Dehydration synthesis and crystal structure of terbium oxychloride, TbOCl

Terbium oxychloride, TbOCl, was synthesized via the simple heat-treatment of TbCl₃·6H₂O and its structure was determined by refinement against X-ray powder diffraction data. TbOCl crystallizes with the matlockite (PbFCl) structure in the tetra­gonal space group P4/nmm and is composed of alternating (001) layers of (TbO)_n and n Cl⁻. The unit-cell parameters, unit-cell volume, and density were compared to the literature data of other isostructural rare-earth oxychlorides in the same space group and showed good agreement when compared to the calculated trendlines.

1. Chemical context

Rare-earth oxychlorides, REOCl, are promising materials for various applications including use as catalysts, sensors, and phosphors (Podkolzin et al., 2007; Au et al., 1997; Peringer et al., 2009; Marsal et al., 2005,; Kim et al., 2019; Berdowski et al., 1984; Imanaka et al., 2001a,b; Okamoto et al., 2002; Kim et al., 2014). LaOCl is a stable catalyst for converting methane to methyl chloride (Podkolzin et al., 2007) and can be used as a sensor material to detect CO₂ and Cl₂ gases (Marsal et al., 2005; Imanaka et al., 2001b). The EuOCl catalyst showed high efficiency in converting ethyl­ene to vinyl chloride (Scharfe et al., 2016). The luminescent properties of REOX (RE = La, Eu; X = F, Cl, Br, I) can be controlled to emit a wide range of visible light from blue to red by changing the crystal symmetries and compositions (Kim et al., 2014, 2019). As part of our studies in this area, we now describe the dehydration synthesis and structure of the title compound.

2. Structural commentary

The structural parameters of REOCl (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho) in the literature and current study are summarized in Table 1. All these REOCl compounds crystallize in the matlockite (PbFCl; Bannister, 1934) structure within the tetra­gonal P4/nmm space group. The crystal structure of TbOCl contains alternating (001) layers of (TbO)_n and n Cl⁻ (Fig. 1a). The Tb cation is coordinated by five chloride ions and four oxygen atoms, forming a mono-capped TbO₄Cl₅ square anti­prism (Fig. 1b and 1c). The RE—Cl and RE—O bond lengths in the REOCl compounds are provided in Table 1. With larger RE cations in the structures, the RE—Cl and RE—O bond lengths increase (Fig. 2).

Table 1
Structural parameters of REOCl compounds

All compounds crystallize in the P4/nmm space group. For the RE—Cl bond lengths, the first value refers to one neighboring Cl atom, and the second number refers to four neighboring Cl atoms. Densities are calculated from crystallographic data.

RE	a(Å)	c(Å)	V (Å^3)	Density(g cm^−3)	RE—O(Å)	RE—Cl(Å)	Cl⋯Cl(Å)	Cl⋯O(Å)	O⋯O (Å)	ICSD/PDF
Ho	3.893	6.602	100.1	7.182	2.247	3.04, 3.05	3.24	3.12	2.753	76171 (Templeton & Dauben, 1953)
Dy	3.91	6.62	101.2	7.023						00–047-1725 (Kirik et al., 1996)
Tb	3.9269	6.648	102.5	6.815						00–048-1648 (Kirik et al., 1996)
Tb	3.9279	6.6556	102.7	6.804	2.2649	3.064, 3.082	3.271	3.151	2.7774	Current study
Gd	3.9495	6.6708	104.1	6.661	2.2839	3.036, 3.098	3.267	3.176	2.7927	59232 (Meyer & Schleid, 1986)
Gd	3.9698	6.7008	105.6	6.564	2.28	3.212, 3.071	3.428	3.089	2.8071	77820 (Hölsä et al., 1996)
Eu	3.9646	6.695	105.2	6.42	2.286	3.08, 3.11	3.3	3.17	2.8034	28529 (Bärnighausen et al., 1965)
Eu	3.9668	6.6955	105.4	6.412	2.2901	3.062, 3.1103	3.289	3.183	2.80492	54682 (Schnick, 2004)
Sm	3.982	6.721	106.6	6.289	2.296	3.09, 3.12	3.31	3.19	2.8157	26581 (Templeton & Dauben, 1953)
Nd	4.04	6.77	110.5	5.882	2.359	3.114, 3.11	3.428	3.165	2.86	31665 (Zachariasen, 1949)
Nd	4.0249	6.7837	109.9	5.914	2.3362	3.082, 3.141	3.343	3.221	2.84603	59231 (Meyer & Schleid, 1986)
Pr	4.053	6.799	111.7	5.723	2.3674	3.128, 3.116	3.441	3.178	2.866	31664 (Zachariasen, 1949)
Ce	4.0866	6.8538	114.5	5.558	2.3687	3.1190, 3.1846	3.3942	3.2572	2.8897	412069 (Schnick, 2004)
Ce	4.0785	6.8346	113.7	5.596	2.36413	3.103, 3.180	3.38	3.254	2.88393	72154 (Wołcyrz & Kepinski, 1992)
La	4.109	6.865	115.9	5.454	2.39	3.14, 3.18	3.45	3.24	2.9055	24611 (Sillen & Nylander, 1941)
La	4.117	6.881	116.6	5.42	2.3866	3.126, 3.2046	3.416	3.2751	2.9112	40297 (Brixner & Moore, 1983)
La	4.1351	6.904	118.1	5.355	2.395	3.165, 3.209	3.457	3.268	2.92397	77815 (Hölsä et al., 1996)
La	4.1162	6.8746	116.5	5.428	2.3832	3.138, 3.201	3.425	3.265	2.9106	84330 (Hölsä et al., 1997)
La	4.12	6.882	116.8	5.412						00–008-0477 (Swanson et al., 1957)

Figure 1 (a) Crystal structure of TbOCl, (b) the coordination environment of Tb, and (c) polyhedron representation of the Tb environment.

Figure 2 The RE—Cl and RE—O bond lengths in the REOCl compounds listed in Table 1 as a function of RE crystal radius (coordination = 9) according to Shannon (1976). Where multiple values were available, averages and standard deviations are included for the datapoints. For (a), 1-nd and 4-nd denote 1 and 4 neighbor distances, respectively

The shortest Cl⋯Cl separation in TbOCl is 3.271 (4) Å, which compares with the van der Waals diameter of a Cl⁻ ion of about 3.62 Å. The Cl⋯Cl distances of other REOCl compounds are also short, ranging from 3.24 to 3.46 Å on going from Ho³⁺ to La³⁺. With non-bonded vectors shorter than the van der Waals separation, strong inter­actions between atoms are expected in the structure (Maslen et al., 1996). Templeton & Dauben (1953) mention the presence of weaker anion–anion repulsion between Cl atoms in REOCl structures. The structural parameters of TbOCl were compared with the trendlines calculated using the values from Table 1 (Fig. 3). The unit-cell parameters and volumes increase linearly with the larger RE cations (Shannon, 1976) whereas the densities decrease non-linearly, fitting well to a 2nd order polynomial trend.

Figure 3 (a, b) Unit-cell parameters (a and c, respectively), (c) unit-cell volumes, and calculated unit-cell densities as a function of the crystal radius of the RE (coordination = 9) according to Shannon (1976) compared to literature values provided in Table 1.

3. Synthesis and crystallization

The title compound was synthesized by a simple heat treatment of TbCl₃·6H₂O (Alfa Aesar, 99.99%). About 0.5 g of TbCl₃·6H₂O was placed in an alumina crucible, heated to 400°C at 5°C min⁻¹, held for 8 h, and then cooled to room temperature at 5°C min⁻¹. This synthesis method was used in our previous study (Riley et al., 2018). The resulting product was a light-brown powder, which was ground in a mortar and pestle for X-ray powder diffraction analysis.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. The unit-cell parameters were obtained using TOPAS (version 4.2; Bruker, 2009) by refining the GdOCl pattern (ICSD 77820) with geometrical and chemical resemblance as a starting model. The Rietveld refinement was performed using JANA2006 (Petříček et al., 2014) with the obtained unit-cell parameters as initial values. A pseudo-Voigt function with other peak-shape parameters were used to fit peaks, and the background was modeled with a Chebychev polynomial. The plot of the Rietveld refinement result is shown in Fig. 4. The final refinement converged at R_wp = 3.22%.

Table 2
Experimental details

Crystal data
Chemical formula	TbOCl
Mr	210.4
Crystal system, space group	Tetragonal, P4/nmm
Temperature (K)	293
a, c (Å)	3.9279 (2), 6.6556 (5)
V (Å^3)	102.68 (1)
Z	2
Radiation type	Cu Kα, λ = 1.54188 Å
Specimen shape, size (mm)	Cylinder, 25 × 25
Data collection
Diffractometer	Bruker D8 Advance
Specimen mounting	Packed powder pellet
Data collection mode	Reflection
Scan method	Step
2θ values (°)	2θmin = 5, 2θmax = 68.977, 2θstep = 0.019
Refinement
R factors and goodness of fit	Rp = 0.020, Rwp = 0.032, Rexp = 0.009, R(F) = 0.033, χ^2 = 13.690
No. of parameters	17
Computer programs: XRD Commander (Kienle & Jacob, 2003), TOPAS (Bruker, 2009), SUPERFLIP (Palatinus & Chapuis, 2007), JANA2006 (Petříček et al., 2014), VESTA (Momma & Izumi, 2011) and publCIF (Westrip, 2010).

Figure 4 Measured, calculated, and difference XRD patterns of TbOCl.

Supporting information

Computing details

Data collection: XRD Commander (Kienle & Jacob, 2003); cell refinement: TOPAS (Bruker, 2009); program(s) used to solve structure: SUPERFLIP (Palatinus & Chapuis, 2007); program(s) used to refine structure: JANA2006 (Petříček et al., 2014); molecular graphics: VESTA (Momma & Izumi, 2011); software used to prepare material for publication: publCIF (Westrip, 2010).

Terbium oxychloride

Crystal data

TbOCl	Z = 2
Mr = 210.4	Dx = 6.804 Mg m^−3
Tetragonal, P4/nmm	Cu Kα radiation, λ = 1.54188 Å
a = 3.9279 (2) Å	T = 293 K
c = 6.6556 (5) Å	light brown
V = 102.68 (1) Å^3	cylinder, 25 × 25 mm

Data collection

Bruker D8 Advance diffractometer	Data collection mode: reflection
Radiation source: sealed X-ray tube	Scan method: step
Specimen mounting: packed powder pellet	2θmin = 5°, 2θmax = 68.977°, 2θstep = 0.019°

Refinement

Rp = 0.020	17 parameters
Rwp = 0.032	Weighting scheme based on measured s.u.'s
Rexp = 0.009	(Δ/σ)max = 0.030
R(F) = 0.033	Background function: 8 Chebyshev polynoms
3292 data points	Preferred orientation correction: March & Dollase
Profile function: Pseudo-Voigt

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

	x	y	z	Uiso*/Ueq
Tb1	0.5	0	0.3305 (2)	0.002
Cl1	0	0.5	0.1298 (9)	0.002
O1	1	0	0.5	0.002

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Tb1	0.002	0.002	0.002	0	0	0
Cl1	0.002	0.002	0.002	0	0	0
O1	0.002	0.002	0.002	0	0	0

Geometric parameters (Å, º)

Tb1—Tb1^i	3.5784 (13)	Tb1—O1^v	2.2649 (7)
Tb1—Tb1^ii	3.5784 (13)	Tb1—O1	2.2649 (7)
Tb1—Tb1^iii	3.5784 (13)	Tb1—O1^vi	2.2649 (7)
Tb1—Tb1^iv	3.5784 (13)	Tb1—O1^vii	2.2649 (7)
Tb1^i—Tb1—Tb1^ii	66.57 (2)	Tb1^iii—Tb1—O1^vii	99.31 (4)
Tb1^i—Tb1—Tb1^iii	66.57 (2)	Tb1^iv—Tb1—O1^v	99.31 (4)
Tb1^i—Tb1—Tb1^iv	101.82 (4)	Tb1^iv—Tb1—O1	37.817 (14)
Tb1^i—Tb1—O1^v	37.817 (14)	Tb1^iv—Tb1—O1^vi	99.31 (4)
Tb1^i—Tb1—O1	99.31 (4)	Tb1^iv—Tb1—O1^vii	37.817 (14)
Tb1^i—Tb1—O1^vi	37.817 (14)	O1^v—Tb1—O1	120.25 (6)
Tb1^i—Tb1—O1^vii	99.31 (4)	O1^v—Tb1—O1^vi	75.63 (3)
Tb1^ii—Tb1—Tb1^iii	101.82 (4)	O1^v—Tb1—O1^vii	75.63 (3)
Tb1^ii—Tb1—Tb1^iv	66.57 (2)	O1—Tb1—O1^vi	75.63 (3)
Tb1^ii—Tb1—O1^v	37.817 (14)	O1—Tb1—O1^vii	75.63 (3)
Tb1^ii—Tb1—O1	99.31 (4)	O1^vi—Tb1—O1^vii	120.25 (6)
Tb1^ii—Tb1—O1^vi	99.31 (4)	Tb1—O1—Tb1^viii	120.25 (4)
Tb1^ii—Tb1—O1^vii	37.817 (14)	Tb1—O1—Tb1^iii	104.37 (2)
Tb1^iii—Tb1—Tb1^iv	66.57 (2)	Tb1—O1—Tb1^iv	104.37 (2)
Tb1^iii—Tb1—O1^v	99.31 (4)	Tb1^viii—O1—Tb1^iii	104.37 (2)
Tb1^iii—Tb1—O1	37.817 (14)	Tb1^viii—O1—Tb1^iv	104.37 (2)
Tb1^iii—Tb1—O1^vi	37.817 (14)	Tb1^iii—O1—Tb1^iv	120.25 (4)

Symmetry codes: (i) −x+1/2, y−1/2, −z+1; (ii) −x+1/2, y+1/2, −z+1; (iii) −x+3/2, y−1/2, −z+1; (iv) −x+3/2, y+1/2, −z+1; (v) x−1, y, z; (vi) −y+1/2, x−3/2, z; (vii) −y+1/2, x−1/2, z; (viii) x+1, y, z.