Successive metal-insulator transitions in (DIMET)2BF4 were studied
by chopped-light ac calorimetry and EPR measurement.
An anomaly due to the metal-insulator transitions has been
clearly detected in ac calorimetry between 20 and 35 K.
In contrast to other double-column salts (DMET)2BF4
successive metal--insulator transitions detected in
transport properties are shown to be thermodynamic phase transitions.
The EPR detected an anomaly only for the lowest transition around 12 K.
The properties of the metal--insulator transitions
are compared among the DIMET and DMET salts having the double-column structure.
A frequency-dependent step in heat capacity was
detected around 110 K.
The step-like anomaly is attributed to a glass transition
due to freezing of the intramolecular motion of
the ethylene group in the
ET moiety within a DIMET molecule.
(J. Phys. Soc. Jpn., 70, 1635-1641 (2001))
The electrical resistivity under hydrostatic pressure was measured for the double-column organic conductors, (DMET)2BF4,
(DMET)2ClO4 and (DIMET)2BF4.
Each of these conducting salts undergoes three phase transitions at
and below the metal-to-semiconductor transition temperature of about
30 K at o kbar. The temperatures of the M-I transition and two other
ones were determined and the temperature-pressure phase diagrams were
constructed. It was found that the second up to 15 kbar and third
transition temperatures of (DMET)2BF4 and
(DMET)2ClO4 are almost independent of pressure.
The characteristics of the phase transitions are discussed by comparing the
pressure dependence of the transition temperatures with that of
single-column organic conductors.
(Synth. Metals, 96, 19-27 (1998))
An organic conductor, (DMET)2ClO4, has two types of
DMET columns along the a- and the b-axes of the crystal
lattice, respectively. The salt undergoes a metal-insulator transition
at 32 K. Its resistivity and thermopower both along the a and
b-axes show anomalies at 18 and 7 K in addition to that at 32 K.
Magnetoresistance along the c*-axis was measured while rotating the
magnetic field within the ab plane above and below temperature of
the anomalies. The identification of the column that is mainly related to
each anomaly is tried using the change in the peak position of the
field-orientation dependence of the magnetoresistance. The resistivity along
the c*-axis and thermopower along the c-axis show anomalies at
70 and 110 K in addition to those at low temperatures.
(Synth. Metals, 82, 83-88 (1996))
Organic conductors, (DMET)2BF4,
(DMET)2ClO4 and (DIMET)2BF4
(DMET = dimethyl(ethylenedithio)diselenadithiafulvalene), have two types of
donor columns, which are almost perpendicular to each other.
Resistivity and thermopower were measured along the two types of columns
under ambient pressure between 4.2 and 300 K. All the salts show metal-
insulator transition at about 30 K. Unusual stepwise increase in resistivity
and corresponding changes in thermopower were found for the salts below
30 K. The stepwise changes suggests that a few transitions occur in
the semiconducting state.
(Synth. Metals, 72, 141-146 (1995))
A method for determining the band-filling (the degree of oxidation of the donor or accepter molecule) is proposed in cases where plural types of the column exist in crystal. The method assume the anisotropy (ta/tb in terms of
the transfer integrals) is more insensitive than the magnitude of
t's themselves obtained under some assumption. The
anisotropy in the thermopower and the reflectance spactra are measured on
(DMET)2BF4, where two types of the columns
of the donor molecule exist along the two crystallographic axes
almost perpendicular to each other. The band-fillings are determined
so as the ratio of the transfer integrals obtained from the two quantities
to coincide to each other. The band-filling thus obtained is favorably
compared with that obtained from the Raman spactra.
(Synth. Metals, 55-57, 1756-1761 (1993))
See also Glass Transition in Organic Conductor
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