Polymorphism and Polytypism of α‑LiNH4SO4 Crystals. Monte Carlo Modeling Based on X‑ray Diffuse Scattering

The crystal structure and phase transformations of α-LiNH4SO4 were investigated using powder and single-crystal samples over the temperature range of 120–713 K. An α → β reconstructive phase transition was observed between 431 and 457 K. The average crystal structure of α-LiNH4SO4 was refined at 120, 300, and 383 K. X-ray diffuse scattering, associated with structural disorder existing in the planar structure of α-LiNH4SO4 was registered and modeled by the Monte Carlo technique using two spinlike Ising models of disorder: one- and two-dimensional (1D and 2D, respectively). The nature of polytypism in α-LiNH4SO4 was determined. The 1D model allowed for the simulation of a continuum of disordered polytypic structures reported previously: from a disordered 2O structure, different types of 4O polytypes with duplicate c axis, to 6O disordered structures with a triplicate c axis. The local structure of the measured crystal was described as a disordered 2O polytype with the very small addition of a 4O polytypic order. It was proven that there was no general structure of α-LiNH4SO4. The crystals, due to their metastability, can crystallize with different layer sequences, which can be easily changed during growth. Each crystal can be a specific realization of a polytypic structure ordered to a varying degree.