The Covalent Model of Intermediate Valence of Transition Ions in Metallic Compounds

摘要

The effect of mixed valency (MY) is considered as the result of competition of metallic, covalent, and ionic bonds. Amplitudes #zeta#_n of state probabilities of rare-earth (RE) ions with different numbers (n) of covalent electrons are introduced as expansion coefficients of the wave functions on many-electron operator spinors (MEOS). The entropy of chemical-bonds' fluctuations (CBE) and strong dependence of covalent-bonds' energies #GAMMA# on the lattice parameter determine the phase diagrams of RE metals and compounds. Calculated spectra of CBF and band electrons as well as their correlators depend functionally on #GAMMA#_n, temperature (T), and pressure (P). Variation method is used to the thermodynamic potentials F(#GAMMA#_n,). Small value #GAMMA#_3~2 0.1 is determined for the metallic (M) phase of Sm_(1-x)Gd_xS. This value is equivalent to MV of Sm ions and correlates with Gd critical concentration (x <= 0.15) on the P-T-line of semiconductor (s/c)->M transition. The theory interprets the decrease ( approx 20 times) of charge-carriers' mobility at s/c->M transition in SmS. Calculated phase P-T-x-diagrams are agreed with experiment.