Current Inorganic Chemistry (v.4, #2)

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 canbe 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 offreedom in a statistical manner, one can gain detailed knowledge on the microscopic level on the basis of precise thermodynamicinvestigations. Among them, heat capacity calorimetry is an extremely useful tool to investigate thermal properties ofmaterials, in particular at low temperatures.For correct understanding of functionality of materials, it is crucially important to complementarily adopt both spectroscopicand/or structural methods leading to microscopic aspects of materials and thermodynamic methods revealing energeticaspects. The aim of this thematic issue is to review calorimetric studies on functional inorganic materials to show the importantroles played by thermodynamic studies.The topics picked up in Part II of this thematic issue are the following six functional materials: [1] Mixed-valence metalcomplexes M(II)M(III)X (M = Pt, Ni) and [2] Mixed-valence metal complex Fe(II)Fe(III)(dto)3: Electron transfer betweenthe mixed-valence metal ions provide a variety of phase transitions in which the magnetic and electronic properties are dramaticallyaltered [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 electronicproperties 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 givevarious metallic compounds. The electron correlations and electron-phonon coupling occuring in them produce a variety ofphase transitions such as metal-insulator, magnetic, and superconductive transitions. Thermodynamic information related tolow-energy excitations of itinerant electrons gives clue for understanding the mechanism of them [6]. Relaxors: The relaxor ischaracterized by a large, broad and frequency-dependent dielectric constant peak extending a wide temperature range. The giantelectromechanical response in ferroelectric relaxors is of great importance for a number of ultrasonic and medical applicationsas well as in telecommunications. Papers concerning calorimetry of other functional materials will be published in the forthcomingPart III.We would like to thank Prof. Yann Garcia, the Editor-in-Chief of Current Inorganic Chemistry, who invited us to edit athematic issue concerning calorimety of inorganic materials. We would like to acknowledge all the authors who accepted ourinvitation to contribute to this thematic issue, as well as the reviewers who invested their valuable time to ensure the high scientificquality of all contributions.

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

In mixed-valence assembled metal complexes whose spin states are situated in the spin-crossover region, multifunctionalproperties induced by the synergetic effect between spin and charge are expected. Based on this viewpoint, wehave 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 = 3and 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 regardedas a spin-entropy driven phase transition to minimize the Gibbs energy in the whole system. The appearance ofCTPT and the Curie temperature for (n-C2H2n+1)4N[FeIIFeIII(dto)3] significantly depend on the size of (n-CnH2n+1)4N+. Inthe case of (SP-Me)[FeIIFeIII(dto)3] (SP = spiropyran), the photo-isomerization of SP-Me by UV light irradiation inducesthe 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 areclassified as molecule-based magnets. Copper octacyanotungstate (1) has a 3-dimensional network structure. As the temperaturedecreases, 1 exhibits a magnetic dimensional crossover from 2-dimensional Heisenberg-type magnetic orderingto 3-dimensional Heisenberg-type magnetic ordering, which originates from the switching of the dominant superexchangeinteraction from the intralayer to the interlayer. Cobalt octacyanoniobate (2) exhibits humidity-sensitive magnetism fromferromagnetism to antiferromagnetism as the humidity decreases. Copper octacyanotungstate (3), whose crystal structurediffers from 1, displays an alcohol-sensitive magnetism with an extensive change in magnetization. The diverse spatialconfigurations and flexible coordination geometries are important to observe such magnetic functionalities because thesefeatures 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 experimentsto study the thermodynamic properties of this reentrant M-I-M transition and have discussed the mechanism of thetransition in the DCNQI-Cu system. Measurements of specific heat and latent heat of samples at various effective pressureswere 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 differencebetween the metallic and insulating phases. We succeeded in drawing the phase diagram that reproduced the experimentalresults quite well. We conclude that the competition between the free energy of free electrons in the metallic phaseand that of localized electrons with spin s = 1/2 in the insulating phase essentially drives this characteristic reentrant M-IMtransition. The reentrant M-I-M transition observed in this composite system of conduction electrons and localizedelectrons is a universal behavior of Fermion particles in which free Fermion particles localize to have spin freedom.

Investigations on Electronic States of Molecule-based Compounds by High-Pressure AC Calorimetry by Yasuhiro Nakazawa, Ryo Yoshimoto, Shuhei Fukuoka, Satoshi Yamashita (122-134).
We review pressure-controlled calorimetric investigations for molecular compounds mainly focusing on thetwo-dimensional networked magnetic systems consisting of [Mn4] single molecule magnets (SMM) and the organic superconductivecompounds with κ-(BEDT-TTF)4X composition, where BEDT-TTF is bisethylenedithiotetrathiafulvaleneand X denotes counter anions. The former system shows sensitive magnetic features against external stimuli such as magneticfields and pressures. The latter is known as typical dimer-Mott systems in effectively half-filled bands. At first, weintroduce the idea and constructional details of the high-pressure calorimetry system designed for single crystal samples ofmolecule compounds weighing about 50 µg -1.0 mg. The adoption of extremely small resistance chips as a heater andthermometer parts made it possible for four-terminal detection of the temperature modulation with enough sensitivity evenat 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 anomalyand its non-monotonous pressure dependence observed in [Mn4(hmp)4Br2(OMe)2{N(CN)2}2](ClO4)2¨2THF¨0.5H2O underpressures prompt us to consider that the competitive nature of the frustration effect due to the nearly orthogonal tilting ofanisotropic spin axes and multi-body effects to form 2D Ising-type ordering. We also studied thermal anomaly at the superconductivetransitions in dimer-based compounds with κ-(BEDT-TTF)2X. We claim there exists a gradual crossover ofthe electronic state inside the superconductive phase and the coupling strength between electrons to make a gap structuretends to vary from metal-insulator boundary to normal metal region.

Heat Capacity Study of Relaxors by Hitoshi Kawaji (135-144).
The thermodynamic properties of various relaxors of PMN, PMT, PZN, PST, PMN-PT, PZN-PT, and BTZhave been reviewed and the excess thermodynamic properties are discussed. In lead-containing complex perovskite, excessheat capacity owing to the formation of PNR and glass-like low-temperature excitations are clearly observed. Studiesof solid solutions indicate the difficulty of forming PNR with rhombohedral symmetry in the ferroelectric phase and thepossible growth of a polar region with tetragonal symmetry in the compounds. Thus, the lead-free Ba-based relaxorBTZ35 can be considered as the frozen state of a displacive-type phase transition without disorder.