Single Crystal High Entropy Perovskite Oxide Epitaxial Films

The first 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 ABO3 perovskite, Ba(Zr0.2Sn0.2Ti0.2Hf0.2Nb0.2)O3, epitaxially on SrTiO3 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.


INTRODUCTION
The ABO 3 perovskite oxide structure and its derivatives are of broad interest to the study and application of magnetism 1 , energy conversion and storage 2,3 , superconductivity 4 , topology 5 , ferroics [6][7][8] , and a host of other phenomena. The perovskite's ability to produce such a varied range of functionalities lies in its structural and chemical flexibility, which can enable mixing cation combinations of vastly different character on the two different cation sublattices.
Consequently, substitutional electron or hole doping on the A and B sites allows for a wide variety of charge and distortion states to be tuned through synthesis, with the cation size variance balanced by internal changes to Jahn-Teller distortions and octahedral tilts and rotations. This substitutional approach is a central pillar of materials design strategies-with the search for new functionally relevant materials often beginning with a parent ABO 3 ternary compound which is then partially substitutionally doped to an [A x A' 1-x ]BO 3 or A[B x B' 1-x ]O 3 quaternary compound of superior character or novel physical behavior 9,10 . Substitutional doping to quinary or higher states can provide further functional tunability or unexpected physics in strongly correlated systems, such as colossal magnetoresistance and emergent phase coexistence in (La x Pr y Ca 1-xy )MnO 3 11,12 .
Intentional modification of long range structure through global isovalent substitution, transient pressure, heterostructuring, or strain effects [13][14][15][16] are widely used to induce changes to conduction pathways, spin states, and orbital degeneracies. Since many perovskite oxides have strongly correlated electrons, short and long range distortions can have different and dominating influences on behavior owing to the inherent spin-charge-lattice-orbital order parameter coupling length scales. The influence of local configurational disorder on mesoscopic properties is not well studied, but the existing examples are promising. As in the example of the quinary manganite above, modifying the type and magnitude of disorder can be a powerful tuning parameter in designing transition temperatures and magnetic phase compositions 17 . Further, manipulation of local structural disorder is a known route to restoring access to hidden quantum critical point phase spaces for the fundamental study of emergent behaviors 18,19 . The ability to create single crystal perovskites with very high levels of configurational disorder would open many new possibilities for materials design beyond simple electronic doping. 3 The lack of experimental studies on quinary or higher rank oxide perovskites is in large part due to the difficulty of stabilizing homogeneously doped single crystals. Typically, increasing the number of elements results in a higher probability of the formation of multiple phases or complicated microstructures. Approaches to predicting crystal stabilities are developing but are typically built upon calculations made at 0 K, which can be a severe limitation if entropy were to play a role in the stabilization process [20][21][22] . It was recently shown that entropy stabilized quinternary oxides, or high entropy oxides (HEOs), possessing a single cation sublattice could be synthesized 23 . The random distribution of constituent elements into the cation sublattice enhances the configurational entropy in such oxide solutions-analogous to the more wellknown metallic high entropy alloys (HEAs) 24 34 . Here, the rocksalt structure was stabilized in epitaxial thin film form and, driven by the inherent local disorder of the HEO, shown to induce an order of magnitude increase in exchange coupling response at a ferromagnetic nickel-iron alloy interface. If such large disordermediated responses can be utilized in this relatively simple structure, the perovskite structure may offer even greater novelty of response due to its often extreme sensitivity to disorder.
In this work, we demonstrate the first example of a single crystal high entropy perovskite oxide STO has in-plane lattice constants a = b = 3.905 Å which are significantly smaller than the expected cubic bulk lattice parameter found in the ceramic HEPO of a = b = 4.115Å 35 . This lattice mismatch of ~ 6% means that the films are unlikely to be coherently strained. Figure 1(b) shows that the all three film thicknesses align near the same value with the 7 nm film displaying some slight asymmetry in peak shape. To map films' coherency relationship to the underlying substrate, X-ray reciprocal space mapping (RSM) measurements were performed around the asymmetric (204) Bragg's reflection of the film and substrate (Fig. 2(a)-(c)). As expected, all films are relaxed from the substrate, as shown in the lack of vertical alignment of (204) film with respect to the substrate peak. Calculating lattice parameters from the x-ray data, we find the in- we see some strain passing to at least 7 nm (see also the magnified image in Fig. 3(b)). The STEM observations are also consistent with the XRD findings that the film is uniform and epitaxial. To confirm the local chemical homogeneity and distribution of B-site cations in the film, chemical analysis using atomically resolved EELS (STEM) was performed. Figure 3 The thermal transport properties were studied using time-domain thermoreflectance (TDTR).
Utilizing a two-tint pump-probe setup 43 Fig. 5 The fitting is based on a three component model 46  an ultralow, or glass-like, thermal conductivity. This is an interesting observation since the material still possesses a configurationally ordered A-site sublattice. This suggests that the theoretical predictions that highly configurationally disordered single crystals could provide an avenue to circumvent the amorphous limit may be valid 50 . By introducing configurational disorder on the A-site sublattice, thermal conductivity might be decreased to values well below the amorphous limit by further driving the necessary combined changes to local strain field and sublattice site-to-site mass differences which drive phonon scattering and limit heat flow.

CONCLUSIONS
Laser molecular beam epitaxy is shown to be an effective route to obtaining high quality single behavior. Time-domain thermoreflectance measurements show that this material has a thermal conductivity which is an order lower than the configurationally ordered BaTiO 3 parent material and approaches BaTiO 3 's amorphous limit in a single crystal form even though it possesses a fully configurationally ordered A-site sublattice.
Since ABO 3 -type perovskites show such a wide range of physical properties, further studies of HEPOs are likely to lead to new functionalities due to their distinct highly-tunable chemistries.
The ability to use entropy stabilization to introduce extreme configurational disorder opens new possibilities for designing materials from a much broader combinatorial cation pallet and should be of particular interest to fundamental studies in strongly correlated quantum materials where local disorder can play a critical role in determining macroscopic properties. As a final comment, the tunability of cation sizes should also allow very fine tuning of lattice parameters which may lead to the development of a new means of creating tailored substrates for epitaxial film growth.