Spin Susceptibilities of Organic Systems on a Narrow-Band Model

Abstract

A narrow‐band model is used to explain the temperature dependence of the spin susceptibilities of organic free‐radical solids. It is shown that for temperatures in excess of the bandwidth the spin susceptibility is half the expected Curie susceptibility, while at low temperatures exponentially vanishing susceptibilities are obtained through the splitting of the bands due to the asymmetry of the crystal structure. A consideration of the effects of nuclear motion on the band structure leads to a “Curie–Weiss” law behavior at high temperatures, with a quite different interpretation of the Weiss constant. The results of the calculations are compared with available experimental data for DPPH (diphenylpicrylhydrazil), PAC (picrylaminocarbazyl), and D(NO₂)₂ [bis(p‐nitrophenyl)picrylhydrazyl]. Very good agreement with experiment is obtained. Further applications of the narrow‐band model are discussed.