Topotactical growth of thick perovskite oxynitride layers by nitridation of single crystalline oxides

Thick films of the perovskite-related oxynitrides LaTiO2N, NdTiO2N, SrNbO2N and SrTaO2N were synthesised by nitridation of single crystals of the corresponding oxides with general composition ABO3.5. The oxide crystals were obtained by optical floating zone growth. They correspond to n = 4 member of the AnBnO3n+2 family of layered perovskites and were reacted at temperatures between 900 °C and 1050 °C to form the oxynitrides. Electron probe microanalysis proved the presence of nitrogen in a surface layer of a few micrometer thickness. Cross-section SEM revealed additional thin stripes of oxynitride within the bulk of the crystals, indicating that nitrogen is incorporated preferably parallel to the perovskite-type layers, which in turn are connected in a zipper-type mechanism. The formation of the desired perovskite-type oxynitrides was confirmed by X-ray diffraction. Pole figure measurements proved an epitaxial orientation ABO2N (110)[001] ‖ ABO3.5 (001)[100]. The mosaicity of the oxynitrides both in polar and azimuthal direction was very small (<2°) indicating a nearly single crystalline quality of the surface layer. The nitridation of the crystals results in a dramatic change in colour. Optical spectroscopy revealed shifts of the absorption edge by more than 200 nm to longer wavelengths with respect to the parent oxides, corresponding to a reduction of the band gap energies by 1.4–1.8 eV.

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