Current Inorganic Chemistry (Discontinued) - Volume 4, Issue 2, 2014

Various experimental tools have been applied for the characterization of materials and evaluation of their functionalities. Among them, thermodynamic methods are unique in the sense that the energetic and entropic aspects inherent in materials can be directly observed. Physical quantities obtained from thermodynamic measurements reflect macroscopic aspects of materials. However, because those quantities are closely related to the microscopic energy schemes of all kinds of molecular degrees of freedom in a statistical manner, one can gain detailed knowledge on the microscopic level on the basis of precise thermodynamic investigations. Among them, heat capacity calorimetry is an extremely useful tool to investigate thermal properties of materials, in particular at low temperatures. For correct understanding of functionality of materials, it is crucially important to complementarily adopt both spectroscopic and/or structural methods leading to microscopic aspects of materials and thermodynamic methods revealing energetic aspects. The aim of this thematic issue is to review calorimetric studies on functional inorganic materials to show the important roles played by thermodynamic studies. The topics picked up in Part II of this thematic issue are the following six functional materials: [1] Mixed-valence metal complexes M(II)M(III)X (M = Pt, Ni) and [2] Mixed-valence metal complex Fe(II)Fe(III)(dto)3: Electron transfer between the mixed-valence metal ions provide a variety of phase transitions in which the magnetic and electronic properties are dramatically altered [3]. Assembled bimetallic complex: By changing the bimetallic ions, various dimensional assemblies are established. Some complexes exhibit dimensional crossover by temperature change. Dramatic changes in the magnetic and/or electronic properties are sensitively reflected on thermodynamic quantities [4]. Organic conductor (DMe-DCNQ)2Cu and [5] Organic super conductors: The charge transfer complexes consisting of organic molecules and inorganic counter ions give various metallic compounds. The electron correlations and electron-phonon coupling occuring in them produce a variety of phase transitions such as metal-insulator, magnetic, and superconductive transitions. Thermodynamic information related to low-energy excitations of itinerant electrons gives clue for understanding the mechanism of them [6]. Relaxors: The relaxor is characterized by a large, broad and frequency-dependent dielectric constant peak extending a wide temperature range. The giant electromechanical response in ferroelectric relaxors is of great importance for a number of ultrasonic and medical applications as well as in telecommunications. Papers concerning calorimetry of other functional materials will be published in the forthcoming Part III. We would like to thank Prof. Yann Garcia, the Editor-in-Chief of Current Inorganic Chemistry, who invited us to edit a thematic issue concerning calorimety of inorganic materials. We would like to acknowledge all the authors who accepted our invitation to contribute to this thematic issue, as well as the reviewers who invested their valuable time to ensure the high scientific quality of all contributions.

The authors’ publications on M2(RCS2)4I are summarized with some additional discussion. Because of the nature of heat capacity calorimetry at relatively high temperatures, emphases are put not on the properties of respective ground state of Q1D electron systems on MMX main chain but on dynamical nature of ligands and its coupling to the electron systems.

In mixed-valence assembled metal complexes whose spin states are situated in the spin-crossover region, multifunctional properties induced by the synergetic effect between spin and charge are expected. Based on this viewpoint, we have developed a ferromagnetic mixed-valence system, A[FeIIFeIIII(dto)3] (A = (n-CnH2n+1)4N, spiropyran; dto = C2O2S2). In (n-C2H2n+1)4N[FeIIFeIII(dto)3], we observed a charge transfer phase transition (CTPT) at 122.4 K and 142.8 K for n = 3 and 4, respectively, where a thermally induced charge transfer occurs reversibly between the FeII and FeIII sites. In contrast, the CTPT does not occur for n = 5 and 6 at ambient pressure. From the analysis of heat capacity, the CTPT is regarded as a spin-entropy driven phase transition to minimize the Gibbs energy in the whole system. The appearance of CTPT and the Curie temperature for (n-C2H2n+1)4N[FeIIFeIII(dto)3] significantly depend on the size of (n-CnH2n+1)4N+. In the case of (SP-Me)[FeIIFeIII(dto)3] (SP = spiropyran), the photo-isomerization of SP-Me by UV light irradiation induces the CTPT on the two-dimensional [FeIIFeIII(dto)3] layer and the remarkable change of Curie temperature.

This review describes unique magnetic functionalities observed in cyano-bridged bimetallic assemblies that are classified as molecule-based magnets. Copper octacyanotungstate (1) has a 3-dimensional network structure. As the temperature decreases, 1 exhibits a magnetic dimensional crossover from 2-dimensional Heisenberg-type magnetic ordering to 3-dimensional Heisenberg-type magnetic ordering, which originates from the switching of the dominant superexchange interaction from the intralayer to the interlayer. Cobalt octacyanoniobate (2) exhibits humidity-sensitive magnetism from ferromagnetism to antiferromagnetism as the humidity decreases. Copper octacyanotungstate (3), whose crystal structure differs from 1, displays an alcohol-sensitive magnetism with an extensive change in magnetization. The diverse spatial configurations and flexible coordination geometries are important to observe such magnetic functionalities because these features enable bimetallic assemblies to be altered using external stimuli.

The (DMe-DCQNI)2Cu system exhibits the low-dimensional mysterious reentrant metal-insulator-metal (M-IM) transition driven by the competition between conduction electrons and localized electrons. We have conducted experiments to study the thermodynamic properties of this reentrant M-I-M transition and have discussed the mechanism of the transition in the DCNQI-Cu system. Measurements of specific heat and latent heat of samples at various effective pressures were carried out with thermal relaxation (0.5 < T < 40 K), adiabatic (20 < T < 100 K), and differential thermal analysis (1.5 < T < 200 K) methods. Using the results of the thermal experiments, we evaluated the Gibbs free energy difference between the metallic and insulating phases. We succeeded in drawing the phase diagram that reproduced the experimental results quite well. We conclude that the competition between the free energy of free electrons in the metallic phase and that of localized electrons with spin s = 1/2 in the insulating phase essentially drives this characteristic reentrant M-IM transition. The reentrant M-I-M transition observed in this composite system of conduction electrons and localized electrons is a universal behavior of Fermion particles in which free Fermion particles localize to have spin freedom.

We review pressure-controlled calorimetric investigations for molecular compounds mainly focusing on the two-dimensional networked magnetic systems consisting of [Mn4] single molecule magnets (SMM) and the organic superconductive compounds with κ-(BEDT-TTF)4X composition, where BEDT-TTF is bisethylenedithiotetrathiafulvalene and X denotes counter anions. The former system shows sensitive magnetic features against external stimuli such as magnetic fields and pressures. The latter is known as typical dimer-Mott systems in effectively half-filled bands. At first, we introduce the idea and constructional details of the high-pressure calorimetry system designed for single crystal samples of molecule compounds weighing about 50 µg -1.0 mg. The adoption of extremely small resistance chips as a heater and thermometer parts made it possible for four-terminal detection of the temperature modulation with enough sensitivity even at low temperatures. Using this system, we measured single crystalline samples of 2D networked systems of [Mn4(hmp)6{N(CN)2}2](ClO4)2 (hmp- = 2-hydroxymethylpyridinate) under pressures up to 1.1 GPa and [Mn4(hmp)4Br2(OMe)2{N(CN)2}2](ClO4)2·2THF·0.5H2O up to 0.86 GPa. From the rather broad peak of thermal anomaly and its non-monotonous pressure dependence observed in [Mn4(hmp)4Br2(OMe)2{N(CN)2}2](ClO4)2·2THF·0.5H2O under pressures prompt us to consider that the competitive nature of the frustration effect due to the nearly orthogonal tilting of anisotropic spin axes and multi-body effects to form 2D Ising-type ordering. We also studied thermal anomaly at the superconductive transitions in dimer-based compounds with κ-(BEDT-TTF)2X. We claim there exists a gradual crossover of the electronic state inside the superconductive phase and the coupling strength between electrons to make a gap structure tends to vary from metal-insulator boundary to normal metal region.

The thermodynamic properties of various relaxors of PMN, PMT, PZN, PST, PMN–PT, PZN–PT, and BTZ have been reviewed and the excess thermodynamic properties are discussed. In lead-containing complex perovskite, excess heat capacity owing to the formation of PNR and glass-like low-temperature excitations are clearly observed. Studies of solid solutions indicate the difficulty of forming PNR with rhombohedral symmetry in the ferroelectric phase and the possible growth of a polar region with tetragonal symmetry in the compounds. Thus, the lead-free Ba-based relaxor BTZ35 can be considered as the frozen state of a displacive-type phase transition without disorder.