Compounds magnetic transitions

The magnetic term is associated with changes in internal energy related to magnetic transitions. Its thermal dependence leads to the magnetic entropy and enthalpy determination. The Schottky term comes from the excitation of higher lying crystal field levels in compounds with locahzed 5 f levels. 

Paramagnetism results from unpaired electrons. As a result, most compounds containing transition, rare-earth, and actinide elements, including oxides, nitrides, carbides, and borides, exhibit paramagnetism. Such ceramics are generally not of importance due to their paramagnetism alone, since they often exhibit other types of magnetism, as well. 

In the magnetic transitional-metal compounds to be described in subsequent chapters the conduction electrons lie wholly in a d-band. This does not of course mean that there is no hybridization with 4s-electrons such hybridization must always be present to some extent, and will be responsible for any observable Knight shift. It does mean, however, that the d-band remains separate from the conduction band and contains an integral number of electrons per atom. As we... 

Two electrons with opposite spins in the same orbital are described as paired. When extended to molecules, the exclusion principle allows us to understand the pairing of electrons in covalent bonds. The net spin angular momentum of a pair of electrons is zero. If not all electrons are paired in a molecule or solid, magnetic properties arise, as happens with many compounds of transition metals. 

Thermodynamic (specific-heat) and transport (resisitivity) studies for the U(Pti xPdx)3 compounds have revealed magnetic transitions for x-values of 0.02 and higher, in contrast to the compounds below x = 0.01 that did not show anomalies in specific heat and resistivity that point to magnetic order. The sharpest transition in the specific heat studies is found for x = 0.05. For this compound, the resistivity v.v temperature curve exhibits a Cr-type of anomaly just below the magnetic ordering temperature suggesting the presence of a spin-density wave below TN. A few results from neutron diffraction studies are shown in fig. 6. 

In recent years, the design and synthesis of functional molecular materials is a focus of active research in the fields of chemistry and materials science. Polyoxometalate anions constitute a distinctive class of compounds which are receiving increasing interest in catalysis, medicine and materials science. Their chemical, structural and electronic versatility make them act as electron acceptors and accommodate magnetic transition metal centers in their structures. 

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