Soft-mode anisotropy in the negative thermal expansion material ReO3

We use a symmetry-motivated approach to analyse neutron pair distribution function data to investigate the character of the soft phonon modes in negative thermal expansion (NTE) material ReO3. This analysis shows that its local structure is dominated by an in-phase octahedral tilting mode and that the octahedral units are far less flexible to scissoring type deformations than in the related NTE compound ScF3. The lack of flexibility in ReO3 restricts the NTE-driving phonons to a smaller region of reciprocal space, limiting the magnitude and temperature range of NTE. These results support the idea that structural flexibility is an important factor in NTE materials. Surprisingly, our results show that the local fluctuations, even at elevated temperatures, respect the symmetry and order parameter direction of the initial pressure induced phase transition in ReO3. The result indicates that the dynamic motions associated with rigid unit modes are highly anisotropic in these systems

Metal cluster terminated "molecular wires"

The gold complexes Au(C≡CC6H4C≡CC6H4Me)(PPh3) (3) and {Au(PPh3)}2(μ-C≡CC6H4C≡CC6H4C≡CC6H4C≡C) (6), prepared from the reaction of AuCl(PPh3) with the corresponding terminal or trimethylsilyl protected alkynes, react readily with Ru3(CO)10(μ-dppm) to afford phenylene ethynylene derivatives featuring the Ru3(μ-AuPPh3)(μ-C2R)(CO)7 cluster “end-caps”. The hydrido cluster Ru3(μ-H)(μ-C2C6H4C≡CC6H4Me)(CO)7 (4a) has also been obtained. There are significant differences in the absorption spectra of the organic precursors, the gold complexes and the clusters indicate a mixing of electronic states between the cluster and phenylene ethynylene moieties, while the presence of the Ru3 and in particular Ru3(μ-AuPPh3) cluster end-caps leads to a quenching of the phenylene ethynylene centred emission. The crystallographically determined structures of 3, 4a and Ru3(μ-AuPPh3) (μ-C2C6H4C≡CC6H4Me)(CO)7 (4b) are reported