Maximum-entropy-method charge densities based on structure-factor extraction with the commonly used Rietveld...

Area of Science:

• Crystallography
• Materials Science
• Computational Chemistry

Background:

• Rietveld refinement is crucial for analyzing crystal structures from diffraction data.
• Electron-density distributions (EDDs) provide insights into chemical bonding and material properties.
• Maximum Entropy Method (MEM) is a common technique for calculating EDDs from structure factors.

Purpose of the Study:

• To evaluate structure-factor extraction from common Rietveld programs (FullProf, Jana2006, GSAS).
• To assess the impact of these extractions on subsequent Maximum Entropy Method (MEM) electron-density distribution (EDD) calculations.
• To investigate a potential new approach for optimizing MEM EDDs.

Main Methods:

• Collected synchrotron powder X-ray diffraction data for NaGaH(4) at 90 K.
• Collected neutron powder diffraction data on the deuterated NaGaD(4) sample.
• Extracted structure factors (F(obs)) using FullProf, Jana2006, and GSAS, analyzing background and peak-shape models.
• Calculated EDDs using MEM and analyzed results within the quantum theory of atoms in molecules.

Main Results:

• Observed structure factors (F(obs)) were corrected for anomalous dispersion and scaled.
• Significant differences were found in the estimated standard deviations (σ(obs)) generated by different Rietveld programs.
• These variations in σ(obs) led to substantial differences in the resulting MEM EDDs.

Conclusions:

• The choice of Rietveld refinement program and its associated parameters significantly influences the accuracy of MEM EDDs.
• Estimated standard deviations (σ(obs)) are critical for reliable MEM EDD calculations.
• A novel method is proposed to select an optimized MEM EDD, mitigating variations caused by σ(obs) differences.