Incommensurate modulations in a hollandite phase Bax(Al, Fe)2xTi8–2xO16 intended for the storage of radioactive wastes: a (3+1) dimension structure determination

Intensive researches on specific treatments of radioactive cesium, occurring in high level nuclear wastes, are in progress in France. Nowadays, a cesium immobilization in host matrices with a high chemical durability seems to be the favourite option. In that perspective, titanium based upon hollandite is a good candidate as a host matrix because of its high cesium incorporation ability, its excellent chemical stability, and its capacity to support charge compensation during the Cs + into Ba 2+ transmutation process. In this study we report the preparation and crystal growth in a BaF 2 -B 2 O 3 flux melt of the Ba x Cs y M z Ti 8-z O 16 hollandite phase and its characterization by single crystal X-ray diffraction. An attention was focused on materials with M = Al or Al + Fe and with barium only since single crystals including cesium could not be obtained from the flux method. An incommensurate modulation of the Ba cation site occupancy, with I4/m(00γ)00 as a superspace group, was observed for all the single crystals under study.