Electron charge distribution of CaAl 2−xZn x: Maximum entropy method combined with Rietveld analysis of high-resolution-synchrotron X-ray powder diffraction data

Abstract

Using short wavelength X-rays from synchrotron radiation (SPring-8), high-resolution powder diffraction patterns were collected. In order to study both the structural relationship and the mechanism of stability in the CaAl 2− x Zn x system, among the Laves phases (MgCu 2 and MgNi 2 type) and KHg 2-type structures, the charge density distribution of CaAl 2− x Zn x as a function of x was obtained from the diffraction data by Rietveld analysis combined with the maximum entropy method (MEM). In the MEM charge density maps overlapping electron densities were clearly observed, especially in the Kagomé nets of the Laves phases. In order to clarify the charge redistribution in the system, the deformation charge densities from the densities formed by the constituent free atoms are discussed. In the ternary MgNi 2-type phase, partial ordering of Al and Zn atoms is observed, a finding that is supported by ab-initio total energy calculations.