The model-free quest for disordered protons in solids

Abstract

It is commonplace to assert that neutrons are a good probe for protons and deuterons in solids, all the more so when all the remaining atoms (backbone) are reasonably well known from previous accurate X-ray measurements. When the protons are disordered, however, good is not good enough without a suitable unconventional data analysis. By (i) removing most of the truncation effects, (ii) allowing for the use of prior chemical knowledge, (iii) making use of the experimental error bars and (iv) taking advantage of the negative scattering length of the proton which overrides the huge incoherent cross-section of the latter (at least in the case of single-crystal diffraction), maximum entropy asserts itself as an ideal tool for model-free computer-assisted proton vision that easily supersedes the commonly used Fourier and difference Fourier syntheses. A typical example will be discussed.