Magnetic Field Effects in Crystal Field Excitations and Sound Velocity in Paramagnetic Insulators

Structural phase transitions from tetragonal to monoclinic crystal symmetry induced by an external magnetic field are studied on the basis of a simple threelevel model. It is argued that the model is capable of describing the phenomena in f-electrons rare earth insulators and solid oxygen. The applied theoretical method is the equation of motion for the spin and phonon Green functions. The width of the crystal-field excitation line and the soft-mode phonon velocity are numerically studied and preseInted in diagrams.