Structure and microstructure of the high pressure synthesised misfit layer compound [Sr2O2][CrO2]1.85

Abstract

The strontium chromium oxide [Sr2O2][CrO2]1.85 misfit layer compound has been synthesised at high pressure and high-temperature conditions. Electron diffraction patterns and high-resolution transmission electron microscopy images along [001] show the misfit character of the different layers composing the structure with a supercell along the incommensurate parameter b=7b1=13b2. The modulated crystal structure has been refined within the superspace formalism against single-crystal X-ray diffraction data, employing the (3+1)-dimensional superspace group Cnmb(0s20)0 0 s. The compound has a composite structure with lattice parameters a1=5.182(1)A˚ , b1=5.411(1)A˚ , c1=18.194(3)A˚ for the first, SrO, subsystem and the same a and c, but with b2=2.925(1)A˚ for the second, CrO2, subsystem. The layer stacking is similar to that of orthorhombic PbS(TiS2)1.18, but with a much stronger intersubsytem bonding in the case of the oxide. The intersubsystem lattice mismatch is mainly handled by displacement modulations of the Sr atoms, correlated with modulations of the valence, the coordination and the anisotropic displacement parameters.