Far-Infrared Transmission Measurements of N-Methyl-N-Ethy-Morpholinium-Ditetracyanoquinodimethanide[MEM (TCNQ)_2] at Temperatures above and below Spin-Peierls Transition

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Far-infrared transmission spectra of a single crystal N-methyl-N-ethyl-morpholiniumditetracyanoquinodimethanide, MEM(TCNQ)., was measured for the first time over therange from 15 to 200 cm ' at 1.5 K (below spin-Peierls transition temperature T.,= 17.4-lK) and 23 K (above T.,). We found that the absorption lines 130.5 and 150.5 cm at 23 Kshift to 126 and 156 cm ' at 1.5 K. With an aid of calculations of phonons above andbelow T it is concluded that the 126, 150.5 and 156 cm ' lines are phonons and theSP ?150.5 cm ' line at 23 K changes to the 156 cm ' line at 1.5 K. The 126 cm ' line at 1.5 K isdue to the lattice distortion caused by the spin-Peierls transition and can be interpreted bythe folding mechanism. The assignment of the 130.5 cm ' line at 23 K is remained in thepresent stage.
1985-11-15