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Description
The simultaneous reaction of oxygen, its heavier homologs, and metals usually lead to the formation of substances characterized by the presence of covalent O-Ch (Ch = S, Se) bonds within complex anions like sulfates or selenides. Examples for compounds containing both, O$^{2-}$ and Ch$^{2-}$ without any attractive interaction between these species are far less known. Only a few examples for chalcogenide oxides of transition (T) and rare-earth metals (RE) are described in the literature so far, though exhibiting a relatively high variety in composition and crystal structures.
Salt-flux assisted reactions of rare-earth metals and their oxides, transition metals and selenium lead to the discovery of the mixed anionic selenide oxides RE$_2$ZrSe$_2$O$_3$ (RE = Ce-Nd, Sm) and Ce$_7$TiSe$_5$O$_7$. All these substances appear as thin needle-like crystals with diameters of 5 μm and less which are extraordinarily sensitive to mechanical stress. Consequently, structure determination had to be realized using synchrotron radiation. First investigations of Ce$_7$TiSe$_5$O$_7$ were carried out at the ESRF, beamline ID11, using micro-focused synchrotron radiation. Small crystals were pre-selected and pre-characterized using transmission electron microscopy including EDX spectroscopy [1]. The substance crystallizes with the La$_7$VSe$_5$O$_7$ structure type [2], space group Cmcm, suggesting the presence of Ti$^{3+}$ cations.
The first Zr containing selenide oxides RE$_2$ZrSe$_2$O$_3$ (= 2 RE$^{3+}$Zr$^{4+}$2Se$^{2-}$3O$^{2-}$) were investigated at PETRA III, beamline P24, and the ESRF, beamline ID11. The compounds were found to crystallize with their own structure type, space group C 2/m, characterized by the presence of large cavities extended along [010] formed by Se atoms (see Fig. 1).
Fig. 1: a-c) Crystal structure, bulk material, and SEM-SE image of Nd$_2$ZrSe$_2$O$_3$. d) TEM-BF image of the Pr$_2$ZrSe$_2$O$_3$ single-crystal investigated at ERSF, beamline ID11. The crystal part used for structure determination is emphasized.
[1] F. Fahrnbauer, T. Rosenthal, T. Schmutzler, G. Wagner, G. B. M. Vaughan, J. P. Wright, O. Oeckler, Angew. Chem. Int. Ed. 2015, 54, 10020.
[2] S. Peschke, L. Gamperl, V. Weippert, D. Johrendt, Dalton Trans. 2017, 46, 6230-6243.
