Theoretical Studies of the Pressure Effects for β-Phase of p-NPNN

Abstract

Molecular orbital calculations were carried out for model clusters of paranitrophenyl nitronyl nitroxide (p-NPNN) whose geometrical structures were based on its crystal structure of the β-phase. The effective exchange integrals between p-NPNN were calculated for qualitative understanding of the recently discovered pressure-induced transition from ferro- to antiferro-magnetic state. Taking the high pressure condition into consideration, p-NPNN in the clusters was gradually shifted along several lines or was rotated around appropriate axes. From these model calculations, it was found that the ferromagnetic exchange interaction changed to antiferromagnetic one by the structure deformations. Qualitative models of the pressure effects on the magnetic orders in the β-phase were presented on the basis of these calculated results.