CdBiO₂Cl: synthesis and powder structure solution

The title compound, cadmium bismuth dioxide chloride, CdBiO₂Cl, was obtained as a white powder by reaction of solid BiOCl with CdO at 973 K. Ab initio crystal structure determination was carried out using X-ray powder diffraction techniques, including direct methods for atom location and Rietveld fitting for the final refinement. Being monoclinic, the crystal structure can be related to tetragonal Sillen layered phases. The main structural elements present are CdBiO₂⁺ pleated metal-oxy­gen layers alternating with Cl layers along the c axis, whereas along the b axis, all atoms are on mirror planes. The formation of a strong Cd-Cl bond draws the layers together, causing layer deformation and a monoclinic distortion in the layer arrangement.

S1. Comment

At present, there is much information on Bi-based mixed oxides with layered structures, where infinite Bi₂O₂²⁺ charged layers alternate with slabs of other metal oxides (Alexandrov & Beznosikov, 1997). Much less attention has been paid to a related family of layered compounds known as Sillen's phases, revealed in 1939 (Sillen, 1939; Dolgih & Holodkovskay, 1992). In these compounds, the space between the Bi₂O₂²⁺ layers is filled with halogen and metal-halogen slabs. For many members of this family, the crystal-structure data were reported by Sillen and they have not been revised since.

It is a characteristic of both these classes of compound that Bi₂O₂²⁺ layers exhibit a surprising ability to complement the variety of oxygen-metal slabs, while keeping the O atoms strictly in the plane and allowing the Bi to be substituted by other metals. In particular, Sillen (1942) described three compounds, with the compositions Cd_1.25Bi_1.5O₂Cl₃, Cd_1.1Bi_1.6O₂Cl₃ and Cd_1.03Bi_1.65O₂Cl₃, and reported unit cell constants of a = 3.865 and c = 21.08 Å, a = 3.862 and c = 21.14 Å, and a = 3.920 and c = 22.42 Å, respectively, space group P4/mmm. The suggested model of the crystal structure consists of two layers of [Cd_xBi_{2 - x}O₂]^{(2 - x)+} separated by a three-layered slab of [Cd_{1 - y}Cl₃] with the NaCl structure. Our attempt to follow the published synthesis produced the title substance, (I), with the composition CdBiO₂Cl in the monoclinic system. \sch

The crystal structure of (I) (Fig. 1) consists of infinite double metal-oxygen CdBiO₂²⁺ layers alternating with Cl sheets. The metal-oxygen layers in turn consist of two metal sublayers with O atoms in between. Bi and Cd atoms are located at the vertices of pyramidal BiO₄ and CdO₄ units linked by shared edges. Interatomic Bi—O and Cd—O distances correspond well with the literature values (Wells, 1984). It should be noted that the M—O distances are not equal. The Cd—O2 bond [2.453 (14) Å] differs most from the average value (2.26 Å), but is still in the acceptable range.

A distortion of the oxygen plane is a characteristic of (I) which differentiates the structure from other members of the family. The O—O distances are in the range 2.72–2.98 Å. The O2—O1—O2 angle of 175° also characterizes a distortion of the oxygen plane.

Although Bi³⁺ and Cd²⁺ cations are rather similar in size, the metal atoms in the sublayers of (I) are well ordered. Conditionally, the arrangement of the metal atoms can be regarded as a plane, with the rows of Cd or Bi along the 2₁ axes. The degree of distortion of this metal-atom plane can be characterized by the Cd—Bi—Cd angle of 172°.

The nearest neighbour to Cd besides the four O atoms is the Cl atom, at a distance of 2.49 Å. This is about 1 Å less than regular M-halogen distances in Sillen phases. It is reasonable to suppose that Cd—Cl bonds cause deformation of both the metal-oxygen and Cl layers, as well as the shift in the layers leading to monoclinic distortion.

The title compound differs from those described by Sillen, mainly in the structure of the halogen layer. There is a limited resemblance to the halogen layer in Bi₃O₄Cl and the series M_xBi_{3 - x}O₄Cl (Nurgaleev, 1983; Aurivillius, 1984). However, the metal-oxygen layers in these phases are of triple thickness.

In summary, the main structural elements of CdBiO₂Cl, double metal-oxygen layers alternating with Cl layers, are typical for Sillen phases. However, formation of a strong Cd—Cl bond draws the layers closer together, causing layer deformation and distortion in the layer arrangement, from tetragonal to monoclinic.

Related literature

For related literature, see: Alexandrov & Beznosikov (1997); Aurivillius (1984); Brauer (1956); Dolgih & Holodkovskay (1992); Kirik et al. (1979); Nurgaleev et al. (1983); Sillen (1939, 1942); Visser (1969); Wells (1984).

S2. Experimental

CdBiO₂Cl was synthesized as a white powder by the reaction of BiOCl and CdO. High purity starting materials were used as received from Reachim. BiOCl was obtained from Bi₂O₃ according to the method of Brauer (1956) and its purity was checked by X-ray diffraction. A stoichiometric mixture of BiOCl with CdO was homogenized and heated at 823 K in air for 1 h. An alternative synthesis consists of heating a stoichiometric mixture of Bi₂O₃, CdO and CdCl₂ at 973 K.

S3. Refinement

The sample was prepared by top-loading the standard quartz sample holder and cutting the excess of the well grained substance. Corundum was used as the external standard. The powder pattern of CdBiO₂Cl is presented in Fig. 2. The cell parameters were obtained using programs described by Kirik et al. (1979) and Visser (1969). Analysis of the systematic absences gave two possible space groups, P2₁ or P2₁/m; the latter was used. Errors given in the tables report primarily the precision of the measurement rather than the accuracy.

Computing details

Data collection: DRON-4 data collection software; cell refinement: local modification of DBWM (Wiles & Young, 1981); data reduction: XDIG (local program); program(s) used to solve structure: EXPO (Altomare et al., 1995); program(s) used to refine structure: local modification of DBWM; molecular graphics: DIAMOND (Brandenburg, 1999).

Figures

	Fig. 1. The crystal structure of CdBiO2Cl.
	Fig. 2. The Rietveld plot for CdBiO2Cl.

Bismuth cadmium dioxide chloride

Crystal data

CdBiO2Cl	Z = 2
Mr = 388.84	Cu Kα radiation, λ = 1.540562, 1.544390 Å
Monoclinic, P121/m1	T = 293 K
a = 7.5878 (7) Å	Particle morphology: thin powder
b = 4.1397 (4) Å	white
c = 6.0594 (6) Å	circular flat plate, 20.0 × 20.0 mm
β = 101.529 (11)°	Specimen preparation: Prepared at 973 K
V = 186.50 (3) Å^3

Data collection

DRON-4 powder diffractometer	Data collection mode: reflection
Radiation source: conventional sealed X-ray tube, BSV-28	Scan method: step
Graphite monochromator	2θmin = 10°, 2θmax = 90°, 2θstep = 0.02°
Specimen mounting: packed powder pellet

Refinement

Refinement on F^2	Excluded region(s): none
Least-squares matrix: full	Profile function: Pearson VII
Rp = 0.076	40 parameters
Rwp = 0.106	0 restraints
Rexp = 0.074	5 constraints
RBragg = 0.034	Weighting scheme based on measured s.u.'s
R(F^2) = 0.030
χ^2 = 2.045	Preferred orientation correction: March-Dollase correction
4000 data points

Special details

Refinement. R_prof-backgr = 0.076

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

	x	y	z	Uiso*/Ueq
O1	0.864 (2)	0.2500	0.513 (2)	0.0199 (5)*
O2	0.400 (2)	0.2500	0.555 (2)	0.02115 (9)*
Cl	0.8182 (9)	0.7500	0.036 (1)	0.00106 (11)*
Cd	0.9092 (4)	0.7500	0.6637 (4)	0.00798 (11)*
Bi	0.5954 (2)	0.2500	0.3101 (2)	0.00139 (6)*

Geometric parameters (Å, º)

Bi—O1	2.160 (14)	Cd—O1^iii	2.201 (16)
Bi—O2	2.298 (15)	Cd—O1	2.261 (5)
Bi—O2^i	2.223 (5)	Cd—O1^iv	2.259 (5)
Bi—O2^ii	2.222 (5)	Cd—Cl^v	2.488 (7)
Cd—O2^i	2.453 (14)
Bi···Cl^vi	3.320 (6)	Bi···Cl^viii	3.408 (6)
Cd···Cl^vii	3.220 (5)
Bi—O1—Cd^vii	117.6 (5)	O1—Cd—O1^iii	82.8 (4)
Bi—O2—Cd^ii	108.8 (4)	O1—Cd—O1^iv	132.6 (4)
O2^i—Bi—O2	74.2 (4)	O1—Bi—O2	106.8 (4)
O1—Cd—O2^i	74.4 (4)	O2^ii—Bi—O2^i	137.2 (4)

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