  Over the last few years of the heyday of hybrid halide perovskites, so many
metal cations additives have been tested to improve their optoelectronic
properties that it is already difficult to find an element that has not yet
been tried. In general, the variety of these approaches is united under the
name "doping", however, there is currently no clear understanding of the
mechanisms of the influence of the metal ion additives on the properties of the
lead halide perovskite materials. For many ions there is even no consensus on
the most fundamental questions: what lattice position does a given ion occupy
and is it incorporated in the structure at all? Here, we derived a system of
effective radii of different metal ions in the iodine environment for the set
of iodide compounds and reveal their crystal chemical role in the APbI3
perovskites. We analysed the possible lattice positions for 40 most common
monovalent, divalent, and trivalent metals to reveal whether they could
successfully enter into the perovskite structures. We show that, at most, three
parameters - effective size, electronegativity and the softness of metal ions
are the main ones for crystal chemical analysis of the possibility of metal
doping of hybrid halide perovskites. Our results provide a useful theoretical
guidance to rationalize and improve current doping strategies of hybrid halide
perovskites with metal ions.
