Heat Capacity Curves Research Articles

First-Principles Bottom-Up Study of 1D to 3D Magnetic Transformation in the Copper Pyrazine Dinitrate S = 1/2 Antiferromagnetic Crystal

On the basis of magnetic susceptibility and heat capacity data, copper pyrazine dinitrate crystal [abbreviated CuPz(NO(3))(2)] has long been considered a good prototype for S = (1)/(2) antiferromagnetic (AFM) Heisenberg chain behavior down to 0.05 K. However, a recent muon-spin rotation experiment indicated the presence of a previously unnoticed 1D to 3D magnetic transition below 0.107 K. Our aim in this work is to perform a rigorous quantitative study of the mechanism of this 1D-3D magnetic transformation, by doing a first-principles bottom-up study of the CuPz(NO(3))(2) crystal at 158 K, where the magnetic properties are clearly 1D, and at 2 K, at which the neutron structure (reported in this work) is considered nearly identical with that below 0.1 K (due to small thermal effects). A change in the magnetic topology is found between these two structures: at 158 K, there are isolated AFM spin chains (J(intra) = -5.23 cm(-1)), while at 2 K, the magnetic chains (J(intra) = -5.96 cm(-1)) weakly interact (the largest of the J(inter) parameters is -0.09 cm(-1)). This change is caused by thermal contraction upon cooling (no crystallographic phase transition is detected down to 2 K, and one will not likely occur below that temperature). The computed and experimental magnetic susceptibility chi(T) curves are nearly identical. The calculated heat capacity C(p)(T) curve has a maximum at 6.92 K, close to the 5.20 K maximum found in the experimental curve at zero external field. In spite of the 3D magnetic topology of the crystal at low temperature, the magnetic susceptibility and heat capacity curves behave as a pure 1D AFM chain in all regions because of the large J(intra)/J(inter) ratio (66.2 in absolute value) and the effect of including the J(inter) interactions will not be easily appreciated in any of these experiments. The impact of the presence of odd- and even-membered regular AFM finite chains in the CuPz(NO(3))(2) crystal has also been evaluated. Odd-membered interacting chains produce an increase in both chi(T) and C(p)(T) curves when the temperature is very close to zero, in agreement with the experimental observations, while even-membered chains produce a small shoulder in the C(p)(T) curve between 0.8 and 5 K. No changes are seen in the remaining regions. Concerning the spin gap, odd-membered chains present a quasi-zero gap but the finite even-membered chains still have a sizable one. Finally, the effect of increasing the magnitude of J(inter) was investigated by fixing the value of J(intra) to that found for the 2 K CuPz(NO(3))(2) crystal. The magnetic susceptibility and heat capacity curves remain practically unchanged.

Unprecedented Layered Inorganic−Organic Hybrid Compound Mn3Sb2S6(C6H18N4) Composed of Mn4Sb2S6 Double-Cubane Units Showing Magnetic Long-Range Order and Frustration

The title compound Mn(3)Sb(2)S(6)(C(6)H(18)N(4)) (C(6)H(18)N(4) = triethylenetetramine) was obtained under solvothermal conditions by reacting Mn, Sb, S, and the amine at 140 degrees C for 7 days. The compound crystallizes in the triclinic space group P1 with a = 6.645(1) A, b = 8.667(1) A, c = 9.660(1) A, alpha = 90.82(2) degrees , beta = 109.70(2) degrees , gamma = 110.68(2) degrees , Z = 1, and V = 484.4(1) A(3). The Mn(4)Sb(2)S(6) double-heterocubane unit is the main motif in the structure of the title compound, which results from the interconnection of two SbS(3) trigonal pyramids, two MnS(6) octahedra, and two MnS(4)N(2) octahedra. The two N atoms completing the environment of the latter Mn(2+) ions belong to the tetradentate amine; i.e., the amine acts in a bidentate manner. The Mn(4)Sb(2)S(6) groups are joined by corner sharing of the MnS(6) octahedra, yielding one-dimensional linear Mn(3)Sb(2)S(6) rods along [100]. The two other N atoms of the amine molecule act in a bidentate manner to Mn(2+) ions of neighboring rods, thus producing layers within the (010) plane. Within the rods, the arrangement of the Mn(2+) ions in triangles leads to a chain of Mn(2+) diamonds connected via opposite corners. For the magnetic properties, each edge connecting the Mn(2+) ions represents a superexchange path due to coupling of the Mn(2+) centers via S bridges. The resulting Mn(2+) triangles give rise to substantial competing interaction and magnetic frustration. Below about 100 K, a gradual buildup of short-range antiferromagnetic correlations is observed. At lower temperatures, long-range antiferromagnetic interactions occur with T(N) = 2.90 K, as indicated by a lambda-type anomaly in the heat capacity curve. The analysis of the magnetic and heat capacity data evidences that the magnetic properties are essentially determined by the one-dimensional character of the Mn(3)Sb(2)S(6) chain. In addition, significant magnetic frustration due to the arrangement of the Mn(2+) ions in a triangular configuration cannot be neglected.

Structural changes in the water tetramer. A combined Monte Carlo and DFT study

The heat capacity curve of the water tetramer has been calculated at various levels of theory and over a broad range of temperatures, T = 50-200 K. Parallel-tempering and multiple-histogram Monte Carlo methods have been used and combined with the Density Functional Theory calculations of intra-cluster interactions via the Boltzmann-reweighting approach. It is demonstrated that such a combination can yield well converged thermodynamic data even for a modest number of sample configurations, which makes the methodology particularly appropriate for the inclusion of quantum chemistry calculations in Monte Carlo simulations. The B97-1 exchange-correlation functional has been used in the present work together with augmented correlation-consistent basis sets of atomic orbitals up to triple-zeta quality. A structural change has been detected in the water tetramer in the temperature range considered. It is demonstrated that the change corresponds to the onset of rotations of the water molecules around hydrogen bonds with an almost rigid skeleton of the cluster (square cycle) conserved.

Thermodynamic Investigation of Electrolytes of the Vanadium Redox Flow Battery (II): A Study on Low-Temperature Heat Capacities and Thermodynamic Properties of VOSO4·2.63H2O(s)

The low-temperature heat capacities of VOSO4·2.63H2O(s) which is a key component in the electrolyte of the vanadium redox flow battery were measured by adiabatic calorimetry in the temperature range of (78 to 388) K, and the experimental values of the molar heat capacities in the temperature regions of (78 to 372) K were fitted to a polynomial equation. The polynomial fitted values of the molar heat capacities and fundamental thermodynamic functions of the compound relative to the standard reference temperature of 298.15 K were calculated at intervals of 5 K. Additionally, a dehydration process occurred in the temperature range of (372 to 382) K, and the peak temperature, Tde = (378.895 ± 0.309) K, molar enthalpy of dehydration, ΔdeHm = (116.446 ± 0.148) kJ·mol−1, and molar entropy of dehydration, ΔdeSm = (307.329 ± 0.141) J·K−1·mol−1, were determined in terms of the heat capacity curve.

Investigation on Phase Transitions and Thermodynamic Properties of 1-Dodecylamine Hydrochloride (C12H25NH3Cl)(s) by an Improved Adiabatic Calorimeter

1-Dodecylamine hydrochloride was synthesized by the solvent-thermal method. The structure and composition of the compound were characterized by chemical and elemental analyses, the X-ray powder diffraction technique, and X-ray crystallography. The main structure and performance of an improved automated adiabatic calorimeter are described. Low-temperature heat capacities of the title compound are measured by the new adiabatic calorimeter over the temperature range from 78 K to 397 K. Two solid-to-solid phase transitions have been observed at the peak temperatures of (330.78 ± 0.43) K and (345.09 ± 0.16) K. The molar enthalpies of the two phase transitions of the substance were determined to be (25.718 ± 0.082) kJ · mol−1 and (5.049 ± 0.055) kJ · mol−1, and their corresponding molar entropies were calculated as (77.752 ± 0.250) J · mol−1 · K−1 and (14.632 ± 0.159) J · mol−1 · K−1, respectively, based on the analysis of heat–capacity curves. Experimental values of heat capacities for the title compound have been fitted to two polynomial equations. In addition, two solid-to-solid phase transitions and a melting process of C12H25NH3Cl(s) have been verified by differential scanning calorimetry.

HEAT CAPACITY OF 13- AND 19-ATOM PD–CO BINARY CLUSTERS: PARALLEL TEMPERING MONTE CARLO STUDY

Alloy nanoclusters are of interest because of their novel properties compared to bulk alloys. In this study, the thermal behavior of 13- and 19-atom Pd–Co binary clusters has been investigated by parallel tempering Monte Carlo technique using the Sutton–Chen many body potential. Clear changes in heat capacity curve are observed as a function of Pd–Co composition. We also found that, the 13-atom cluster melts in two stages and 19-atom cluster melts as a whole.

Synthesis, Characterization and Thermodynamic Study of the Solid State Coordination Compound Ni(Nic)2·H2O(s) (Nic=Nicotinate)

Abstract A novel compound‐monohydrated nickel nicotinate was synthesized by the method of room temperature solid phase synthesis and ball grinder. FTIR, chemical and elemental analysis, TG/DTG, and X‐ray powder diffraction technique were applied to characterize the structure and composition of the coordination compound. Low‐temperature heat capacities of the solid coordination compound have been measured by a precision automated adiabatic calorimeter over the temperature range from 78 to 386 K. A solid‐solid phase transition occurred in the temperature range of 328–358 K in the heat capacity curve, and the peak temperature, the molar enthalpy and molar entropy of the phase transition were determined to be Ttrs=(356.759±0.697) K, ΔtrsHm=(13.650±0.408) kJ· mol−1, and ΔtrsSm= (38.279±0.086) J·K−1·mol−1, respectively. The experimental values of the molar heat capacities in the temperature ranges of 78–328 K and 358–386 K were fitted to two polynomials, respectively. The polynomial fitted values of the molar heat capacities and fundamental thermodynamic functions of the sample relative to the standard reference temperature 298.15 K were calculated and tabulated at the intervals of 5 K.

Prediction of distinct surface segregation effects due to coordination-dependent bond-energy variations in alloy nanoclusters

The first implementation of a recently introduced method based on the extraction of the coordination dependence of surface bond-energy variations (CBEV) from density-functional theory (DFT) computed pure-metal surface-energy anisotropy is reported. In particular, polynomial functions fitted to DFT data computed previously for Pt, Pd, and Rh are used as input energetics for statistical-mechanical computations of Pt-Pd 923-atom cuboctahedron-cluster compositional structures (and Pt-Rh(111) as a test case) using the free-energy concentration expansion method (FCEM). The major findings concern the roles of preferential strengthening of intrasurface and surface-subsurface interlayer bonds leading to quite unique segregation characteristics: (i) strong Pt segregation at certain (111) surface sites of the Pt-Pd clusters, accompanied, at relatively high overall Pt composition, by weaker Pt segregation forming Pt-Pd ordered (100) structure, whereas Pd segregates mainly at the edge and vertex sites; (ii) dominant Pd subsurface segregation. The high computation efficiency of the CBEV/FCEM approach allows the determination of the complete temperature dependence of atomic-exchange processes among surface sites, as well as between subsurface and deeper sites, reflected in the corresponding configurational heat-capacity curves. Compared to other approaches, the high transparency of this method helps to elucidate the origin of the distinct bond-energy-variation effects on site-specific segregation in alloy nanoclusters.

Melting behaviors of Co N ( N = 13, 14, 38, 55, 56) clusters

Constant-temperature molecular dynamics simulations and the Gupta-type many-body interatomic potential are used to study the melting behaviors of Co N ( N = 13, 14, 38, 55, 56) clusters. The atomic equivalence indexes are used to describe the structural changes of clusters at different temperatures. Other quantities like the Lindemann index, the heat capacity and caloric curve are also employed. The lag phenomena (the rapid change of Lindemann index is not consistent with that of the heat capacity curve during the cluster melting) are discussed in detail for the Co 13 and Co 14 clusters. Our results show that the Lindemann index fails to capture the melting transition while the heat capacity does for the Co 13 or Co 14 clusters. All these clusters show premelting behaviors and the surface melting are found for the Co 14 and Co 56 clusters. The Co 38 cluster undergoes distinct isomerization transitions prior to its full melting.

Adsorption and pinning of multiblock copolymers on chemically heterogeneous patterned surfaces

The results of Monte Carlo simulations carried out on a system of multiblock copolymers having two different types of monomer units adsorbed on checker board surface configurations are presented here. We investigated the adsorption behavior for a series of different chess board square dimensions and also for various block lengths of the copolymer. We find that the specific heat capacity curves for adsorption indicate double peaks showing a two stage pattern recognition of the copolymer on the surface. It is also seen that the transition that happens at lower temperature corresponds to pinning where the junction point of the different blocks gets pinned to the interface between the different surface sites. It is interesting to see that the multiblock copolymers form multiple pinning sites on the boundary between different kinds of surface sites. There exists an intermediate size of the square on the board where the recognition and pinning are most favored and for smaller and larger size of the board and block length, the adsorption proceeds like in homopolymer on homogeneous surfaces. Unlike in the case of the simplest model of diblock copolymer on stripe-patterned surfaces [K. Sumithra and E. Straube, J. Chem. Phys. 125, 154701 (2006)], here the recognition is stronger and the average adsorption energy and the perpendicular component of the radius of gyration show distinct changes corresponding to the two transitions. The conformational properties of the multiblock copolymer near the checkered surface show interesting effects with the perpendicular component showing strong deviations from the standard behavior.

Characterization of granular phase changing composites for thermal energy storage using the T-history method

The present article reports on the characterization of granular phase changing composites using the T-history method. Further modifications and improvements of the method are employed to handle granular materials undergoing phase change over a temperature range. The accuracy of the T-history method is shown to be limited by the assumption of temperature-independent specific heats and the difficulty of determining the limits of solid and liquid phases. The concept of enthalpy and its relationship with temperature has been employed in the analysis to overcome these difficulties. Enthalpy–temperature and apparent heat capacity curves similar to those obtained using DSC have been developed. These characteristic curves are necessary for accurate design, modeling, and optimization of latent heat thermal energy storage systems. Experiments have been also carried out to measure the transient temperature distribution inside a cylindrical packed bed using phase changing granulates. Analysis of temperature variation along the bed shows good agreement with the measured phase change characteristics. Copyright © 2009 John Wiley & Sons, Ltd.

Stability of gold cages (Au16 and Au17) at finite temperature

We have employed ab initio molecular dynamics to investigate the stability of the smallest gold cages, namely Au16 and Au17, at finite temperatures. First, we obtain the ground state structure along with at least 50 distinct isomers for both the clusters. This is followed by the finite temperature simulations of these clusters. Each cluster is maintained at 12 different temperatures for a time period of at least 150 ps. Thus, the total simulation time is of the order of 2.4 ns for each cluster. We observe that the cages are stable at least up to 850 K. Although both clusters melt around the same temperature, i.e. around 900 K, Au17 shows a peak in the heat capacity curve in contrast to the broad peak seen for Au16.

Enthalpy Relaxation upon Glass Transition and Kinetic Fragility of Molecular Liquids

Relaxation enthalpies in the vicinity of glass transitions were recorded on the basis of the heat capacity curves measured in nine glass-forming molecular liquids and solutions in the well defined cooling and subsequent heating cycles. It is observed that enthalpy hysteresis in the cooling/heating cycles is less significant in liquids with increasing fragility. A notable correlation of the fragility index m with relaxation enthalpy DeltaH(R), glass transition temperature, and heat capacity jump from glasses to liquids is demonstrated. The introduction of DeltaH(R) favors the thermodynamic evaluation of fragility in the glass forming multicomponent systems such as solutions and alloys.

The heat capacities and thermodynamic properties of NiAl 2O 4 and CoAl 2O 4 measured by adiabatic calorimetry from T = (4 to 400) K

The low-temperature heat capacity of NiAl 2O 4 and CoAl 2O 4 was measured between T = (4 and 400) K and thermodynamic functions were derived from the results. The measured heat-capacity curves show sharp anomalies peaking at around T = 7.5 K for NiAl 2O 4 and at T = 9 K for CoAl 2O 4. The exact cause of these anomalies is unknown. From our results, we suggest a standard entropy for NiAl 2O 4 at T = 298.15 K of (97.1 ± 0.2) J · mol −1 · K −1 and for CoAl 2O 4 of (100.3 ± 0.2) J · mol −1 · K −1.

Insights into protein–polysorbate interactions analysed by means of isothermal titration and differential scanning calorimetry

Therapeutic proteins formulated as liquid solutions at high protein concentration are very sensitive to chemical and physical degradation. Especially avoiding the formation of protein aggregates is very crucial for product quality. In order to stabilize the colloidal properties of protein therapeutics various excipient are used. Especially the detergents polysorbate 20 and 80 are common. However, the mechanism upon which the detergents protect the protein from aggregation is not really known. The present study investigates the interaction of polysorbate 20 and 80 with different proteins: lysozyme, bovine serum albumin (BSA) and an immunoglobulin. The interaction and binding of the detergents to the proteins is investigated by isothermal titration calorimetry (ITC). From ITC the thermodynamic parameters (DeltaH: change in enthalpy, DeltaS: entropy and DeltaG: free energy) upon binding are derived as well as the binding constant K (a). The thermal stability of the proteins in the presence of the detergent is assessed by differential scanning calorimetry (DSC). The results show that both detergents bind to BSA with K (a) between 8 and 12 x 10(3) M(-1) with DeltaH -50 to -60 kJ/mol (25 degrees C). One to two detergent molecules bind to BSA. The presence of both detergents induces a weak stabilisation of the thermal denaturation properties of BSA. However, the interaction of polysorbate 20 and 80 with lysozyme and the immunoglobulin is quite negligible. The presence of the detergents up to a concentration of 2 mM has no impact on the heat capacity curve neither a destabilisation nor a stabilisation of the native conformation is observed.

Heat capacity at constant pressure and thermodynamic properties of phase transitions in PbMO3 (M=Ti, Zr and Hf)

The heat capacity of PbMO3 (M=Ti, Zr and Hf) at constant pressure was measured using a differential scanning calorimeter (DSC) from room temperature up to 870 K. Large anomalies were found in the heat capacity curves, corresponding to the ferroelectricparaelectric phase transition in PbTiO3 (PT), the antiferroelectric-paraelectric phase transitions in PbZrO3 (PZ) and PbHfO3 (PH). The transition entropies were estimated as 7.3 J K−1 mol−1 (PT), 9.9 J K−1 mol−1 (PZ) and 9.3 J K−1 mol−1 (PH). These values of transition entropies are much larger than that of a typical displacive-type phase transition.

Magnetocaloric properties of Mn3Sn2 from heat capacity measurements

The heat capacity of the Mn3Sn2 compound was measured between 2 and 300 K in magnetic fields up to 7 T. The isothermal magnetic entropy change (ΔSM) and the adiabatic temperature change (ΔTad) were calculated from these data. In agreement with previous magnetic measurements, three transitions are detected in the zero-field heat capacity versus temperature curve, two λ-like peaks close to TC1∼262 K and TC2∼227 K due to ferromagneticlike second-order transitions and a third anomaly at Tt∼197 K ascribed to the development of a weak antiferromagnetic interaction. The Debye temperature is estimated to be ∼303 K from the low-temperature data. The thermal dependences of ΔSM and ΔTad present a two-peak response with peak maxima near TC1 and TC2. For a field variation of 5 T, the values of ΔTad are found to be ∼2.1 and ∼1.6 K, respectively. The results are discussed and compared to previously published data concerning other magnetic refrigerants working near room temperature.

The high-temperature heat capacity of ThPd 3 and UPd 3

The enthalpy increments of UPd 3 and ThPd 3 have been measured using the drop calorimetry for temperature range (480 to 1290) K. The heat capacity curves have been derived using the simultaneous linear regression of our data and the low temperature heat capacity data from previous studies. Furthermore the Schottky contributions for UPd 3 were determined by comparing the ThPd 3 and UPd 3 heat capacity functions and correlated with the theoretical values calculated from the crystal field energy levels.

Synthesis, Characterization, and Thermodynamic Study of the Solid State Coordination Compound Cu(Nic)2·H2O(s)

AbstractA novel compound–monohydrated cupric nicotinate was synthesized by the method of room temperature solid phase reaction and a ball grinder. FTIR, chemical and elemental analyses and X‐ray powder diffraction techniques were applied to characterize the structure and composition of the complex. Low‐temperature heat capacities of the solid coordination compound have been measured by a precision automated adiabatic calorimeter over the temperature range from 78 to 400 K. A melting process occurred in the temperature range of 326–346 K in the heat capacity curve, and the melting point, molar enthalpy and entropy of the fusion were determined to be Tfus= (341.290±0.873) K, ΔfusHm= (13.582±0.012) kJ·mol−1, ΔfusSm= (39.797±0.067) J·K−1·mol−1. The experimental values of the molar heat capacities in the temperature regions of 78–326 K and 346–400 K were respectively fitted to two polynomial equations. The polynomially fitted values of the molar heat capacities and fundamental thermodynamic functions of the sample relative to the standard reference temperature 298.15 K were calculated and tabulated at an interval of 5 K.

Thermochemical Behavior of Crystalline RE(Val)Cl3·6H2O (RE = Nd, Er, Val = Valine)

Heat capacities of two solid complexes of rare-earth elements with valine RE(Val)Cl3·6H2O (RE = Nd, Er, Val = Valine) have been measured with a high-precision automatic adiabatic calorimeter over the temperature range from (78 to 376) K. An anomaly of a large step in the heat capacity curve was detected in the temperature range of roughly (308 to 314) K and (297 to 300) K for the Nd and Er complex, respectively, which may be attributed to a glass transition corresponding to freezing of structural disorder. Thermal decomposition of the two complexes was studied by a thermogravimetric (TG) technique, and the possible mechanism for the decomposition was suggested.