Fig. 3. Coordination of U in the structure of Rh6U6S15.Figure optionsDownload full-size imageDownload as PowerPoint slide
[Cs2M2U6Q15]: Cs2Ti2U6Se15, Cs2Cr2U6Se15, and Cs2Ti2U6Te15. Single crystals of these compounds were synthesized from the reaction of U, M, and Q in a CsCl flux at 1173 K.
The structure of Cs2M2U6Q15 is analogous to that of the Rh6U6S15 parent structure with a three-dimensional framework made up of M, U, and Q atoms that create three-dimensional channels. However, the substitution of M2+ for Rh6+ leads to the three-dimensional C646 being variably filled by Cs1 (4/mm.m) and Cs2 (4m.m) atoms ( Fig. 4).
Fig. 4. Framework structure of Cs2M2U6Q15 (M=Ti, Cr, Q=Se, Te) with Cs1 atoms in the channels.Figure optionsDownload full-size imageDownload as PowerPoint slide
The axial dimension (Table 1) for Cs2Ti2U6Se15-2 is significantly shorter at 13.9062(8) Å than diaphragm of 14.0029(4) Å for Cs2Ti2U6Se15-1. The Cs positions in Cs2Ti2U6Se15-1 are fully occupied whereas those in Cs2Ti2U6Se15-2 (and also Cs2Cr2U6Se15) are not. The result is expansion of the overall framework. This difference may be the result of the slower rate of cooling in the synthesis of Cs2Ti2U6Se15-1 compared with those of Cs2Ti2U6Se15-2 and Cs2Cr2U6Se15. Note that despite the same heating profile as Cs2Ti2U6Se15-2 and Cs2Cr2U6Se15, Cs2Ti2U6Te15 has fully occupied Cs positions. The increase in size from Se to Te expands the dimensions of the framework so perhaps a slower cooling rate is not required for preferential occupation of the Cs1 site.