Structure evolution in "114" oxides CaBaZn₂Ga(2-x)Al(x)O₇ (x = 0, 1, 2) and layered cationic ordering in tetrahedral sites for CaBaZn₂Al₂O₇.

Abstract

Rietveld refinements were performed on CaBaZn2Ga(2-x)Al(x)O7 (x = 0, 1, 2) to investigate the site preference of cations in tetrahedral cavities. Tri-valent Ga(3+)/Al(3+) preferred the tetrahedral sites (T1-sites) within the triangular layers. Moreover, a layered type cationic ordering was observed in CaBaZn2Al2O7, where all the T1-sites were occupied by Al(3+). This represents the first example of a cationic ordering in "114" compounds. By substituting Al(3+) into CaBaZn2Ga2O7, the major change in the structure was the shrinkage of the T1O4 tetrahedrons, especially the shortening of the T1-O2 bond distance along the c-axis, which led to an anisotropic shrinkage of the unit cell. In the literature, such an anisotropic change of unit cell would induce a structure distortion, as well as a decrease in the symmetry from P6₃mc to Pbn2₁. All the title compounds crystallized in P6₃mc, which is unexpected. A comparison with the selected "114" magnetic oxides help us to confirm that the strong antiferromagnetic interactions of magnetic lattices would also be beneficial for symmetry lowering. Overall, the factors affecting cationic ordering on T1-sites and symmetry lowering are discussed for the title compounds, which may also be applicable to other "114" oxides.