Alkali-metal (M) dihydrogen phosphates MH$_{\mathrm{2}}$PO$_{\mathrm{4~}}$stability at superprotonic behavior temperatures.

Alkali-metal (M) dihydrogen phosphates MH$_{\mathrm{2}}$PO$_{\mathrm{4~}}$(solid acids) have a particular behavior, at high temperatures they show a huge proton conductivity increase (1000-fold) (e.g. RbH$_{\mathrm{2}}$PO$_{\mathrm{4}}$~and CsH$_{\mathrm{2}}$PO$_{\mathrm{4~}}$(CDP)) upon heating above a certain temperature TS. The so called~superprotonic~behavior observed in CDP~at temperatures above~TS$=$237C has been previously reported; and this behavior is particularly attractive to use alkali-metal dihydrogen phosphates as a fuel cell electrolytes in the intermediate temperatures range. ~A major drawback is that~the cubic CDP~phase (which have been previously debated to be the main cause to generate the superprotonic behavior in the solid acids) is not stable~under ambient humidity and pressure conditions at TS temperatures, becoming a huge barrier to mass production. Indeed,~CDP dehydrates and decomposes chemically after its polymorphic transition from the room temperature (RT) monoclinic phase to the HT cubic.~ ~In order to treat the CDP and become more stable, dopants (such as SiO$_{\mathrm{2}})$ in combination of different humidity conditions have been tried to delay the dehydration of the CDP after the superprotonic its been achieved.