Surface Layer Effects on the Dielectric Behaviour of Polycrystalline Ammonium Tetra Fluoberyllate

抄録

The dielectric constant of compressed powder of (NH₄)₂BeF₄ has been measured at 1 MHz as a function of temperature for different particle sizes ranging from 105 μm to 710 μm over a wide range of temperature to cover ferro- and para-electric phases. Dielectric anomaly at −97°C indicates the ferroelectric behaviour. It is found that the dielectric constant corrected for porosity decreases with decreasing particle size. The measured dielectric constant for the biggest particle size is lower than the theoretically predicted value for a polycrystalline sample. Such behaviour can be explained by postulating a defective surface layer of thickness 50 μm with dielectric constant 15.