Insights into varying dimension structures for deep-UV optical crystals NaBa2Al(P2O7)2 and NaBaAl(PO4)2 constructed separately from unique [Al(P2O7)2] chains and [Al(PO4)2] layers
Two new metal aluminium phosphate millimetre-crystals NaBa2Al(P2O7)2 and NaBaAl(PO4)2 were successfully grown by using high-temperature molten method. The crystals NaBa2Al(P2O7)2 and NaBaAl(PO4)2 are located in space groups Pbar1 and P21/c, respectively. NaBa2Al(P2O7)2 displays a [Al(P2O7)2] 1D chain structure constructed from corner-sharing linkage of novel AlP4O169- anion groups, and the AlP4O169- group is formed by edge-sharing connection of two P2O7 dimers and one AlO6 octahedron. However, NaBaAl(PO4)2 exhibits a new [Al(PO4)2] 2D layer structure composed of AlP2O107- anion groups via corner-sharing, and the AlP2O107- group is generated by alternate linkages of corner-sharing PO4 and AlO4 tetrahedra. Analysis based on Pauling's electrostatic rule and Brown's bond valence model finds that structural feature of the two anion groups is mainly responsible for their different dimension structures in addition to structure-directed role of large Ba2+. The two crystals manifest a deep-UV optical transparency below 190 nm and good thermal stability, making them a potential optical window material. First-principles calculations reveal that the optical bandgaps of both crystals are determined by the Ba-O groups, rather than Al-O and P-O common groups.