The High-temperature And High-pressure X-ray Diffraction And First-principles Theoretical Calculation Of SrMnO₃

ABX₃-type perovskites have attracted lots of attention in ranges of areas, including physics, solid-state chemistry, physical chemistry and earth sciences. The SrMnO3 solid solutions is a kind of typical A-site substituted perovskite, which has a hexagonal crystal system.The high-temperature powder X-ray diffracrion （HTXRD） is used to identify the structural transformation of the P63/mmc SrMnO3. The HTXRD results reveal that SrMnO3 experiences a P63/mmc （<1260℃）-P63/mmc+Pm3m （1260-1370℃）-Pm3m （> 1370℃） transformation （upmost to 1450℃）. In the experimental process, the expansibility of a axis is bigger than c axis. And after the heating process, the structure of SrMnO3 is recovered to hexagonal, which means the transformation is reversible.The in-situ high-pressure behavior of SrMnO3 has been investigated using the synchrotron radiation X-ray diffraction and diamond anvil cell device （DAC）. The experiments shows that all peaks gradually move to the high angle （right） from 32.8 GPa to 47.8 GPa, and back to the low angle （left） during releasing the pressure to room condition. A decomposition is observed with MnO2 and SrO being the decomposition product, and a small amount of SrMnO3 sample. The cell parameters of MnO2 is a=13.6646,b=2.9312 and c=4.3934 （P=0.3 GPa）. The Murnaghan equation of state was used to fit the P-V data, and the isothermal bulk modulus K298 =111.5（5.6） GPa, V0=160.9（11.1） A3 were obtained.Furthermore, based on Density Functional Theory （DFT） and Local Density Approximation （LDA）, we used VASP software to calculate the structural stability of two candidated SrMnO3 and possible decomposition under high pressure. The first principle calculation confirms that P63/mmc and C2221 phase have similar structural stability from zero to 100 GPa. SrMnO3, and P63/mmc decomposes to MnO2 and SrO when the pressure is above 91GPa.