Heat Capacity and
Magnetic Phase Transitions of
the Mixed-Valence Prussian Blue Complex
K0.2MnII0.66MnIII1.44 [FeII0.2FeIII0.8(CN)6]-
O0.66(CH3COO)1.32 · 7.6H2O

Heat capacities of the mixed-valence Prussian blue complex K0.2MnII0.66MnIII1.44 [FeII0.2FeIII0.8(CN)6] O0.66(CH3COO)1.32 · 7.6H2O were measured by adiabatic calorimetry and relaxation method under magnetic fields. Two heat capacity peaks were observed at 7.5 K and 2.1 K, which correspond to ferri- and ferromagnetic phase transitions, respectively. The uniaxial zero-field splitting parameter due to MnIII was estimated to be D/kB = 14.7 K. The zero-field magnetic entropy amounted to 29.2 J K−1 mol−1, which is close to the expected value R(0.66ln6 + 1.44ln5 + 0.2ln1 + 0.8ln2) (= 33.7 J K−1 mol−1). Additionally, a glass transition was found at 194 K, which is presumably due to freezing of the orientational motion of the H2O molecules present in the complex.

(by Y. Miyazaki)

	Fig. 1. Heat capacity of K0.2MnII0.66MnIII1.44 [FeII0.2FeIII0.8(CN)6] O0.66(CH3COO)1.32 · 7.6H2O by adiabatic calorimetry.
	Fig. 2. Heat capacities (upper) and magnetic heat capacities (lower) of K0.2MnII0.66MnIII1.44 [FeII0.2FeIII0.8(CN)6] O0.66(CH3COO)1.32 · 7.6H2O under magnetic fields. Solid curve indicates the lattice heat capacity. Broken curve shows the spin wave heat capacity with an energy gap.
	Fig. 3. Heat capacities divided by temperature (upper) and temperature drift rates (lower) of K0.2MnII0.66MnIII1.44 [FeII0.2FeIII0.8(CN)6] O0.66(CH3COO)1.32 · 7.6H2O.

Anomalous Enhancement of
Electronic Heat Capacity Coefficient of
(DI-DCNQI)2 Ag1−xCux
with One-Dimensional Structure

Thermodynamic investigation on the organic alloying system of (DI-DCNQI)2 Ag1−xCux was performed to study the variation from the charge-ordered insulating state to the π-d hybridized metallic state. In the metallic region between x = 0.7 and 1.0, the coexistence of π-d hybridization and inter-site Coulomb interaction (V) which induces localized character in the π-electrons was found to work cooperatively. The low-temperature electronic heat capacity coefficient, γ is enhanced up to about 64 mJ K−2 mol−1 at the concentration of x = 0.90. This value is 1.5 times the larger than the Cu 100% samples, which is already known as the enhanced metals through magnetic susceptibility and heat capacity measurements. The electron correlation effects in the π-electrons plays a role to enhance the electron mass in the hybridized band.

(by Y. Nakazawa)

Fig. 1. Conceptual phase diagram of (DI-DCNQI)2 Ag1−xCux studied by Itou et al. (Synth. Met. 120, 835 (2001).)

Fig. 2. Temperature dependence of heat capacity of (DI-DCNQI)2 Ag0.10Cu0.90 shown in a CpT −1 vs T 2 plot.

Heat Capacity Measurements of
θ-(BEDT-TTF)2 CsZn(SCN)4
with Applying Electric Currents

θ-(BEDT-TTF)2 CsZn(SCN)4 shows peculiar lattice heat capacity at low temperatures originating from strong charge fluctuations due to strong electron correlation. The fluctuations produce low-energy phonon modes through the electron-phonon coupling and show a peak structure of CpT −3 around 3 K. Since the charge fluctuation are known to be affected by the electric fields, heat capacity measurement of θ-(BEDT-TTF)2 CsZn(SCN)4 with applying electric currents is performed as a steady state calorimetry. We have introduced an analytic model to evaluate the resistance change during the measurement process. The obtained result of θ-(BEDT-TTF)2 CsZn(SCN)4 indicates a possibility of slight decrease of lattice heat capacity by applying currents.

(by K. Hino & Y. Nakazawa)

	Fig. 1. Typical temperature profile against time in a relaxation type measurement. T0 is the temperature of the heat sink, T1 and T2 is the temperature of sample stage in steady state.
	Equation.
	Fig. 2. Heat capacity of θ-(BEDT-TTF)2 CsZn(SCN)4 with applying electric currents (c axis direction).

Low-Temperature Heat Capacity of
Charge Transfer Salts of
BEDT-TTF with θ-Type Structure

Heat capacity measurements of θ-(BEDT-TTF)2 MZn(SCN)4 (M = Rb, Cs), which is known as charge-ordered insulators with two-dimensional structure were performed by the thermal relaxation technique. The M = Rb salt shows a characteristic behavior of disordered metals in which electrons are weakly localized in mesoscopic domains. The electronic heat capacity coefficient with a comparable magnitude with those of metallic systems consisting of BEDT-TTF was detected, while that of slowly cooled sample is almost vanishing due to the opening of charge gap. In the case of M = Cs salt, the strong charge fluctuations seem to be coupled with lattice vibrations and gives large peak of CpT −3 around 3 K. The electronic state of this charge fluctuated system is discussed.

(by Y. Nakazawa)

	Fig. 1. CpT −1 vs T 2 plot of θ-(BEDT-TTF)2 MZn(SCN)4 (M = Rb, Cs). The rapidly cooled M = Rb salt has a finite γ value, while the slowly cooled M = Rb and M = Cs salts do not.
	Fig. 2. CpT −3 vs T plot of θ-(BEDT-TTF)2 MZn(SCN)4 (M = Rb, Cs). The large peak of CpT −3 is observed around 3 K. This peak demonstrates the strong coupling of charge fluctuations and lattice vibration.