On the tetragonal → monoclinic and superprotonic RbH2PO4 phase transitions: are they physical transformations?

Abstract

During the last quarter century, the high-temperature phase transition (HTPT) at $${\text{72 < T}}_{{\text{p}}} { < 130 }$$ °C on $${\text{RbH}}_{{2}} {\text{PO}}_{{4}}$$ has become a controversial issue; while most researchers state that this phenomenon corresponds to a physical transformation (tetragonal → monoclinic structural phase transition), others support its chemical nature. On the other hand, it has been established that if the salt is heated above $${\text{T}}_{{\text{p}}}$$ , a superprotonic phase transition at $${\text{72 < T}}_{{\text{p}}} {\text{ < 130}}$$ °C takes place. By using thermal analyses (conventional and modulated differential scanning calorimetry, thermogravimetric analysis, simultaneous thermogravimetric analysis and differential scanning calorimetry) and temperature evolution of X-ray diffraction measurements, we found solid evidence that none of these transformations is physical in nature; instead, they correspond to chemical thermal decompositions. Therefore, according to these results, when $${\text{RbH}}_{{2}} {\text{PO}}_{{4}}$$ is submitted to a heating process, it keeps its room-temperature tetragonal-phase until the sample reaches its total thermal decomposition. Consequently, we believe that the tetragonal → monoclinic phase transition and the transition to the superprotonic phase do not take place.