Phase stability of the SrMnO3 hexagonal perovskite system at high pressure and temperature

SrMnO 3 is a perovskite compound which, unlike most perovskites, can be synthesized in three different but closely related polymorphs. In this paper, an experimental equation of state of the 6H polymorph is reported. The experimentally determined bulk modulus of SrMnO 3 increases from 115.6(11) GPa in the 4H polymorph to 143.7(17) GPa in the 6H polymorph, while density functional theory (DFT) calculations predict a further increase to 172.5(4) GPa in the 3C polymorph. In situ observations of transformations between the three known polymorphs, under high pressure and high temperature conditions, are also reported. The results are compared with extensive DFT calculations and literature, and it is demonstrated that the 6H polymorph is the thermodynamically stable phase between 5.9(3) and 18.1(2) GPa at 0 K. The effect of possible oxygen substoichiometry is also explored, using DFT. Finally, the findings are combined with the existing knowledge of the phase behavior in this system to outline where further knowledge needs to be collected before a pressure/temperature (PT) phase diagram can be constructed for this system.