Structure of the network glass-former ZnCl2: From the boiling point to the glass

The structure of the network glass-forming material ZnCl2 was measured using both neutron and high-energy x-ray diffraction for the glass at 298(1) K and for the liquid over the temperature range 601(1)–977(2) K. Intermediate range order, as manifested by the appearance of a first-sharp diffraction peak in the measured diffraction pattern for the glass at a scattering vector kFSDP≃1Å−1, is retained in the liquid state even at temperatures close to the boiling point. The correlation lengths associated with both the intermediate and extended range ordering are found to be inversely proportional to temperature. The reverse Monte Carlo (RMC) method was used to model the material, and the results at two different temperatures are compared to those obtained from RMC models based on the partial structure factors measured by using the method of isotope substitution in neutron diffraction. The models show temperature dependent structural variability in which there is an interplay between the fractions of corner-sharing versus edge-sharing ZnCl4 tetrahedra. Corner-sharing motifs are predominant in the glass, and edge-sharing motifs become more numerous in the liquid as the temperature is increased. The appearance of a first-sharp diffraction peak in the Bhatia–Thornton concentration–concentration partial structure factor is discussed in the context of classifying the different network types for glass-forming materials. •Structure of a network glass-former investigated from the boiling point to the glass.•Measured correlation lengths are inversely proportional to temperature.•Models show temperature dependent structural variability.•Appearance of edge-sharing structural motifs consistent with melt fragility.•Revised classification scheme proposed for network glass-forming materials.