Importance of multiple diffraction avoidance for charge density observation

Abstract

It might not be well recognized, most reflections are contaminated by multiple diffractions (MD). Therefore high redundancy data could not coincide with high accuracy data when MDs are not avoided. We collected both data set of MD-avoided and no MD-avoided ones and investigated its effectiveness in electron density measurement. For data collection, four-circle diffractometer at KEK-PF BL14A (Tsukuba, Japan) was used. In MD-avoided measurement, each reflection is collected at angle setting of least of MD contamination which calculated by psi-scan simulation software MDC [1]. In no MD-avoided measurement, usual bisect setting were used. In no MD-avoided measurement, intensities of forbidden reflections of YMn2O5 are more than 10 times largely observed than for MD-avoided one, and resulting residual density map is also highly contaminated reflecting the tendency of Fo>>Fc which is typical for reflections of weak intensity. Figure 1 shows this situation. Figure 2 is the deformation density of YTiO3 for MD-avoided data. Where model density of without Ti-3d1 valence electrons is subtracted from experimentally observed electron density. In the figure, quenching of angular momentum of Ti-3d1 electron is clearly observed. Although Rint could not be an ideal indicator of data accuracy since it cannot perceive Fo>>Fc, Rint(F) of MD-avoided measurement for YTiO3 is significantly reduced to ~0.5%. For no MD-avoided one, Rint(F) is ~1.2%. Since accuracy of MD-avoidance technique is confirmed, the next step is to exploit informations of only a few numbers of valence electrons among F(000) electrons. To accomplish this, wave function based refinement such as XAO [2] should be applied and studied.