Abstract text (incl. figure legends and references)

Crystallisation of zinc gallogermanate glasses is of interest for the synthesis of new transparent ceramics. In this context, we were interested in the solid solution Ca₃Ga_2-2xZn_xGe_4+xO₁₄ (0≤x≤1) and its detailed structural characterization. Powder diffraction was used to determine that this new compound adopt the trigonal langasite structure type, offering three possible crystallographic sites for the coordination of isoelectronic Zn²⁺, Ga³⁺ and Ge⁴⁺. The distribution determination through the different sites of the three isoelectronic cations for the complex intermediate member Ca₃GaZn_0.5Ge_4.5O₁₄ is not possible using only Rietveld refinement from neutron diffraction. For this compound, we have developed an original approach using quantitative 2D analysis of atomic-resolution STEM-EDS maps.¹

In order to visualise the 2D structural projection of Ca₃GaZn_0.5Ge_4.5O₁₄ and to distinguish the three type of crystallographic site, the sample was oriented along the [001]* direction and imaged in STEM-HAADF mode at the atomic scale (Fig a-b). However, as Ge, Ga and Zn are of similar atomic number, it is not possible to determine the distribution of these 3 cations using only the intensities of the STEM-HAADF images. Consequently, EDS mapping at the atomic scale was carried out and show that one site is clearly visible and composed only of Ca as expected, whilst another one contains only Ge, and the last two are mixed occupied (Ga³⁺/Ge⁴⁺/Zn²⁺) (Fig c-d). In order to gain more information and determine the Ga, Ge and Zn cation ratio on the different sites, a 1D profile analysis was first performed by extracting an intensity profile through the sites from the EDS maps, using a Gaussian decomposition and assuming that the area under the curve is proportional to the element content. In a second time, to reduce certain limitations of this approach such as the application of a Wiener filter on the elementary maps, which can induce a bias, as well as the inability to integrate the signal through the entire sites due to the small distance between the neighbouring, a two-dimensional fitting approach has been tested (Fig e-f).^2-3 The results were then compared with those obtained by Rietveld refinement of NPD data whose model was built following the STEM-EDS mapping analyses. The quantitative 2D refinement of atomic-resolution STEM-EDS maps could be applicable to materials where multiple cations with poor scattering contrast are distributed over different crystallographic sites in a crystal structure.

¹ H. Bazzaoui et al., Inorg. Chem. 2022, 61, 9339-9351

² P. Lu et al., Microsc. Microanal.2014, 20, 1782-1790

³ P. Lu et al., Sci. Rep. 2014, 4, 3945

Figure a) Scheme of the Ca₃GaZn_0.5Ge_4.5O₁₄ structure projection along [001]. b) Corresponding STEM-HAADF image. c) and d) Overlaid EDS maps of "Ca (blue), Ge (red), Ga (green)" and "Ca (blue), Ge (red), Zn (green)". e) and f) 3-D representation of the Ge signal extracted from the STEM-EDS 2D-maps and the corresponding fit.