The nuclear quadrupole resonance of nitrogen-14 was observed for potassium, ammonium, rubidium, and thallium lead hexanitrocuprates(II) at various temperatures below the second highest transition temperature of each complex. All the compounds except the ammonium salt yielded two practically same sets of the quadrupole parameters, eQq/h and η, indicating the presence of at least two crystallographically different but very similar kinds of nitrogen atoms in the crystals. For the ammonium salt, three phase transitions at 94.5, 287, and 316 K were located from the experiments of DTA and NQR. The crystals of the highest and the second highest temperature phases were proved to be i8omorphous with those of the remaining complexes in their highest and intermediate temperature phases, respectively. The salt yielded two sets of the quadrupole parameters between 287 and 94.5 K, and only one sot below 94.5 K down to 4.2 K, indicating that a structural phase transition takes place at 94.5 K. No resonance could be observed in the highest and the second highest temperature phases of all the complexes studied. The covalency of copper-nitrogen bonds in lead hexanitrocuprates(II) is estimated at about 10% from the quadrupole parameters observed at 4.2 K. The structure of the complex anion is discussed in relation to the Jahn-Toller distortion.
