Synthesis and single-crystal structural study of an original low-temperatureform α of BaTbF6

Abstract

BaTbF ₆ is the first terbium(iv) fluoride for which a polymorphic phase transition at high temperature is evidenced. Whereas its high temperature form is isostructural with β-BaZrF ₆ , 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. α-BaTbF ₆ crystallizes in the triclinic system with space group P and unit-cell parameters a=7.344(1), b=8.4476(6), c=8.6770(9) Å, α=101.63(8), β=96.89(1), γ=114.62(1)° and Z=4.  The three-dimensional structure may be regarded as a stacking along the c direction of alternated corrugated layers of Ba polyhedra and layers of Tb polyhedra built from [Ba ₄ F ₃₀ ] ^22- and [Tb ₄ F ₂₆ ] ^10- tetrameric complex anions respectively. The [Tb ₄ F ₂₆ ] ^10- complex anion results from the association of four square antiprisms by sharing corners and edges.