J. W. Stout

Heat Capacity and Entropy of CuCl2 and CrCl2 from 11° to 300°K. Magnetic Ordering in Linear Chain Crystals

The heat capacities of CuCl2 and CrCl2 have been measured between 11° and 300°K. The contributions to the entropy and heat capacity arising from the ordering of the magnetic moments are evaluated. Maxima in the heat capacity are found at 23.91±0.1°K for CuCl2 and 16.06±0.05°K for CrCl2. Both compounds show gradual maxima in the magnetic contribution to the heat capacity at higher temperatures. These are interpreted as short‐range one‐dimensional ordering arising from relatively strong antiferromagnetic coupling within the chains of atoms which make up these crystals. At lower temperatures relatively weak interactions between atoms in different chains cause the development of long‐range order. By use of the Ising model to describe interactions within a chain and a molecular field to describe the secondary interactions, an approximate theoretical treatment is given. Smooth values of the heat capacity, entropy, enthalpy, and free energy are tabulated at selected temperatures. The values of the entropy and en...

Heat Capacity of Zinc Fluoride from 11 to 300°K. Thermodynamic Functions of Zinc Fluoride. Entropy and Heat Capacity Associated with the Antiferromagnetic Ordering of Manganous Fluoride, Ferrous Fluoride, Cobaltous Fluoride, and Nickelous Fluoride

The heat capacity of ZnF2 has been measured calorimetrically between 11 and 300°K. In addition to the direct experimental data, values of heat capacity, entropy, enthalpy, and free energy are tabulated at selected temperatures. The values of entropy, and enthalpy at 298.16°K are S0=17.61±0.03 cal deg—1 mole—1 and H0–H00=2827±5 cal mole—1. The heat capacity of ZnF2 has been used, with a corresponding states argument, to estimate the lattice contributions to the entropy and heat capacity of the isomorphous fluorides MnF2, FeF2, CoF2, and NiF2 which exhibit heat capacity maxima associated with antiferromagnetic ordering at 66.5, 78.35, 37.70, and 73.22°K, respectively. The values of electronic entropy at the maximum in heat capacity are: MnF2, 0.85R ln6; FeF2, 0.87R ln5; CoF2, 0.80R ln2; NiF2, 0.71R ln3. The electronic entropies and heat capacities are compared with molecular field and spin wave theories of antiferromagnetism. At very low temperatures the electronic heat capacity on NiF2 is varying approxima...

Heat Capacity of α‐NiSO4·6H2O between 1 and 20°K. Electronic Energy Levels of the Ni++ Ion

The heat capacity of α‐NiSO4·6H2O has been measured over the temperature range between 1 and 20°K. The heat capacity passes through a rounded maximum of 1.51 cal deg—1 mole—1 at 2.58°K. The magnetic contribution to the heat capacity is well fitted by that calculated for the three spin states associated with the singly degenerate ground orbital level of the Ni++. From the heat‐capacity data the energy spacing between the lowest and middle spin states is calculated to be 4.48±0.07 cm—1 and between the lowest and highest spin states 5.05±0.07 cm—1. The magnetic susceptibilities in the directions of the three principal axes of a Ni++ ion are calculated and, by comparison with literature data on the magnetic anisotropy and single‐crystal magnetic susceptibilities, the orientations of these axes relative to the crystal axes are deduced. Data of other investigators on the magneto‐optical rotation and high field magnetization are interpreted.

Heat Capacity and Entropy of CoCl2 and MnCl2 from 11° to 300°K. Thermal Anomaly Associated with Antiferromagnetic Ordering in CoCl2

The heat capacities of MnCl2 and CoCl2 have been measured between 11° and 300°K. By the use of the measurements of Murray on MnCl2 at lower temperatures the lattice and magnetic contributions to the heat capacity and entropy of MnCl2 have been evaluated. CoCl2 has a lambda peak in heat capacity at 24.71±0.05°K associated with the cooperative ordering of the magnetic moments of the cobaltous ions. The entropy change associated with this cooperative ordering is R ln2. Smooth values of the heat capacity, entropy, enthalpy, and free energy are tabulated at selected temperatures. The values of the entropy and enthalpy at 298.15°K are: MnCl2, S° = 28.26±0.05 cal deg—1 mole—1, H°–H0° = 3602±8 cal mole—1; CoCl2, S° = 26.09±0.05 cal deg—1 mole—1, H°–H0° = 3375±8 cal mole—1.

The Magnetic Anisotropy of α—NiSO4·6H2O between 13 and 295°K. A Torsion Balance for Magnetic Anisotropy Measurements

The magnetic anisotropy of single crystals of α—NiSO4·6H2O has been measured at temperatures between 13 and 295°K. Two crystals of different origin gave concordant results. The difference in magnetic susceptibility between directions parallel and perpendicular to the tetragonal crystallographic axis is given by χ⊥−χ‖=1.37×10−5+1.502×10−2/T+1.686/T2. The susceptibility difference was independent of field strength up to 10,000 gauss over the temperature range investigated. The results are discussed in connection with the theoretical treatment of the susceptibility of nickel salts by Schlapp and Penney. A torsion balance for the magnetic anisotropy measurements is described.

The Magnetic Anisotropy of Manganous Fluoride between 12 and 295°K

The magnetic anisotropy of single crystals of MnF2, a typical antiferromagnetic substance, has been measured over the temperature range 12 to 295°K. At temperatures above 100°K the anisotropy is of the order of 0.1 percent of the susceptibility and the greater susceptibility is in the direction of the c axis of the tetragonal crystal. Below 70°K the anisotropy becomes extraordinarily large and the susceptibility in the direction of the c axis is smaller than that perpendicular to this axis. Within the limit of experimental error, the anisotropy was independent of field strength up to 10,000 gauss. From the present measurements, combined with those of previous investigators on the powder susceptibility, the directional susceptibilities parallel and perpendicular to the c axis have been calculated. As the temperature is lowered from 70 to 14°K, the perpendicular susceptibility increases by about 12 percent and the parallel susceptibility approaches zero at zero temperature. These results are discussed in co...

Heat Capacity and Entropy of FeF2 and CoF2 from 11 to 300°K. Thermal Anomalies Associated with Antiferromagnetic Ordering

The heat capacities of FeF2 and CoF2 have been measured between 11 and 300°K. Both salts show anomalies in heat capacity associated with the antiferromagnetic ordering of the magnetic ions. The heat capacity of FeF2 exhibits a sharp maximum of 17.8 cal deg—1 mole—1 at 78.35°K and that of CoF2 has a maximum of 5.64 cal deg—1 mole—1 at 37.70°K. Values of the heat capacity, entropy, enthalpy, and free energy are tabulated at selected temperatures. The values of the entropy and enthalpy at 298.16°K are: FeF2, S0=20.79±0.04 cal deg—1 mole—1, H0–H00=3049±6 cal mole—1; CoF2, S0=19.59±0.04 cal deg—1 mole—1, H0–H00=2978±6 cal mole—1.

Heat Capacity of NiF2 from 12 to 300°K. Thermodynamic Functions of NiF2. The Thermal Anomaly Associated with the Antiferromagnetic Ordering

The heat capacity of NiF2 has been measured calorimetrically between 12 and 300°K. There is an anomaly in heat capacity rising to a maximum of 9.23 cal deg—1 mole—1 at 73.22°K. The anomaly is associated with the antiferromagnetic ordering of the magnetic moments of the nickel ions. Values of heat capacity, entropy, enthalpy, and free energy are tabulated at selected temperatures. The entropy at 298.16°K is 17.59±0.04 cal deg—1 mole—1. For the reaction NiF2+H2=Ni+2HF, ΔH00=29.56±0.20 kcal and ΔH0=30.06±0.20 kcal at 298.16°K.

Heat capacity and entropy of MnF2 from 10 to 300 °K. Evaluation of the contributions associated with magnetic ordering

The heat capacity of a sample of MnF2 consisting of crystals larger than 2 mm has been measured between 10 and 300 °K. The peak in heat capacity associated with the antiferromagnetic ordering is at 67.30 °K. Measurements with small temperature rise show the shape of the heat capacity curve near the anomaly. The corresponding states approximation for evaluating the lattice heat capacity is tested in regions where the magnetic contributions are calculable. Magnetic contributions to the heat capacity and entropy are tabulated between 10 and 100 °K. Smooth values of the total heat capacity, entropy, enthalpy, and Gibbs energy are tabulated between 10 and 300 °K. Values at 298.15 °K are C°P=15.96 cal °K−1 mole−1, S°=22.04 cal °K−1 mole−1, H°−H°0=3106 cal mole−11.

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 A, c=3.237±0.005 A, 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 val...