Experimental investigation of the electronic structure of Gd5Si2Ge2 by photoemission and x-ray absorption spectroscopy

The electronic structure of the magnetic refrigerant Gd5Ge2Si2 has been experimentally investigated by photoemission and x-ray absorption spectroscopy. The resonant photoemission and x-ray absorption measurements performed across the Gd N4,5 and Gd M4,5 edges identify the position of Gd 4f multiplet lines, and assess the 4f occupancy (4f7) and the character of the states close to the Fermi edge. The presence of Gd 5d states in the valence band suggests that an indirect 5d exchange mechanism underlies the magnetic interactions between Gd 4f moments in Gd5Ge2Si2. From 175 to 300 K the first 4 eV of the valence band and the Gd partial density of states do not display clear variations. A significant change is instead detected in the photoemission spectra at higher binding energy, around 5.5 eV, likely associated to the variation of the bonding and antibonding Ge(Si) s bands across the phase transition

Phonon precursors to the high temperature martensitic transformation in Ti50_{50}Pd42_{42}Cr8_8

Inelastic neutron scattering measurements were carried out on the Ti$_{20}$Pd$_{50-\rm x}$Cr$_{\rm x}$ alloy, which is a high temperature shape memory material. For $\rm x=0$, the transformation temperature is ~800K and for the composition studied ($\rm x=8$ at %) M$_{\rm s}\sim 400$K. The majority of the measurements were performed in the parent, B2-phase, up to 873K. Most of the phonons propagating along the three symmetry directions [ $\zeta$00] , [ $\zeta \zeta \zeta$] , and [ $\zeta\zeta$0] were well defined with the exception of the [ $\zeta\zeta$0] transverse acoustic mode with displacements along the [ -$\zeta\zeta$0] corresponding to the $\rm C'=l/2 (C_{11}-C_{12}$) etastic constant in the $\zeta \to 0$ limit. These phonons are well defined for small $\zeta$, but for $\zeta > 0.10$ they are strongly overdamped near the transition temperature and become better defined at higher temperatures. An elastic peak develops in the cubic phase at $\zeta =0.22$ and increases in intensity as M$_{\rm s}$ is approached. However, the dispersion curves show no anomaly at this particular wavevector, in marked contrast to the lattice dynamic studies of other systems exhibiting Martensitic transformations