Temperature dependence of the nuclear positions and spontaneous polarization in pyroelectric ${\mathrm{B}\mathrm{a}\left(\mathrm{N}{\mathrm{O}}_2\right)}_2$·${\mathrm{H}}_2$O

The nuclear positions in a single crystal of ${\mathrm{B}\mathrm{a}\left(\mathrm{N}{\mathrm{O}}_2\right)}_2$·${\mathrm{H}}_2$O at 20, 102, and 298 K have been measured by neutron diffraction, and the change in spontaneous polarization between 20 and 300 K has been continuously determined. A polarization change of -30 × ${10}^{-4\mathrm{}}$ ${\mathrm{Cm}}^{-2\mathrm{}}$ between 102 and 298 K is calculated from the nuclear positions, based upon an electric dipole moment of 6.2 × ${10}^{-30\mathrm{}}$ Cm for ${\mathrm{H}}_2$O and 6 × ${10}^{-30\mathrm{}}$ Cm for N${\mathrm{O}}_2^{-}$ and the charge on N${\mathrm{O}}_2^{-}$ as midway between the oxygen atoms. Integral ionic charges are assumed. The measured polarization change between 102 and 298 K is -26 × ${10}^{-4\mathrm{}}$ ${\mathrm{Cm}}^{-2\mathrm{}}$. The calculated polarization change between 20 and 102 K is -5 × ${10}^{-4\mathrm{}}$ ${\mathrm{Cm}}^{-2\mathrm{}}$; the experimental value is +3 × ${10}^{-4\mathrm{}}$ ${\mathrm{Cm}}^{-2\mathrm{}}$. The error in the calculated spontaneous polarization is estimated at 10 × ${10}^{-4\mathrm{}}$ ${\mathrm{Cm}}^{-2\mathrm{}}$.