X-ray diffuse scattering and conductivity studies of the N-methylphenazinium-tetracyanoquinodimethanideIAform

Abstract

The x-ray diffuse scattering study of the highly conducting form ($\mathrm{I}A$) of N-methylphenazinium-tetracyanoquinodimethanide [(NMP)(TCNQ)] shows that the methyl groups, although less ordered than in the less highly conducting form ($\mathrm{I}B$), still display substantial local order of the same type along the stacking $a$ direction. In addition, it shows two types of one-dimensional (1-D) scattering bearing resemblance to the Kohn anomalies earlier observed in (TTF)(TCNQ). A first 1-D scattering is observed at room temperature at the wave vector $0.33a^{*}$; it couples three-dimensionally below approximately 200 K. Further diffuse scattering is observed below 70 K at half the previous wave vector ($0.165a^{*}$). As expected in an intrinsically disordered system (as potassium cyanoplatinate [${\mathrm{K}}_2$Pt${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_4$ ${\mathrm{Br}}_{0.30}$·3${\mathrm{H}}_2$O (KCP)] no long-range 3-D ordering is observed down to 20 K. Our results cannot ascribe unambiguously the $0.33a^{*}$ and a $0.165a^{*}$ scattering to $4k_F$ and $2k_F$ anomalies (as was done for tetrathiafulvalenium-tetracyano-$p$-quinodimenthanide [(TTF)(TCNQ)], but strongly suggests that the charge transfer in (NMP)(TCNQ) is $\frac{2}{3}$ electron, and is quite far from unity as previously deduced from other work. Temperature-dependent dc conductivity measurements after x-ray study unequivalocally associate the highly conducting form with these properties.