Orientational ordering, tilting and lone-pair activity in the perovskite methylammonium tin bromide, CH₃NH₃SnBr₃

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).

Computing details

(Synchr)

Crystal data

CH6Br3NSn	V = 407.32 (3) Å^3
Mr = 390.47	Z = 2
Orthorhombic, Pmc21	Dx = 3.134 Mg m^−3
Hall symbol: P 2c -2	Synchrotron radiation, λ = 0.700136 Å
a = 5.89406 (17) Å	T = 215 K
b = 8.3862 (2) Å	dark red
c = 8.2406 (2) Å

Data collection

X3B1 diffractometer	Data collection mode: reflection
Si(111) monochromator	Scan method: step
Specimen mounting: 15 mm wide flat plate	2θmin = 14.600°, 2θmax = 41.748°, 2θstep = 0.004°

Refinement

Refinement on Inet	Excluded region(s): 15.7040-16.1200
Least-squares matrix: full	33 parameters
Rp = 0.093	1 restraint
Rwp = 0.072	0 constraints
Rexp = 0.062	H-atom parameters not defined
RBragg = 0.069	Weighting scheme based on measured s.u.'s
6683 data points

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

	x	y	z	Uiso*/Ueq
SN1	0.0	0.7488 (3)	0.03174 (10)	0.0106 (6)*
BR2	0.0	0.5217 (6)	0.3596 (9)	0.0396 (15)*
BR3	0.0	1.0743 (5)	0.2563 (10)	0.0446 (15)*
BR4	0.5	0.7248 (7)	0.01624 (10)	0.0567 (18)*
C5	0.5	0.308 (3)	0.065 (2)	0.021 (6)*
N6	0.5	0.183 (3)	−0.057 (2)	0.021 (6)*

Geometric parameters (Å, º)

SN1—BR2	3.306 (5)	BR2—SN1^iv	2.675 (6)
SN1—BR2^i	2.675 (6)	BR3—SN1	3.298 (6)
SN1—BR3	3.298 (6)	BR3—SN1^v	2.712 (6)
SN1—BR3^ii	2.712 (6)	BR4—SN1	2.9566 (6)
SN1—BR4^iii	2.9566 (6)	BR4—SN1^vi	2.9566 (6)
SN1—BR4	2.9566 (6)	C5—N6	1.457 (8)
BR2—SN1	3.306 (5)	N6—C5	1.457 (8)
BR2^i—SN1—BR3^ii	91.16 (18)	BR3^ii—SN1—BR4	90.06 (13)
BR2^i—SN1—BR4^iii	85.38 (13)	BR4^iii—SN1—BR4	170.8 (3)
BR2^i—SN1—BR4	85.38 (13)	SN1—BR4—SN1^vi	170.8 (3)
BR3^ii—SN1—BR4^iii	90.06 (13)

Symmetry codes: (i) x, −y+1, z−1/2; (ii) x, −y+2, z−1/2; (iii) x−1, y, z; (iv) x, −y+1, z+1/2; (v) x, −y+2, z+1/2; (vi) x+1, y, z.