Neutron diffraction study of high-pressure-induced structural changes in the ammonium halogenides ND4Br and ND4Cl

Structural changes in the deuterated ammonium halogenides ND4Br and ND4Cl have been studied by neutron time-of-flight diffraction up to pressures of 45 and 35 kbar, respectively. Data on the equations of state and pressure dependence of the deuterium position parameter have been obtained. A comparison with the hydrogen-containing analogs showed that isotopic substitution of deuterium for hydrogen affects only slightly the compressibility of the systems under study, although the effect is noticeable for ND4Cl. It has been established that the order-disorder transition from the phase with random deuterium distribution (CsCl cubic structure, space group Pm3m) to the ordered phase (same structure, space group \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$P\overline {43} m$$ \end{document}) occurs in both compounds at the same critical value of the position parameter u=0.153±0.002, which is apparently the same for all ammonium halogenides, and, possibly, for other systems of this structural type as well.