Observation of a phase transition in KTaO$_3$ induced by residual niobium impurities

TL;DR

This study reports a residual niobium impurity-induced ferroelectric phase transition in KTaO$_3$, observed via microwave cavity resonances near 134 K, with implications for tunable device applications.

Contribution

It demonstrates that residual niobium impurities can induce a ferroelectric phase transition in KTaO$_3$, revealing impurity-driven ferroelectricity in a typically paraelectric material.

Findings

Phase transition occurs near 134 K in Nb-doped KTaO$_3$

Resonant mode frequencies decrease then increase across transition

Lower Nb concentration crystals do not show transition but have a loss peak

Abstract

We report the observation of a phase transition in a KTaO$_3$ crystal, corresponding to a paraelectric-to-ferroelectric transition. The crystal was placed inside a copper cavity to form a dielectric-loaded microwave cavity, and the transition was observed to occur near 134 K. As the cavity was cooled, the frequencies of both transverse electric and transverse magnetic resonant modes decreased (corresponding to an increase in permittivity). The mode frequencies converge at the transition temperature (near 134 K) and, below this point, reverse their tuning direction, increasing their frequency with decreasing temperature. This behaviour corresponds to a decrease in dielectric permittivity and is atypical for pure KTaO$_3$. To investigate further, we conducted impurity analysis using Laser Ablation inductively coupled mass spectrometry (LA-ICPMS), revealing a significant concentration ($\sim$ 7\%) of niobium (Nb) in the crystal. This suggests that the observed phase transition is driven by residual Nb impurities, which induce ferroelectricity in an otherwise paraelectric host. Similar crystals with a lower concentration ($<$ 2\%) did not undergo a phase transition but exhibited a loss peak at this temperature. These findings have practical implications for the design of tunable devices, for example, resonator-based dark matter detectors, where low-loss material phase stability and tunability are crucial.