The 122 iron arsenideunconventional superconductors are part of a new class of iron-based superconductors. They form in the tetragonal I4/mmm, ThCr2Si2 type, crystal structure. The shorthand name "122" comes from their stoichiometry; the 122s have the chemical formula AEFe2Pn2, where AE stands for alkaline earth metal (Ca, Ba Sr or Eu) and Pn is pnictide (As, P, etc.).[1][2][3] These materials become superconducting under pressure and also upon doping.[4][5][6][7] The maximum superconducting transition temperature found to date is 38 K in the Ba0.6K0.4Fe2As2.[8] The microscopic description of superconductivity in the 122s is yet unclear.[9]
^Kreyssig, A.; Green, M. A.; Lee, Y.; Samolyuk, G. D.; Zajdel, P.; Lynn, J. W.; Bud'ko, S. L.; Torikachvili, M. S.; Ni, N.; Nandi, S.; Leão, J. B.; Poulton, S. J.; Argyriou, D. N.; Harmon, B. N.; McQueeney, R. J.; Canfield, P. C.; Goldman, A. I. (2008). "Pressure-induced volume-collapsed tetragonal phase of CaFe2As2 as seen via neutron scattering". Physical Review B. 78 (18): 184517. arXiv:0807.3032. Bibcode:2008PhRvB..78r4517K. doi:10.1103/PhysRevB.78.184517. S2CID118703521.