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 NaBa 2 Al(P 2 O 7 ) 2 and NaBaAl(PO 4 ) 2 were successfully grown by using high-temperature molten method. The crystals NaBa 2 Al(P 2 O 7 ) 2 and NaBaAl(PO 4 ) 2 are located in space groups P ¯1 and P 2 1 /c, respectively. NaBa 2 Al(P 2 O 7 ) 2 displays a [Al(P 2 O 7 ) 2 ] 1D chain structure constructed from corner-sharing linkage of novel AlP 4 O 16 9− anion groups, and the AlP 4 O 16 9− group is formed by edge-sharing connection of two P 2 O 7 dimers and one AlO 6 octahedron . However, NaBaAl(PO 4 ) 2 exhibits a new [Al(PO 4 ) 2 ] 2D layer structure composed of AlP 2 O 10 7− anion groups via corner-sharing, and the AlP 2 O 10 7− group is generated by alternate linkages of corner-sharing PO 4 and AlO 4 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 Ba 2+ . 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.