ac Calorimetry and EPR Study on Metal-Insulator Transition and Glass Transition in an Organic Conductor with Double-Column Structure, (DIMET)2BF4

Kazuya SAITO, Hiroki AKUTSU, Koichi KIKUCHI, Hiroyuki NISHIKAWA, Isao IKEMOTO and Michio SORAI

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 and (DMET)2ClO4, 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))

Temperature-Pressure Phase Diagrams of Double-Column Organic Conductors: (DMET)2BF4, (DMET)2ClO4 and (DIMET)2BF4

Harukazu YOSHINO, Kazuya SAITO, Tatsuo SATO, Hiroyuki NISHIKAWA, Koichi KIKUCHI, Keiji KOBAYASHI and Isao IKEMOTO

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))

Field-Orientation Dependence of Magnetoresistance of a Double-Column Organic Conductor, (DMET)2ClO4

Harukazu YOSHINO, Kazuya SAITO, Koichi KIKUCHI, Hiroyuki NISHIKAWA, Isao IKEMOTO and Keiji KOBAYASHI

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))

Metal-Insulator Transitions in Double-Column Salts: (DMET)2BF4, (DMET)2ClO4 and (DIMET)2BF4

Harukazu YOSHINO, Kazuya SAITO, Koichi KIKUCHI, Isao IKEMOTO and Keiji KOBAYASHI

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))

Determination of Band-filling in Radical Salts with Plural Types of Columns from Anisotropy


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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