X-ray diffraction study of the phase transitions of (CH₃)₄NCdCl₃ between 293 and 80 K: a quantitative analysis of the ferroelastic domains distribution below 118 K

X-ray diffraction patterns of [N(CH)][CdCl], tetramethyl­ammonium trichlorocadmate(II), have been investigated in the temperature range 80-293 K, which includes two phase transitions at 118 and 104 K, respectively. The main interest in this compound is to establish the mechanism of the structural phase transitions common to other members of the isostructural family [(CH)N][MX]. It is supposed to be related to the ordering of the organic part together with some small distortion of the inorganic chains. The origin of the order-disorder mechanism would be the orientationally disordered distribution of the tetramethylammonium tetrahedra at room temperature. Maximum Entropy Methods suggest that the most probable distribution of the organic groups can be described through the so-called two-well model, in which one threefold axis of the tetramethylammonium tetrahedron coincides with the crystallographic threefold axis of the structure. Below 118 K the reflections are split. However, the splitting cannot be fully explained by the ferroelastic domains expected to appear after the phase transitions. Recent NMR results [Mulla-Osman et al. (1998). J. Phys. Condensed Matter, 10, 2465-2476] corroborate the existence of more domains than expected from symmetry considerations. A model of ferroelastic domains which is in agreement with both X-ray diffraction diagram and NMR measurements is proposed.

Computing details

Data collection: CAD4VPC; cell refinement: CAD4VPC; data reduction: xtal_DIFDAT_ABSORB_ADDREF_SORTRF; program(s) used to solve structure: Xtal; program(s) used to refine structure: xtal_CRYLSQ; molecular graphics: Xtal; software used to prepare material for publication: xtal_BONDLA_CIFIO.

(tmcc)

Crystal data

C4CdCl3H12N	Dx = 2.008 Mg m^−3
Mr = 292.62	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63/m	Cell parameters from 25 reflections
Hall symbol: -P_6c	θ = 4–25°
a = 9.126 (5) Å	µ = 2.99 mm^−1
c = 6.718 (2) Å	T = 293 K
V = 484.5 (6) Å^3	Hexagonal prism, colourless
Z = 2	0.29 × 0.12 × 0.09 mm
F(000) = 284

Data collection

CAD4 diffractometer	791 reflections with F____ > _.000_σ(F___)
Radiation source: xray_tube	Rint = 0.021
Graphite monochromator	θmax = 44.9°, θmin = 4.0°
θ/2θ scans	h = 0→15
Absorption correction: analytical (Alcock, 1974)	k = 0→15
Tmin = 0.709, Tmax = 0.788	l = −13→13
2979 measured reflections	3 standard reflections every 60 min
1406 independent reflections	intensity decay: 3%

Refinement

Refinement on F	0 restraints
Least-squares matrix: full	0 constraints
R[F^2 > 2σ(F^2)] = 0.045	H-atom parameters not defined
wR(F^2) = 0.032	Weighting scheme based on measured s.u.'s
S = 0.67	(Δ/σ)max = 0.363
791 reflections	Δρmax = 1.51 e Å^−3
29 parameters	Δρmin = −4.81 e Å^−3

Special details

Refinement. Sfls: F σ weight full matrix

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

	x	y	z	Uiso*/Ueq
Cd	0.00000	0.00000	0.00000	0.0319 (2)
Cl	0.1006 (1)	−0.1552 (1)	0.25000	0.0388 (5)
N	0.33330	0.66660	−0.25000	0.035 (2)
C1	0.532 (2)	0.153 (2)	0.212 (3)	0.075 (9)
C2	0.578 (3)	0.219 (2)	0.415 (2)	0.19 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd	0.0387 (2)	0.0387 (2)	0.0183 (2)	0.01934 (9)	0.00000	0.00000
Cl	0.0505 (7)	0.0460 (6)	0.0304 (5)	0.0319 (6)	0.00000	0.00000
N	0.034 (2)	0.034 (2)	0.036 (3)	0.017 (1)	0.00000	0.00000
C1	0.086 (8)	0.035 (5)	0.06 (2)	−0.006 (5)	−0.04 (1)	−0.006 (7)
C2	0.28 (5)	0.25 (3)	0.13 (2)	0.21 (3)	0.15 (2)	0.17 (2)

Geometric parameters (Å, º)

N^i—C1	1.50 (1)	N^i—C1^iv	1.50 (1)
N^i—C2^ii	1.46 (2)	N^i—C2^v	1.46 (2)
N^i—C1^iii	1.50 (2)	N^i—C1^vi	1.50 (2)
N^i—C2	1.46 (3)	N^i—C2^iv	1.46 (3)
N^i—C1^ii	1.50 (3)	N^i—C1^v	1.50 (3)
N^i—C2^iii	1.46 (4)	N^i—C2^vi	1.46 (4)
C1—N^i—C2^ii	118 (1)	C2—N^i—C2^v	142 (2)
C1—N^i—C1^iii	117 (1)	C2—N^i—C1^vi	102 (1)
C1—N^i—C2	59 (1)	C2—N^i—C2^iv	99 (2)
C1—N^i—C1^ii	117 (1)	C2—N^i—C1^v	100 (2)
C1—N^i—C2^iii	115 (1)	C2—N^i—C2^vi	142 (3)
C1—N^i—C1^iv	20 (1)	C1^ii—N^i—C2^iii	118 (2)
C1—N^i—C2^v	102 (1)	C1^ii—N^i—C1^iv	121 (1)
C1—N^i—C1^vi	121 (1)	C1^ii—N^i—C2^v	40 (1)
C1—N^i—C2^iv	40 (1)	C1^ii—N^i—C1^vi	121 (2)
C1—N^i—C1^v	121 (1)	C1^ii—N^i—C2^iv	100 (2)
C1—N^i—C2^vi	100 (1)	C1^ii—N^i—C1^v	20 (1)
C2^ii—N^i—C1^iii	115 (1)	C1^ii—N^i—C2^vi	102 (2)
C2^ii—N^i—C2	68 (2)	C2^iii—N^i—C1^iv	100 (1)
C2^ii—N^i—C1^ii	59 (1)	C2^iii—N^i—C2^v	142 (1)
C2^ii—N^i—C2^iii	68 (2)	C2^iii—N^i—C1^vi	40 (2)
C2^ii—N^i—C1^iv	102 (1)	C2^iii—N^i—C2^iv	142 (3)
C2^ii—N^i—C2^v	99 (1)	C2^iii—N^i—C1^v	102 (2)
C2^ii—N^i—C1^vi	100 (1)	C2^iii—N^i—C2^vi	99 (2)
C2^ii—N^i—C2^iv	142 (2)	C1^iv—N^i—C2^v	118 (1)
C2^ii—N^i—C1^v	40 (1)	C1^iv—N^i—C1^vi	117 (1)
C2^ii—N^i—C2^vi	142 (1)	C1^iv—N^i—C2^iv	59 (1)
C1^iii—N^i—C2	118 (2)	C1^iv—N^i—C1^v	117 (1)
C1^iii—N^i—C1^ii	117 (2)	C1^iv—N^i—C2^vi	115 (1)
C1^iii—N^i—C2^iii	59 (2)	C2^v—N^i—C1^vi	115 (1)
C1^iii—N^i—C1^iv	121 (1)	C2^v—N^i—C2^iv	68 (2)
C1^iii—N^i—C2^v	100 (1)	C2^v—N^i—C1^v	59 (1)
C1^iii—N^i—C1^vi	20 (1)	C2^v—N^i—C2^vi	68 (2)
C1^iii—N^i—C2^iv	102 (1)	C1^vi—N^i—C2^iv	118 (2)
C1^iii—N^i—C1^v	121 (2)	C1^vi—N^i—C1^v	117 (2)
C1^iii—N^i—C2^vi	40 (2)	C1^vi—N^i—C2^vi	59 (2)
C2—N^i—C1^ii	115 (2)	C2^iv—N^i—C1^v	115 (2)
C2—N^i—C2^iii	68 (1)	C2^iv—N^i—C2^vi	68 (1)
C2—N^i—C1^iv	40 (1)	C1^v—N^i—C2^vi	118 (2)

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