A new interstitial Cu2+ center was found in crystals which had been grown in an acidic solution (pH less than 0.2). It was not observed when the solution pH was about 1.6. The X-band electron paramagnetic resonance at 293K indicated that the symmetry was tetragonal and that the spin Hamiltonian parameters were: g|| = 2.004, g = 2.188, A|| = 0, A = 0.0083 and A = 0.0030. The model, H2O-Cu2+-H2O, was proposed after studying the appearance of 3 types of center in crystals which had been grown at pH values of between about 8, and less than 0.2. A re-examination of a previous study of NH4Br (pH of about 1.6) indicated that the center which was observed there may have been of the form, NH3-Cu2+-H2O. This could also be ascribed to the center, in NH4Cl, which was known as center III.

E.Di Mauro, W.Sano: Journal of Physics - Condensed Matter, 1994, 6[48], L755-7