Ferroelectric-ferroelastic properties of the Na5W3O9F5family

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Abstract
X-ray diffraction, optical, calorimetric, dielectric, pyroelectric and nonlinear optical studies have shown that A5M3O9F5 oxyfluorides with an atomic arrangement related to that of chiolite, Na5Al3F14, form a new ferroelectric-ferroelastic family. The first crystals grown were obtained from the melt, with composition Na5W3O9F5 and Curie temperature TC = 800K. The crystals are heavily microtwinned at room temperature. Detwinning was accomplished ferroelastically by the application of compressive stress at high temperatures. The best present fit to the diffraction data is obtained with a monoclinic unit cell, space group Pn, with a = 7.3620 ± 0.0006, b = 10.6345 ± 0.0022, c = 7.3620 ± 0.0008Å and β = 90.82 ± 0.02°. The pyroelectric coefficient P2 increases sharply after detwinning, suggesting that the ferroelasticity is coupled to the ferroelectric behavior. The ferroelectric-ferroelastic Curie temperature is a function of ionic substitution in Na5W3O9F5, and may be reduced to 88K in the case of the composition Na4 Lu(WNb2)O9F5.

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