Order-disorder nature of the antiferroelectric transition in Pb(2)MnWO(6)

The nature of the para-antiferroelectric phase transition in Pb(2)MnWO(6) was investigated by x-ray absorption spectroscopy. Extended x-ray absorption spectra (EXAFS) and x-ray absorption near-edge structure (XANES) at the Pb L(3)-edge, W L(3)-edge and Mn K-edge were measured at temperatures above and below the transition temperature, T(c)∼445 K. The temperature dependence of the local geometrical and electronic structure around each of the three atoms was quantitatively determined. XANES spectra confirm the expected formal valence state as Pb(2+), W(6+) and Mn(2+) both above and below T(c). There are no significant changes of the local structure around the three atoms, which remains distorted in the whole temperature range. This indicates the existence of dynamic local distortions with a short-range order in the paraelectric phase. Below T(c), the local distortions (and the electrical dipoles) are static and their ordered arrangement gives rise to the orthorhombic crystallographic structure. Above T(c), these distortions are dynamically disordered, causing an average cubic crystallographic structure. Hence, the phase transition is of the order-disorder type.