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-
