CALCUL OF THE BAFCL SPECIFIC-HEAT BY USING THE SHELL-MODEL

The shell model has been developped by Dick, Overhauser [1], and Cochr an [2,3,4]. It takes account of the electronic density deformation dur ing the ions movement. The layer electrons form a shell that follow in stantaneously the hearts of ions. The movement equations are obtained by considering the heart-heart, heart-shell and shell-shell interactio ns. The application of this model requires the introduction of sixteen parameters (ten parameters for the short-range interactions, three pa rameters for the shell charges Y-i and three spring constants K-i betw een the core and the shell of the same ion i). The force constants are determined by an adjustement of the dynamical matrix eigenvalues and the experimental values of the different modes in the Brillouin zone c enter. This has allowed US to plot the dispersion curves following two principal directions (Lambda(0,0,q(z)) and Delta(0,q(y),0)), to plot the density of states and to plot the specific heat.