Chemical and X-ray studies of ferroelectric domains in KNiPO₄

Abstract

Crystals of KNiPO₄ have space group Pna2₁ at room temperature (a = 8.6358(4), b = 9.2594(5) and c = 4.9096(3) in [A]). This compound has two structural phase transitions, at 485[ddot]C and 580[ddot]C. Ferroelectric 180[ddot] domains are revealed by chemical etching with a 0.1 M HC1 solution on {001} cuts. A single domain was cut from an etched crystal and a X-ray study of the absolute structure showed that the (001) facets are preferentially dissolved by the etchant. Another crystal having the two ferroelectric domain orientations in a ratio 31.3:68.7 was also cut from the same etched sample and studied by X-ray diffraction. The absolute structure parameter xabs, available in the Xtal 3.2 program was refined together with the other variables. This parameter gives the ratio of the crystal which has its absolute structure reversed with respect to the model. A final value of xabs = 33.2(1.8)% for the twinned crystal was found.

Key Words:

• Ferroelectric domains
• X-ray diffraction
• KNiPO₄
• chemical etching
• absolute structure