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Synchrotron powder diffraction data from methylammonium tin bromide, CH3NH3SnBr3, taken as a function of temperature, reveal the existence of a phase between 230 and 188 K crystallizing in Pmc21, a = 5.8941 (2), b = 8.3862 (2), c = 8.2406 (2) Å. Strong ferroelectric distortions of the octahedra, associated with stereochemical activity of the Sn 5s2 lone pair, are evident. A group analysis and decomposition of the distortion modes of the inorganic framework with respect to the cubic parent is given. The primary order parameters driving this upper transition appear to be an in-phase tilt (rotation) of the octahedra coupled to a ferroelectric mode. The precise nature of the lower-temperature phase remains uncertain, although it appears likely to be triclinic. Density-functional theory calculations on such a triclinic cell suggest that directional bonding of the amine group to the halide cage is coupled to the stereochemical activity of the Sn lone pair via the Br atoms, i.e. that the bonding from the organic component may have a strong effect on the inorganic sublattice (principally via switching the direction of the lone pair with little to no energy cost).

Supporting information


Crystallographic Information File (CIF)
Contains datablocks global, Synchr


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CIF of model 1


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CIF of model 2


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CIF of model 3


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CIF of model 4


Rietveld powder data file (CIF format)
Contains datablock Synchr

CCDC reference: 788844

Computing details top

(Synchr) top
Crystal data top
CH6Br3NSnV = 407.32 (3) Å3
Mr = 390.47Z = 2
Orthorhombic, Pmc21Dx = 3.134 Mg m3
Hall symbol: P 2c -2Synchrotron radiation, λ = 0.700136 Å
a = 5.89406 (17) ÅT = 215 K
b = 8.3862 (2) Ådark red
c = 8.2406 (2) Å
Data collection top
Data collection mode: reflection
Si(111) monochromatorScan method: step
Specimen mounting: 15 mm wide flat plate2θmin = 14.600°, 2θmax = 41.748°, 2θstep = 0.004°
Refinement top
Refinement on InetExcluded region(s): 15.7040-16.1200
Least-squares matrix: full33 parameters
Rp = 0.0931 restraint
Rwp = 0.0720 constraints
Rexp = 0.062H-atom parameters not defined
RBragg = 0.069Weighting scheme based on measured s.u.'s
6683 data points
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
SN10.00.7488 (3)0.03174 (10)0.0106 (6)*
BR20.00.5217 (6)0.3596 (9)0.0396 (15)*
BR30.01.0743 (5)0.2563 (10)0.0446 (15)*
BR40.50.7248 (7)0.01624 (10)0.0567 (18)*
C50.50.308 (3)0.065 (2)0.021 (6)*
N60.50.183 (3)0.057 (2)0.021 (6)*
Geometric parameters (Å, º) top
SN1—BR23.306 (5)BR2—SN1iv2.675 (6)
SN1—BR2i2.675 (6)BR3—SN13.298 (6)
SN1—BR33.298 (6)BR3—SN1v2.712 (6)
SN1—BR3ii2.712 (6)BR4—SN12.9566 (6)
SN1—BR4iii2.9566 (6)BR4—SN1vi2.9566 (6)
SN1—BR42.9566 (6)C5—N61.457 (8)
BR2—SN13.306 (5)N6—C51.457 (8)
BR2i—SN1—BR3ii91.16 (18)BR3ii—SN1—BR490.06 (13)
BR2i—SN1—BR4iii85.38 (13)BR4iii—SN1—BR4170.8 (3)
BR2i—SN1—BR485.38 (13)SN1—BR4—SN1vi170.8 (3)
BR3ii—SN1—BR4iii90.06 (13)
Symmetry codes: (i) x, y+1, z1/2; (ii) x, y+2, z1/2; (iii) x1, y, z; (iv) x, y+1, z+1/2; (v) x, y+2, z+1/2; (vi) x+1, y, z.

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