Sharp Heat Research Articles

Critical heat capacity of antiferroelectric liquid crystal 12BIMF10

Abstract Heat capacity of the antiferroelectric liquid crystal 12BIMF10 has been measured with both an ac method and a relaxation method. A very sharp heat capacity peak was observed at the Sm-Cα*-Sm-A transition. The excess heat capacity for this transition showed a non-Landau critical behavior. The exponent α determined by a fit with a preasymptotic power-law equation lied 0.10-0.21. The data were also fitted acceptably well with 3D XY exponents when the correction-to-scaling terms up to the second order have been included. These results strongly suggest that the present data reflect a crossover from the 3D XY behavior to the tricritical behavior.

Metal-insulator transition at 230 K in a new thiospinel CuIr2S4

Abstract A new thiospinel CuIr2S4 has been synthesized. We have discovered a metal-insulator transition at 230 K. The physical properties of this compound have been investigated with special emphasis on heat capacity measurements in the range 13·8 K < T < 300K using an adiabatic calorimeter. The metal-insulator transition is accompanied by a structural phase transition from tetragonal symmetry (at the lower temperature) to cubic symmetry (at the higher temperature). The phase transition has been observed to be first order. A sharp heat capacity anomaly is exhibited at 230·8 K. The enthalpy and entropy of the transition are 3·50 kJ mol−1 and 15·6 J K−1 mol−1 respectively. A hysteresis effect at the phase transition was detected clearly in the heat capacity measurements. The metallic phase of CuIr2S4 shows Pauli paramagnetism which indicates a rather high density of states at the Fermi level, D(∊F) = 0·67 states/eV atom. The insulating phase of CuIr2S4 reveals semiconductive behaviour with an activation ene...

Calorimetric study on the spin-crossover phenomenon between 3T1 and 5E in trans-bis[1,2-bis(diethylphosphino) ethane]diiodochromium(II), [CrI 2(depe) 2

The molar heat capacity of [CrI 2(depe) 2], where depe = trans-bis[1,2-bis(diethylphosphino) ethane], has been measured with an adiabatic calorimeter in the 14–300 K range, and infrared spectra have been recorded at 300 and 91 K in the 4000− ndash;30 cm −1 range. A sharp heat capacity anomaly arising from the spin-state transition from the low-spin 3 T 1 to the high-spin 5 E electronic ground state was found at 171.45 K. The enthalpy and entropy of the phase transition are Δ trs H = 6686 ± 33 J mol −1 and Δ trs S = 39.45 ± 0.22 JK −1 mol −1, respectively. The entropy gain can be well accounted for in terms of the contributions from the change in the spin manifold (10%), the change in the metal-ligand skeletal vibrations (~ 75%) and the change in the barrier heights hindering the internal rotation of the eight methyl moieties of the ethyl groups attached to the phosphine ligands (~ 15%). The extremely sharp phase transition observed for this complex has been explained in terms of the large polarizability and thus large compressibility of the iodide ligands. The “cooperativity” often used for spin-crossover phenomena is discussed.

Low temperature phase transition in K 2ZnCL 4 at 146 K

The heat capacity of K 2ZnCl 4 has been measured between 10 and 300 K using an adiabatic calorimeter. The low temperature phase transition is strongly influenced by the small amount of water remaining in the crystals which were grown from an aqueous solution. A sharp heat capacity anomaly is observed at 146.24 K for the sample dried under vacuum above 100°C for 30 h, and the enthalpy and entropy of transition are 57.16J mol −1 and 0.387 J K −1 mol −1, respectively.

High heat flow anomalies over salt structures on the Texas Continental Slope, Gulf of Mexico

We made 74 closely spaced (&lt; 2 km apart) heat flow measurements around and over two salt structures on the Texas continental slope, Gulf of Mexico. The values outlined the shape of the heat flow anomalies over both structures. Based on a preceding high resolution seismic survey, we interpreted these structures to be a cylindrical plug and a salt tongue extending from the crest of a wall‐shaped feeder. The heat flow observations clearly reflect differences between the two features and are consistent with the prior structural interpretation. The values over the salt plug are nearly all greater than 70 mW/m². The measurements over the salt tongue have a sharp heat flow peak of 90 mW/m² associated with the presumed feeder and rather uniform values around 60 mW/m² over the remainder. The variation of heat flow over both structures is smooth and shows no apparent scatter. Heat flow values off these features are uniformly low, around 30 mW/m². Thermal effects from bottom water temperature fluctuation, slope sedimentation, diapiric movement of the salt body, and pore fluid migration appear unable to provide a satisfactory explanation for the observations. However, thickness variations of a highly conductive salt body can easily account for the heat flow anomalies. We suggest that modeling of the conductive anomaly should provide substantial constraints on the bottom geometry of the salt.

Magnetic phase transition in adsorbed 3He films

In view of a recent finding of a sharp specific heat anomaly at 2.5 mK in a second layer of absorbed 3He on grafoil by Greywall and Busch, we study a lattice gas model or Hubbard model on a triangular lattice. A novel type of the magnetic ordering of a vortex lattice is found, which is non-collinear with triple Q vectors and holds three-fold symmetry, for nearly half fillings. The size of the unit cell varies with 3He coverages. This is a generic periodic pattern for neatly accommodating the defects of 3He atoms which are always present in an experiment. We compare the present result with that in the square lattice case.

A study of the split transition in YBa2Cu3O7- delta by specific heat and resistivity measurements

Fully oxygenated ceramic YBa2Cu3O7- delta ( delta approximately 0) exhibits a double transition with a sharp specific heat peak at 88 K (Tc1) and a mean field like step at 92.5 K (Tc) indicative of a multicomponent order parameter. With small increase in delta , Tc-Tc1 decreases systematically (and reversibly) and is zero at delta =0.05. Several interpretations are considered and they conclude that the evidence favours an unconventional pair interaction. From a preliminary analysis of the fluctuation component of the specific heat and resistivity they find a fourfold decrease in coherence volume as delta increases from 0 to 0.1.

Magnetic structure of vortex lattice state in the adsorbed second layer of 3He films

Abstract Stimulated by a recent discovery of a sharp specific heat anomaly at 2.5mK in a second layer of adsorbed 3He on grafoil by Greywall et al., we study a lattice gas model or Hubbard model on a triangular lattice within the mean field approximation. In the case of nearly half fillings, we find a novel type of the magnetic structure forming vortex lattice states, which is non-collinear and characterized by triple Q vector keeping the three-fold symmetry.

Heat capacity study of the abrupt valence-detrapping phase transition of mixed-valence hexakis(acetato)oxotris(pyridine)trimanganese.pyridine

The heat capacity of the mixed-valence complex [Mn 3 O(O 2 CCH 3 ) 6 (py) 3 ].py, where py is pyridine, has been measured with an adiabatic calorimeter between 13 and 300 K. A phase transition with a sharp heat capacity peak has been found at 184.65 K. The enthalpy and entropy of the phase transition are ΔH=6460 J mol −1 and ΔS=35.77 J K −1 mol −1 . From a comparison of the present calorimetric results with the results of single-crystal X-ray diffraction, magnetic susceptibility, and solid-state 2 H NMR studies, it is concluded that the phase transition is associated with the onset of rapid intramolecular electron transfert in the mixed-valence Mn 3 O complexes and the orientational disordering of the pyridine solvate molecules

In vitro collagen fibril assembly: thermodynamic studies.

The in vitro fibril assembly of calf skin collagen was examined as a function of ionic strength and temperature. In a 0.03 M NaPi, pH 7.0, buffer, fibril assembly required a minimum critical concentration of collagen. At nearly physiological ionic strengths and temperatures, the critical concentration was less than 1 microgram/mL and required a very sensitive method for measurement. Raising the ionic strength of the buffer resulted first in higher and then lower critical concentrations. Raising the temperature led to lower critical concentrations. A van't Hoff plot of the fibril growth constant calculated from the critical concentration gave positive enthalpy changes and positive heat capacity changes which indicate that the fibril growth is driven by both hydrophobic and ionic inter-collagen interactions. Sedimentation equilibrium studies showed the collagen to be monomeric at subcritical concentrations. Differential scanning microcalorimetric studies showed only one very sharp heat absorption peak for the fibril assembly which coincided with the appearance of solution turbidity. Within experimental error, the enthalpy changes of the fibril assembly measured with the microcalorimeter were of the same magnitude as the van't Hoff enthalpy changes. These results are discussed in light of a cooperative nucleation-growth mechanism of collagen fibril assembly proposed earlier.

Low Dimensional Magnetism in the High Tc Superconductor LnBa2Cu3O7-y (Ln=Nd, Sm, Gd, Ho, Er)

Heat capacities have been measured for the high Tc superconductors LnBa2Cu3O7-y (Ln=Y, Nd, Sm, Gd, Ho, Er) between 0.4K and 100K. A cooperative heat capacity peak has been found at Tm=0.585K for SmBa2Cu3O7-y, 2.21K for GdBa2Cu3O7-y and 0.60K for ErBa2Cu3O7-y. In addition, broad heat capacity anomalies are found between 1K and 5K for all the compounds of magnetic rare earths studied. A semiquantitative fit can be obtained assuming that there exist two nearly independent systems of rare earth ions in LnBa2Cu3O7-y, namely, a 2–d Ising system which is responsible for the sharp heat capacity peak and a 1–d Ising system which causes the rounded maximum.

Effects of Compression and Magnetic Dilution on the First and Second Order Phase Transitions in Mn(HCOO)2·2H2O

Effects of compression and magnetic dilution on the first and second order phase transitions have been examined with Mn(HCOO) 2 ·2H 2 O, which exhibits both of the spin axis reorientation at T R =1.70 K and the antiferromagnetic transition at T N = 3.69 K. The application of hydrostatic pressure ( p ) makes a clear shift of T N ( p ) (d T N ( p )/d p = +0.022 T N ( p 0 ) K·kbar -1 ; p 0 ≃0 kbar) preserving a sharp heat capacity peak. However, the transition at T R ( p 0 ) = 1.70 K becomes quite obscure in the pressure with a broadening of the heat capacity peak around T R ( p )(d T R ( p )/d p = +0.078 T R ( p 0 ) K·kbar -1 ). These phenomena are compared with the phase transitions in the magnetically diluted system with Zn 2+ ions.

On the thermal structure of the southern Tibetan crust

Summary We present a quantitative analysis of the heat flow measurements recently made in southern Tibet by Francheteau et al. The data document a sharp heat flow anomaly of 54 ± 12 mW m-2. The sediment ages now available and reported here show that thermal effects due to sedimentation are negligible. Three physical effects must be taken into account in the interpretation: heat refraction across the complex structures of the Tibetan crust. erosion in transient thermal conditions and unsteady lateral heat transfer. We show that the refraction of heat may be responsible for up to 30 per cent of heat flow variations. We derive a new formula for the heat flow erosion correction and show that the correction which is usually applied can be a gross overestimate in thermally perturbed regions. Accounting for the possible magnitude of heat refraction across the geological features of southern Tibet, the amplitude of the anomaly could be as large as 70 mW m-2. The anomaly can only be due to a shallow and recent heat source. We show that in such a case erosion effects are small (less than 10 per cent), whatever the erosion rate is, and therefore do not affect the above values significantly. Using both the amplitude and the spatial characteristics of the anomaly, we show that the source must be younger than 1 Myr and shallower than 10 km. This is in good agreement with the results of magnetotelluric soundings carried out in the same area. The source could well be as young as 50 000 yr, which would make it a likely origin for the widespread geothermal activity observed further north on the Plateau. These results are evidence that melting processes are active today in the Tibetan crust near the Yalu-Zangbo suture zone, more than 45 Myr after the India-Asia collision and more than 200 km away from the site of underthrusting of the Indian lithosphere.

Heat capacity studies on single crystal annealed Fe3O4.

Single crystals of magnetite were grown and annealed under a controlled oxygen atmosphere to produce homogeneous, single phase samples. Heat capacity studies revealed a single, sharp first order heat capacity transition at the Verwey transition ${T}_{V}$\ensuremath{\simeq}121 K, no anomaly at 10 K, no evidence for short range order, and an entropy of transition of \ensuremath{\Delta}${S}_{v}$=R ln2 per mole of ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$. A model accounting for this numerical result is discussed.

Heat Capacity Studes on Magnetite

Abstract Single crystals of magnetite were grown and annealed under a controlled oxygen atmosphere to produce homogeneous, single phase samples. Heat capacity studies revealed a single, sharp first order heat capacity transition at the Verwey transition, no anomaly at 10 K, and an entropy of transition of ΔS = R In 2 per mole of Fe3O4.

Interacting phase transitions in Fe1+xCr2−xO4 (0≤x≤0.4)

The system Fe1+xCr2−xO4(0≤x≤0.4), in which a cooperative Jahn–Teller transition and a magnetic transition take place in the same temperature range, has been investigated by x-ray, magnetization, Mössbauer effect, and specific heat measurements. When 0.00≤x≤0.25, the Jahn–Teller transition temperature TD is higher than that of the magnetic transition TN, two peaks being observed in the specific heat. When x increases TD decreases slightly, whereas TN increases steeply, so that they coincide at x=0.275 with TD=TN=130 K, exhibiting a very sharp lambda type heat anomaly. When x&amp;gt;0.30, TD is expected to be lower than TN, but no distinct heat anomaly was observed in the expected temperature range. This indicates that the two cooperative phase transitions can couple when the higher temperature transition introduces a crucial part of the entropy lowering which would be expected after the lower temperature transition.

Heat capacities of lad x (2.5 ⩽ x ⩽ 3) from 1 to 300 K

The heat capacities of LaD x (2.5 ⩽ x ⩽ 3) were measured in the temperature range from 1 to 300 K. Four sharp heat capacity peaks were found in LaD 3.0 at 211, 230.5, 233.5 and 274 K indicating the existence of five polymorphic forms. Only one sharp heat capacity peak was observed at 251 K for LaD 2.91 and LaD 2.76, while no peaks were observed in the heat capacity of LaD 2.53. The electronic specific heat constant γ was found to be zero for LaD x (2.76 ⩽ x ⩽ 3), indicating that at low temperatures these alloys are semiconductors or semimetals with a small number of conduction electrons. The non-zero γ value for LaD 2.53 indicates the existence of a few conduction electrons. The thermodynamic functions S 298.15 o and (H 298.15 o−H 0) o) T were determined.

Thermal analysis of bacteriophage T4

The process of bacteriophage T4 morphogenesis was studied using a heat leakage scanning calorimeter. Thermograms of defective mutant 49 (am NG727) in permissive and non-permissive cells of Escherichia coli showed a difference in thermal properties between packaged and non-packaged DNA molecules. In vivo , non-packaged DNA carried out their thermal transition at 85°C, the same temperature as that of T4 DNA melting measured in the standard saline citrate buffer, while the packaged DNA gave a sharper peak at 87°C due to some interaction with the head shell structure. Empty head shells showed a sharp heat absorption peak at 89°C both in vivo and in vitro , indicating the high degree of cooperativity in their conformational changes.

Energetics of the Thermal Transitions of RNA 1 and RNA 4 of Alfalfa‐Mosaic Virus in the Presence and Absence of Coat Protein

Transition enthalpies of one genomic RNA, RNA 1. and the subgenomic RNA 4 of alfalfa mosaic virus were determined by scanning microcalorimetry in the absence and presence of various amounts of coat protein in a buffer containing 10 mM NaH2PO4, 1 mM EDTA, 1mM NaH3, pH 7.0. Analogous measurements were performed with the top and bottom components of alfalfa mosaic virus in the same buffer and with the coat protein in water. A comparison of the various enthalpies showed that the energetics of the protein–nucleic‐acid interaction are different for RNA 1 and RNA 4. The thermally induced transition of the coat protein in water is characterized by an enthalpy of 418 ± 67 kJ (100 ± 16 kcal) mol−1 at 38.9°C and a heat capacity change of approximately 6.7 kJ (1.6 kcal) mol−1 K−1. The unfolding pattern of the native bottom component of alfalfa mosaic virus, which consists of RNA 1 encapsulated by 240 coat protein molecules, is similar to that of the protein alone in exhibiting a single sharp heat capacity maximum and a heat capacity increase after the transition. The corresponding transition enthalpy is 523 ± 42 kJ (125 ± 10 kcal) · (mol protein)−1, the heat capacity change amounts to approximately 8 ± 1.7 kJ (1.9 ± 0.4 kcal) · (mol protein)−1, · K−1. An analogous transition curve results from unfolding of native top component fragments of alfalfa mosaic virus, which comprise two RNA 4 molecules per 132 protein molecules. The enthalpies involved in the unfolding process are 489 ± 42 kJ (117 ± 10 kcal) · (mol protein)−1, the heat capacity change 9.6 ± 2.5 kJ (2.3 + 0.6 kcal) · (mol protein)−1· K−1. The noticeable differences to the transition of the pure protein are the transition temperatures of 49.5°C and 47.9°C for the bottom and top components, respectively. The transition enthalpies of all protein · nucleic‐acid complexes are smaller than the sum of the transition enthalpies of the single components. This finding has been used to estimate enthalpies of interaction per mole of protein. The characteristic feature of these values is the fact that they all are positive, which indicates a decrease of the internal energy of the system on protein–nucleic‐acid interaction.

Librational and reorientational specific heats of NH+4 in NH4ZnF3 and NH4CoF3

Specific heat data on NH 4 CoF 3 and NH 4 ZnF 3 are presented. At room temperature these compounds crystallize in the cubic perovskite structure. At T c = 124.5 ± 0.1K and 115.1 ± 0.1 K, respectively, structural changes are found to occur, the main distortion being a tetragonal elongation of about 1.8%. For the Co 2+ compound it is accompanied by the onset of antiferromagnetic order. After subtracting the estimated lattice-phonon contribution and, for NH 4 CoF 3 also the magnetic contribution, we obtain the part of the specific heat associated with the NH + 4 movements. We find a nonanomalous contribution having a broad maximum near T ≌ 200 K ( C max R ≌ 2.3 ) which is ascribed to hindered rotations of the NH + 4 ions in the electrostatic potential of the surrounding ions. From a quantum-mechanical calculation of the rotational-energy spectrum for a rigid NH 4 tetrahedron in a potential appropriate for the cubic NH 4 ZnF 3 perovskite, we obtain a prediction that agrees quite well with the experimental specific heat. The fit yields a maximum energy barrier that is in good agreement with the calculated electrostatic barrier of 1857 K. In addition we find extremely high and sharp specific heat peaks at T c for both compounds, which are ascribed to a reorientational transition for the NH + 4 ions, accompanying the structural change. The associated transitional entropies are found to amount to R ln 3, implying a reduction by a factor of three in the number of orientational degrees of freedom.