The matlockite-type praseodymium(III) oxide bromide PrOBr

Pia Talmon-Gros; Christian M Schurz; Thomas Schleid

doi:10.1107/S1600536811048227

. 2011 Nov 23;67(Pt 12):i74. doi: 10.1107/S1600536811048227

The matlockite-type praseodymium(III) oxide bromide PrOBr

Pia Talmon-Gros ^a, Christian M Schurz ^a, Thomas Schleid ^a,^*

PMCID: PMC3238581 PMID: 22199472

Abstract

The crystal structure of the praseodymium(III) oxide bromide, PrOBr, can be best described with layers of agglomerated square antiprisms [PrO₄Br₄]⁹⁻. These slabs are stacked along the c axis and linked via two different secondary contacts between Pr³⁺ and Br⁻. The Pr³⁺ cations occupy the Wyckoff site 2c with 4mm symmetry and carry four O²⁻ anions as well as four primary Br⁻ anions, yielding a coordination number of 8. While the Br⁻ anions exhibit the same site symmetry as the Pr³⁺ cations, the oxide anions are located at the Wyckoff position 2a with site symmetry Inline graphic m2 and have four Pr³⁺ cations as neighbours, defining a tetrahedron.

Related literature

For prototypic PbFCl (mineral name: matlockite), see: Nieuwenkamp & Bijvoet (1932 ▶) and for an early powder study, see: Mayer et al. (1965 ▶). For other PrOX structures, see: Baenziger et al. (1950 ▶) for X = F, Zachariasen (1949 ▶) for X = Cl, and Potapova et al. (1977 ▶) for X = I. For data used for a comparison of the unit-cell dimensions, see: Shannon (1976 ▶) for ionic radii and Biltz (1934 ▶) for volume increments. For a proper classification of primary and secondary contacts, see: MAPLE (Hoppe, 1975 ▶) and for the bond-valence method, see: Brown (2002 ▶). For a comparison of intended synthesis attempts, see: Mattausch & Simon (1996 ▶); Lulei (1998 ▶).

Experimental

Crystal data

PrOBr
M _r = 236.82
Tetragonal,
a = 4.0671 (3) Å
c = 7.4669 (5) Å
V = 123.51 (2) Å³
Z = 2
Mo Kα radiation
μ = 35.52 mm⁻¹
T = 293 K
0.11 × 0.07 × 0.02 mm

Data collection

Bruker–Nonius KappaCCD diffractometer
Absorption correction: numerical (X-SHAPE; Stoe & Cie, 1999 ▶) T _min = 0.049, T _max = 0.535
1621 measured reflections
113 independent reflections
111 reflections with I > 2σ(I)
R _int = 0.082

Refinement

R[F ² > 2σ(F ²)] = 0.026
wR(F ²) = 0.059
S = 1.20
113 reflections
10 parameters
Δρ_max = 1.14 e Å⁻³
Δρ_min = −2.52 e Å⁻³

Data collection: COLLECT (Nonius, 1998 ▶); cell refinement: SCALEPACK (Otwinowski & Minor, 1997 ▶); data reduction: SCALEPACK and DENZO (Otwinowski & Minor, 1997 ▶); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: DIAMOND (Brandenburg, 2006 ▶); software used to prepare material for publication: SHELXL97.

Supplementary Material

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S1600536811048227/fi2117sup1.cif

e-67-00i74-sup1.cif^{(13.9KB, cif)}

Structure factors: contains datablock(s) I. DOI: 10.1107/S1600536811048227/fi2117Isup2.hkl

e-67-00i74-Isup2.hkl^{(6.5KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Selected bond lengths (Å).

Pr—O 4	2.3496 (3)
Pr—Br 4	3.2457 (8)
Pr—Br	3.6083 (14)
Pr—Br	3.8586 (14)

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Acknowledgments

This work was supported by the State of Baden-Württemberg (Stuttgart) and the German Research Foundation (DFG, Bonn) within the funding programme Open Access Publishing. We thank Dr Sabine Strobel for the data collection.

supplementary crystallographic information

Comment

With the exception of PrOF (Baenziger et al. 1950) all praseodymium(III) oxide halides of the general composition PrOX (X = Cl – I; Zachariasen 1949, Potapova et al. 1977) crystallize with the matlockite-type structure (Nieuwenkamp & Bijvoet, 1932). The tetragonal crystal structure of the here presented praseodymium(III) oxide bromide PrOBr can be best described with layers of agglomerated square antiprisms [PrO₄Br₄]^9– (d(Pr³⁺–O^2-) = 234.96 (4) pm, d(Pr³⁺–Br^-) = 324.57 (8) pm, d(Pr³⁺···Br^-) = 360.8 (1) and 385.9 (1) pm; Figure 1). These slabs are stacked along the c-axis and linked via two different secondary contacts between Pr³⁺ and Br^- (Figure 2). According to the ionic radii (r_Cl = 180 pm, r_Br = 195 pm, r_I = 220 pm; Shannon, 1976) of the halide anions involved the expansion of the unit-cell dimensions occurs in quite an usual range, but the c-axes become significantly longer than the a-axes (a-axes: from 405.3 pm to 408.6 pm; c-axes: from 679.9 pm to 916.2 pm) along the Cl^-–Br^-–I^- track. The lattice parameters of single crystalline PrOBr (a = 406.71 pm, c = 746.69 pm) fit almost perfectly with that from a previous powder diffraction study (a = 407.1 pm, c = 748.7 pm; Mayer et al. 1965). Differences in the molar volumes of the PbFCl-type praseodymium(III) oxide halides (V_m(PrOCl) = 33.6 cm³/mol, V_m(PrOBr) = 37.2 cm³/mol, V_m(PrOI) = 46.1 cm³/mol) correspond well with the differences of the molar volumes of the respective halide anions (V_m(Cl^-) = 16.3 cm³/mol, V_m(Br^-) = 19.2 cm³/mol, V_m(I^-) = 24.5 cm³/mol; Biltz 1934). However, the Pr³⁺ cations occupy the Wyckoff site 2c (symmetry: 4mm) and bond four O^2- anions as well as four+one+one Br^- anions ending up with a total coordination number of 8+1+1 (Figure 1). While the Br^- anions exhibit the same site symmetry as the Pr³⁺ cations, the oxide anions are located at Wyckoff position 2a with the site symmetry 4m2. Bond-Valence and MAPLE calculations support the interpretation of one important (d(Pr³⁺···Br^-) = 360.8 (1) pm) and one less important secondary contact (d(Pr³⁺···Br^-) = 385.9 (1) pm): The valency and ECoN for the first bond amounts to values of about 0.08 (with R₀ = 267 pm, b = 37 pm; Brown, 2002) and 0.12 (Hoppe, 1975), but almost nil for the second one, since this next nearest contact to bromide has only very low influence on the effective coordination sphere of the Pr³⁺ cations (ECoN = 0.03).

Experimental

Pale green, transparent, plate-shaped single crystals of PrOBr were obtained as by-product from a mixture of 0.06 g Pr, 0.38 g PrBr₃ and 0.01 g NaN₃, along with 0.30 g NaBr added as a flux. The mixture was kept at 800 °C for 7 days in an evacuated, sealed fused-silica vessel designed to produce the praseodymium(III) nitride bromide Pr₃NBr₆in analogy with La₃NBr₆ (Lulei, 1998) and Ce₃NBr₆ (Mattausch & Simon, 1996).