SYNTHESIS AND SINGLE-CRYSTAL STRUCTURAL STUDY OF AN ORIGINAL LOW-TEMPERATURE FORM A OF BATBF6

BaTbF6 is the first terbium(IV) fluoride for which a polymorphic phase transition at high temperature is evidenced. Whereas its high tempera ture form is isostructural with beta-BaZrF6, the crystal structure of its low-temperature polymorph has been determined from single-crystal X-ray diffraction data and refined to conventional R = 0.044 (R-w = 0. 053) for 2880 independent reflections and 149 variable parameters. alp ha-BaTbF6 crystallizes in the triclinic system with space group P (1) over bar and unit-cell parameters a = 7.344(1), b = 8.4476(6), c = 8.6 770(9) Angstrom, alpha = 101.63(8), beta = 96.89(1), gamma = 114.62(1) degrees and Z = 4. ' The three-dimensional structure may be regarded a s a stacking along the c direction of alternated corrugated layers of Pa polyhedra and layers of Tb polyhedra built from [Ba4F30](22-) and [ Tb4F26](10-) tetrameric complex anions respectively. The[Tb4F26](10-) complex anion results from the association of four square antiprisms b y sharing corners and edges.