CRYSTAL-STRUCTURE OF KTBF5 AND EARLY CRYSTAL-CHEMICAL DATA FROM SINGLE-CRYSTAL WORKS FOR 8-COORDINATED TB4+ ION

KTbF5 is triclinic (P (1) over bar) with a = 7.446(1) Angstrom, b = 8. 567(1) Angstrom, c = 10.834(3) Angstrom, alpha = 76.02(2)degrees, beta = 72.51(2)degrees, gamma = 67.03(1)degrees and Z = 6. The crystal str ucture was refined from 3609 reflections to a conventional R = 0.035 ( R-w = 0.039). This structure is built of infinite layers of edge and c orners shared (TbF8)(4-) polyhedra further linked together by the K+ i ons. KTbF5 is isostructural with KZrF5. A discussion about the 8-coord ination of the Tb4+ ion in fluorides is presented. A bond valence para meter for the Tb-F bond is proposed from single crystals available dat a. An experimental ionlc radius evaluation of the Tb4+ ion in 8-coordi nation is given.