Journal of Chemical Physics, Vol.108, No.18, 7758-7762, 1998
Raman intensity study of scaling properties of Na+, K+, Ag+, and Pb2+ doped NH4NO3 crystals during phase transitions
It is observed that, for molecular crystals, during phase transitions the Raman intensities of the vibrational modes obey the power law \T-T-c\(beta). The exponent beta contains much critical information and is mode dependent. The scaling properties of the doped materials are imbedded in beta which is found to be very sensitive to very small degrees of doping. One of the eminent phenomena is the so-called crossover point where beta of the totally symmetric nu(1) mode equals the nonsymmetric nu(4) mode of the nitrate ion. The scaling is expressed through the scaled coordinate d/M-1/2 where d is the averaged interdoped-cationic distance and M is the mass of the cation. beta of the nu(1) mode behaves as (d/M-1/2)(gamma) with exponent gamma rather independent of the transitions and the kinds of doping. Resonant modes due to doped cations are employed to explain the origin of the scaling.