Substitution-Induced Structure Evolution and Zn²⁺/Ga³⁺ Ordering in “114” Oxides MAZn₂Ga₂O₇ (M = Ca²⁺, Sr²⁺; A = Sr²⁺, Ba²⁺)

The "114" oxides LnBa(Co/Fe)4O7+δ represent a new family of materials that exhibits intriguing physical properties, including geometrically frustrated magnetism, oxygen storage, and magnetoelectric couplings. Various chemical substitutions have been conducted to modify their crystal and magnetic structures as well as physical properties. However, the principles beneath the substitution-induced structural evolution and charge/cationic ordering have not yet been understood. Thus, in this contribution, two complete solid solutions of MAZn2Ga2O7 ( M = Ca2+, Sr2+; A = Sr2+, Ba2+) were designed, synthesized, and characterized by Rietveld refinements based on high-resolution X-ray diffraction (XRD) and neutron diffraction (ND) data. The structure symmetry of MAZn2Ga2O7 is determined by the cationic size mismatch between M and A cations that can be defined by the tolerance factor t, i.e., symmetry transitions from P63 mc ( t > 0.87) to P31 c (0.87 > t > 0.75) and to Pna21 ( t < 0.75) were observed for MAZn2Ga2O7, associated with the rotation of T1O4 tetrahedra in the triangular layers. The Zn2+/Ga3+ ordering at T sites is also a consequence of the increase or decrease of the average sizes of M and A cations. A small concentration of interstitial oxygen ions can be obtained in Sr2Zn2- xGa2+ xO7+ x/2 ( x = 0.1, 0.2); however, no oxygen ionic conduction was observed at high temperatures, indicating the migration ability of the interstitial oxygen was very limited.