Saturation of Spontaneous Polarization Charge in Pyroelectric Crystals   of LiNbO3, LiTaO3 and CsNO3 at Low Temperature Above 4.2 K
  Experimental observations of the change in the polarization charge of pyroelectric crystals of LiNbO3, LiTaO3 and CsNO3, as the temperature of the crystal is changed from about 300K to a lower limit of 4.2K, are described. It was found that the rate of change of the polarization charge slowed down considerably and, perhaps, reached a zero rate of change. The onset of this saturation of the polarization charge occurred at low temperatures, above the temperature of liquid helium, which were different for each of the three types of crystal. 
James D. Brownridge and Sol Raboy