Time-resolved nanoscale domain imaging in (K,Na)NbO₃

We investigate the temperature-driven structural phase transition from the low-temperature monoclinic Mc-phase to the high-temperature orthorhombic c-phase in (K,Na)NbO₃ via time-resolved scanning X-Ray Diffraction (XRD) imaging. In the Mc-phase, our sample displays a hierarchical order of domains and superdomains on length scales of 50 nm and few µm, respectively. Absorption of nanosecond laser pulses triggers the structural transition from the Mc- to c-phase and subsequent (re)condensation. We monitor this process by time-resolved XRD and derive the time-dependent volume fractions of both phases. We show that, to transform the same volume ratio to the high-temperature phase, nanosecond heating requires the generation of almost 5 times higher temperatures compared slowly to increasing the temperature with a furnace. This agrees with similar experiments on NiMnGa Heussler alloys, where we have monitored the overheating as well as the energy-dependent speed of the martensitic phase transition [1]. While time-resolved XRD can provide the transient ratio of the Mc- and c-phase, the technique is unable to monitor the domain morphology. Such information is extremely valuable, since the mechanism behind the hierarchical order of domains and superdomains is still not understood. To investigate this phenomenon, we employ nanofocus scanning XRD imaging in combination with time-resolved XRD measurements. The experiments were performed at the ID01 beamline of the European Synchrotron ESRF. In our contribution, we report on two distinct experiments. First, we monitor the domain morphology before and after exposing our sample to nanosecond laser pulses of different fluence. We find that the sample favors (re)condensation in its initial domain morphology and that a disproportionally higher thermal energy is required to change the morphology in the same way as the change in volume fraction would suggest. Second, we measure the transient domain morphology with nanosecond temporal resolution.

[1] S. Schwabe et al., STAM, https://doi.org/10.1080/14686996.2022.2128870