Single-crystal high entropy perovskite oxide epitaxial films

Examples of single-crystal epitaxial thin films of a high entropy perovskite oxide are synthesized. Pulsed laser deposition is used to grow the configurationally disordered $AB{\mathrm{O}}_3$ perovskite $\mathrm{Ba}\left(\mathrm{Z}{\mathrm{r}}_{0.2}\mathrm{S}{\mathrm{n}}_{0.2}\mathrm{T}{\mathrm{i}}_{0.2}\mathrm{H}{\mathrm{f}}_{0.2}\mathrm{N}{\mathrm{b}}_{0.2}\right){\mathrm{O}}_3$ epitaxially on $\mathrm{SrTi}{\mathrm{O}}_3$ and MgO substrates. X-ray diffraction and scanning transmission electron microscopy demonstrate that the films are single phase with excellent crystallinity and atomically abrupt interfaces to the underlying substrates. Atomically resolved electron-energy-loss spectroscopy mapping shows a uniform and random distribution of all $B$-site cations. The ability to stabilize perovskites with this level of configurational disorder offers new possibilities for designing materials from a much broader combinatorial cation pallet while providing a fresh avenue for fundamental studies in strongly correlated quantum materials where local disorder can play a critical role in determining macroscopic properties.

Figure 1

X-ray diffraction results of the HEPO films on STO(001): (a) $\theta$-$2\theta$ XRD scans where the HEPO peaks are clearly labeled. (b) Enlarged view of XRD patterns around the 002 peaks of HEPO and STO. (c) XRR patterns of HEPO films with different thicknesses. (d) A rocking curve about the HEPO 002 peak $(\omega =21.84)$. (e) $\varphi$ scans of the 222 reflections of both film and substrate.

Figure 2

RSM around the 204 reflections: (a) 7 nm, (b) 26 nm, and (c) 72 nm.

Figure 3

STEM imaging and spectroscopy. (a) Cross-sectional HAADF-STEM observation of a HEPO/STO heterostructure along the [100] STO direction. (b) Magnified image of the film at the interface. A smooth interface with regular HEPO layers is observed. The inset shows a highest magnification with overlaid structural model and atomic columns. (c) HAADF survey image from the HEPO film grown on STO(001) where EELS mapping was performed with EELS elemental maps of Ti-L, Hf-M, Nb-M, Sn-M, and Zr-M EELS signal from the region. The EELS mapping shows homogeneous distribution of $B$-site cations.

Figure 4

X-ray diffraction of HEPO films on MgO(001). (a) $\theta$-$2\theta$ XRD scans where the HEPO peaks are labeled. (b) Enlarged view of XRD patterns around the 002 peaks of HEPO and MgO. (c) XRR patterns of HEPO films with different thicknesses. (d) RSM of 62-nm-thick HEPO films taken around the 204 reflection.

Figure 5

Lattice vibration and thermal transport characteristics: (a) Room-temperature unpolarized micro-Raman spectrum of 62-nm-thick HEPO film grown on MgO(001). (b) Comparison of TDTR of HEPO films on STO and MgO substrates. The ratio of the in-plane to out-of-plane signals at three different modulation frequencies for each sample is plotted as a function of the delay time ($t$) between pump and probe. The symbols are experimental measurements and the lines are the corresponding theoretical fitting results for a set thermal conductivity ($k$) value as the one fitting parameter.