Characterization of Radium Sulphate

This paper examines the crystal structure of radium sulphate and compares its structure to barium sulphate, strontium sulphate, and lead sulphate. The radium sulphate powder was measured by both powder X-ray diffraction and EXAFS. The unit cell was determined to be orthorhombic, belonging to the Pnma (no. 62) space group with the cell parameters ？？, ？？, ？？, and ？？3. These data support the fact that radium sulphate is isostructural with barium, strontium, and lead sulphate. The bond distances were determined using EXAFS. The mean Ra–O and S–O bond distances were found to be 2.96(2)？？ and 1.485(8)？？, respectively, and the Ra–O–S bond angle was . Findings of EXAFS data are quite consistent and support the XRD data. These findings show that it is possible for radium to coprecipitate with barium, strontium, and lead in sulphate media to form a substitutional solid solution. 1. Introduction There are many crystals that are isostructural; that is, they have the same space groups, and the atoms of the compound are placed in an identical way in the unit cells. For two compounds to form a substitutional solid solution they must be isostructural [1]. The coprecipitation of radium, barium, and strontium is believed to take place due to the formation of a substitutional solid solution. To be able to determine whether this is true or not it is necessary to make further investigations of the structural parameters of radium sulphate and compare them with those reported for barium, strontium, and lead(II)sulphate. These facts allow for possible coprecipitation between these elements. From the point of, for example, a repository for used nuclear fuel, radium release seems to dominate the radiotoxicity to man in a longer time perspective [2]. Thus, if coprecipitation with, for example, barium occurs, the released radium levels can be kept low. The crystal structures of strontium sulphate (celestite), barium sulfate (barite), and lead(II)sulphate (anglesite) are well determined in a series of studies (see Tables S1 and S2 in supplementary material available online at http://dx.doi.org/10.1155/2013/940701). The unit cell parameters of radium sulphate have been determined in powder diffraction study [3] and show that radium sulphate is isostructural with strontium, barium, and lead(II)sulphate. The crystal structures of barite (BaSO4), celestite (SrSO4), and anglesite (PbSO4) show that the metal ion is surrounded by twelve oxygens from the sulphate ions in a very broad bond distance distribution, (Table S1) [4]. The mean M–O bond distances, 2.826, 2.948, and 2.864？？ for Sr,