Phase stability of ferroelectric KNO3 switching devices during polarization aging

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Abstract
The persistent observation of polarization aging during repeated room‐temperature switching of metastable thin‐film ferroelectric potassium nitrate capacitors has led to the conjecture that the observed polarization decay is a direct consequence of a concomitant phase III to phase II structural transformation. Appropriate large area thin‐film devices have been fabricated to perform this x‐ray diffraction study of the phase stability of ferroelectric KNO3 devices during in situ electrical switching. As the initial remanent polarization of 7.5 μC/cm2 decreases to 2 in 6 polarization reversals, the volume fraction of III‐KNO3 remains constant, and no reflections from II‐KNO3 appear. After complete polarization loss, however, the initial d spacings measured for III‐KNO3 have increased an average 0.4%. Examination of the line shapes during aging reveals that the observed shift is not a continuous transition, but a convolution of reflections centered at the initial and final d spacings. This characteristic modification to the original cell, here designated III’‐KNO3, occurs throughout the switched volume. The simultaneous loss of polarization and appearance of the expanded unit cell suggest that III’‐KNO3 either pins the polarization or is itself nonferroelectric.