Abstract: The quest for novel materials with enhanced properties is ongoing. High entropy oxides (HEOs) have transformed material design by providing a vast compositional space and remarkable property tunability. These are multicomponent systems that consist of five or more cations randomly distributed within a solid solution. Since their discovery in 2015, HEOs have garnered significant attention for their potential applications such as ionic conductors, magnetic materials, ferroelectrics, thermoelectrics, and various other functional materials [1-3]. A notable property observed in HEOs is low thermal conductivity [3]. This is attributed to their enhanced phonon scattering because of the presence of local ionic charge disorder [4]. As the lattice vibrations, i.e. the phonon modes play a crucial in understanding the thermal conductivity of a material, it is necessary to investigate the phonons in HEOs.
The vibrational response of materials can be measured using Fourier Transform Infrared Spectroscopy (FTIR), neutron scattering, or Raman spectroscopy for bulk materials [5]. However, there is a need to probe the phonon modes at the nanoscale resolution to better understand the role of microstructural inhomogeneities or interfaces. With advancements in monochromators and spectrometers, Scanning/Transmission Electron Microscopy combined with Electron Energy Loss Spectroscopy (EELS) has now become an ideal tool for probing the phonon dynamics at the atomic scale. Recently, energy resolution in advanced electron microscopes have improved to 4.2meV, expanding the applications of STEM-EELS to probe phonons, excitons, band gaps, and more [6].
In this study, we utilize ultra-high energy resolution STEM-EELS combined with theoretical calculations to investigate the vibrational modes of the prototypical HEO called J14: (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O, as well as six component HEO thin films (J14+Mn and J14+Cr). These films are grown on MgO substrates using Pulsed Laser Deposition (PLD). Due to the presence of aliovalent cations, local structural variations are observed in J14Mn thin film [7]. Figure 1 shows the phonon spectra of J14Cr HEO in comparison to the MgO substrate, acquired in the dark-field EELS geometry (to probe impact phonon scattering and thus study the localized vibrational response of the system at the atomic scale [8]). The phonon spectrum of J14Cr exhibits a peak around 18 meV, which is not observed in the parent oxide (MgO). Between 40 meV and 70 meV, MgO shows a peak around 48 meV, while J14Cr has a peak around 60 meV, indicating a blue shift compared to the MgO peak. We use FTIR and theoretical analysis to investigate the origin of spectral changes and assign the corresponding phonon modes. This investigation focuses on understanding the influence of composition on the phonon resonances in HEOs. Additionally, the variation in vibrational properties resulting from local structural nuances will also be explored using STEM-EELS data [9].
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