Crystal structure, morphotropic phase transition and luminescence in the new cyclosilicates Sr{sub 3}R{sub 2}(Si{sub 3}O{sub 9}){sub 2}, R=Y, Eu-Lu

A new series of promising luminescent materials, cyclosilicates Sr{sub 3}R{sub 2}(Si{sub 3}O{sub 9}){sub 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 [Si{sub 3}O{sub 9}] rings located in layers are basic building units and stack with Sr/R layers along the [1 0 1{sup Macron }] 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 [Si{sub 3}O{sub 9}] 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 Sr{sub 3}Y{sub 2}(Si{sub 3}O{sub 9}){sub 2}:Eu{sup 3+} under ultraviolet (UV) excitation has been discussed. - Graphical abstract: The change of the [Si{sub 3}O{sub 9}] ring conformation in Sr{sub 3}R{sub 2}(Si{sub 3}O{sub 9}){sub 2}, R=Eu, Er, Tm and Lu. Highlights: Black-Right-Pointing-Pointer A new group of cyclosilicates Sr{sub 3}R{sub 2}(Si{sub 3}O{sub 9}){sub 2}, R=Y, Eu-Lu, was first synthesized. Black-Right-Pointing-Pointer Isolated [Si{sub 3}O{sub 9}] rings located in layers are basic building units and stack with Sr/R layers along the [1 0 1{sup Macron }] direction. Black-Right-Pointing-Pointer The rare earth atoms are distributed among three independent Sr/R sites coordinated by 6, 7 and 8 oxygen ligands. Black-Right-Pointing-Pointer A step-like change of the unit cell parameters is observed in the Sr{sub 3}R{sub 2}(Si{sub 3}O{sub 9}){sub 2} structure for R=Er{yields}Tm. Black-Right-Pointing-Pointer Sr{sub 3}Y{sub 2}(Si{sub 3}O{sub 9}){sub 2}:Eu{sup 3+} demonstrates intense multiband orange-red photoluminescence.