Locally resolved Knight shifts and the order-disorder phase transition in the organic conductor N,N-dimethyl-thiomorpholinium bis-tetracyanoquinodimethane [DMTM(TCNQ]
Phys. Rev. B 46, 674 – Published 1 July, 1992
DOI: https://doi.org/10.1103/PhysRevB.46.674
Abstract
The electronic and dynamic changes through the phase transition in the organic conductor N,N-dimethyl-thiomorpholinium bis-tetracyanoquinodimethane [DMTM(TCNQ] have been investigated with use of nuclear magnetic resonance. Proton Knight shifts have been measured with multiple-pulse, magic-angle spinning NMR as a function of temperature. Three inequivalent TCNQ lines were observed above the phase transition at 272 K, and eight below, all with negative Knight shifts, ranging from -20 to -75 ppm, in the temperature range 190–300 K, representing an extremely large observed Knight shift. The isotropic hyperfine coupling constants and corresponding spin densities were calculated, and an estimate of the charge redistribution at , based upon a tentative assignment of the lines, is made. The cation dynamics were also investigated with broadline NMR which indicate a slowing of the DMTM-cation motion upon cooling through the phase transition, while relaxation measurements in the rotating frame indicate a cooperative phenomenon involving slow motions at . All of these results are considered in terms of the structural changes at the phase transition.