Single‐crystal X‐ray diffraction investigation of the reversible order‐disorder phase transition in iron‐deficient TlFe 2–x Se 2

A reversible phase transition was found for iron‐deficient TlFe 2–x Se 2 ( x ≈ 0.4) and investigated on single crystals using high temperature synchrotron radiation X‐ray diffraction. Full data sets have been collected at room temperature and at 200 °C to evaluate the structures. The high‐temperature form (space group I 4/ mmm , lattice parameters a = 3.8919(3) Å, c = 14.027(3) Å) shows complete statistical disorder of iron positions and vacancies in a ThCr 2 Si 2 type structure. The low‐temperature form corresponds to a √5 × √5 superstructure of the ThCr 2 Si 2 type ( I 4/ m , a = 8.6966(2) Å, c = 14.009(9) Å) and shows partial ordering of iron positions and vacancies. A group‐subgroup relation can be compiled between both structures.