Determination of the spin-Peierls distortion in N-methyl-N-ethyl-morpholinium ditetracyanoquinodimethanide [MEM(TCNQ)2]: Neutron diffraction study at 6 K

We have carried out a crystallographic study of the spin-Peierls distorted phase ($T_c=17.4\mathrm{}$ K) of N-methyl-N-ethyl-morpholinium ditetracyanoquinodimethanide [MEM${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_2$]. Neutron-diffraction data were collected at 6 K from a deuterated sample. At the transition the unit cell axis along the dimerized TCNQ stack is doubled, hence giving rise to tetramers. Refinement of rigid molecules converged to $R=0.162\mathrm{}$ for 340 super-reflections. The structural change consists of a transverse shift $\Delta \overrightarrow{\nu }=0.17\mathrm{}\overrightarrow{\mathrm{L}}+0.10\mathrm{}\overrightarrow{\mathrm{M}}$ of every second dimer in the stack. Calculations of transfer integrals confirmed the significance of the distortion.