Neutron total scattering of crystalline materials in the gigapascal regime

Phase behaviour of ammonium bromide-d4under high pressure and low temperature; an average and local structure study

We revisit the pressure-induced order-disorder transition between phases II and IV in ammonium bromide-d₄using neutron diffraction measurements to characterise both the average and local structures. We identify a very sluggish transition that does not proceed to full conversion and local structure correlations indicate a slight preference for ammonium cation ordering along ⟨110⟩ crystallographic directions, as pressure is increased. Simultaneous cooling below ambient temperature appears to facilitate the pressure-induced transition. Variable-temperature, ambient-pressure measurements across the IV → III → II transitions show slower conversion than previously observed, and that phase III exhibits metastability above ambient temperature.

Recovering local structure information from high-pressure total scattering experiments

High pressure is a powerful thermodynamic tool for exploring the structure and the phase behaviour of the crystalline state, and is now widely used in conventional crystallographic measurements. High-pressure local structure measurements using neutron diffraction have, thus far, been limited by the presence of a strongly scattering, perdeuterated, pressure-transmitting medium (PTM), the signal from which contaminates the resulting pair distribution functions (PDFs). Here, a method is reported for subtracting the pairwise correlations of the commonly used 4:1 methanol:ethanol PTM from neutron PDFs obtained under hydro-static compression. The method applies a molecular-dynamics-informed empirical correction and a non-negative matrix factorization algorithm to recover the PDF of the pure sample. Proof of principle is demonstrated, producing corrected high-pressure PDFs of simple crystalline materials, Ni and MgO, and benchmarking these against simulated data from the average structure. Finally, the first local structure determination of α-quartz under hydro-static pressure is presented, extracting compression behaviour of the real-space structure.

Calculation of pair distribution functions for multiphase systems

The total scattering method is becoming increasingly popular because of its ability to investigate the structures of disordered crystalline and amorphous materials. Also, in recent years, significant development of total scattering instruments and sample environments has allowed for the study of increasingly complex materials, including multiphase samples. The total scattering formalism has already been well described in the paper by Keen [J. Appl. Cryst. (2001), 34, 172–177] but it was limited to the single phase case. In the present paper the formulae for multiple phase samples (consisting of a physical mixture of two or more distinct phases) are derived for the calculation of pair distribution functions for analysis using reverse Monte Carlo and other methods. The equations for conversion between different representations of the pair distribution function are also provided.