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Abstract
The values of EFG tensors at sodium nuclei were measured by NMR under hydrostatic pressure of 1000 kg°Cm−2. Shift in the upper transition temperature determined from satellite splitting of a Na NMR line was 10.6°C/1000 kg°Cm−2. The result was coincident with the value obtained by Ps measurement. The change of EFG tensors under hydrostatic pressures is equivalent to that produced by decrease in temperature in the upper paraelectric and ferroelectric phases. The difference of OH(5) orientations in the two sublattices under 1000 kg°Cm−2 was estimated to be 20 degree from a calculation of EFG tensors in the ferroelectric phase. A part of value of Ps 0.20 μC/cm2 was attributed to the displacement of H(5) from the one sublattice to the other.