The phase Cs3Sc2Cl9 has the Cs3Tl2Cl9 type structure (R3¯c, a = 12.707 (2) Å, c = 18.117 (4) Å) rather than the chromium(III) structure type common to the remainder of the 3d elements. The 45Sc NMR spectra of ScCl3 and of Cs3Sc2Cl9, which has been slowly cooled in comparison with that quenched from the melt and annealed, establish that the ternary compound melts somewhat incongruently. Reduction of the scandium(III) phase at elevated temperatures by metal gives CsScCl3 which has the CsNiCl3 structure (hexagonal perovskite, P63mmc, a = 7.350 (2) Å, c = 6.045 (3) Å). The close relationship between the two structures, strings of scandium pairs alternating with vacancies in Cs3Sc2Cl9 and fully occupied strings of metal in the reduced limit, allows for an apparent region of nonstoichiometry between the two without X-ray evidence for superstructure ordering of vacancies. Values of R = 0.055 and Rw = 0.064 were obtained by least-squares refinement of 108 independent reflections from a single crystal with a refined average scandium occupancy of 0.79 (3). Limited refinement of 3m-symmetry diffraction data from the same crystal reveals that the scandium atoms are probably unevenly distributed, namely, as fully occupied pairs with random fractional occupancy of the third site. Precise Guinier powder diffraction data suggest that all scandium sites become crystallographically equivalent above about 87-90% average occupancy. The blue CsScCl3 is probably a semiconductor rather than a metallic compound.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry