Present General Status of Understanding of Heteropoly Electrolytes and a Tracing of Some Major Highlights in the History of Their Elucidation

Synthesis, Characterization, and Hydrotreating Activity of Several Iron Group Transition Metal Phosphides

Magnetocaloric materials: The search for new systems

The chemistry of metals in low valence states is marked by the frequent occurrence of metal clusters, which are easily recognizable when they occur as molecular units. Many metal-rich compounds of transition metals with p-elements (3rd to the 6th main groups) are closely related to the corresponding halides, since they are built up from metal clusters of the same type. The clusters are however, linked together (condensed) by metal-metal bonds. This principle of construction holds particularly well in the case of the novel reduced halides of the lanthanoids.

Condensed Metal Clusters

Polyoxometalates (POMs) are a large group of anionic polynuclear metal-oxo clusters with discrete and chemically modifiable structures. In most aqueous POM solutions, numerous, and often highly negatively charged, species of different nuclearities are formed. It is rather difficult to determine the dominant POM species or their combination, which is responsible for the specific POM activity, during a particular application. Thus, the identification of all individual speciation profiles is essential for the successful implementation of POMs in solution applications. This review article summarizes species that are present in isopoly- and heteropolyvanadates, -niobates, -molybdates and -tungstates aqueous solutions and covers their stability and transformations. The ion-distribution diagrams over a wide pH range are presented in a comprehensive manner. These diagrams are intended for the targeted use of POMs, and in a clear form shows species that are in equilibrium at the given pH value. Thus, the data accumulated in this review can serve as both a starting point and a complete reference material for determining the composition of POM solutions. Some examples are highlighted where the POM speciation studies led to a detailed understanding of their role in applications. In doing so, we aim to motivate the POM community for more speciation studies and to make the subject more comprehensible, both for synthetic POM chemists and for scientists with different backgrounds interested in applying POMs in biological, medical, electrochemical, supramolecular and nanochemistry fields, or as homogeneous catalysts and other water-soluble materials.

Polyoxometalates in solution: speciation under spotlight

Determination of the homogeneity range and refinement of the crystal structure of Fe2P

The magnetic properties of stoichiometric Fe2P and non-stoichiometric Fe2-xP (0 < × ≤ 0.06) have been studied by means of magnetic susceptibility. Measurements on pure stoichiometric Fe2P samples show that there is a first order magnetic phase transition from a ferro- to a paramagnetic state at 216 ± 1 K. The transition is accompanied by a discontinuous change in the dimensions of the hexagonal unit cell with a decrease in the a-axis of 0.06% and an increase in the c-axis of 0.08% for increasing temperature. The transition is interpreted to be of magnetoelastic origin with an exchange energy critically sensitive to the interatomic spacing. Measurements on single crystals show that the spins are directed along the hexagonal c-axis with an exceedingly high axial anisotropy. The saturation moment is 1.46 μB/iron atom and the anisotropy energy as estimated from low field data is ~ 2.5 × 106 J m-3 (2.5 × 107 erg cm-3). The transition behaviour is very sensitive to the magnitude of the external field as well as to the presence of impurities and deviations from the ideal stoichiometry in the samples.

http://iopscience.iop.org/article/10.1088/0031-8949/17/1/008/pdf

First Order Magnetic Phase Transition in Fe2P

First order magnetic transition, magnetic structure, and vacancy distribution in Fe2P

Measurements of the magnetic susceptibility, electrical resistivity and Hall effect have been made on FeP2 single crystals prepared by a chemical transport reaction. FeP2 is a diamagnetic semiconductor with a nearly temperature independent susceptibility of -(8.8±0.7)10-6SI (at room temperature), and a band gap of 0.37 eV.

http://iopscience.iop.org/article/10.1088/0031-8949/4/3/010/pdf

Magnetic and Electric Properties of FeP2 Single Crystals

Susceptibility, resistivity and thermal expansion measurements have been performed on single crystals of FeP. The temperature ranges were 4.2 K-300 K, 15 K-300 K and 95 K-300 K respectively. The susceptibility measurements show a magnetic transition at 120 K, which is due to an antiferromagnetic ordering in the ab-plane. Above TN there is a considerable amount of anisotropy. The susceptibility curves representing the three principal directions have maxima around 220 K. The magnetic structure is interpreted in terms of a linear chain model. This interpretation of the experimental results is based on the theoretical work by Kallel et al. Resistivity measurements, carried out with the van der Pauw method, give an anisotropic resistivity tensor. The resistivity vs. temperature curves have points of inflexion at 115 K. The thermal expansion coefficient of the b-axis is much larger than the coefficients of the other axes.

Bertil Carlsson

Papers

Determination of the homogeneity range and refinement of the crystal structure of Fe2P

First Order Magnetic Phase Transition in Fe2P

First order magnetic transition, magnetic structure, and vacancy distribution in Fe2P

Magnetic and Electric Properties of FeP2 Single Crystals

Magnetic Susceptibility Resistivity and Thermal Expansion Measurements on FeP