Ion Segregation and Electrostatic Imbalance in Metal Hydrides

Crystal chemical indices of ion segregation and electrostatic imbalance after Beck help to rationalize inorganic crystal structures and sometimes even to predict them. Metal hydrides, from hydrido‐aluminates and ‐gallates, to complex transition metal hydrides, ternary ionic magnesium hydrides and mixed anionic hydrides, were investigated using these tools. Beck's ion segregation and electrostatic imbalance indices are found to work well in explaining crystal structures of most metal hydrides, although the electronegativity differences between cations and hydride anions are often much smaller in ionic metal hydrides, as compared to metal fluorides and oxides. This includes complex transition metal hydrides, despite the fact that formal oxidation states do not describe the actual charges properly. Rare cases of violations can be explained by the chemical bonding situation, e.g. the presence of weak metal–metal bonds in Li₃RhH₄.