  We present a thermodynamic analysis of the recently discovered nitride
ferroelectric materials using the classic Landau-Devonshire approach. The
electrostrictive and dielectric stiffness coefficients of Al1-xScxN with
wurtzite structure (6mm) are determined using a free energy density function
assuming a hexagonal parent phase (6/mmm), with the first order phase
transition based on the dielectric stiffness relationships. The results of this
analysis show that the strain sensitivity of the energy barrier is one order of
magnitude larger than that of the spontaneous polarization in these novel
wurtzite ferroelectrics, yet both are less sensitive to strain compared to
classic perovskite ferroelectrics. These analysis results reported here explain
experimentally reported sensitivity of coercive field to elastic strain/stress
in Al1-xScxN films, and would enable further thermodynamic analysis via phase
field simulation and related methods.
