Synthesis and structural characterization of Ca12Ga14O33

Abstract

Ca12Ga14O33 was successfully synthesized using a wet chemistry technique to promote the homogenous mixing of the Ca and Ga cations. Rietveld refinements on X-ray and neutron powder diffraction data confirm that the compound is isostructural to Ca12Al14O33, however, with a significantly larger lattice parameter allowing for the cages that result from the framework arrangement to expand. In naturally occurring Ca12Al14O33, the mineral mayenite, these cages are occupied by O2− anions, however, experimental studies exchanging the O2− anions with other anions has led to a host of applications, depending on the caged anion. The functional nature of the structure, where framework distortions coupled with cage occupants, are correlated to electronic band structure and modifications to the framework could lead to interesting physical properties. The phase evolution was tracked using thermogravimetric analysis and high temperature X-ray diffraction and showed a lower formation temperature for the Ca12Ga14O33 analogue compared to Ca12Al14O33 synthesized using the same wet chemistry technique. Analyzing both X-ray and neutron powder diffraction using the Rietveld method with two different starting models results in one structural model, with one Ca position and the caged O on a 24d special position, being preferred.

Phase Evolution, Electrical Properties, and Conduction Mechanism of Ca12Al14–xGaxO33 (0 ≤ x ≤ 14) Ceramics Synthesized by a Glass Crystallization Method

“…Therefore, replacing Al atoms with Ga ions with less rigidity and meanwhile with a larger ionic radius, which would expand the width of the windows, is expected to result in higher conductivity. Unfortunately, as observed in our experiments, the conductivities of these high-level Ga-doped samples did not show apparent enhancement, although the width of the windows between cages increased with the gallium content, as displayed in Figure S14, and similar expansion was also observed for the cages in Ca 12 Ga 14 O 33 with the mayenite structure when compared to that of the Ca 12 Al 14 O 33 material . It may indicate that the rigidity of the framework cations and the width of the windows between cages are not key factors for improving oxide-ion conductivity in the mayenite Ca 12 Al 14 O 33 -based materials.…”

“…McCoy et al recently reported that the composition of Ca 12 Ga 14 O 33 with the mayenite-type structure can be obtained through a polymer assisted sol–gel synthesis method . The amorphous precursor would crystallize into the pure mayenite phase only within a narrow temperature range of 750–975 °C.…”

“…McCoy et al recently reported that the composition of Ca 12 Ga 14 O 33 with the mayenite-type structure can be obtained through a polymer assisted sol−gel synthesis method. 47 The amorphous precursor would crystallize into the pure mayenite phase only within a narrow temperature range of 750−975 °C. However, here, in our work, for Ga-dominated compositions (x > 7 of Ca 12 Al 14−x Ga x O 33 ), the mayenite phase had been yielded only for the x = 8 composition when annealed at 800 °C (resulted in a mixed mayenite phase and glassy substances) or 850 °C (lead to minor mayenite phase and dominated by a Ca 5 Ga 6 O 14 -based phase).…”

“…Unfortunately, as observed in our experiments, the conductivities of these high-level Ga-doped samples did not show apparent enhancement, although the width of the windows between cages increased with the gallium content, as displayed in Figure S14, and similar expansion was also observed for the cages in Ca 12 Ga 14 O 33 with the mayenite structure when compared to that of the Ca 12 Al 14 O 33 material. 47 It may indicate that the rigidity of the framework cations and the width of the windows between cages are not key factors for improving oxide-ion conductivity in the mayenite Ca 12 Al 14 O 33based materials. Therefore, optimizing the conductivity of mayenite Ca 12 Al 14 O 33 material by changing its free oxygen content through aliovalent substitution on cation sites may be a better way in future work.…”