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Anisotropy of thermopower in N-methyl-N-ethylmorpholinium bistetracyanoquinodimethane, MEM ( TCNQ ) 2 , in the region of the high-temperature phase transitions

1984; American Physical Society; Volume: 30; Issue: 5 Linguagem: Inglês

10.1103/physrevb.30.2839

1095-3795

Manuel Almeida, Luís Alcácer, S. Oostra,

Molecular Junctions and Nanostructures

Measurements of the thermoelectric power along the three crystallographic axes of N-methyl-N-ethylmorpholinium bistetracyanoquinodimethane, ${\mathrm{MEM}(\mathrm{TCNQ})}_{2}$, in the temperature range 280-360 K are reported. The first-order metal-insulator phase transition at 338 K is observed as a discontinuous change between the temperature-independent values ${S}_{a}=\ensuremath{-}50$ \ensuremath{\mu} V ${\mathrm{K}}^{\ensuremath{-}1}$, ${S}_{b}=\ensuremath{-}35$ \ensuremath{\mu} V ${\mathrm{K}}^{\ensuremath{-}1}$, and ${S}_{c}=\ensuremath{-}65$ \ensuremath{\mu} V ${\mathrm{K}}^{\ensuremath{-}1}$ in the metallic phase ($T>338$ K) and the values ${S}_{a}=\ensuremath{-}7$ \ensuremath{\mu} V ${\mathrm{K}}^{\ensuremath{-}1}$, ${S}_{b}=\ensuremath{-}48$ \ensuremath{\mu} V ${\mathrm{K}}^{\ensuremath{-}1}$, and ${S}_{c}=+35$ \ensuremath{\mu} V ${\mathrm{K}}^{\ensuremath{-}1}$. In the semiconducting phase, ${S}_{b}$ is almost temperature independent, but both ${S}_{a}$ and ${S}_{c}$ increase upon cooling. At 314 K, a strong anomaly is made evident as a drastic change in the slope of ${S}_{a}$ and ${S}_{c}$ versus temperature. The electrical conductivity accurately measured along the three crystallographic axes, also shows an anomalous decrease for $T<314$ K. The thermopower and conductivity anomalies at 314 K are consistent with a second-order phase transition which originated in a change of cation motion, and is associated mainly with a decrease in the mobility of the charge carriers, the role of a possible increase in the gap being secondary. For $T<314$ K there is a predominant decrease of the electron mobility mainly along the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$ axis but also along the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}$ axis, while, along the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{b}}$ axis, both the electron and hole mobilities decrease, remaining equal to each other.