STRUCTURAL-CHANGES IN SILVER BROMIDE AT THE MELTING-POINT

A neutron diffraction study has been performed on AgBr at a series of temperatures from 293 K to 706 K (melting point, T(m) = 701 K), including measurements as close as 0.3+/-0.05-degrees from T(m). Analysis of the structure factors using the reverse Monte Carlo modelling technique indicates that the motion of the Ag+ ions about their rocksalt lattice sites is anisotropic, with increasing occupancy of the (1/4 1/4 1/4) tetrahedral interstitial site as the melting point is approached. It is concluded that the silver sub-lattice is undergoing a second-order transition to a fast-ion phase in which both tetrahedral and octahedral sites arc occupied (possibly similar to the fast-ion transition in the high pressure FCC phase of AgI) although melting prevents complete transition. There is a clear discontinuity in Bragg scattering at T(m), suggesting that the first-order nature of the melting transition is unaffected.