  Entropic contributions to the stability of solids are very well understood
and the mixing entropy has been used for forming various solids, for instance
such as inverse spinels. A particular development was related to high entropy
alloys in which the configurational disorder is responsible for forming simple
solid solutions and which are thoroughly studied for various applications
especially due to their mechanical properties but also electrical properties,
hydrogen storage, magnetic properties. Many unexplored compositions and
properties still remain for this class of materials due to their large phase
space. In a recent report it has been shown that the configurational disorder
can be used for stabilizing simple solid solutions of oxides, which should
normally not form solid solutions, these new materials were called
"entropy-stabilized oxides". In this pioneering report, it was shown that
mixing five equimolar binary oxides yielded, after heating at high temperature
and quenching, an unexpected rock salt structure compound with statistical
distribution of the cations in a face centered cubic lattice. Following this
seminal study, we show here that these high entropy oxides (named HEOx
hereafter) can be substituted by aliovalent elements with a charge compensation
mechanism. This possibility largely increases the potential development of new
materials by widening their (already complex) phase space. As a first example,
we report here that at least one HEOx composition exhibits colossal dielectric
constants, which could make it very promising for applications as large-k
dielectric materials.
