Bond length compressibility in hard ReB2 investigated by x-ray absorption under high pressure.

This work describes x-ray absorption measurements under high pressure in ReB(2), complemented by ab initio calculations. The EXAFS analysis yields the average Re-B bond compressibility, which turns out to be χ(ReB) = 5.6(9) × 10(-4) GPa(-1). Combining this information with previous x-ray diffraction experiments we have characterized the network of covalent bonds responsible for the rigidity of the structure. The main conclusion is that the compression is anisotropic and nonhomogeneous, reflecting bonding differences between Re-B and B-B bonds and also between nonequivalent Re-B bonds. The layer defined by boron atoms tends to become flatter under high pressure. As a consequence, the structural rigidity, necessary to attain high hardness values, is compromised.

Pressure tuning of electron-phonon coupling: the insulator to metal transition in manganites.

An extended temperature and pressure-dependent investigation is carried out on a La0.75Ca0.25MnO3 sample exploiting the infrared absorption technique coupled to a diamond anvil cell. The pressure dependence of the insulator to metal transition temperature T(IM) is determined for the first time up to 11.2 GPa. The T(IM)(P) curve we propose to model the present data has an exponential-like behavior with an associated characteristic pressure P* playing the role of a decay constant. It is found that the equivalence between an external and an internal (chemical) pressure holds over a limited range of pressure, namely, P< or =2P*. Moreover, a certain universality character is associated with the proposed model curve in its ability to account for a large class of low-disorder manganites characterized by intermediate electron-phonon coupling.

Anomalous high pressure dependence of the Jahn-Teller phonon in La(0.75)Ca(0.25)MnO(3).

Raman spectra of La(0.75)Ca(0.25)MnO(3) have been collected for the first time over a wide pressure range (0-14 GPa) using a diamond anvil cell. The frequency range explored (200-1100 cm(-1)) and the very good quality of the data allowed us to carefully analyze the pressure evolution of the phonon modes of the MnO(6) octahedra. The results show an abrupt transition at approximately 7.5 GPa with an evident deviation from the linear trend of the frequency of the Jahn-Teller phonon versus the applied pressure, accompanied by a strong phonon broadening. This behavior disagrees with the predicted insulator to metal transition and, on the contrary, indicates the occurrence of a new unpredicted phase in the very high pressure regime.