Phase transitions rare earth elements

The rare earth elements are different from other elements because the optical transitions between levels of the fn configuration are inherently very sharp-lined and have well-resolved structure characteristic of the local crystal fields around the ion. In minerals, this characteristic provides an excellent probe of the local structure at the atomic level. Examples will be shown from our work of how site selective laser spectroscopy can be used to determine the thermal history of a sample, the point defect equilibria that are important, the presence of coupled ion substitution, the determination of multiple phases, and stoichiometry of the phase. The paper will also emphasize the fact that the usefulness and the interpretation of the rare earth luminescence is complicated by the presence of quenching and disorder in mineral samples. One in fact needs to know a great deal about a sample before the wealth of information contained in the site selective luminescence spectrum can be understood. 

The term Zintl phase is applied to solids formed between either an alkali- or alkaline-earth metal and a main group p-block element from group 14, 15, or 16 in the periodic table. These phases are characterized by a network of homonuclear or heteronuclear polyatomic clusters (the Zintl ions), which carry a net negative charge, and that are neutralized by cations. Broader definitions of the Zintl phase are sometimes used. Group 13 elements have been included with the Zintl anions and an electropositive rare-earth element or transition element with a filled d shell (e.g. Cu) or empty d shell (e.g. Ti) has replaced the alkali- or alkaline-earth element in some reports. Although the bonding between the Zintl ions and the cations in the Zintl phases is markedly polar, by our earlier definition those compounds formed between the alkali- or alkaline-earth metals with the heavier anions (i.e. Sn, Pb, Bi) can be considered intermetallic phases. 

The presence of early transition metals or rare earth elements is not necessary for the stabilization of metal-rich compounds. This is shown by the metal-rich nickel-tin sulfides Ni6SnS2 and Ni9Sn2S2, which were found during investigations of phase relations in the ternary Ni-Sn-S... 

Two types of fundamental metal hydride electrodes comprising the AB, and ABj classes of alloys are currently of interest. The AB, alloys with A = rare earth or mischmetal, B = Ni and/or other transition metal are investigated. LaNi, has been well-investigated because of its utility in conventional hydrogen storage applications, but it is very expensive and corrodes rapidly. The commercial AB, electrodes use mischmetal, a low-cost combination of rare earth elements, as a substitute for La. The partial substitution of Ni by Co, Ce, Mn and Al increases the thermodynamic stability of the hydride phase, the corrosion resistance and hence the cycle life. However, the substitution reduces the hydrogen storage capacity. 

Antipcrovskites are known to form with rare earth elements, most notably the series ATTAIN (Af = C c. La. Nd, Pr, Sm [227], Those with the actual perovskite structure are oxynitrides LaWO(, N, 4 and NdWOu N, 4 [22.8] and have been the subject of some theoretical discussion [2291. The magnetic phase transitions can be explained by assuming that the Fermi level lies near a singularity in the electronic density of states [230], XANF.S and heat capacity measurements confirm these magnetic transitions [231, 232]. 

Vasylechko, L. O. Crystal chemistry and phase transitions in complex oxides of rare earth elements with perovskite structure. Dr. Sd. Thesis. The Ivan Franko Natl. Univ. of Lviv, 2005, 343. (in Ukrainian). 

All f 2Cu2ln indides order ferromagnetically with Curie temperatures between 26.7 and 85.5 K. Strong crystal field anisotropies result in Tc values that do not scale with the de Getmes frmction of the rare earth elements (Fisher et al., 1999). Dy2Cu2ln shows a second phase transition to an antiferromagnetic ground state at 7n = 22 K. 

---