Strong coupling between magnetic and structural order parameters in SrFe2As2

X-ray and muon spin-relaxation experiments performed on ${\text{SrFe}}_{2}{\text{As}}_{2}$ polycrystals confirm a sharp first-order transition at ${T}_{0}=205\text{ }\text{K}$ corresponding to an orthorhombic phase distortion and to a commensurate antiferromagnetic Fe ordering with a larger distortion and larger size of the ordered moment than reported for ${\text{BaFe}}_{2}{\text{As}}_{2}$. The structural and the magnetic order parameters present a remarkable similarity in their temperature dependence from ${T}_{0}$ down to low temperatures, showing that both phenomena are intimately connected. Accordingly, the size of the ordered Fe moments scales with the lattice distortion when going from ${\text{SrFe}}_{2}{\text{As}}_{2}$ to ${\text{BaFe}}_{2}{\text{As}}_{2}$. Full-potential band-structure calculations confirm that the columnar magnetic order with propagation vector (1,0,1) and the orthorhombic lattice distortion are intrinsically tied to each other.