	2.928 (6)
Tl1—F3^i	3.212 (6)
F1—Be1—F1^iv	109.74 (19)	F1—Be1—F3	108.33 (15)
F1—Be1—F2	109.79 (14)	F2—Be1—F3	110.8 (3)

Symmetry codes: (i) x−1/2, −y+1/2, −z+1/2; (ii) −x+1/2, −y, z−1/2; (iii) −x+1/2, y+1/2, z−1/2; (iv) x, −y+1/2, z; (v) x−1/2, y, −z+1/2; (vi) x+1/2, −y+1/2, −z+1/2; (vii) −x+1/2, −y+1, z−1/2; (viii) −x, y+1/2, −z+1; (ix) −x+1/2, −y, z+1/2; (x) −x, −y, −z+1; (xi) −x+1/2, y+1/2, z+1/2; (xii) −x, y−1/2, −z+1; (xiii) x−1/2, −y+1/2, −z+3/2.

(I_100K) Thallium Fluoroberyllate

Crystal data

Tl2BeF4	V = 463.75 (1) Å^3
Mr = 493.77	Z = 4
Orthorhombic, Pnma	Dx = 7.072 Mg m^−3
Hall symbol: -P 2ac 2n	Neutron radiation, λ = 1.2251 Å
a = 7.68265 (11) Å	T = 100 K
b = 5.83059 (8) Å	white
c = 10.35278 (15) Å	cylinder, 50 × 10 mm

Data collection

3T2 Line diffractometer	Data collection mode: transmission
Radiation source: nuclear reactor	Scan method: step
Ge 335 monochromator	2θmin = 6.30°, 2θmax = 125.45°, 2θstep = 0.05°
Specimen mounting: vanadium can

Refinement

Rp = 0.048	Profile function: pseudo-Voigt
Rwp = 0.062	35 parameters
Rexp = 0.076	0 restraints
RBragg = 0.063	Weighting scheme based on measured s.u.'s
R(F) = 0.037	(Δ/σ)max = 0.01
χ^2 = 0.666	Background function: Fourier filter
2384 data points	Preferred orientation correction: Not used
Excluded region(s): 0° to 10°; no reflection region with a complex background shape

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Tl1	0.1667 (3)	0.25000	0.0901 (3)	0.0113 (6)*
Tl2	−0.0108 (3)	0.25000	0.6930 (3)	0.0079 (6)*
Be1	0.2364 (3)	0.25000	0.4198 (3)	0.0082 (6)*
F1	0.3085 (4)	0.0336 (5)	0.3523 (3)	0.0160 (7)*
F2	0.0352 (5)	0.25000	0.4139 (5)	0.0180 (9)*
F3	0.3016 (5)	0.25000	0.5612 (4)	0.0126 (9)*

Geometric parameters (Å, º)

Be1—F1	1.545 (3)	Tl1—F3^ii	2.9407 (6)
Be1—F2	1.547 (4)	Tl1—F3^vii	2.9407 (6)
Be1—F3	1.547 (5)	Tl2—F1^viii	2.861 (4)
Tl1—F1	3.185 (4)	Tl2—F1^ix	2.805 (4)
Tl1—F1^i	3.086 (4)	Tl2—F1^x	2.861 (4)
Tl1—F1^ii	2.972 (4)	Tl2—F1^xi	2.805 (4)
Tl1—F1^iii	2.972 (4)	Tl2—F2	2.911 (6)
Tl1—F1^iv	3.185 (4)	Tl2—F2^xii	3.124 (2)
Tl1—F1^v	3.086 (4)	Tl2—F2^viii	3.124 (2)
Tl1—F2	3.501 (6)	Tl2—F3	2.761 (5)
Tl1—F2^vi	2.831 (4)	Tl2—F3^xiii	2.925 (5)
Tl1—F3^i	3.213 (5)
F1—Be1—F1^iv	109.50 (15)	F1—Be1—F3	108.17 (11)
F1—Be1—F2	109.90 (11)	F2—Be1—F3	111.2 (3)

Symmetry codes: (i) x−1/2, −y+1/2, −z+1/2; (ii) −x+1/2, −y, z−1/2; (iii) −x+1/2, y+1/2, z−1/2; (iv) x, −y+1/2, z; (v) x−1/2, y, −z+1/2; (vi) x+1/2, −y+1/2, −z+1/2; (vii) −x+1/2, −y+1, z−1/2; (viii) −x, y+1/2, −z+1; (ix) −x+1/2, −y, z+1/2; (x) −x, −y, −z+1; (xi) −x+1/2, y+1/2, z+1/2; (xii) −x, y−1/2, −z+1; (xiii) x−1/2, −y+1/2, −z+3/2.

(I_50K) Thallium Fluoroberyllate

Crystal data

Tl2BeF4	V = 460.96 (1) Å^3
Mr = 493.77	Z = 4
Orthorhombic, Pnma	Dx = 7.115 Mg m^−3
Hall symbol: -P 2ac 2n	Neutron radiation, λ = 1.2251 Å
a = 7.67874 (9) Å	T = 50 K
b = 5.81248 (7) Å	white
c = 10.32788 (12) Å	cylinder, 50 × 10 mm

Data collection

3T2 Line diffractometer	Data collection mode: transmission
Radiation source: nuclear reactor	Scan method: step
Ge 335 monochromator	2θmin = 6.30°, 2θmax = 125.45°, 2θstep = 0.05°
Specimen mounting: vanadium can

Refinement

Rp = 0.053	Profile function: pseudo-Voigt
Rwp = 0.068	35 parameters
Rexp = 0.076	0 restraints
RBragg = 0.061	Weighting scheme based on measured s.u.'s
R(F) = 0.033	(Δ/σ)max = 0.01
χ^2 = 0.807	Background function: Fourier filter
2384 data points	Preferred orientation correction: Not used
Excluded region(s): 0° to 10°; no reflection region with a complex background shape

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Tl1	0.1668 (3)	0.25000	0.0895 (2)	0.0056 (5)*
Tl2	−0.0112 (3)	0.25000	0.6924 (2)	0.0023 (5)*
Be1	0.2375 (3)	0.25000	0.4202 (3)	0.0047 (5)*
F1	0.3114 (3)	0.0321 (4)	0.3532 (3)	0.0098 (6)*
F2	0.0366 (4)	0.25000	0.4118 (4)	0.0082 (7)*
F3	0.3016 (5)	0.25000	0.5621 (3)	0.0074 (7)*

Geometric parameters (Å, º)

Be1—F1	1.551 (3)	Tl1—F3^ii	2.9300 (5)
Be1—F2	1.545 (4)	Tl1—F3^vii	2.9300 (5)
Be1—F3	1.546 (4)	Tl2—F1^viii	2.868 (3)
Tl1—F1	3.202 (3)	Tl2—F1^ix	2.793 (3)
Tl1—F1^i	3.066 (3)	Tl2—F1^x	2.868 (3)
Tl1—F1^ii	2.945 (3)	Tl2—F1^xi	2.793 (3)
Tl1—F1^iii	2.945 (3)	Tl2—F2	2.921 (5)
Tl1—F1^iv	3.202 (3)	Tl2—F2^xii	3.1052 (16)
Tl1—F1^v	3.066 (3)	Tl2—F2^viii	3.1052 (16)
Tl1—F2	3.476 (5)	Tl2—F3	2.753 (4)
Tl1—F2^vi	2.840 (4)	Tl2—F3^xiii	2.915 (4)
Tl1—F3^i	3.212 (4)
F1—Be1—F1^iv	109.51 (12)	F1—Be1—F3	107.85 (11)
F1—Be1—F2	109.89 (9)	F2—Be1—F3	111.8 (3)

Symmetry codes: (i) x−1/2, −y+1/2, −z+1/2; (ii) −x+1/2, −y, z−1/2; (iii) −x+1/2, y+1/2, z−1/2; (iv) x, −y+1/2, z; (v) x−1/2, y, −z+1/2; (vi) x+1/2, −y+1/2, −z+1/2; (vii) −x+1/2, −y+1, z−1/2; (viii) −x, y+1/2, −z+1; (ix) −x+1/2, −y, z+1/2; (x) −x, −y, −z+1; (xi) −x+1/2, y+1/2, z+1/2; (xii) −x, y−1/2, −z+1; (xiii) x−1/2, −y+1/2, −z+3/2.

(I_1.5K) Thallium Fluoroberyllate

Crystal data

Tl2BeF4	V = 459.53 (1) Å^3
Mr = 493.77	Z = 4
Orthorhombic, Pnma	Dx = 7.137 Mg m^−3
Hall symbol: -P 2ac 2n	Neutron radiation, λ = 1.2251 Å
a = 7.67735 (10) Å	T = 2 K
b = 5.80377 (8) Å	white
c = 10.31316 (13) Å	cylinder, 50 × 10 mm

Data collection

3T2 Line diffractometer	Data collection mode: transmission
Radiation source: nuclear reactor	Scan method: step
Ge 335 monochromator	2θmin = 7.30°, 2θmax = 125.45°, 2θstep = 0.05°
Specimen mounting: vanadium can

Refinement

Rp = 0.067	Profile function: pseudo-Voigt
Rwp = 0.085	35 parameters
Rexp = 0.093	0 restraints
RBragg = 0.069	Weighting scheme based on measured s.u.'s
R(F) = 0.036	(Δ/σ)max = 0.01
χ^2 = 0.834	Background function: Fourier filter
2364 data points	Preferred orientation correction: Not used
Excluded region(s): 0° to 10°; no reflection region with a complex background shape

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Tl1	0.1668 (3)	0.25000	0.0906 (2)	0.0000 (4)*
Tl2	−0.0115 (3)	0.25000	0.6930 (2)	0.0018 (4)*
Be1	0.2385 (3)	0.25000	0.4196 (3)	0.0045 (5)*
F1	0.3140 (4)	0.0284 (5)	0.3536 (3)	0.0080 (6)*
F2	0.0361 (4)	0.25000	0.4123 (4)	0.0061 (7)*
F3	0.2974 (5)	0.25000	0.5636 (3)	0.0042 (7)*

Geometric parameters (Å, º)

Be1—F1	1.566 (3)	Tl1—F3^ii	2.9281 (5)
Be1—F2	1.556 (4)	Tl1—F3^vii	2.9281 (5)
Be1—F3	1.552 (4)	Tl2—F1^viii	2.870 (4)
Tl1—F1	3.208 (4)	Tl2—F1^ix	2.766 (4)
Tl1—F1^i	3.053 (4)	Tl2—F1^x	2.870 (4)
Tl1—F1^ii	2.934 (3)	Tl2—F1^xi	2.766 (4)
Tl1—F1^iii	2.934 (3)	Tl2—F2	2.918 (5)
Tl1—F1^iv	3.208 (4)	Tl2—F2^xii	3.1042 (16)
Tl1—F1^v	3.053 (4)	Tl2—F2^viii	3.1042 (16)
Tl1—F2	3.466 (5)	Tl2—F3	2.721 (4)
Tl1—F2^vi	2.835 (4)	Tl2—F3^xiii	2.908 (4)
Tl1—F3^i	3.251 (4)
F1—Be1—F1^iv	110.39 (15)	F1—Be1—F3	107.93 (11)
F1—Be1—F2	110.40 (11)	F2—Be1—F3	109.7 (3)

Symmetry codes: (i) x−1/2, −y+1/2, −z+1/2; (ii) −x+1/2, −y, z−1/2; (iii) −x+1/2, y+1/2, z−1/2; (iv) x, −y+1/2, z; (v) x−1/2, y, −z+1/2; (vi) x+1/2, −y+1/2, −z+1/2; (vii) −x+1/2, −y+1, z−1/2; (viii) −x, y+1/2, −z+1; (ix) −x+1/2, −y, z+1/2; (x) −x, −y, −z+1; (xi) −x+1/2, y+1/2, z+1/2; (xii) −x, y−1/2, −z+1; (xiii) x−1/2, −y+1/2, −z+3/2.