We have shown that the molecular ionic solids MI3 (M=Tl, Cs, Tl, NH4) undergo structural phase transitions below 3.4 GPa at RT to give the same new high pressure phase (HP) with symmetric I3- ions. The HP phase of CsI3 is an electrical conductor at RT and it has been shown that the HP phases of CsI3 and RbI3 can be recovered to ambient pressure at a low temperature ( ~ 140 K). We now propose to study the effect of pressure and temperature upon the crystal structure of ND4I3, with particular emphasis on determining the role played by hydrogen bonding (HB) in the HP phase, which is believed to give rise to the extremely asymmetric I3- ions in the LP phase of this system. From this and our previous work on the heavy metal binary triiodides we will obtain a clearer understanding of the pertinent interactions which influence the structural behaviour observed in this series of compounds.