Sharp Peak In Heat Capacity Research Articles

The structural and magnetic properties of Pr1−xErxAl2

We report on the effect of Er addition to PrAl2 on the lattice parameters, magnetic behavior, heat capacity, and magnetocaloric effect by using x-ray diffraction, magnetization, and heat capacity measurements. Unlike Pr0.6Er0.4Al2, other alloys we studied in the pseudobinary (Pr1−xErx)Al2 system do not exhibit a sharp peak in heat capacity with the application of magnetic field. Both the cubic lattice parameter and the Curie temperature decrease with increasing Er concentration. The nuclear specific heat coefficient decreases from 660 mJ K mol−1 for x = 0.05 to a nearly negligible value for x = 0.95. The magnetic entropy and adiabatic temperature change varies from 2 to 4 J mol−1 K−1 and 2.5 to 5 K at ΔH = 20 kOe for x = 0.05 to 0.95, respectively. These values of the magnetocaloric effect are comparable to those of the other rare-earth dialuminides systems.

Investigation of Size Effects on the Al Nanoclusters Physical Properties via Molecular Dynamics Simulations

Molecular dynamics (MD) simulation has been carried out for study of size effects on physical properties of Al nanoclusters with different sizes (N = 256, 500, 864, 1372, 2048 and 4000) in the temperature range 300 K ≤ T ≤ 1200 K for both free and periodic boundary conditions. Energy per site for periodic and free boundary conditions was calculated for the mentioned sizes and temperature. By increasing size of Al nanocluster, the energy per atom was convergence to that of the bulk. By use of molecular dynamics some physical properties such as melting point, surface energy, sublimation energy, thermodynamic limit and pair distribution function have been obtained. Melting point of bulk state is 875 K which proportional the jump of energy and sharp peak in heat capacity. Our calculated sublimation energy (ΔHs-calculated = 330 kJ/mol) was in good agreement with experimental result (ΔHs-experimental = 318 kJ/mol).

Magnetism of Ho1−xTbxAl2alloys: Critical dependence of a first-order transition on Tb concentration

HoAl${}_{2}$ exhibits a first-order spin reorientation transition at 20 K, which is manifested as a sharp peak in the heat capacity. When Ho is partially replaced by only 5$%$ of Tb, the sharp heat-capacity peak in ${\mathrm{Ho}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}{\mathrm{Al}}_{2}$ ($x$ = 0.05) disappears, and then reappears again for $x$ \ensuremath{\ge} 0.07. For $x$ = 0.05, the anomaly corresponding to the spin reorientation transition is barely seen in the heat capacity, but as $x$ exceeds 0.07 the weak anomaly transforms to a sharp peak. The spin reorientation transition temperature increases to 29 K for $x$ = 0.05, and as $x$ increases further the transition shifts to lower temperature and returns to \ensuremath{\sim}20 K for $x$ = 0.25. The transition is no longer observed when $x$ exceeds 0.60. Temperature-dependent x-ray powder-diffraction data confirm the first-order nature of the spin reorientation transition for the alloy with $x$ = 0.40, and indicate that the compound retains the room-temperature cubic structure within the sensitivity of the technique. Experimental observations are discussed considering the easy magnetization directions of HoAl${}_{2}$ and TbAl${}_{2}$.

Accurate heat capacity data at phase transitions from relaxation calorimetry

Extracting accurate heat capacities by conventional relaxation calorimetry at first-order or very sharp second-order phase transitions is extremely difficult. The so-called “scanning method” provides a key to overcome this challenge. Here, we introduce new corrections in the data analysis of this method. Critical examinations of the improvements are made experimentally by investigating the well-studied first-order ferroelectric phase transitions of KH 2PO 4 and BaTiO 3 using a commercial relaxation calorimeter Physical Property Measurement System (PPMS) supplied by Quantum Design. The results for KH 2PO 4 are shown to be excellent; a very sharp peak in heat capacity is obtained and the absolute values are shown to agree well with the previous results obtained by adiabatic calorimetry on much larger samples. The critical behavior of the heat capacity in the vicinity of the transition temperature, as well as the thermodynamic quantities such as the transition enthalpy and entropy, also agrees very well with the previous results. For BaTiO 3, clear hysteretic behavior of the transition is observed for heating and cooling curves.

Heat Capacity Study of the Doping Effect of Paramagnetic Radicals on the Organic Spin–Peierls System (p-CyDOV Radical Crystal)

The doping effect of paramagnetic organic radical impurities ( p -CyDTV) on the organic spin–Peiels (SP) system of p -CyDOV (3-(4-cyanophenyl)-1,5-dimethyl-6-oxoverdazyl; the SP transition temperature T SP =15.0 K) was studied by the heat capacity measurement down to 0.1 K. At low temperatures below 1 K, quite a sharp heat capacity peak was detected for each system with small impurity concentration ( x ) at the temperature T N ( x ); T N (0)=0.135 K (±0.002 K), T N (0.01)=0.290 K and T N (0.07)=0.164 K. The appearance of the peak for the system with x =0 is considered to be due to minute and inevitable impurities in the pure p -CyDOV. From the dependence of T N ( x ), T N (0)< T N (0.01), and the heat capacity behavior on x , the peaks for x =0 and 0.01 are explained to be accompanied with the three-dimensional (3D) antiferromagnetic (AFM) transition in the SP phase. While the decrease of T N ( x ) for higher impurities x =0.07 than that for x =0.01 may be interpreted as merely a 3D AFM transition from the paramagnetic state in the quasi-1D systems with the interchain interaction | z J '/ J |=6×10 -4 . Although no anomaly of the heat capacity was found around T SP ( x )=13–15 K in the present systems, where the characteristic decrease of the magnetic susceptibility is reported, we observed a distinct hump of the heat capacity at T SP '(0)=5.6 K and T SP '(0.01)=5.3 K with magnetic entropy consumption up to 3%, suggesting that this transition may relate to SP transition.

Phase transition and glass transition concerning configurational order/disorder of ions in crystalline (TMA)2[Sr{Ni(pro)2}6](ClO4)4 and (TMA)[Sm{Ni(pro)2}6](ClO4)4

Crystalline (TMA)2[Sr{Ni(pro)2}6](ClO4)4 was synthesized newly and its structure was determined, where TMA and pro denote tetramethylammonium and L-prolinato, respectively. Heat capacities of crystalline (TMA)2[Sr{Ni(pro)2}6](ClO4)4 and (TMA)[Sm{Ni(pro)2}6](ClO4)4 were measured at low temperatures by using an adiabatic calorimeter. In the former compound, a phase transition of the first order was found to occur at (160±1)K with a sharp heat-capacity peak. The enthalpy and entropy of the transition were estimated to be (11.0±0.2) and (69.4±1.5)JK−1mol−1, respectively. In the latter compound, a phase transition of the first order and a glass transition were found at (190±1) and (162±2)K, respectively. The entropy of the phase transition was estimated to be in the range 20–45JK−1mol−1. The phase transitions were attributed to the orientational order/disorder process of perchlorate ions ClO4−, and it was suggested that each ClO4− ion has six and three distinguishable, reasonably stable orientations in the high-temperature disordered phase for the SrII and SmIII complex compounds, respectively. The glass transition was interpreted as a freezing-in phenomenon of the reorientational motion of ClO4− ions, and the activation energy for the motion was estimated to be (53±1)kJmol−1 and less than 39kJmol−1 for the SmIII and SrII complex compounds, respectively; the removal of one of the two TMA ions neighboring to the ClO4− ion leads to an increase in the activation energy. It is discussed that the cooperative interaction between the orientations of the ClO4− ions operates through the orientational and positional shifts of TMA ions, and thus the lattice deformation in the relevant region, associated with the orientational change of the ClO4− ions. Then it is noted that the position of the ClO4− ion itself would shift to form preferable ionic interaction, for example through a kind of hydrogen bond of C–Hδ+⋯δ−O–Cl, for each permissible orientation. The shift in total results in a large thermal parameter of the chlorine atom in the disordered phase.

Magnetic heat capacity in lanthanum manganite single crystals

The heat capacity of single crystal La0.7D0.3MnO3, where D=Ca, Sr, has been measured through the Curie point in fields up to 70 kOe. The magnetic contribution of the Ca sample exhibits a sharp heat capacity peak at TC≃218 K in zero field. The peak broadens and decreases in height with increasing field but, unlike an ordinary ferromagnet, the peak shifts substantially in temperature. As a consequence, the heat capacity data cannot be collapsed into a single scaling function. These features indicate that the transition is not an ordinary second-order ferromagnetic transition. Preliminary heat capacity data from the Sr-doped single crystal, with TC≃360 K, do not exhibit the same shift in peak position with applied field. We attribute the difference in behavior between Ca- and Sr-doped samples to a change in the nature of the phase transition as TC lowers.

Heat capacity of a 2DEG

We review recent heat capacity experiments performed on GaAs/AlGaAs heterostructures containing two-dimensional electron layers. Near ν=1, the strong coupling to the nuclear spin system attributed to Skyrmion-induced low-energy spin excitations has a dramatic effect on the heat capacity which rises by orders of magnitude. We show that this giant heat capacity is consistent with the Schottky nuclear heat capacity of Ga and As atoms in the quantum wells, except at very low temperature where heat capacity exhibits a remarkably sharp peak suggestive of a phase transition in the electronic system. We also report quasi-adiabatic experiments providing quantitative evidence that the sharp peak in heat capacity is due to an enhanced nuclear-spin diffusion from the GaAs quantum wells into the AlGaAs barriers. Finally, we present the latest heat capacity results which probe the presence or the absence of Skyrmions at large Zeeman energy.

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.

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

Heat capacities of LaDx and LaHx (1.9

The heat capacities of lanthanum hydrides in the LaD{sub {ital x}} and LaH{sub {ital x}} (1.9{le}{ital x}{le}3.0) series have been measured over the temperature range from 1.2 to 300 K. The electronic specific-heat coefficients showed nearly the same variation for both LaD{sub {ital x}} and LaH{sub {ital x}} over the entire composition range, having the values of {gamma}=0.81{plus minus}0.01 and 0.038{plus minus}0.01 mJ/g-at. K{sup 2} at the stoichiometric compositions of LaD{sub 2} (LaH{sub 2}) and LaD{sub 3} (LaH{sub 3}), respectively. Some different variations were found in the composition dependence of the Debye temperatures of the LaD{sub {ital x}} and LaH{sub {ital x}} series, but, nevertheless, the same values of {ital FTHETA}{sub {ital D}}=348{plus minus}2 and 381{plus minus}2 K were found at the compositions of LaD{sub 2} (LaH{sub 2}) and LaD{sub 3} (LaH{sub 3}), respectively. The variations of {gamma} and {ital FTHETA}{sub {ital D}} do not show a monotonic behavior but depend on the formation of the three hydrogen-ordering phases, namely, LaD(H){sub 2.25}, LaD(H){sub 2.5}, and LaD(H){sub 2.75}. One sharp heat-capacity peak was found in LaD{sub {ital x}} between {ital x}=2.65 and 2.90, associated with octahedral hydrogen ordering centered at LaD{sub 2.75}, whereas four sharp peaks were found in LaD{sub 2.95}more » at 215.5, 265.5, 286.8, and 293.8 K and were correlated with the four transitions observed in LaD{sub 3} by Ito {ital et} {ital al}. The metal-to-semiconductor transition, which is found in the alloys between 2.75{le}{ital x}{le}3.0, is discussed on the basis of the electronic specific-heat coefficient and recent theoretical work. We also report that LaH{sub 2.01} becomes a superconductor at 0.17 K and, if LaD{sub 2.06} goes superconducting, it does so below 0.06 K.« less

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.

Growth mode and phase transitions of multilayer nitrogen on graphite.

The nitrogen-on-graphite growth process above the monolayer was studied by ac heat-capacity technique. The evolution of the heat-capacity peaks due to the orientational-ordering transitions as a function of coverage indicates regions of coexistence of the single and bilayer films and of the bilayer and trilayer films. Between regions of coexistence pure bilayer and trilayer films are found. The data also show clear evidence that near 30 K, only a maximum of three layers can form on the graphite surface when the bulk clusters start to grow. In addition, a sharp heat-capacity peak at 23.4 K was found for coverages above two layers. This peak is related to structural transition in the three-layer system and is probably a layering transition from a two-layer--plus--bulk system for T23.4 K to a three-layer--plus--bulk system for T&gt;23.4 K.

Commensurate-uniaxial-incommensurate transition of monolayer nitrogen on graphite.

Heat-capacity measurements were made to study the orientational-ordering transition of ${\mathrm{N}}_{2}$ on graphite in commensurate and incommensurate phases. Whereas a sharp heat-capacity peak was observed in the commensurate (C) and striped or uniaxial-incommensurate (UI) phases, only a broad and weak anomaly was found in the triangular-incommensurate phase. These heat-capacity traces show regions of coexistence indicating the C-UI transition to be first order.

Rb 2ZnCl 4: three heat-capacity anomalies due to soft-mode and commensurate-incommensurate transitions

There are three phase transitions for Rb 2ZnCl 4. The transition temperatures, the enthalpies of transition, and the entropies of transition are (74.6±0.15) K, (30±1) J·mol −1, (0.42±0.01) J·K −1·mol −1; (195.2±0.05) K, (6.2±0.9) J·mol −1, (0.032±0.005) J·K −1·mol −1; (303.2±0.3) K, (222±7) J·mol −1, (0.66±0.14) J·K −1·mol −1. The second transition shows a very sharp heat-capacity peak but the other two are much broader, being consistent with the soft-mode mechanism. Thermodynamic functions of Rb 2ZnCl 4 are tabulated. A comparison is made with the behaviour of ferroelectrics of the same type: K 2SeO 4 and Rb 2ZnBr 4.

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.

ChemInform Abstract: CRYSTALLINE VANADIUM(II) FLUORIDE, VF2. PREPARATION, STRUCTURE, HEAT CAPACITY FROM 5 TO 300 K AND MAGNETIC ORDERING

AbstractDie Darstellung des tiefblaue Nadeln bildenden VF2 erfolgt durch Reduktion von VF3 in einer H2/HF(3:1)‐Atmosphäre bei 1100°C. Seine Kristallstruktur ist vom Rutil‐Typ (P42/mnm, Z=2).

Crystalline vanadium (II) fluoride, VF2. Preparation, structure, heat capacity from 5 to 300 °K and magnetic ordering

Crystalline VF2 was prepared by the reduction of VF3 in an atmosphere of 3 H2:1 HF at 1100°C. The VF2 formed deep blue crystalline needles. The crystal structure is of the rutile type, space group P42/mnm. Parameters determined by x-ray diffraction are a=4.804±0.005 Å, c=3.237±0.005 Å, u=0.306±0.005. The low temperature heat capacity of VF2 is reported from 5 to 300 °K. A sharp peak in heat capacity associated with the development of long-range magnetic order is observed at 7 °K. Smoothed values of the total heat capacity, entropy, enthalpy, and Gibbs energy are tabulated between 5 and 300 °K. Values at 298.15 °K are: C °P=15.10 cal °K−1 mole−1, S °=18.217 cal °K−1 mole−1, H°−H°0=2661.0 cal mole−1. By a corresponding states approximation the magnetic contributions to the heat capacity and entropy are calculated. The magnetic heat capacity has a gradual maximum at 26.8±1 °K. The heat capacity curve agrees with that calculated for a one-dimensional chain with antiferromagnetic Heisenberg interactions. A value of the exchange constant, J/k=−9.4±0.4 °K is obtained.