Study of the Crystal Structures and Nonstoichiometry in the System Cs₃Sc₂Cl₉-CsScCl₃

The phase Cs₃Sc₂Cl₉ has the Cs₃Tl₂C_l9 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 R_w = 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.