The crystal structure adopted by a material is strongly dependent on the chemical composition and synthesis conditions. In this sense, high pressure and high temperature syntheses (HPHT) have been widely used not only to prepare new materials but also for inducing phase transformations.In particular, the (HPHT) method is traditionally used to synthesise metastable structures, as it is the case of the highly distorted perovskites and the tetragonal zircon-type (S. G. I41/amd) of RCrO4 oxides (R = rare earth element); transforms into the tetragonal scheelite-type structure (S. G. I41/a). In the case of the perovskites HPHT conditions allow the stabilisation of small cations at the A-perovskite sites, even for chemical compositions where the tolerance factor t is below 0.75. Among the ABO3 systems treated under HPHT conditions, Mn-containing double perovskites have attracted an especial interest during the last years, due to its spin, charge and orbital degrees of freedom, which induce complex orderings featuring their properties. Mn can be accommodated into the A or B sites depending on its oxidation state,. for instance, Mn4+ and Mn3+ (rVI = 0.53 Å and r VI = 0.645 Å, respectively are only stable at the B position, but Mn2+ can be stabilised under high pressure conditions in the highly coordinated A site of the perovskite structure despite its relatively small ionic radius .By combining A and B site orders in AA'BB'O6 given rise to a so called double-double perovskites (DDPv) obtained in . our previous results on the HP synthesis of the stabilisation of small A–site cations has lately motivated the study of Mn2+2RSbO6 DPv polymorphs. Within this family of oxides several types of magnetic and transport properties appear. Research concerning the polymorphism on the RCrO4 phases (R= rare earth) under HPHT conditions have been studied in our research group. We have reported that the ferromagnetic RCrO4-zircon phases show large magnetocaloric effect at low temperatures which make these compounds potential candidates to be used in magnetic refrigeration processes.