Pressure Effects on the Quasi-1-D Molecular Ferromagnet 2-(4,5,6,7-Tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl

Abstract

Crystallographic lattice compression and magnetic variations in the quasi-1-D ferromagnet 2-(4,5,6,7-tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (F4BImNN) were studied under hydrostatic pressure for polycrystalline samples. The crystallographic c-axis—along which F4BImNN forms hydrogen bonds—was compressed by 3% at 10 kbar and by 4% at 17.8 kbar. The overall lattice volume contracts by 12% from ambient pressure to 17.8 kbar. The axis of the hydrogen bonded chain propagation is compressed, since the bent F4BImNN hydrogen bonds accommodate geometric compression. The magnetic susceptibility measured over 1.8–300 K showed an increase in ferromagnetic exchange interactions as pressure increased. Curie–Weiss 1/χ vs T analyses showed an increase in Weiss constant from (+)10.4 K to (+)15.3 K from ambient pressure to 9.85 kbar. Fitting of the χT vs T data indicated predominantly 1-D Heisenberg chain type behavior at all pressures, increasing from J/k = (+)19 K at room pressure to (+)25 K at 4.77 kbar and (+)30 K at 9.85 kbar. The increase in exchange strength is attributed to pressure-increased overlap of spin orbitals in the hydrogen bonded chains, which favors 1-D ferromagnetic interaction. There was no indication of a phase change, either crystallographically or in discontinuities of the magnetic behavior.