Empirical evaluation of the thermodynamic and magnetic properties from the atomic distances of NaCl-type actinide compounds

Some thermodynamic and magnetic properties of NaCl-type MX compounds (M: actinide, X: metalloid) were evaluated from their bonding nature. The number of electrons in a molecule participating in bonding, N_B, was determined from the lattice parameter using the Pauling's equation. The binding energy defined as the enthalpy changes accompanied with the reaction of MX(s)=M(g)+X(g) at 298 K showed a linear relationship against bonding electron density. The number of electrons not-participating in bonding, N_L, was obtained by subtracting N_B from the total number of valence electron in a molecule. When the effective magnetic moments were plotted against N_L values ranging from zero to about eight, they fitted the curve described as applying L-S coupling model and Hunt's rule to ground state 5f electron. This implies that N_L corresponds to the number of localized 5f electron in a molecule. From these findings, it is certified for the valence of actinides as follows. U, Np and Pu in their chalcogenides are divalent. In the pnictides, Np, Pu, Am and Cm are trivalent but Bk and Cf are divalent. Further, the magnetic contribution to entropy of these compounds was discussed. (author)