Heat Capacity of Intercalated Layered Materials FexNbS2 at Low Temperature

The heat capacities and magnetic susceptibilities of powdered samples of Fe_xNbS₂ (x=0.14, 0.21 and 0.30) were measured at temperatures from 8 to 303 K and from 5 to 300 K, respectively. For Fe_0.14NbS₂, the magnetic susceptibility exhibited an anomaly as a shoulder at about 57 K, but no heat capacity anomaly was observed at this temperature, indicating the appearance of a spin-glass state. Anomalies in the heat capacity for Fe_xNbS₂ (x=0.21 and 0.30) occurred at 100.5 and 45.0 K, respectively, where the magnetic susceptibility displayed a maximum, corresponding to an antiferro-paramagnetic phase transition. The electronic state of the iron atom is discussed on the basis of entropy analysis.This revised version was published online in November 2005 with corrections to the Cover Date.     

Heat Capacity and Thermodynamic Properties of p'-Substituted p-n-Hexyloxybenzylideneaniline. I. p-n-Hexyloxybenzylideneamno-p'-Benzonitrile (HBAB)

Abstract

The heat capacity of the nematogenic liquid crystal, HBAB, has been measured between 15 K and 385 K by using an adiabatic calorimeter. The crystal-crystal phase transition has been discovered at 27 K below the crystal-nematic phase transition temperature. The transition temperatures, the enthalpies and the entropies of the three phase transitions have been determined: T ₁ = 306.98 K, ΔH _t = 5.11 kJ mol^?1, ΔS _t = 16.7 JK^?l; T _m = 334.05 K, ΔH _m = 23.77 kJ mol^?1, ΔS_m = 71.2 J K^?l mol^?1; and T _c = 375.10 K, ΔH _c = 1.75 kJ mol^?1, ΔS _c = 3.2 J K^?1 mol^?1, respectively. The thermodynamic functions of HBAB from 0 K to 385 K have been determined from the heat capacity data and the enthalpies of the transitions. Two crystal modifications, one yellow and granular form and the other white and needle-like form, have been obtained during the course of the preparation of the sample. It turned out that the yellow form was the stable crystal and the white the metastable modification. The crystal-crystal phase transition has been discussed as an onset of partial melting from the entropy consideration. In this connection the total entropies of the transitions, 91.1 J K^?1 mol^?1 has been proposed to be an important measure of melting.     

Phase diagram of lithium-doped copper oxide, Cu1−xLixO

Heat capacities of lithium-doped samples of CuO have been measured below room temperature by adiabatic calorimetry. The antiferromagnetic ordering transition to incommensurately modulated state was detected as a step in the heat capacity. Its concentration dependence was compatible with existing reports based on Li-NMR. The incommensurate-commensurate transition of lithium-doped copper oxide was clearly detected for the first time. The magnetic phase diagram of Cu_1−xLi_xO was thus constructed. The suppression of both transition temperatures by the Li doping is nearly twice as strong as that expected from mean-field and percolation theories.     

Heat capacity and magnetic phase transitions of rare-earth orthoferrite HoFeO3

Heat capacity of two rare-earth orthoferrites HoFeO₃ and LuFeO₃ were measured between 1.8 and 200 K. A distinctly large and two small heat capacity anomalies were detected for HoFeO₃ under zero magnetic field around 3.3, 53 and 58 K, respectively. The low-temperature anomaly with a peak at 3.3 K is due to the ordering of Ho³⁺ ions and the estimated magnetic entropy for this transition was favorably compared with the expected (R ln 2). Application of magnetic field significantly affects the positions and the magnitudes of the anomaly at 3.3 K. Energies of low-lying levels of the lowest J-term of Ho³⁺ ion were roughly estimated through analysis of the Schottky heat capacity.     

Cubic phase of 4'-n-Hexadecyloxy-3'-cyanobiphenyl-4-carboxylic acid (ACBC-16)

The cubic phase structure of 4'-n-hexadecyloxy-3'-cyanobiphenyl-4-carboxylic acid (ACBC-16) was examined by X-ray diffraction. Unlike the octadecyloxy homologue showing an Im3m-type cubic phase, the cubic phase of ACBC-16 was of Ia3d type, both on heating and on cooling, similarly to the corresponding nitro-substituted analogue (ANBC-16). The lattice dimension a at 453 K was a = 11.0 nm, 2.5% larger than the value for ANBC-16 and rather close to the value of ANBC-17 or -18. It is expected that the appearance of the cubic phase type, as a function of the number of carbon atoms n in the alkoxy chain in the ACBC-n series, is essentially the same as in the ANBC-n series, but shifted towards shorter n by 1 or 2. In the latter ANBC-n series, the cubic phase type is Ia3d for 15≤n≤18, while an Im3m type is formed for 19≤n≤21, both on heating and on cooling.     

Calorimetric study of the relationship between molecular structure and liquid-crystallinity of rod-like mesogens I. Heat capacities and phase transitions of 4'-propylbiphenyl-4-carbonitrile and trans, trans-4'-propylbicyclohexyl-4-carbonitrile

The heat capacities of the rod-like compounds 4'-propylbiphenyl-4-carbonitrile (3-BBCN) and trans, trans-4'-propylbicyclohexyl-4-carbonitrile (3-CCCN) have been measured with an adiabatic calorimeter between 12 and 383 K. 3-BBCN is not mesogenic and melts into an isotropic liquid at 338·77 K, whereas 3-CCCN is mesogenic and its melting and clearing points are 330·73 K and 353·80 K, respectively. The enthalpy and entropy of fusion of 3-BBCN are 22·7 kJ mol^-1 and 67·0 J K^-1 mol^-1, respectively, while those of 3-CCCN are 27·0 kJ mol^-1 and 81·7 J K^-1 mol^-1, respectively. The enthalpy and entropy gain due to the nematicisotropic transition of 3-CCCN are 1·8 kJ mol^-1 and 5·0 J K^-1 mol^-1. 3-CCCN shows a mesomorphic transition from a smectic to the nematic state at 329·62 K, which occurs as a metastable state, its transition enthalpy and entropy are 5·4 kJ mol^-1 and 16·4 J K^-1 mol^-1, respectively. The temperature dependence of the molar entropies of both compounds shows that molecular arrangement in the crystal is more ordered in 3-CCCN than in 3-BBCN. This fact may be related to the stability of mesophases. Finally, Eidenschink's theoretical model for the nematicisotropic transition has been applied to 3-CCCN. As far as the present mesogen is concerned, the transition enthalpy estimated according to this model agrees well with the observed value.     

Heat capacity of a cubane-tetramer, [Ni(OCH3) (SAL) (CH3OH)]4, between 0.4 AND 288 K: spin-spin interactions and tunnel-splitting of the internal rotation of methyl-groups2

The heat capacity of a cubane-tetramer, tetra-μ₃-methoxy-tetrakis [salicylaldehydato (methanol) nickel (II)] (abbreviated as [Ni(OCH₃) (SAL) (CH₃OH)]₄), has been measured from 0.4 to 288 K. A broad peak centered at 0.85 K was observed. Subtraction of the lattice heat capacity from the observed one brought about another broad peak centered around 13.5 K. The magnetic anomaly is accounted for by a nearest-neighbor intracluster ferromagnetic exchange interaction characterized by $\text{J}\text{k}\text{= 3.8 K}$ and a single-ion zero-field splitting due to biaxial crystalline anisotropy$\text{(}\text{D}\text{K}\text{= 3.8 K}$ and$\text{E}\text{k}\text{= 1.9 K).}$ The tunnel-splitting of the ground vibrational-rotational state is δ = 2.4 J mol^?1, which corresponds to a potential barrier of V₀ = 2.3 kJ mol^?1 when a three-fold symmetry of a methyl-rotator is assumed. Comparison of the heat capacities between the present compound and a similar cubane-tetramer, [Ni(OCH₃) (acac) (CH₃OH)]₄, indicates that among eight methyl-groups the four belonging to methoxy-ligands are concerned with the tunnel-splitting at low temperatures.     

Towards the molecular-statistical modelling of the optically isotropic mesophase in neat systems: from the thermodynamic point of view

Phase diagrams of some compounds showing optically isotropic mesophases are compared as a function of the number of paraffinic carbon atoms per molecular core. In spite of a wide range of the transition temperatures, the number of paraffinic carbon atoms required for the appearance of the isotropic mesophases is limited within a narrow range, irrespective of molecular structure and/or intermolecular interaction. Combining this finding with the previous results obtained from thermodynamic studies, a possible framework towards molecular-statistical modelling is proposed.     

Heat capacities of the electron acceptor 7,7,8,8-tetracyano- quinodimethane (TCNQ) and its radical-ion salt NH4(TCNQ) showing spin-Peierls transition

Heat capacities of the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its radical-ion salt NH₄-TCNQ have been measured at temperatures in the 12-350 K range by adiabatic calorimetry. A λ-type heat capacity anomaly arising from a spin-Peierls (SP) transition was found at 301.3 K in NH₄-TCNQ. The enthalpy and entropy of transition are Δ_trsH=(667±7) J mol⁻¹ and Δ_trsS=(2.19±0.02) J K⁻¹ mol⁻¹, respectively. The SP transition is characterized by a cooperative coupling between the spin and the phonon systems. By assuming a uniform one-dimensional antiferromagnetic (AF) Heisenberg chains consisting of quantum spin (S=1/2) in the high-temperature phase and an alternating AF nonuniform chains in the low-temperature phase, we estimated the magnetic contribution to the entropy as Δ_trsS_mag=0.61 J K⁻¹ mol⁻¹ and the lattice contribution as Δ_trsS_lat=1.58 J K⁻¹ mol⁻¹. Although the total magnetic entropy expected for the present compound is R ln 2 (=5.76 J K⁻¹ mol⁻¹), a majority of the magnetic entropy (∼4.6 J K⁻¹ mol⁻¹) persists in the high-temperature phase as a short-range-order effect. The present thermodynamic investigation quantitatively revealed the roles played by the spin and the phonon at the SP transition. Standard thermodynamic functions of both compounds have also been determined.