SiBr₄ – prediction and determination of crystal structures

For SiBr₄ no crystal structures have been reported yet. In this work the crystal structures of SiBr₄ were predicted by global lattice-energy minimizations using force-field methods. Over an energy range of 5 kJ mol⁻¹ above the global minimum ten possible structures were found. Two of these structures were experimentally determined from X-ray synchrotron powder diffraction data: The low-temperature β phase crystallizes in P2₁/c, the high-temperature α phase in . Temperature-dependant X-ray powder diffraction shows that the phase transition occurs at 168 K.

Computing details

(alpha-SiBr4) Silicontetrabromide

Crystal data

Br4Si	Z = 8
Mr = 347.69	Dx = 3.308 Mg m^−3
Cubic, Pa3	Melting point: 245 K
Hall symbol: -P 2ac 2ab	Synchrotron radiation, λ = 0.92103 Å
a = 11.17701 (7) Å	T = 237 K
V = 1396.29 (3) Å^3	cylinder, ? × ? × ? mm

Data collection

'beamline X7B, NSLS,Brookhaven National Laboratory,Brookhaven, USA' diffractometer	Data collection mode: transmission
Silicon double monochromator	Scan method: step
Specimen mounting: '0.3 mm glass capillary'	2θmin = 4.0°, 2θmax = 36.0°, 2θstep = 0.02°

Refinement

Rp = 0.010	Excluded region(s): 0 to 4 and 36 to 38 degrees unreliable due to beam dump
Rwp = 0.017	Profile function: Fundamental parameters
Rexp = 0.020	25 parameters
R(F^2) = 0.00560
χ^2 = NOT FOUND	Background function: 'Chebyshev polynomial'
? data points

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

	x	y	z	Biso*/Beq
Si1	0.131 (1)	0.131 (1)	0.131 (1)	5.22 (4)
Br1	0.2426 (6)	0.2426 (6)	0.2426 (6)	5.22 (4)
Br2	0.0255 (2)	0.0129 (6)	0.2475 (3)	5.22 (4)

Geometric parameters (Å, º)

Si1—Br1	2.16 (2)	Si1—Br2	2.201 (9)
Br1—Si1—Br2	108.5 (5)	Br2—Si1—Br2	110.5 (5)

(beta-SiBr4) Silicontetrabromide

Crystal data

Br4Si	V = 678.16 (4) Å^3
Mr = 347.69	Z = 4
Monoclinic, P21/c	Dx = 3.406 Mg m^−3
Hall symbol: -P 2ybc	Melting point: 245 K
a = 10.1319 (4) Å	Synchrotron radiation, λ = 0.92103 Å
b = 6.7002 (2) Å	T = 147 K
c = 10.2389 (3) Å	cylinder, ? × ? × ? mm
β = 102.665 (2)°

Data collection

'beamline X7B, NSLS,Brookhaven National Laboratory,Brookhaven, USA' diffractometer	Data collection mode: transmission
Silicon double monochromator	Scan method: step
Specimen mounting: '0.3 mm glass capillary'	2θmin = 4.0°, 2θmax = 36.0°, 2θstep = 0.02°

Refinement

Rp = 0.013	Excluded region(s): 0 to 4 and 36 to 38 degrees unreliable due to beam dump
Rwp = 0.023	Profile function: Fundamental parameters
Rexp = 0.019	49 parameters
R(F^2) = 0.02541
χ^2 = NOT FOUND	Background function: 'Chebyshev polynomial'
? data points

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

	x	y	z	Biso*/Beq
Si1	0.2458 (4)	0.5798 (10)	0.6408 (3)	3.17 (14)
Br1	0.191587	0.886384	0.583029	3.17 (14)
Br2	0.294500	0.421193	0.472209	3.17 (14)
Br3	0.076449	0.433943	0.700362	3.17 (14)
Br4	0.420504	0.577761	0.807720	3.17 (14)

Geometric parameters (Å, º)

Si1—Br1	2.175 (3)	Si1—Br3	2.175 (3)
Si1—Br2	2.175 (3)	Si1—Br4	2.175 (3)
Br1—Si1—Br2	109.47	Br2—Si1—Br3	109.47
Br1—Si1—Br3	109.47	Br2—Si1—Br4	109.47
Br1—Si1—Br4	109.47	Br3—Si1—Br4	109.47