Experimental charge density study of (E)-4-(2,4-diisopropylphenyl)-4-oxo-2-butenoic acid

Oxygen-ionic conductors are very important materials for various kinds of applications such as fuel cells and oxygen-gas sensors. In 1995, Nakayama found new type of oxygen-ionic conductor; Ln-Si-O apatites (Ln = Lanthanoids). [1,2] These compounds show better oxygen ionic conductivity in middle range of temperature compared with other famous oxygen-ionic conductors (ZrO2 and/or CeO2-based materials). Recently, Yoshioka reported that Mg-doped apatites show the best ionic conduction. [3] However, in all of the apatite system, details of mechanism of ionic conduction are still unknown. Therefore, we carried out high-temperature diffraction study using synchrotron radiation to suppose mechanism from a change of electronic density. In this study, we selected 20 % Mg-doped compound, which showed the highest conductivity. Intensity data was measured by powder method at BL-19XU beam line, SPring-8, Japan, and the temperature range was from room temperature to 1283 K. The crystal structures were refined by Rietveld method using RIETAN-FP [4], the electron distributions were investigated by whole-pattern fitting approach based on the maximum-entropy method (MEM) using PRIMA [5], and the structure and electron distributions were drawn by VESTA [6,7]. In whole temperature range, this compound belongs to P63/m. The Rietveld refinement suggested an interstitial oxygen site O5 located at (-0.01,-0.003,-0.002). Electron density map derived from the MEM analyses revealed that oxide ions O4 located at 2a site were diffused along the caxis. In addition, a short-range curved diffusion path through the interstitial site was clearly visualized between two oxygen atoms O3 within the SiO4 tetrahedra.