Neutron diffraction study of structural variations in ND4I and ND4F ammonium halides under high pressures

The structural variations in ammonium iodide ND4I under pressures up to 52 bar and ammonium fluoride under pressures up to 30 bar (the phases with CsC l-type structures) are studied by the neutron diffraction method. The equat ions of state and the pressure dependences of the positional parameters of deuterium are obtained. It is confirmed that the orientational phase transi tions in ammonium halides under high pressures can be described with the us e of the unified critical values of the positional parameter. Possible reas ons of such behavior are discussed. The compressibilities of the F-, Cl-, B r-, and I- ions are discussed as well as the behavior of their electron pol arizabilities under high pressures.