Structure instability of A2Al2B2O7 (A=K, Na) crystals

A theoretical study on the stabilities of the crystal structures of K2Al2B2O7 (KABO) and Na2Al2B2O7 (NABO) has been carried out using density functional theory with generalized-gradient corrections. All structures have been optimized by minimizing the total energies with respect to lattice constants and to the atomic coordinates within the unit cell. In the case of KABO, the structure with space group P321 always takes energy advantage over the whole volume range studied, whereas on both volume expansion and contraction, the original NABO structure becomes unstable, taking a structure transition from its P3¯1c space group to the P321 space group of KABO structure. The theoretical predictions were well realized in the experimental results on the structure changes of the (K1−xNax)2Al2B2O7 solid solution system.