Applications of low temperature calorimetry in material research

Low temperature calorimetry has been used not only to obtain heat capacity, entropy, enthalpy and Gibbs free energy, but also to investigate and understand lattice vibrations, metals, superconductivity, electronic and nuclear magnetism, dilute magnetic systems and structural transition involved in material research.     

Evidence of a Mixed Magnetic Phase in EuMgGe: A Semi­metallic Zintl Compound with TiNiSi Structure Type

The ternary Zintl phase EuMgGe was synthesized from the elements, and its structure solved by single‐crystal X‐ray diffraction. Chemical bonding is discussed, by means of electronic structure calculations at the DFT level and its physical properties characterized with respect to electronic conductivity, magnetic susceptibility, specific heat capacity, and magnetoresistivity. The compound may be interpreted according to the Zintl‐Klemm concept as (Eu²⁺)(Mg²⁺)(Ge^4–) with isolated germanium anions. Resistivity measurements reveal a semimetallic character, which is consistent with the vanishing energy gap obtained from our calculations. The magnetic susceptibility and the specific heat indicate that two consecutive transitions take place, at 9 and 16 K, and they show evidence of magnetic frustration. A possible physical scenario for this magnetic behavior is discussed based on known models of partially frustrated magnets.     

Heat capacity of superfine oxides of iron under applied magnetic fields

Heat capacity is one of the most characteristic and important properties when the peculiarities of magnetic nanosystems are studied. In these systems the magnetic ordering becomes obvious due to the thermal effects such as heat capacity anomalies. It was considered earlier that heat capacity change under magnetic fields applied is slight and it cannot be taken into account in thermodynamic calculations. However the experimental heat capacity data for ferrofluids under magnetic fields applied show that field and temperature heat capacity dependences have a complicated behavior and in magnetic fields an essential heat capacity change takes place. On the other hand in the literature the contradictory data about heat capacity of nanoparticles appear. According to some papers nanoparticles heat capacity can exceed heat capacity of a bulk material a few times.     

The Magnetocaloric Effect and Heat Capacity of Suspensions of High-Dispersity Samarium Ferrite

The magnetocaloric effect and specific heat capacity of an aqueous suspension of samarium ferrite were determined calorimetrically over the temperature range 288–343 K in magnetic fields of 0–0.7 T. The data obtained were used to calculate changes in the magnetic component of the molar heat capacity and entropy of the magnetic phase and changes in the enthalpy of the process under an applied magnetic field. The magnetocaloric effect was found to increase nonlinearly as the magnetic field induction grew. The corresponding temperature dependences contained a maximum at 313 K related to the second-order magnetic phase transition at the Curie point. The field and temperature dependences of heat capacity contained a maximum in fields of 0.4 T and a minimum at the magnetic phase transition temperature.     

The magnetocaloric effect and the heat capacity of aqueous suspensions of porphyrin complexes of rare earth elements according to microcalorimetric data

The magnetocaloric effect (MCE) and heat capacity during the magnetization process of (5,10,15,20-tetraphenylporphyrinato)acetatogadolinium(III), (AcO)GdTPP, and (5,10,15,20-tetraphenylporphyrinato)chlorogadolinium(III) complexes, (Cl)GdTPP, in the form of 6%-aqeous suspensions are determined by the microcalorimetric method in a range of temperatures from 278 to 318 K and magnetic fields from 0 to 1 T. It is found that MCE for all the complexes are positive, i.e., at applying a magnetic field in the adiabatic conditions temperature of a suspension of complex increases. It is established that MCE increases with an increase in magnetic induction at all temperatures and decreases with an increase in temperature at all magnetic fields. It is shown that the substitution of chloride ligand by acetate in (X)GdTPP leads to a significant increas in MCE and its temperature dependence; in the case of (Cl)GdTPP actually MCE does not depend on temperature. Relationships between magnetothermal properties and structure of the complexes are analyzed. The conclusion is argumented that the reason of changes in magnetothermal properties after the replacement of axial ligand in gadolinium complexes and complexes of lanthanides with an unsymmetrically filled f-shell is non-planar geometry of the coordination site and specific electronic properties of the central ion. It is concluded that heat capacity of the complexes slightly increases with an increase in temperature and more noticeably in the case of (AcO)GdTPP; a magnetic component of heat capacity is revealed only in (AcO)GdTPP at temperatures above 298 K, which is connected perhaps with a temperature change in the crystal lattice of the complex and influence of the magnetic properties of gadolinium ion on this change.     

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.     

Thermodynamic properties of nm-sized Pd and Ni particles

With decreasing size the density of electronic states of a metal particle becomes different from that of the bulk. This significantly changes the behaviour of the thermodynamic properties of such a particle. We present specific heat and spin susceptibiblity measurements on a series of chemically synthesized Pd particles with diameters of 2.4–15 nm and magnetic measurements on a Ni colloid with an average diameter of about 2.8 nm. Pronounced size dependence of the electronic magnetic properties was found which is accompanied by only weak size dependence of the electronic specific heat. This result can be understood in terms of a size-dependence of the Stoner enhancement factor, which only enters into the susceptibility and not into the specific heat.     

Magnetocaloric effect and the heat capacity of ferrimagnetic nanosystems in magnetic fluids

The specific heat capacity of a magnetite-based magnetic fluid and changes in the magnetic part of the molar heat capacity of its magnetic phase in magnetic fields of 0–0.7 T were determined calorimetrically over the temperature range 288–353 K. The temperature dependence of changes in the magnetic part of entropy in an applied magnetic field was calculated. It was found that the field dependence of heat capacity had a maximum in fields of 0.3–0.4 T, and the temperature dependences of changes in the magnetic part of heat capacity ΔC _p (H) and entropy ΔS _m(H) had maxima at the magnetic phase transition temperature.     

Electronic structure and thermodynamic properties of dimeric mixed-valence clusters

A many-electron theory is proposed for exchange and tunneling levels of dimeric mixed-valence clusters. Transition-metal ions are examined, and it is established that the parameters of the Heisenberg and double-exchange interactions depend on the configuration of the d_n-electronic states. Temperature dependences are constructed for the effective magnetic moments and the electronic heat capacity.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 2, pp. 148–157, March–April, 1987.     

V4S9Br4: a novel high-spin vanadium cluster thiobromide with square-planar metal core

The novel vanadium thiobromide, V4S9Br4, with a square-planar metal cluster core was synthesized and characterized by single-crystal X-ray diffraction, measurements of magnetic properties and the heat capacity, and DFT calculations of the electronic structure. At the room temperature, the compound displays paramagnetic properties with an independent spin on each V atom and with a weak exchange constant (J approximately 10 cm(-1)). The paramagnetic state is transformed into a low-spin state (AF-type ordering) at low temperatures. This change is accompanied by a heat-capacity anomaly. The observed magnetic and heat-capacity anomalies can be explained by the thermal excitation of electrons on the closely spaced molecular energy levels in the presence of the Jahn-Teller effect.     

Thermal properties of ferromagnetic clusters

The magnetic moments and heat capacities of small iron, cobalt and nickel clusters in a molecular beam have been measured as a function of their size and temperature. The magnetic moment of nickel and cobalt clusters decreases with increasing size and with increasing temperature, slowly converging to the bulk magnetization curve as the cluster size increases. Heat capacity measurement of the clusters in the beam reveals that the magnetic contribution to the heat capacity is well approximated using mean field concepts, combined with measured magnetic properties. In contrast, iron clusters show anomalies, both in the magnetic moments as well as in the heat capacities. We speculate that these effects reflect a sensitivity of the magnetic order on structure as in bulk iron.     

Heat capacity, enthalpy, entropy, and gibbs energy of terbium diboride as determined from calorimetric measurements within 5–300 K

The heat capacity at constant pressure C _p (T) of terbium diboride synthesized from elements via an intermediate hydride phase was studied experimentally within 5–300 K. A ferromagnetic phase transition manifests itself in the C _p (T) dependence as a sharp maximum at 142.4 ± 0.1 K. The C _p (T) dependence was used to calculate the tempreature dependences of the enthalpy, entropy, and the Gibbs energy and to determine the parameters of the electronic, lattice, and magnetic contributions to the heat capacity of TbB₂.     

Unusual low-temperature thermodynamic properties of pellet samples of (DMe-DCNQI)<Subscript>2</Subscript>M (<Emphasis Type="Italic">M</Emphasis>=Li,Ag)

Summary We have performed low-temperature heat capacity measurements on pellet samples of (DMe-DCNQI)₂M (M=Li, Ag) which is known to show spin-Peierls transitions at 51 and 86 K, respectively. A linearly temperature-dependent term in the low-temperature heat capacity has been observed for both the samples: It is attributable to the spin-wave excitations induced by the inhomogeneous pressure effects produced in the pellet forming process. Although the temperature dependences of the magnetic susceptibility in both materials are almost the same, the coefficient of T-linear term of the Ag salt becomes three times larger that that of the Li salt. The peculiar electronic state originating from the competition of the spin-Peierls mechanism and the Coulomb repulsion is suggested.     

Magnetic phase transition of Li0.75CoO2 compared with LiCoO2 and Li0.5CoO2

Magnetic and thermodynamic properties of the LiCoO₂ positive-electrode material used in lithium-ion battery were first examined. Partially deintercalated LiCoO₂ that is Li_0.75CoO₂, showed definite anomaly in the magnetic susceptibility at T=ca. 175 K probably related to magnetic phase transition which was supported by observation of a weak anomaly in heat capacity. On the other hand, LiCoO₂ did not show such magnetic phase transition as expected, whereas Li_0.5CoO₂ a weak one in the similar temperature range. These behaviors are discussed in association with the mixing of Co³⁺ and Co⁴⁺ electronic structures.     

The heat capacity of NdPO4

The heat capacity of neodymium orthophosphate has been measured in the temperature range (0.6 to 1570) K. From the results the excess (electronic) heat capacity has been derived by subtracting the lattice heat capacity derived from the heat capacity of the isostructural LaPO₄ and GdPO₄, reported earlier. The calculated excess heat capacity thus obtained is in reasonable agreement with that calculated from the crystal field energies.     

Phase transitions and thermodynamic properties of lanthanide compounds LnAO4 (A = P,V, Nb)

Russian Journal of General Chemistry - The influence of structural phase transitions, features of electronic structure, and magnetic transformation on the temperature dependence of the heat...     

A microcalorimeter for studying the magnetocaloric effect and the heat capacity in magnetic fields

The design of an automated microcalorimeter for determining the magnetocaloric effect and the heat capacity of suspensions and magnetic colloids in magnetic fields of from 0 to 1 T over a wide temperature range and the corresponding experimental procedure were described.     

Low-temperature heat capacity and thermal behavior of Zn<Subscript>0.98</Subscript>Co<Subscript>0.02</Subscript>O in the high-temperature region

The low-temperature heat capacity of Zn _0.98Co_0.02O oxide was measured by adiabatic calorimetry. The formation of a solid solution was shown to be accompanied by a change in the entropy by 0.4 J/(K mol). No anomalies in the heat capacity or the thermal behavior confirming the phase transformations found earlier by other methods were observed. A heat capacity anomaly was revealed below 15 K and tentatively attributed to a change in the magnetic properties of the substance.     

Heavy fermion behavior in Cu3Au-structure compounds: CePb3 and U(In,Sn)3

The recent discovery of anomalous properties associated with extremely narrow f-band character in the electronic density of states in the vicinity of the Fermi energy has stimulated considerable interest in these systems. Some particularly exciting recent discoveries have emerged from studies of the magnetic and electronic properties of cerium- and uranium-based Cu₃Au intermetallic compounds which form with group IIIB and group IVB elements. It has been reported that CePb₃ is a heavy fermion antiferromagnetic system with a Néel temperature of 1.1K. In addition, this system displays field-induced superconductivity for magnetic fields greater than 15 T and temperatures below 0.5K. Studies of (Ce,La)Pb₃ clearly indicate that the heavy fermion behavior reported for this system is primarily a reflection of single-ion Kondo behavior. In addition, studies of U(In,Sn)₃ and Ce(In,Sn)₃ explicitly indicate that the heavy fermion and mixed-valent behavior reported for these systems are a result of a similar hybridization-driven mechanism. The CeX₃ and UX₃ systems where X is a group IIIB or group IVB element display many of the features of current interest to magnetic moment formation and allow a unique opportunity to study these phenomena systematically.     

比热和直流磁化率证明N+H…O-氢键的电子自旋翻转在D-和L-丙氨酸单晶中的不对称相变

为了解决D-和L-丙氨酸在约270K相变的分岐和机理,对其单晶、多晶粉末及原料利用微分扫描量热仪测定比热.用三线法以蓝宝石作校正,并与手册的D-和L-丙氨酸标准比热值比较.在单晶中,实验观察到吸热相变峰最高处时的温度及热焓为:D-丙氨酸,Tc=272.02K,△H=1.87J·mol-1;L-丙氨酸,Tc=271.85K,△H=1.46J·mol-1;热焓差为0.41J·mol-1.参比晶体D-缬氨酸,Tc=273.59K,△H=1.75J·mol-1;L-缬氨酸,Tc=273.76K,△H=1.57J·mol-1;热焓差为0.18J·mol-1.实验发现已测量过的单晶磨成多晶粉末后再测,相变峰消失.说明相变与晶格有关.变温中子衍射排除了D→L的构型相变,但发现N+H…O-氢键沿D-和L-丙氨酸单晶的c轴反向变化.变温偏振拉曼散射反映相变机制与N+H…O-中电子的轨道磁偶极矩相关,观察到偏振光的不对称散射.在外加磁场强度H为+1T和-1T下,变温测定D-和L-丙氨酸晶体的直流磁化率,证明在270K有电子自旋翻转的相变.电子自旋的向上或向下,取决于晶格中NH+3的扭曲振动及N+H…O-氢键沿晶体c轴的方向.由于自旋的定轴性,可以解释单晶和多晶粉末比热结果的分岐.