Knowledge Bank

We have calculated the specific heat $Cy$ for $NH4Br$ near its $\lambda$-phase transition using our Raman frequency data of the $\nu_{5}LO_{r} $ ($177cm-1$) phonon mode of this crystal. For the $Cv$ calculation we have used an Ising model superimposed on an Einstein and/or Debye model. The specific heat $CVL$ due to the lattice vibrations (Einstein model) and $CVI$ due to the spin interactions (Ising model) were calculated according to $CV=CVL+CVI$ at various temperatures using our observed Raman frequencies for the $177cm-1$ mode of $NH4Br$. From our analysis it is concluded that an Ising model superimposed on an Einstein and/or Debye model explains adequately the observed behaviour in the lattice region of the $NH4Br$ crystal.