Citation: Himcinschi, C.; Drechsler, F.; Walch, D.S.; Bhatnagar, A.; Belik, A.A.; Kortus, J. Unexpected Phonon Behaviour in BiFe x Cr 1−x O 3 ,a Material System Different from Its BiFeO 3 and BiCrO 3 Parents. Nanomaterials 2022, 12, 1607. https:// doi.org/10.3390/nano12091607 Academic Editor: Sam Lofland Received: 31 March 2022 Accepted: 5 May 2022 Published: 9 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Article Unexpected Phonon Behaviour in BiFe x Cr 1-x O 3 , a Material System Different from Its BiFeO 3 and BiCrO 3 Parents Cameliu Himcinschi 1, * , Felix Drechsler 1 , David Sebastian Walch 2,3 , Akash Bhatnagar 2,3 , Alexei A. Belik 4 and Jens Kortus 1 1 Institute of Theoretical Physics, TU Bergakademie Freiberg, D-09596 Freiberg, Germany; felix.drechsler@student.tu-freiberg.de (F.D.); jens.kortus@physik.tu-freiberg.de (J.K.) 2 Zentrum für Innovationskompetenz SiLi-nano, Martin-Luther-Universität Halle-Wittenberg, D-06120 Halle (Saale), Germany; david.knoche@physik.uni-halle.de (D.S.W.); akash.bhatnagar@physik.uni-halle.de (A.B.) 3 Institut für Physik, Martin-Luther-Universität Halle-Wittenberg, D-06120 Halle (Saale), Germany 4 International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Ibaraki, Tsukuba 305-0044, Japan; alexei.belik@nims.go.jp * Correspondence: himcinsc@physik.tu-freiberg.de Abstract: The dielectric function and the bandgap of BiFe 0.5 Cr 0.5 O 3 thin films were determined from spectroscopic ellipsometry and compared with that of the parent compounds BiFeO 3 and BiCrO 3 . The bandgap value of BiFe 0.5 Cr 0.5 O 3 is lower than that of BiFeO 3 and BiCrO 3 , due to an optical transition at ~2.27 eV attributed to a charge transfer excitation between the Cr and Fe ions. This optical transition enables new phonon modes which have been investigated using Raman spectroscopy by employing multi-wavelengths excitation. The appearance of a new Raman mode at ~670 cm −1 with a strong intensity dependence on the excitation line and its higher order scattering activation was found for both BiFe 0.5 Cr 0.5 O 3 thin films and BiFe x Cr 1−x O 3 polycrystalline bulk samples. Furthermore, Raman spectroscopy was also used to investigate temperature induced structural phase transitions in BiFe 0.3 Cr 0.7 O 3 . Keywords: Raman spectroscopy; thin films; spectroscopic ellipsometry; charge transfer; phase transition; multiferroics; BiFe x Cr 1−x O 3 1. Introduction Bi 2 FeCrO 6 was first theoretically proposed as a bismuth-based multiferroic material having large spontaneous magnetisation and polarisation by means of first principle studies by Baettig et al. [1,2]. Following this theoretical prediction, Bi 2 FeCrO 6 epitaxial films have also been synthesised and its multiferroic character demonstrated [3,4]. Consequently, a lot of attention was dedicated to the characterisation of structural, ferroelectric and multiferroic properties of the material, and to the optimisation of the growth parameters for thin films [5–8]. Promising photovoltaic properties, the low bandgap and the possibility to tune it by the growth conditions, cationic ordering, and the domain size have attracted a lot of attention in recent years [9–13]. Various heterostructures or BiCrO 3 /BiFeO 3 superlattices with different thicknesses and repetitions have been proposed for designing efficient ferroelectric photovoltaic devices [14–16]. Raman spectroscopy is an established method to detect subtle structural changes induced by strain or temperature and it has been already very successfully applied to the study of the lattice dynamics in the class of oxides with perovskite structures. However, in the case of Bi 2 FeCrO 6 there are very few Raman or infrared spectroscopic studies reported [17–19]. In the present paper Raman spectroscopy was used in combination with spectro- scopic ellipsometry for the characterisation of BiFe 0.5 Cr 0.5 O 3 thin films and polycrystalline Nanomaterials 2022, 12, 1607. https://doi.org/10.3390/nano12091607 https://www.mdpi.com/journal/nanomaterials