Investigation of the “50 K-transition” in PdH0.73 by γ-ray and neutron diffraction

Rocking curves of the (111)-reflection in a PdH_0.73 single crystal have been measured with 412 keV γ-rays during an ageing experiment at 50 K up to 260 h. The width (variance) shows an overall increase of 1.5% ± 0.3% and the integrated intensity a maximum at 100 h.An investigation at superstructure points in the same PdH_0.73 crystal was made using neutron scattering. An enhancement of the elastic diffuse scattering has been found at the $\text{(1}\text{1}\text{2}\text{0)}$ and (110)-positions after an ageing at 54 K up to 300 h. The intensity ratio to the fundamental (002)-reflection is 10^-4. It is concluded that below the anomaly temperature a local ordering of the hydrogen atoms occurs and the growth of the ordered zones slows down after 100 h ageing.     

Non-linear pressure dependence of phonon frequencies in RbI

The frequencies of a representative sample of phonons with negative Grueneisen parameters, in RbI, were measured at several pressures up to, and slightly above, the thermodynamic equilibrium pressure between the NaCl and CsCl structure phases (3.56 kbar). In general the frequency-pressure curves are linear up to about 3 kbar but show a tendency to saturate around 3.5 kbar.