New compounds and structures in the solid state

Abstract

This chapter reviews the 2007 literature on new compounds and structures in the solid state. HighlightsMany of the highlights of solid state chemistry that were reported in the literature during 2007 arise from studies of structures of surprising simplicity or else from astonishingly complex problems that are at the limits of the analytical techniques. A striking example of the former is the report of the binary compound ReB 2 that can be prepared by direct combination of the elements in a sealed tube. The hardness of this apparently simple material is second only to diamond. Similarly the rutile structure of TcO 2 has been described for the first time and an important chemical consideration of the effect cation doping of Zintl phases has been described. The use of topotactic modification has yielded many new mixed-metal oxides and chalcogenides of increasing complexity, with several new anion-or cation-ordering schemes being observed, as well as stabilising the remarkable perovskite-related phase SrFeO 2 . Complexity has also been a feature of both intermetallic phases and zeolites. The former are exemplified by the 433 000 Å 3 unit cell volume of a new phase in the Li/Na/Ba system whilst the latter are represented by the 24-topologically distinct Si atoms observed in the structure of the industrially-important zeolite IM-5. OxidesThe b-emitter 99 Tc is a long-lived isotope but despite a half life of 2.12 Â 10 5 years the oxide chemistry of technetium has been relatively neglected and the crystal structure of the black binary phase TcO 2 has not previously been reported. Neutron powder diffraction has been employed to complement existing X-ray absorption studies of this compound. Thus the last unknown structure of a simple transition metal oxide has been identified as possessing a distorted rutile arrangement. 1 This compound crystallises in the space group P2 1 /c with lattice parameters (a = 5.6891(1), b = 4.7546(1), c = 5.5195(1) Å and b = 121.453(1)1) in reasonable agreement with a previous indexing of X-ray diffraction data. The rarity of structural studies of technetium compounds is illustrated by necessity of improving the estimated neutron scattering length of 99 Tc from 6.8 fm to 6.00(3) fm. The distortion of the structure is largely invariant at temperatures down to 15 K and arises from Tc-Tc bonding between the edge-sharing TcO 6 octahedra in the