content_image/340421.jpg
<img src='content_image/340421.jpg'>

and ( ζ$_{1}$ + 3 ζ$_{2}$P 2 $_{o}$) f = ω 2 $_{ey}$, the soft phonon frequency along the spontaneous polarisation. With the inverse phonon mass 4 as f = 2 . 7 × 10 24 statC $^{2}$/g cm3 and transversal phonon frequency 19 ω$_{ey}$ = 7.73 THz, this gives ζ$_{1}$ =-10.528 g cm $^{3}$/statC$^{2}$s2 and ζ$_{2}$ = 1 . 934 × 10 -8 erg cm $^{5}$/statC$^{4}$. The suceptibility χ e z is calculated by using the transverse phonon frequency for z polarization $^{4}$, ω$_{ez}$ = 33 . 7 cm − $^{1}$. 

The parameters C$_{c}$ and C$_{αf}$ in Eq.(24) and (33) are converted to frequency units in cm − $^{1}$. 

