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Stabilization of the Trigonal Langasite Structure in Ca3Ga2–2xZnxGe4+xO14 (0 ≤ x ≤ 1) with Partial Ordering of Three Isoelectronic Cations Characterized by a Multitechnique Approach
journal contribution
posted on 2022-06-03, 21:29 authored by Haytem Bazzaoui, Cécile Genevois, Dominique Massiot, Vincent Sarou-Kanian, Emmanuel Veron, Sébastien Chenu, Přemysl Beran, Michael J. Pitcher, Mathieu AllixCrystallization
of oxide glasses rich in Zn2+, Ga3+, and Ge4+ is of interest for the synthesis of
new transparent ceramics. In this context, we report the identification
and detailed structural characterization of a new solid solution Ca3Ga2–2xZnxGe4+xO14 (0 ≤ x ≤ 1). These compounds adopt the trigonal langasite
structure type, offering three possible crystallographic sites for
the coordination of isoelectronic Zn2+, Ga3+, and Ge4+. We used neutron diffraction to determine distributions
of Ga3+/Ge4+ and Zn2+/Ge4+ in the simpler end members Ca3Ga2Ge4O14 and Ca3ZnGe5O14,
while for the complex intermediate member Ca3GaZn0.5Ge4.5O14, we used an original approach combining
quantitative 2D analysis of atomic-resolution STEM-EDS maps with neutron
diffraction. This revealed that, across the solid solution, the tetrahedral
D sites remain fully occupied by Ge4+, while Zn2+, Ga3+, and the remaining Ge4+ are shared between
octahedral B- and tetrahedral C sites in proportions that depend upon
their relative ionic radii. The adoption of the trigonal langasite
structure by glass-crystallized Ca3ZnGe5O14, a compound that was previously observed only in a distorted
monoclinic langasite polymorph, is attributed to substantial disorder
between Zn2+ and Ge4+ over the B and C sites.
The quantitative 2D refinement of atomic-resolution STEM-EDS maps
is applicable to a wide range of materials where multiple cations
with poor scattering contrast are distributed over different crystallographic
sites in a crystal structure.
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relative ionic radiiquantitative 2d refinementpoor scattering contrastoxide glasses richnew transparent ceramicsmultitechnique approach crystallizationdetailed structural characterization2 – 2different crystallographic sitestrigonal langasite structure5 sub3 sub2 sub2 + sup14 sub≤ 1 ).4 sub4 + suptetrahedral c sitesused neutron diffractionc sitesneutron diffraction≤ 1x sub ><crystal structure> sub4 +<wide rangesubstantial disordersolid solutionresolution stempreviously observedpartial orderingmultiple cationseds mapsdetermine distributionsdepend uponcompounds adopt