  The recent reports on high-temperature superconductivity above 190 K in
hydrogen sulfide at 200 GPa pressure, exceeding all previously discovered
superconductors, has greatly invigorated the interest in dense hydrogen-rich
solids. In this paper, we investigate a possible way to optimize the critical
temperature in these compounds using first-principles linear-response
calculations. We construct hypothetical alchemical atoms to smoothly
interpolate between elements of the chalcogen group and study their bonding and
superconducting properties. Our results show that the already remarkable
critical temperatures of H$_3$S could be improved even further by increasing
the ionic character of the relevant bonds, i.e. replacing sulfur partially with
more electronegative elements.
