AG2TI2P2S11 - A NEW LAYERED THIOPHOSPHATE - SYNTHESIS, STRUCTURE DETERMINATION AND TEMPERATURE-DEPENDENCE OF THE SILVER DISTRIBUTION

The new Ag2Ti2P2S11 quaternary phosphosulfide disilver dititanium unde cathiodiphosphate is obtained by heating the elements at ca 850 K in a n evacuated silica tube. It crystallizes in orthorhombic symmetry, Pnm a space group, with a=8.5222 (11), b=6.8359 (10), c=24.142 (4) Angstro m, V=1406.4 (4) Angstrom(3) and Z=4 at 293 K. The refinement of the ro om-temperature structure leads to a reliability factor of R=0.0378 for 1556 independent reflections and 128 variables. Ag2Ti2P2S11 is compos ed of layers, separated by van der Waals gaps. The layers are composed of [Ti2S9] chains built from [TiS7] units (TiS6 octahedra in which a corner has been replaced by an S-2 pair) and linked through regular [P S4] and highly distorted [AgS4] tetrahedra. The formula of the compoun d can be written as (Ag2Ti2P2V)-Ti-I-P-IV(S-2)S--II(9)-II. A Gram-Char lier anharmonic development of the atomic displacement factor for Ag a toms reveals a strong non-harmonic probability density deformation, es pecially in the direction of two empty neighbouring tetrahedra, away f rom the Ti and P cations. The temperature dependence of the effective one-particle potential shows that the Ag distribution is primarily sta tic in nature. At high temperature the tetrahedrally coordinated site is the preferential site. At lower temperature the Ag atoms are redist ributed over the tetrahedral site and the adjacent triangular site. Th e disorder could not be resolved due to a phase transition with splitt ing of spots just below room temperature.