Polymorphic Modifications of Pm₂O₃

Abstract

ROMETHIUM, Z=61, is a synthetic lanthanide formed only P during nuclear reactions such as fission. The most abundant isotope formed in the fission process, 14'Pm, decays by emission of a 225-keV ,&particle with a half-life of 2.6 years, allowing the daughter, '47Sm, to grow in at the rate of 2.2%/month. Gram quantities of promethium are now available, and some of the basic physical and chemical properties of the only known oxide, Pm,O,, have been determined.The lanthanide sesquioxides are known' to exist in one or more crystal forms whose structures are hexagonal ( A ) , monoclinic ( B ) , or cubic (C) . Two additional high-temperature polymorphs reported2 for the rare-earth sesquioxides are designated H and X. The H phase occurs only at high temperature, and the transformation between this phase and the low-temperature C, B, or A form is reversible. The temperature of transformation between the H polymorph and its low-temperature precursor increases with atomic number (decreasing cation radius). The structure of the H phase is reported to be hexagonal and very similar to the A phase, whereas that of the X phase is probably The X phase occurs only in the early members of the rare-earth oxide series; its transformation to the H polymorph is reversible. The cubic form of Pm,O, was identified by Wilson et al.' and confirmed by Weigel and Scherer," who also identified the monoclinic and hexagonal mudifications. The cubic+hexagonal transformation in Pm,O, is irreversible, and long annealing at low temperature will not revert the monoclinic to the cubic form after it has formed. In addition, the intense radioactivity does not alter the stability of the B allotrope after it has crystallized. For example, samples of monoclinic Pmz03 were heated at 700°C for 7 days and then heated at 400°C for an additional 7 days. The monoclinic structure was retained after both treatments. After the samples were stored at room temperature for 6 months, an X-ray pattern of the same material showed the monoclinic form with a slight line shift resulting from Sm ingrowth.To obtain additional information on the cubic+monoclinic transformation temperatures, promethium oxalate was an-

The Binary Rare Earth Oxides

“…In the temperatures above 2000 °C, the types designated as H or X are formed. 35,36 The crystal chemistry of the rare earth sesquioxides was first studied in a systematic manner by Goldschmidt et al 37 Figure 6 35 presents the temperature ranges of stability of the five polymorphic forms. Only in the middle range of ionic radius can all five polymorphic phases be formed by each oxide.…”

“…At temperatures lower than approximately 2000 °C, three types designated as A, B, and C are commonly found. In the temperatures above 2000 °C, the types designated as H or X are formed. , The crystalchemistry of the rare earth sesquioxides was first studied in a systematic manner by Goldschmidt et al…”

High-Pressure Structural Transitions of Sc2O3 by X-ray Diffraction, Raman Spectra, and Ab Initio Calculations

“…It has many applications in various fields of technologies. It can be used as laser optical coatings because of its high refractive index and high damage threshold. − It is also a good candidate for applications in photonics and optoelectronics because of its chemical stability and high ultraviolet cutoff. , In recent years, there have been many studies on the polymorphs of rare-earth sesquioxides. − So far, five structural modifications have been identified in the rare-earth sesquioxides. − Three phases, designated as A (hexagonal, space group P 3̅ m 1) with each cation surrounded by seven anions, B (monoclinic, space group C 2/ m ) with six- or seven-coordinated cation sites, and C (cubic, space group Ia 3̅) with six-coordinated cations, are commonly observed at ambient temperature and pressure. The other two phases denoted as H (hexagonal, space group P 6 3 / mmc ) and X (cubic, space group Im 3̅ m ) are formed at very high temperatures …”

Corrosion Protection Mechanisms of Rare-Earth Compounds Based on Cerium and Praseodymium

“…[23]. The A to H transition is accompanied by a lattice expansion, while the H to X transition results in the lattice contraction [37][38].…”