We have prepared large single crystals of the melilite electroltyes Ln1+xCaxGa3O7+d; (Ln = La and Nd; x = 0 and 0.5) using a floating zone mirror furnace. We have characterised the crystals using single crystal X-ray diffraction and transmission electron microscopy, and this has shown that these material exhibit displacive and occupational incommensurate modulation. The aims of this proposed study are in three aspects:¿ To determine the location and occupancy of the interstitial oxide ions. Although single crystal XRD has been applied, the interstitial oxygen position still cannot be unequivocally determined because of its low occupancy in a heavy cation matrix. ¿ To gain an insight in to the oxygen ion conduction pathway we are hoping to obtain accurate anisotropic ADPs particularly those for the oxygen sites.¿ Collect the weak incommensurate satellite reflections