  An intriguingly complex phase diagram of Na-doped SrFe2As2 is uncovered using
high-resolution thermal-expansion, magnetization and heat-capacity
measurements. The detailed temperature dependence of the orthorhombic
distortion and the anisotropy of the uniform magnetic susceptibility provide
evidence for nine distinct electronic phases near the transition region between
stripe antiferromagnetism and unconventional superconductivity. In particular,
we report the finding of a new magnetic phase which competes surprisingly
strongly with superconductivity. From theoretical studies we propose that this
phase is a double-Q phase consisting of a mixture of symmetry-distinct
commensurate magnetic orders with a peculiar temperature-dependent magnetic
moment reorientation.
