Two-dimensional surface phase diagram of a multicomponent perovskite oxide: ${\mathrm{La}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{MnO}}_3\left(110\right)$

The many surface reconstructions of (110)-oriented lanthanum strontium manganite (${\mathrm{La}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{MnO}}_3$, LSMO) were followed as a function of the oxygen chemical potential (${\mu }_{\text{O}}$) and the surface cation composition. Decreasing ${\mu }_{\text{O}}$ causes Mn to migrate across the surface, enforcing phase separation into $A$-site-rich areas and a variety of composition-related, structurally diverse $B$-site-rich reconstructions. The composition of these phase-separated structures was quantified with scanning tunneling microscopy, and these results were used to build a two-dimensional phase diagram of the LSMO(110) equilibrium surface structures.

Figure 1

Equilibrium surface structures of LSMO(110). Top: $12\times 12{\mathrm{nm}}^2$ STM images; bottom: LEED. ${\mathrm{O}}_2$ pressure ranges (millibar) where each reconstruction is stable at 700 ${}^{\circ }\mathrm{C}$ are reported at the top. (a)–(c) High-pressure (HP) structures, obtained by depositing Mn or La in PLD plus ${\mathrm{O}}_2$ annealing [8]. (d)–(f) Low-pressure (LP) structures, formed by annealing the HP structures at low pressure. The ${(1\times 1)}_{\text{HP/LP}}$ surfaces have the same cation composition. Each structure is identified at the top by its superstructure periodicity and/or by a descriptive, shorthand label used in the text. Relative compositions are given in the bottom axes [1 ML is the number of Mn atoms in an ${\left(A\mathrm{MnO}\right)}_2$ plane of LSMO(110), $4.64\times {10}^{14}{\mathrm{cm}}^{-2}$].

Figure 2

Evolution of the monophase fishbone surface upon annealing at increasingly reducing conditions. (${\mathrm{a}}_1$)–(${\mathrm{d}}_1$), (${\mathrm{a}}_2$)–(${\mathrm{d}}_2$) STM images ($100\times 70{\mathrm{nm}}^2$, and $26\times 26{\mathrm{nm}}^2$, respectively). (a) Monophase fishbone surface. (b) After annealing at $2\times {10}^{-3}$ mbar, small ${(1\times 1)}_{\text{HP}}$ patches appear (orange), and small structural changes occur in the fishbone. (c) Annealing at $2\times {10}^{-5}$ mbar produces ${(1\times 1)}_{\text{LP}}$ areas, and transforms the remaining surface into the oblique structure of Fig. 1. (d) UHV annealing enlarges the ${(1\times 1)}_{\text{LP}}$ areas, and transforms the remaining surface into the stripes of Fig. 1. The process is reversible.

Figure 3

(a) STM quantification of ${(1\times 1)}_{}$ area fractions during the phase separation of LSMO(110) surfaces, starting from different high-pressure phases. Each point averages over 10–12 images acquired on $300\times 300{\mathrm{nm}}^2$ on different sample spots. Orange: Initially ${(1\times 1)}_{\text{HP}}$. Solid black circles: Initially monophase fishbone (Fig. 2). Other symbols: Initial surfaces between fishbone and $(2\times 1)$. Dashed oval: Different ($T$, $p_{{\text{O}}_2}$) combinations, equivalent to the same ${\mu }_{\text{O}}$. The table indicates the annealing parameters for each data point. Lines are a guide for the eye. (b) XPS intensity ratios have no trend with ${\mu }_{\text{O}}$ within the error bars (the data correspond to the experiment of Fig. 2, and are representative of all experiments). (c) Proposed mechanism: Mn travels across the surface to expose ${(1\times 1)}_{}$ and form new Mn-richer structures, preserving the overall cation composition. (d) LEIS spectra of different monophase surfaces. Differences in the relative Sr-to-La signals are related to different sample histories, and do not affect the overall appearance of the corresponding reconstructions [8] (see also Secs. S2 and S4A [51]).

Figure 4

Experimental 2D surface phase diagram of LSMO(110). (a) Evolution of the composition of different phases of LSMO(110) with decreasing ${\mu }_{\text{O}}$, as derived from the data in Fig. 3. (b) Corresponding model. Reconstructions are identified by labels, indicating single and multiphase regions (selected STM images are shown in Sec. S4 [51]). H, L: ${(1\times 1)}_{\text{HP,LP}}$. F: Fishbone family. O: Oblique. S: Stripes.