X-ray and neutron diffraction investigations on electron density distribution in an electride crystals

Graduate School of Engineering,, Nagoya Institute of Technology* Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Japan** Materials and Structures Laboratory, Tokyo Institute of Technology, Japan***
â—‹Kiyoaki Tanaka* Terutoshi Sakakura* Masashi Watanabe** Yukio Noda** Satoru Matsuishi*** Hideo Hosono***

A typical cement material 12CaO.7Al2O3(C12A7) is an first 'electride' stable at room temperature formed by removing two O atoms from cages leaving four electrons ([Ca24Al48O64]4+(4e-)). The unusual crystal was investigated by X-ray and neutron diffraction (ND) methods to find the location of the four electrons in crystallographic cages and to assess the O atoms were extracted from the cage, respectively. The ND study revealed a peak on the 2-fold axis connecting two Ca atoms at about 0.75 A from a Ca atom (Ca1). The crystal structure revealed that small amount of Ca atoms (Ca2) locate there (about 3% of Ca1). X-ray diffraction on the other hand exhibited two large peaks of 10.5 and 6.2 eA-3 on the 2-fold axis at 0.4 (peak A) and 0.75 A from the Ca atom, respectively. The second peak disappeared when Ca2 was assigned to the position but O atoms could not occupy the site indicating they were extracted completely from the cage. X-ray AO analysis as well as anharmonic vibration (AHV) analysis were then carried out dividing each atom into sub-shells, which enabled us to keep the electro-neutrality of the unit cell. It reduced peak A to 6.2eA-3 and made the analysis of temperature factor of Ca2 possible, which further reduced the peak to 3.0eA-3. Significant AHV of Ca1 indicates that peak A is due to electrons which do not belong to any Ca orbitals. The remaining Peak A was integrated to be 0.083e corresponding to 3.98e in the unit cell. These facts supports strongly that the present compound is an electride.