Crystal structure and magnetic properties of 6H-SrMnO3

The 6H-SrMnO₃ polymorph was prepared using a high-pressure and high-temperature technique at 6 GPa and 1773 K. Its crystal structure was refined using synchrotron x-ray powder diffraction data (space group P6₃/mmc (no. 194), Z = 6, a = 5.42892(1) Å, c = 13.40252(3) Å). Magnetic and transport properties of 6H-SrMnO₃ were investigated using direct current magnetization, specific heat, and resistivity measurements. The antiferromagnetic Néel temperature T_N of 6H-SrMnO₃ is 235 K; this value is close to that of cubic-SrMnO₃ (T_N = 240 K) and 4H-SrMnO₃ (T_N = 280 K). 6H-SrMnO₃ and 4H-SrMnO₃ have the same building blocks of face-sharing MnO₆ octahedra. Magnetic susceptibility of 6H-SrMnO₃ exhibits behavior typical for classical three-dimensional antiferromagnets with a maximum near T_N—in contrast to those of cubic-SrMnO₃ and 4H-SrMnO₃, where broad maxima are observed far above T_N. Isothermal magnetization curves of 6H-SrMnO₃ (at 2 K) show an upturn deviation from the linear behavior above 10 kOe, indicating a possible spin-flop magnetic phase transition.