X-ray diffraction and ab initio determinations of the structure of Rb4CdCl6

The structure of Rb₄CdCl₆ has been examined both at the experimental and at the ab initio levels. Experimentally, new X-ray diffraction measurements have been performed on a single crystal grown at room temperature from aqueous solution. The compound is confirmed to crystallize with a rhombohedral unit cell (space group ) but with a distorsion from cubic symmetry noticeably smaller than in a previous study Kristallografiya, 37 (1992) 815]. From the ab initio point of view, three distinct sets of first-principles density functional theory calculations (Perdew-Wang generalized-gradient-approximation (PWGGA), Perdew, Burke and Ernzerhof parametrization and Becke-Lyp exchange-correlation functions) have been carried out. These ab initio results corresponding to the ideal crystal (i.e. infinite, periodic in a definite phase and defect-free) are in correct agreement with our own experimental findings, especially for the distorsion. Nevertheless, due to the relative flatness of the ideal crystal potential energy surface with respect to distorsion around the minimum, it is quite plausible the two presently available X-ray diffraction analyses, made on two different real (imperfect) samples, are both correct even though they come out onto significantly different mean distorsion parameters. The shape, orientation and relative sizes of the thermal vibration ellipsoids are discussed in relation with relevant aspects of the calculated potential energy surface. The PWGGA wavefunction is used to plot valence electronic charge density maps. The comparison of the charge distribution in Rb₄CdCl₆ with that in Rb₂CdCl₄ is discussed in terms of compound structures.