Novel anion conductors – conductivity, thermodynamic stability and hydration of anion-substituted mayenite-type cage compounds C₁₂A₇:X (X = O, OH, Cl, F, CN, S, N)

Abstract

Mayenite (Ca12Al14O33) is a highly interesting functional material not only in view of its unique crystal structure as a cage compound but also for its variety of possible applications. Its ability to incorporate foreign ions into the cage structure opens the possibility to create new types of solid electrolytes and even electrides. Therefore, the conductivity of various anion substituted mayenites was measured as a function of temperature. Due to controversial reports on the stability of mayenite under specific thermodynamic conditions (dry, wet, reducing, and high temperature), a comprehensive study on the stability was performed. Mayenite is clearly not stable under dry conditions (ppm H2O < 100) at temperatures above 1050 °C, and thus, the mayenite phase vanishes from the calcium aluminate phase diagram below a minimum humidity. Two decomposition reactions were observed and are described in detail. To get further insight into the mechanism of hydration of mayenite, the conductivity was measured as a function of water vapour pressure in a range of -5 ≤ lg[pH2O/bar] ≤ -1.6 at temperatures ranging from 1000 °C ≤ θ ≤ 1200 °C. The hydration isotherms are described with high accuracy by the underlying point defect model, which is confirmed in a wide range of water vapour pressure.

High‐pressure synthesis of a 12CaO·7Al2O3–12SrO·7Al2O3 solid solution

“…Conductivities for the x= 0.33, 0.80, and 1.0 samples were one to two orders of magnitude lower than those of the reported C12A7 data . In C12A7, the primary conducting species turned out to be O 2− , not OH − . Therefore, lower concentration of O 2− ions in the cages due to rather low temperatures (≤ ~1000°C) synthesis in wet atmosphere, which enhances a percentage of OH − concentration, is considered to be reasons for the lower conductivities in our samples.…”

“…Thus, one hypothesis for the sudden conductivity enhancement in the x =0 sample (S12A7) is a result of this interstitial (direct) diffusion occurring, which provides a fast ionic conduction. As described above, it is known that bulk ionic conductivity of C12A7 is sensitive to the preparation procedure as well as measurement conditions . From the viewpoint of a physicochemical analogy between C12A7 and S12A7, further optimization of S12A7 conductivity at lower temperature (<600°C) may be possible by applying a similar analogy that was efficient for improving conductivity of C12A7.…”

“…If the oxygen‐site occupancy is taken into account, this crystal structure can be understood as a host‐guest structure: the host is a positively charged [Ca 24 Al 28 O 64 ] 4+ lattice framework constructed of 12 crystallographic local cages; the guest is two oxygen ions (denoted free O 2− ions) randomly distributed in 12 local cages. Recently, new functionalities, such as anion conductivity as well as high ion‐oxide conductivity, light‐induced persistent electronic conduction, insulator‐metal transition, and a catalyst for ammonia synthesis, in C12A7 and C12A7‐derived materials have been reported.…”

Oxidative Degradation of Trichloroethylene over Fe2O3-doped Mayenite: Chlorine Poisoning Mitigation and Improved Catalytic Performance

“…The free oxygen ions can be substituted by other species (Cl − , H − , NH 2 − , etc.) [24,25] and can migrate from the bulk to the surface at temperatures higher than 400 • C [26], thus conferring to the mayenite oxidative properties exploited for many applications [27][28][29]; for instance, as Ni support for the catalytic reforming of tar [30][31][32].…”

Raman imaging as a new approach to identification of the mayenite group minerals

“…The porous nature of the mayenite enforces its economy meaning as a potential water clathrate or modifier of Portland cement 16–18 . This is due to the tendency of its relatively easy hydration as a result of replacing oxygen by hydroxyl group or water molecules 3,5,7,19,20 . However, the presence of H 2 O was described only in natural phases 5,7 .…”