Praseodymium chromate

The monazite structure of CeP04 is very similar to that of zircon. Its space group is P2i n, and the lattice parameters are a = 6.76 A, h = 7.00 A, c = 6.44 A j3 = 103.6°.. Transformations from both the zircon and monazite structures to that of the scheelite have been observed by Stubican and Roy (1%3) for a number of arsenates and vanadates of the rare earths. Stability regions of these phases at standard and high pressures are shown in table 28.13. These reactions occurred under the influence of pressures up to 80 000 atm at 600°C and are accompanied by an 11.5% volume decrease however, transformations between zircon and monazite were not effected because of their similar densities. Praseodymium chromate, prepared by Schwarz (1963a), was a mixture of the monazite and zircon types, and LaCr04 had the monazite structure (Schwarz, 1963b). Presumably other rare earth chromates have either the monazite or zircon structures. 

Chromates of the Rare Earth Metals.—A series of isomorphous yellow chromates, sparingly soluble in water, and of general formula E2 (Cr04)3.8H30, where R=Lanthanum, Praseodymium, Neodymium, or Samarium, has been prepared. ... 

Cr2HKO oCio, Chromate, (ji-hydrido-bis-[pentacarbonyl-, potassium, 23 27 CsCIsSc, Cesium scandium chloride, 22 23 CsCl7Pr2, Cesium praseodymium chloride, 22 2... 

The formation temperatures and the thermal stabilities of the rare earth sulphates, oxides, and oxide sulphates have been investigated. The thermal decomposition of the chromates of several tervalent lanthanides (praseodymium, gadolinium, terbium, dysprosium, holminum, erbium, and ytterbium) has been found to occur with loss of oxygen and reduction of Cr to Cr by a mechanism involving electron transfer from the co-ordinated oxygen to chromium. ... 

If industry adopts rare earth technology for corrosion inhibition, it will result in an increased demand for some of the rare earths, but there is no pubUshed information of the size of the demand nor its influence on supply. Both of these issues will have an impact on the availability and desirability of rare earths as replacements for chromate as an inhibitor. This paper therefore examines the availability and compares the costs of using the rare earths as chromate replacement chemicals. The focus will be on cerium (Ce), lanthanum (La), neodymium (Nd) and praseodymium (Pr), as well as some other candidates for replacement of chromate (Mo, Ti, V and Zr). This chapter explores the issues associated with... 

The chromates(V) are stable to above 600°C, where decomposition with loss of oxygen takes place and chromites(III) are formed. According to Doyle and coworkers the starting temperature of the decomposition reaction, as measured from the TG and DTA curves, is almost invariant in the series Sm - Yb, where the values range from 660 to 692°C. Only praseodymium seems to have lower... 

In aqueous 1 molar non-complexing acid (pH = 0) or 0.1 molar Hi,(pH = 1) there is not the slightest doubt that only R(lll) aqua ions are thermodynamically stable. It is true that the H2 evolution of europium (11) aqua ions and the O2 evolution of the not too well-defined cerium (IV) hydroxo-aqua complexes are so slow (in the absence of suitable catalysts and ultra-violet radiation) that solutions can be kept in practice for several months. Alkaline conditions forming the highly insoluble yellow Ce(OH)4 and many complexing anions (nitrate, phosphate, sulfate, chromate and in roughly neutral solutions peroxide) stabilize Ce(lV). In spite of many attempts to prepare praseodymium (IV) and terbium(IV) from aqueous solution,this does not seem to have succeeded as yet. 

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