Novel high pressure modifications of rare earth trichlorides in the RhF3-type

The results of high pressure investigations on rare earth trichlorides crystallizing with the AlCl3-type arrangement under normal conditions are reported. In addition to the PuBr3-type geometry, formed at higher pressures, a new structure type is found in a lower pressure range. This high pressure phase is isotypic with the RhF3-type arrangement, which so far has only been found with fluorides. It is packed more efficiently and shows a slightly higher primary coordination due to the higher symmetry of the coordination polyhedra compared with that of the original AlCl3-type structure. A model for the mechanism of transformation is presented. (author)