Valence Changes and Structural Distortions in “Charge Ordered” Manganites Quantified by Atomic-Scale Scanning Transmission Electron Microscopy

We investigate the microscopic nature of the “charge ordering” modulation in mixed-valent manganites in real space using scanning transmission electron microscopy. The modulation in ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.4}{\mathrm{Ca}}_{0.1}{\mathrm{MnO}}_3$ has a uniform periodicity appearing as stripes in high angle annular dark field images. Geometric phase analysis shows the modulation to be a displacement wave with transverse amplitude $(0.008\pm 0.001)a$ and longitudinal amplitude $(0.003\pm 0.001)a$. Series of energy loss spectra taken across the stripes show no periodic changes and place an upper bound of $\pm 0.04$ on any valence changes of the Mn ions.

Figure 1

(a) Diffraction pattern from ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.4}{\mathrm{Ca}}_{0.1}{\mathrm{MnO}}_3$. (b) Annular dark field image tilted a small angle away from the [010] zone axis and enhanced by low-pass filtering.

Figure 2

(a) High angle annular dark field image of ${\mathrm{Bi}}_{0.5}{\mathrm{Sr}}_{0.4}{\mathrm{Ca}}_{0.1}{\mathrm{MnO}}_3$ (enlarged in upper inset). Lower inset: power spectrum of the image. White circles indicate masks used for the geometric phase analysis. The analysis was also performed with larger masks which included the $m=2$ reflections, but this made no difference other than to increase the noise. (b) Geometric phase analysis: the color scale indicates the displacement as a fraction of the lattice parameter $a$. (c) Displacement derivative matrix. (d) Profiles in the $a$ direction of $u_c$ and $\partial u_c/\partial a$ averaged along the $c$ direction. The distance $r$ is given in terms of $a$.

Figure 3

(a) Series of energy loss spectra of the Mn-L edge scanning across the charge ordered stripes where the intensity is indicated by false color. (The distance along the scan $r$ is given in terms of $a$.) (c) The series average. (e) Chemical shift for each spectrum in the series. Horizontal lines are shown for pure ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ and dashed vertical lines for the periodicity of the stripes. (b),(d),(f) Same analysis for spectra taken scanning along a single stripe.