title: beta-Tl2SO4.
abstract: The ambient-temperature form of dithallium sulfate, beta-Tl(2)SO(4), is similar to beta-K(2)SO(4) and is characterized by isolated sulfate tetrahedra and two different thallium sites with coordination numbers 9 and 11. All the atoms, except one O atom, lie on mirror planes. In spite of there being a high concentration of Tl(+) cations, the stereochemical activity of the 6s(2) pairs is low, similar to that of isotypic Tl(2)XO(4) compounds (X = Cr and Se). This behaviour is the consequence of both weak Tl-O bonds and strong X-O bonds, because in a Tl-O-X linkage the electronic cloud of the O(2-) anion is strongly distorted and displaced towards X, resulting in a low negative charge in the face of the Tl atom. Consequently, the Coulombic repulsions between the lone pair and the O(2-) anions are weak. All of the Tl(2)XO(4) compounds exhibit the same open packing of A(+) cations and [XO(4)](2-) anions as their isotypic alkali counterparts.
authors.affiliation: Laboratoire de Cristallochimie du Solide - Case 176, Université P. et M. Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France.
authors.forename: Gilles
authors.initials: G
authors.lastname: Wallez
authors.forename: Sylvie
authors.initials: S
authors.lastname: Jaulmes
authors.forename: Ammy
authors.initials: A
authors.lastname: Elfakir
authors.forename: Jean Paul
authors.initials: JP
authors.lastname: Souron
authors.forename: Michel
authors.initials: M
authors.lastname: Quarton