The β to γ transformation in cerium—A twenty year study

Abstract

Cerium has four allotropic modifications at one atmosphere pressure: {alpha}-Ce -- the so called ``collapsed fcc`` phase; {beta}-Ce -- the double hexagonal close packed phase; {gamma}-Ce -- the normal fcc phase; and {delta}-Ce -- the high temperature bcc phase. The phase relationships are transformations between the {alpha}, {beta} and {gamma} forms of cerium are complex and temperature hysteretic in nature, and these are described in great detail by Koskenmaki and Gschneidner. Recently, however, the authors decided to make heat capacity measurements on allotropically pure {beta}-Ce and {gamma}-Ce below 350 K, since the data obtained in the 1950`s were made on samples which contained two or more of the low temperature phases of cerium. The {beta}-Ce sample which was initially used had been prepared in May 1975. When the heat capacity was measured on this sample after it had been cooled down to 77 K to start the run (December 13, 1994), a large peak in the heat capacity was observed at {approximately}180 K, indicating that {alpha}-Ce was transforming primarily to {gamma}-Ce (and perhaps to {beta}-Ce). Since {beta}-Ce is stable down to {approximately}45 K before it transforms to {alpha}-Ce, the question was, where did the {alpha}-Ce come from? The most likely more » answer was that in the 19.6 years since the {beta}-Ce had been prepared, some of it must have transformed to {gamma}Ce while sitting at room temperature (295 {+-} 2 K), and the {gamma}-Ce phase present in the sample transformed to {alpha}-Ce when cooled down below {approximately}110 K. Another piece of the {beta}-Ce starting material was examined by x-rays, which confirmed that this sample contained {approximately}25% {gamma}-Ce. These measurements indicate that at room temperature the true equilibrium phase is {gamma}-Ce, and that the {beta} {l_equilibrium} {gamma} equilibrium temperature must be less than 295 {+-} 2 K. « less