Decastrontium hexagallate Sr10Ga6O19 exhibits very complex structural behaviour as a function of temperatur. The known room-temperature structure is incommensurately modulated. There are four phase transitions between room temperature and 923 K, and only one of the intermediate structures is known. Our interest in this system is two-fold. Firstly, the structural behaviour of Sr10Ga6O19 is an interesting and challenging crystallographic problem. Secondly, doping the material with La produced oxygen-excess La0.2Sr9.8Ga6O19.1, in which the interstitial extra oxide ions appear to be mobile at elevated temperatures (based on conductivity measurements). We propose to study structure-property relationships in these materials and, specifically, determine the role of variable Ga coordination in facilitating the uptake of excess oxygen on doping and the oxide ion mobility.