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−.


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 CO 2 and Cl 2 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(Kim et al., , 2019. As part of our studies in this area, we now describe the dehydration synthesis and structure of the title compound.

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 À (Fig. 1a). The Tb cation is coordinated by five chloride ions and four oxygen atoms, forming a mono-capped TbO 4 Cl 5 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).
The shortest ClÁ Á ÁCl separation in TbOCl is 3.271 (4) Å , which compares with the van der Waals diameter of a Cl À ion of about 3.62 Å . The ClÁ Á ÁCl distances of other REOCl compounds are also short, ranging from 3.24 to 3.46 Å on going from Ho 3+ to La 3+ . 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.

Synthesis and crystallization
The title compound was synthesized by a simple heat treatment of TbCl 3 Á6H 2 O (Alfa Aesar, 99.99%). About 0.5 g of TbCl 3 Á6H 2 O was placed in an alumina crucible, heated to 400 C at 5 C min À1 , held for 8 h, and then cooled to room temperature at 5 C min À1 . 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.

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.   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 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.  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.
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%.