  A new layered iron arsenide oxide (Fe2As2)(Ca4(Mg,Ti)3Oy) was discovered. Its
crystal structure is tetragonal with a space group of I4/mmm consisted of the
anti-fluorite type FeAs layer and blocking layer of triple perovskite cells and
is identical with (Fe2As2)(Sr4(Sc,Ti)3O8) discovered in our previous study. The
lattice constants of (Fe2As2)(Ca4(Mg,Ti)3Oy) are a = 3.877 A and c = 33.37 A.
This compound exhibited bulk superconductivity up to 43 K in susceptibility
measurement without intentional carrier doping. A resistivity drop was observed
at ~47 K and zero resistance was achieved at 42 K. These values correspond to
the second highest Tc among the layered iron-based superconductors after
REFeAsO system.
