Re-examination of the crystal structure of Na2Al2B2O7: stacking faults and twinning

The crystal structure of the layered compound Na2Al2B2O7 is re-examined. It is found that stacking faults can be introduced easily in the sequence of [Al2B2O7]∞2− lamellae. Both rotation and reflection twinning can form in the crystal by introducing appropriate stacking faults. A domain of the Sr2Be2B2O7-type structure might occur at the twin boundary. Single crystal refinement is performed successfully with the twinning model. A similar analysis and discussion is extended to the structures of Sr2Be2B2O7 and AEAl2B2O7 (AE=Alkaline earth elements Ca, Sr and Ba).