The crystal structure of Cl-mayenite (Ca12Al14O32Cl2) is very similar to that of the well-known oxygen ion conductor O-mayenite (Ca12Al14O33), showing zeolite-type cages with partial occupancy by oxygen anions. In Cl-mayenite chlorine ions occupy the cage centers instead of oxygen ions, and it is of interest whether these chlorine ions are mobile and whether Cl-mayenite is a chlorine ion conductor. The answer for these questions is the focus of the paper. High temperature neutron powder experiments and impedance spectroscopy measurements were performed. For information on possible chloride migration pathways and activation energies, quantum-chemical calculations based on density-functional theory were carried out. The behavior of Cl− is in clear contrast to O2− in O-mayenite: even at high temperatures it only shows normal harmonic thermal displacement without indications for long range diffusion between the cages. The total ionic conductivity was found to be very low with a
value of σ ≈ 10−6 S cm−1 at 1073 K. Quantum-chemical calculations result in a very high activation barrier for Cl− migration of 3.07 eV.