Mapping the complete bonding network in KBH₄ using the combined power of powder diffraction and maximum entropy method

The combined power of the maximum entropy method (MEM) and synchrotron powder X-ray diffraction (SPXRD) is exerted to accurately reconstruct the electron density distribution (EDD) of the hydrogen storage material, KBH₄. Its crystal structure features thermally activated disorder among the BH4- moieties, and weak secondary bonding effects occupy a key role in determining the energetic barrier for this dynamical effect. The MEM reconstruction is meticulously optimised and inspected for errors, in what may be envisaged as a general manual for this kind of studies. The successful outcome constitutes an experimental EDD of cutting-edge quality, from which atomic charges and the complete bonding network are mapped by topological descriptors. Remarkably, the chemical insights even extend to the delicate interplay of closed-shell bonding in excellent correspondence with ab initio and two-channel MEM calculations. For the current class of functional materials, access to such subtle electronic features is essential for the fundamental understanding of hydrogen desorption pathways.