Computationally guided epitaxial synthesis of (Sr/Ba)MnO3 films on (Sr/Ba)TiO3 substrates

Epitaxial Stabilization and Persistent Nucleation of the 3C Polymorph of Ba0.6Sr0.4MnO3

“…To grow thick films of a metastable material, a persistent nucleation preference is required throughout the growth of the film. In the ideal epitaxial growth experiment, for systems without solid-phase nucleation back-transformation pathways, every controlled nucleation event would be identical and reproduce the preference for the metastable phase, − allowing for films to be grown to arbitrary thicknesses. However, unless the energetic preference is absolute (that nucleation of the competing phase is not possible), there is some finite probability that the stable phase will nucleate.…”

“…Thus, nucleation should occur in a stochastic fashion such that, after some number of nucleation events, or the thickness of the film, stable phase nucleation should occur . This particular thermodynamic limit is affected by the effective supersaturation, which is related to the number of atoms crystallizing (related to f and E ), temperature, and oxygen pressure (which affect bulk stability). ,− Once a region of the more stable phase nucleates, e.g., 4H in BSMO, the probability of its nucleation in that region immediately becomes preferred (possibly absolute), due to both its inherent bulk stability and its local epitaxial stability upon the preexisting stable phase deposit. − , From that thickness onward, the local probabilities of nucleation vary with respect to location depending on the structure of the pre-existing film in the local area, and ultimately the film will convert completely to the stable (4H) phase with increased thickness, with some thickness range of mixed phase. That 40 nm films can be fabricated using iPLD indicates that any single-phase thickness value in rPLD is likely related to a thickness-dependent change in local nucleation probabilities, in a manner that decreases the preference for the epitaxially stabilized metastable phase. − …”

“…In general, epitaxial stabilization of a bulk metastable phase occurs because there is a change in thermodynamic stability during nucleation, primarily arising from the low-energy (high-energy) interface between the metastable (stable) phase and a substrate or pre-existing deposit. − In addition to the area-specific interface energy, there is the thickness-dependent volumetric (bulk and epitaxial strain) energy. Xu et al and Zhou et al have shown through a first-principle investigation that, respectively, for TiO 2 and (Ba/Sr)­MnO 3 polymorphs, epitaxial stability on differently oriented (Ba/Sr)­TiO 3 substrates can accurately reproduce experimental trends by including an interface energy term along with the bulk formation and strain energies. The stability of the metastable phase is therefore thickness dependent, and there is a crossover point at which the bulk stable phase becomes the lower energy phase for the entire film.…”

“…Epitaxial stabilization is one method that offers an additional structure-directing thermodynamic parameter (the interface with the substrate). − BSMO phases have been epitaxially stabilized in the 3C polytype using regular pulsed laser deposition (rPLD), in synthesis conditions that would otherwise favor hexagonal polymorphs, but only for x ≤ 0.5 . Following the work of Sakai et al, Langenberg et al studied the effect of strain, temperature, and oxygen partial pressure on the epitaxial stability of the 3C phase with the increasing Ba content, describing a feasible route to synthesize metastable phases with novel functional properties .…”

“…Above this maximum thickness, the 4H phase begins to nucleate and film peaks are more difficult to detect via X-ray diffraction (XRD) due to the lower structure factor of the 4H orientation that is stabilized on (100) substrates. Zhou et al computed that 3C BaMnO 3 may be epitaxially stabilized over 4H below 2 stoichiometric layers, indicating that further investigations on the epitaxial stabilization of BSMO with x > 0.5 may be fruitful.…”