Room temperature stable structures and phase transition of Na2Al2B2O7

By Rietveld refinement of the X-ray diffraction (XRD) data of powdered Na2Al2B2O7 samples aged for over 3 months, we found that Na2Al2B2O7 at room temperature is a mixture of two phases with space group P3¯1c and P63/m, respectively. The structures of the two phases can be refined with identical cell parameters of a=4.80760(11) Å, c=15.2684(5) Å and are composed by [Al2B2O7]2−∝ double layers stacking alternatively with Na+ ions along the c-direction, but differ at in-plane bond orientations of the BO3/AlO4 groups within the double layers: in P63/m phase B–O1/Al–O1 bonds of the two layers are perfectly aligned, whereas in P3¯1c phase they are twisted by 46.4/41.6° around c  -axis against each other. It is also found that a freshly prepared sample contains only the P3¯1c phase, but part of the P3¯1c phase will transfer to P63/m phase slowly at room temperature and the transition can be reversed by heating the aged sample above 220 °C.