Optical and Structural Properties of a Eu(II)-Doped Silico-aluminate with Channel Structure and Partial Site Occupation

Abstract

A new barium silico-aluminate phase with the stoichiometry Ba13.35(1) Al30.7Si5.3O70 has been found and characterized. The compound crystallizes in the space group P63/m (No. 176) with a = 15.1683(17), c = 8.8708(6) Å, V = 1767.5(4) Å3, Z = 1, Rw = 0.026, 32 refined parameters. A 3-dimensional matrix of Al/SiO4 tetrahedra with Ba(II) ions located in channels along the c axis builds up the structure. One of these channels is partially filled with Ba(II) ions (CN 6+3) in Wyckoff position 2a, leaving ~ 1/3 of the positions empty. The second and third type of Ba(II) ions occupy channels orientated along the c axis with CN 4+2+2 and 4+3+1, respectively. The structure shows a rare clustered arrangement of six tetrahedra filled exclusively by Al(III) and therefore is an exception to Loewenstein’s rule. The other tetrahedral positions show an Al to Si ratio of ~ 4 : 1. The Al/Si-O bond lengths in the tetrahedral Al/Si positions drawn vs. site occupation show linear behavior similar to the prediction by Vegard’s rule for solid solutions. After doping with Eu(II) the compound shows bright orange-yellow luminescence with an unusual large shift of the Eu(II) emission band.