Rerefinement of the crystal structure of α-ThBr4

The rerefinement of the crystal structure of α-ThBr4 (α-ThCl4 structure type) results in higher precision of the lattice parameters and the atomic coordinates.


Structure description
A crystal of ThBr 4 in its �-modification was isolated as a side product from the reaction of �-ThBr 4 with CuBr at 753 K.
The crystal structure of �-ThBr 4 has been described only once, from a single-crystal X-ray diffraction study at room temperature (Mason et al., 1974), where the authors refer to this modification also as the low-temperature polymorph.They reported the transition temperature at 699 � 5 K and the crystal structure of �-ThBr 4 was assigned to the �-ThCl 4 structure type in the space group I4 1 /a (No. 88, tI20).A comparison of the structural parameters of the original crystal structure refinement and of the current rerefinement is given in Table 1.
Fig. 1 shows the crystal structure based on the current X-ray diffraction data.There is one Th atom (multiplicity 4, Wyckoff letter a, site symmetry 4..) and one Br atom (16f, site symmetry 1) in the asymmetric unit.The Th atom is surrounded by eight Br atoms to form a tetragonal-disphenoidal coordination polyhedron.The Th-Br bond lengths of 4 � 2.9100 (4) A ˚and 4 � 3.0107 (4) A ˚are in good agreement with previously reported values of 2.909 and 3.020 A ˚(no s.u.values or temperature given; Mason et al., 1974), but different compared to those in �-ThBr 4 (space group I4 1 /amd), with values of 2.85 and 3.12 A ˚(no s.u.values or temperature given; Brown et al., 1973).Each Br atom bridges two Th atoms, which results in edge-sharing polyhedra to form the crystal structure.The connection motif of �-ThBr 4 is similar to that in �-ThBr 4 .Although the two polymorphs differ considerably with respect to the two pairs of Th-Br distances, the connectivities in both structures can be described with the Niggli formula 3 1 [ThBr 4/2 Br 4/2 ].The closest Th� � �Th distance of 4.77179 (12) A ˚in �-ThBr 4 is shorter compared to �-ThBr 4 , with a value of 4.8774 A ˚ (Brown et al., 1973).In the crystal structure of �-ThBr 4 , each Th atom is surrounded by eight other Th atoms in the shape of an irregular polyhedron, with Th� � �Th distances of 4 � 4.77179 (12) A ˚and 4 � 6.70680 (19) A ˚.
A silica glass ampoule was loaded with �-ThBr 4 (149 mg, 0.27 mmol) and CuBr (78 mg, 54 mmol, 2.01 equiv.), and sealed under vacuum.The ampoule was heated in a furnace to 753 K at a rate of 1 K min À 1 and kept at this temperature for 480 h for the reaction to take place.Afterwards, it was cooled to 330 K at a rate of 50 K d À 1 .Several colourless crystals of �-ThBr 4 were obtained.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.

Figure 1
Figure 1 Crystal structure of �-ThBr 4 in a projection along [010].Displacement ellipsoids are drawn at the 90% probability level.

Table 2
Experimental details.