Pergamon Solid State Communications, Vol. 91, No. 6, pp. 481-485, 1994 Elsevier Science Ltd Printed in Great Britain 0038-1098/94 $7.00 + .00 0038-1098(94)00326-2 CHARACTERIZATION OF NH4Br:Cu 2+ CRYSTALS: DIRECT EVIDENCE OF NH3 THROUGH RAMAN SPECTROSCOPY M. Couzi Laboratoire de Spectroscopic Mol6culaire et Cristalline (URA 124; CNRS), Universit6 de Bordeaux I, 33405 Talence Cedex, France and M. Moreno and A.G. Brefiosa D.C.I.T.T.Y.M. (Divisi6n Ciencias de Materiales), Facultad de Ciencias, Universidad de Cantabria, 39005 Santander, Spain (Received 20 January 1994 by D. Van Dyck) At variance with the results found for NH4CI : Cu 2+ it is shown here that the same Cu 2+ centre is formed in NH4Br crystals grown either in basic solutions or in acid solutions with a pH clown to 2. In order to definitely clarify the nature of this Cu 2+ species .polarized Raman spectra have been recorded from which the Cu z+ centre can be ascribed to a D4h CuBr4(NH3) 2- species. Aside from detecting the two A lg vibration frequencies of the complex a remarkable feature strongly supporting this model is the clear observation of the internal "umbrella" mode of the NH3 molecule detected at 1283cm -1 and 1293cm -1 for CuBr4(NH3)22- in NH4Br and for CuCI4(NH3) 2- in NH4CI respectively. The detection of three Raman peaks of CuBr4(NH3) 2- in NH4Br crystals containing only a Cu 2+ concentration of ,,~ 3000 ppm is seen to be favoured by the position of the first charge transfer transition lying only at 26000cm -1. This situation gives rise to preresonance phenomena clearly observed in the present work. 1. INTRODUCTION THE INTEREST in studying impurified compounds mainly resides on the new properties to which the impurity gives rise. In this realm a good character- ization of the impurity centre becomes a prerequisite for achieving a proper microscopic understanding of such new properties. This task is, however, not simple to be accomplished and so the nature of a given centre remains sometimes controversial. As a general strategy the use of complementary spectroscopic tools is a good way for reaching a right description about the impurity centre nature. This work concerns with the nature of Cu 2+ centres formed in NH4Br crystals grown by solution [1-5]. At variance with what is found for Cu 2+ doped NH4CI neither the actual nature of centres nor their dependence upon the pH of the mother solution is well established. In the case of NH4CI:Cu 2+ the EPR [6] and specially the ENDOR results carried out by Boettcher and Spaeth [7] definitely proved that for solutions to which NH4OH has been added the CuCl4(NH3)2--complex (D4h geometry) is formed. The model of that centre is depicted in Fig. 1. By contrast, if HCI is added to the solution, the CuCI4(H20)22- (D2h geometry) species becomes the impurity centre [6, 7]. Such centres are also called Cu 2+ (II) and Cu 2+ (I) respectively. For "neutral" solutions (to which neither NH4OH nor HCI has been added) the complex CuCI4(NH3)(H20) 2- can be formed but up to now no procedure has been developed for preparing NH4CI crystals containing only this centre [6, 8]. Coming back to NH4Br:Cu 2+, previous EPR data have shown that a Cu 2+ centre [called Cu 2+ (M)] has been observed indeed by different authors [1-5]. The reported experimental spin-Hamiltonian 481