Crystal chemical aspects of superconductivity in BaFe$_{2}$As$_{2}$ and related compounds

BaFe$_{2}$As$_{2}$ is the parent compound of the 122-type iron arsenides.$^{1}$ Superconductivity can be induced by several kinds of doping$^{2-4}$ or by pressure.$^{5}$ It is widely accepted that superconductivity in iron arsenides is unconventional and a number of experiments agree with the s$\pm $-scenario.$^{6}$ The latter relies on Fermi surface nesting which depends on both the electron count and the lattice. However, the coincidence of doping and pressure effects on the structure of BaFe$_{2}$As$_{2}$ supports the role of the structure.$^{7}$ Another open issue is the co-existence of superconductivity and AF magnetic ordering. Our $^{57}$Fe-M\"{o}ssbauer experiments with underdoped Ba$_{0.8}$K$_{0.2}$Fe$_{2}$As$_{2}$ ($T_{c}$ = 24 K) revealed full magnetic splitting, which indicates such a co-existence.$^{8}$ Compounds like Sr$_{2}$VO$_{3}$FeAs ($T_{c}$ = 37-45 K) are promising candidates for higher $T_{c}$, but their crystal chemistry is not yet understood. In non-superconducting Sr$_{2}$CrO$_{3}$FeAs, we have detected a non-stoichiometry of the Fe-site (Fe$_{0.93(1)}$Cr$_{0.07(1)})$ and $C$-type AF ordering of the Cr$^{3+}$-layers.$^{9}$ The Cr-doping of the FeAs layer is probably detrimental to superconductivity in Sr$_{2}$CrO$_{3}$FeAs, but a similar non-stoichiometry may play a vital role in Sr$_{2}$VO$_{3}$FeAs.\\ -\\ $^{1 }$M. Rotter, M. Tegel, I. Schellenberg, et al., Phys. Rev. B \textbf{78}, 020503 (2008).\\ $^{2 }$M. Rotter, M. Tegel, and D. Johrendt, Phys. Rev. Lett. \textbf{101}, 107006 (2008).\\ $^{3 }$S. Jiang, C. Wang, Z. Ren, et al., J. Phys.: Condens. Matter \textbf{21}, 382203 (2009).\\ $^{4 }$A. S. Sefat, R. Jin, M. A. McGuire, et al., Phys. Rev. Lett. \textbf{101}, 117004 (2008).\\ $^{5 }$P. L. Alireza, Y. T. C. Ko, J. Gillett, et al., J. Phys.: Condens. Matter \textbf{21}, 012208 (2009).\\ $^{6 }$I. Mazin, D. J. Singh, M. D. Johannes, et al., Phys. Rev. Lett. \textbf{101}, 057003 (2008).\\ $^{7 }$M. Rotter, M. Pangerl, M. Tegel, et al., Angew. Chem. Int. Ed. \textbf{47}, 7949 (2008).\\ $^{8 }$M. Rotter, M. Tegel, I. Schellenberg, et al., New J. Phys. \textbf{11}, 025014 (2009).\\ $^{9 }$M. Tegel, Y. Su, F. Hummel, et al., arXiv0911.0450.