Raman spectra,Pippard relations and critical exponents for NH4Cl

The Pippard relations have been extended to correlate the vibrational frequency shifts with the relevant thermodynamic quantities in the vicinity of the λ-point in NH₄Cl. It has been shown that they describe the NH₄Cl system adequately below its λ-transition temperature. (T_λ = 242.8 K). This has been examined using our temperature-dependent frequency data for the ν₅ (174 cm^?1) Raman mode of NH₄Cl at zero pressure by means of the equations developed. The slope of the λ-transition line. (dP/dT)_λ that has been calculated from these equations, is in good agreement with those obtained by previous workers. We have deduced the critical exponents a′ and c′ for the thermal expansion and the isothermal compressibility of NH₄Cl from our temperature and pressure-dependent frequency data for the ν₅ mode. We have identified our exponent value for the thermal expasion with the specific heat exponent, a′, (Pippard relation) to demonstrate the scaling law a′+2b+c′=2, using our value of the critical exponent b for the order parameter from our previous work on NH₄Cl.