The behaviour of the vibrational modes of doped (NH4)(1-x)KxNO3 and (NH4)(1-x)NaxNO3 crystals during phase transitions: a Raman intensity study

The Raman intensities of the vibrational modes of doped (K+, Na+) NH4NO3 du ring phase transitions are shown behaving as power functions of \T - T-c\(b eta). The exponent, beta, is mode and degree-of-doping dependent. Among the modes, the totally symmetric nu(1) mode of the nitrate ion possesses the u nique behaviour of being a function of the degree of doping. The nu(1) mode also possesses the ability to differentiate between the different dopings with K+ and Na+. It is shown that the exponents are very sensitive to the d egree of doping for very dilute doping where the so-called cross-over pheno menon between the nu(1) and other modes is observed. The physical interpret ation is given in terms of the average distance between the doped ions. (C) 1998 Elsevier Science Ltd. All rights reserved.