Crystal structure, morphotropic phase transition and luminescence in the new cyclosilicates Sr3R2(Si3O9)2, R=Y, Eu–Lu

A new series of promising luminescent materials, cyclosilicates Sr3R2(Si3O9)2, R=Y, Eu–Lu, has been synthesized via a solid-state reaction. X-ray and neutron powder diffraction studies show that these oxides crystallize in the monoclinic crystal system (S.G. C2/c , Z =4) and have a morphotropic phase transition between Er and Tm compounds followed by a step-like change of the unit cell constants. Isolated [Si3O9] rings located in layers are basic building units and stack with Sr/R layers along the [1 0 View the MathML source1¯] direction. The rare earth atoms are distributed among three independent Sr/R sites coordinated by 8, 7 and 6 oxygen atoms, and the Sr-R populations change from mixed to 0.5/0.5 over site (1) and full occupation of sites (2) and (3) by Sr and R, respectively, at the transition. Changes of the conformation and mutual arrangement of [Si3O9] rings, as well as exchange of oxygen atoms from the first and the second coordination sphere of two Sr/R sites also feature the phase transition. Luminescence in Sr3Y2(Si3O9)2:Eu3+ under ultraviolet (UV) excitation has been discussed.