“…. For the truth of the conclusions of physical science, observation is the supreme Court of Appeal….” (Sir Arthur Eddington, The Philosophy of Physical Science.) In this paper we shall review the theoretical and experimental results obtained on simple magnetic model systems. We shall consider the Heisenberg, XY and Ising type of interaction (ferro and antiferromagnetic), on magnetic lattices of dimensionality 1, 2 and 3. Particular attention will be paid to the approximation of these model systems in real crystals, viz. how they can be realized or be expected to exist in nature. A large number of magnetic compounds which, according to the available experimental information, meet the requirements set by one or the other of the various models are considered and their properties discussed. Many examples will be given that demonstrate to what extent experiments on simple magnetic systems support theoretical descriptions of magnetic ordering phenomena and contribute to their understanding. It will a...

Experiments on simple magnetic model systems

“All new is well-forgotten old”, the proverb goes. The current “fluorine boom” is news only to a novice in the field: the exceptional importance of fluorinated organic compounds in numerous areas has been known for a long time. The sharpest increase in the number of fluorine-containing pharmaceuticals and agrochemicals is dated back to 30 years ago. Also around that time (1979), the first monograph devoted to industrial applications of organofluorine compounds was published, covering not only fluorine-containing biologically active materials but also refrigerants, propellants, surfactants, textile chemicals, polymers, and dyes. The increasingly broad realization of the key role of organofluorine compounds in numerous areas has recently attracted many new scientists to the field. The development of new methods for the selective introduction of fluorine and fluorinecontaining groups into organic molecules for biologically active and other useful materials has become a hot area. Over 100 reviews, book chapters, and highlights on this subject have appeared in the literature in the past few years. As citing all of these publications in this review article is impossible, we provide references to only the most recent, general, and comprehensive ones. Molecules bearing a trifluoromethyl group constitute one of the most important classes of selectively fluorinated compounds. As early as 1928, Lehmann reported his observations of biological activity of some trifluoromethylated organic derivatives and already in 1959 Yale published a detailed review article entitled “The Trifluoromethyl Group in Medicinal Chemistry”. Since then, numerous books and reviews have appeared in the literature covering various aspects of trifluoromethylated organic and organometallic compounds. Within this family, derivatives bearing the CF3 group on aromatic rings are particularly numerous and important. Some examples of such compounds used as active ingredients of pharmaceuticals and agrochemicals are shown in Scheme 1. Trifluoromethylated building blocks and intermediates are clearly needed to make such molecules. The simplest trifluoromethylated aromatic compound, benzotrifluoride, was originally prepared by Swarts at the end of the 19th century. In his work, Swarts treated benzotrichloride with “two thirds of its weight of antimony fluoride” to obtain a mixture of PhCF2Cl and PhCF3, from which the two were separated and isolated pure by distillation. In the early 1930s, two industrial groups, one from Kinetic Chemicals, Inc. and one from I. G. Farbenindustrie AG patented their discoveries on the successful use of HF instead of SbF3 for the Swarts reaction. These inventions were the starting point for the modern large-scale manufacturing of trifluoromethylated aromatics. Other methods have been developed for conversion of various C1 units on the ring to CF3 with a variety of fluorinating agents. While representing an outstanding discovery and a classic of organic and organofluorine chemistry, the Swarts reaction is nonetheless neither atom-economical nor environmentally benign, as it deals with stoichiometric quantities of hazardous chemicals and generates large amounts of chlorine waste. To convert a CH3 group on the ring to CF3, the methyl is first exhaustively chlorinated to produce 3 equiv of HCl as a

Aromatic Trifluoromethylation with Metal Complexes

REVIEWS OF TOPICAL PROBLEMS: Ferroelectromagnets

Nucleation and Growth of Nanoparticles in the Atmosphere Renyi Zhang,* Alexei Khalizov, Lin Wang, Min Hu, and Wen Xu Department of Atmospheric Sciences andDepartment of Chemistry, Center for Atmospheric Chemistry and Environment, Texas A&M University, College Station, Texas 77843, United States Department of Environmental Science & Engineering and Institute of Global Environment Change Research, Fudan University, Shanghai 200433, China State Key Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China

Nucleation and Growth of Nanoparticles in the Atmosphere

This review is concerned with the properties and structure of silica glass. The following topics are treated: Types of silica glasses; The vitreous state of silica glasses: thermodynamical approach, atomistic approach; Optical properties; absorption and fluorescence, refractive index and homogeneity; Mechanical and thermal properties: specific volume, volume relaxation, volume and pressure, elastic and internal friction behaviour, heat capacity and heat conduction, strength, crystallization.

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Properties and structure of vitreous silica. I

The Thermodynamic Properties of Aqueous Sulfuric Acid Solutions and Hydrates from 15 to 300°K.1

An adiabatic demagnetization−sample isolation calorimeter, in which 3He gas provides equilibrium above 0.3°K, and isolation below 0.2°K as 3He condenses on the sample during demagnetization, has been used to investigate the properties of CZN to 0.8 m°K. The determination of entropy gain during magnetic field changes, for correcting ’’adiabaticas’’ to isentropes, has been discussed. The entropy gain was probably due essentially to heat leak (vibration) arriving at the lower temperatures. The effect of hysteresis at modest rates of field change appeared to be too small to contribute to entropy gain within the limit of error. Heat introduction at zero field has enabled the evaluation of the absolute temperature and heat capacity over the range 0.8−30 m°K. A heat capacity maximum occurred near 1.5 m°K. The heat capacity below 1.4 m°K is given by C = 3.6×1014 T5 gibbs/mole Ce3+. Below 1.7 m°K, the initial magnetic susceptibility is constant at 86.7 cm3/mole Ce3+, which extrapolates to a critical field of 58.5 ...

Magnetothermodynamics of Ce2Mg3(NO3)12· 24H2O. II. The evaluation of absolute temperature and other thermodynamic properties of CMN to 0.6 m°K

The magnetic moment of a 2.44 cm diam spherical single crystal of gadolinium gallium garnet has been measured with stabilized fields of 500, 1000, 2500, 5000, 10 000, 15 000, 25 000, 40 000, 65 000, and 90 000 G along the [100] axis, over the range 0.35 to 4.2 °K. The heat capacity was measured over similar ranges, except that 500 G was omitted and zero field included. The magnetic moment reached saturation at 0.35 °K and 90 000 G. The electronic and nuclear spin components of the heat capacity at 90 000 G could be separated from each other near 1 °K, thus locating a zero of entropy for the electronic system. By means of 34 adiabatics, corrected to isentropes, which crossed most of the isoerstedic heat capacity series, the entropies as a function of field and temperature were evaluated in terms of the zero reference. The sample has (0.4 ± 0.1) mole% excess entropy over the maximum R ln 8 for Gd3+ ion. Thus the sample must have contained 0.4 mole% excess Gd over the stoichiometric amount for Gd3Ga5O12. Usi...

Magnetothermodynamics of gadolinium gallium garnet. I. Heat capacity, entropy, magnetic moment from 0.5 to 4.2 °K, with fields to 90 kG along the [100] axis

Relation of crystalline forms I, III, IV, and V of anhydrous sodium sulfate as determined by the third law of thermodynamics

The thermal conductivity of Pyrex 7740 glass has been measured over the temperature range 0.4 to 4°K and in magnetic fields from 0 to 90 kG. As expected, the conductivity was unaffected by the field. The glass had an iron content of 0.02%. The heat capacity of Pyrex, at constant magnetic fields, has been measured over the above range of temperature and field. There was a considerable field effect due to the iron content. The variation of temperature with field was measured on a series of isentropes and this enabled the evaluation of the entropy. The change of temperature of Pyrex during adiabatic demagnetization at various values of the entropy is shown as a graph for this typical sample. A description of the apparatus used for the measurement is included.

W. F. Giauque

Papers

The Thermodynamic Properties of Aqueous Sulfuric Acid Solutions and Hydrates from 15 to 300°K.1

Magnetothermodynamics of Ce2Mg3(NO3)12· 24H2O. II. The evaluation of absolute temperature and other thermodynamic properties of CMN to 0.6 m°K

Magnetothermodynamics of gadolinium gallium garnet. I. Heat capacity, entropy, magnetic moment from 0.5 to 4.2 °K, with fields to 90 kG along the [100] axis

Relation of crystalline forms I, III, IV, and V of anhydrous sodium sulfate as determined by the third law of thermodynamics

Thermal Conductivity and Heat Capacity of 7740 Pyrex below 4°K and in Magnetic Fields to 90 kG