Order/disorder phenomena in Zn1-xMnxGa2Se4 ordered vacancy compounds: high temperature neutron powder diffraction experiments

We present a study of order-disorder phenomena in the series of tetrahedral ordered vacancy compounds Zn1-xMnxGa2Se4 by means of time-of-flight neutron diffraction at high temperature together with dc magnetic susceptibility, Raman spectroscopy, differential thermal analysis and optical absorption experiments. Samples of nominal composition x = 0, 0.24, 0.5, 0.77 and 1 have been studied. An order-disorder phase transition has been detected, with T-c ranging from 472 to 610 degrees C, which involves a structural change from a defect chalcopyrite phase, with I (4) over bar 4 space group (s.g.) and three different cation sites, to a partially disordered defect stannite, in which Zn, Mn and half of the Ga ions share the 4d site in I (4) over bar 2m s.g. Neither the vacancies nor the Ga ions occupying site 2a are involved in the phase transition. An additional ordering process is observed on approaching the phase transition from below, which is attributed to several factors: the activation of cation diffusion at similar to 300 degrees C, the partially disordered cation distribution exhibited by the as-grown single crystals and the preference of Mn atoms for the 2d crystallographic site in the I (4) over bar structure. The reversibility of the phase transition is analysed with the aid of magnetic, optical and Raman experiments.