text: The crystal structure of APR resembles that of scheelite, with atomic cation is replaced by ammonium. The pertechnetate (NHTcO), periodate (NHIO), tetrachlorothallate (NHTlCl), and tetrachloroindate (NHInCl) follow this motif. It undergoes a molecular orientational ordering transition on cooling without change of space group, but with a highly anisotropic change in the shape of the unit cell, resulting in the unusual property of having a positive temperature and pressure Re NQR coefficient. APR does not give hydrates.