THE RAMAN FREQUENCY SHIFTS RELATED TO THE THERMODYNAMIC QUANTITIES CLOSE TO THE PHASE I - PHASE II TRANSITION IN NH$_4$Br

Abstract

Author Institution: Department of Physics, Middle East Technical University,; Ankara, 06531 - TurkeyWe establish here the modified Pippard relations by relating the specific heat $C_p$ to the frequency shifts $\frac{1}{\nu }\left( {\frac{{\partial \nu }}{{\partial T}}} \right)_P$ and also the thermal expansivity $\alpha_p$ to the $\frac{1}{\nu }\left( {\frac{{\partial \nu }}{{\partial P}}} \right)_T$ for the $\nu$ (140 cm$^{-1}$) Raman mode of NH$_4$Br close to its phase I- phase II transition. From our linear plots of $C_p$ vs. $\frac{1}{\nu }\left( {\frac{{\partial \nu }}{{\partial T}}} \right)_P$ and $\alpha_p$ vs. $\alpha_p$ to the $\frac{1}{\nu }\left( {\frac{{\partial \nu }}{{\partial P}}} \right)_T$, we extract the values of the slope DP/dT, which are close to the experimental value of 28.2 bar/$^irc$C near the phase I-phase II transition in NH$_4$Br