X-ray scattering and theα-α′ phase transition in coherent metal-hydrogen systems

Metallic singlet ground state systems have often anomalous elastic properties which result from the coupling of the phonons to the crystal field energy levels of the rare earth ions. It is shown that structural and magnetic phase transitions should occur in the same way in those systems. They are caused by a sufficiently strong electron-ion interaction. While the orbital part of this interaction is responsible for possible structural phase transitions, the spin dependent part is responsible for magnetic phase transitions. This is demonstrated in detail by considering as an example the aspherical Coulomb charge scattering and the isotropic exchange interaction. We calculate the sound velocity, the sound attenuation and the excitations in the presence of the aspherical Coulomb charge scattering. Furthermore we discuss the mutual coupling of structural and magnetic phase transitions. This includes a consideration of other types of coupling than aspherical Coulomb scattering and isotropic exchange.