Influence of swift heavy ion irradiations on temperature dependent phononic behavior of epitaxial LaNiO3 thin film

The effects of Ag15+ (200 MeV) swift heavy ion irradiations on the structural and phononic properties of epitaxial LaNiO3 (LNO) thin film have been investigated using high resolution x-ray diffraction and Raman spectroscopy. After irradiation, the decrease in the out-of-plane lattice parameter of LNO toward its bulk value indicates the relaxation of epitaxial strain. The temperature dependency of phononic response for different ion irradiation doses was studied by performing the Raman measurements in a temperature range of 80−300 K. For pristine as well as irradiated samples of LNO, the observed phononic modes A1g and Eg shows softening with an increment in the temperature. The temperature coefficient of both modes varies with ion fluence. For the A1g mode, temperature coefficient increases from −0.087 cm−1 K−1 for pristine to −0.092 cm−1 K−1 for a maximum ion fluence of 1012 ions/cm2, while for the Eg mode, it decreases from −0.022 cm−1 K−1 for pristine to −0.015 cm−1 K−1 for 1012 ions/cm2. Raman frequency shift for both the modes shows non-linear behavior with temperature. This temperature dependent behavior was quantitatively analyzed by using a model which suggests that Raman shifts of the A1g mode emerged predominantly due to four phonon processes whereas, for the Eg mode, major contribution came from the thermal expansion effect. Ion irradiation did not change the dominating mechanism resulting in these temperature dependent Raman shifts, although the relative contribution of different processes was altered with ion fluence.