Polymorphism and photoluminescence properties of K₃ErSi₂O₇

Two polymorphs of tripotassium erbium disilicate, K₃ErSi₂O₇, were synthesized by high-temperature flux crystal growth during the exploration of the flux technique for growing new alkali rare-earth elements (REE) containing silicates. Their crystal structures were determined by single-crystal X-ray diffraction analysis. One of them (denoted 1) crystallizes in the space group P6₃/mmc and is isostructural with disilicates K₃LuSi₂O₇, K₃ScSi₂O₇ and K₃YSi₂O₇, while the other (denoted 2) crystallizes in the space group P6₃/mcm and is isostructural with disilicates K₃NdSi₂O₇, K₃REESi₂O₇ (REE = Gd-Yb), K₃YSi₂O₇, K₃(Y_0.9Dy_0.1)Si₂O₇ and K₃SmSi₂O₇. In the crystal structure of polymorph 1, the Er cations are in an almost perfect octahedral coordination, while in the crystal structure of polymorph 2, part of the Er cations are in a slightly distorted octahedral coordination and the other part are in an ideal trigonal prismatic coordination environment. Sharing six corners, disilicate Si₂O₇ groups in the crystal structure of polymorph 1 link six ErO₆ octahedra, forming a three-dimensional network and nine-coordinated potassium cations are located in its holes. In the crystal structure of polymorph 2, the disilicate Si₂O₇ groups connect four ErO₆ octahedra, as well as one ErO₆ trigonal prism. Three differently coordinated potassium cations are situated between them. Different site symmetries of the erbium cations in the crystal structures of polymorphs 1 and 2 affect their photoluminescence properties. Only polymorph 2 exhibits luminescence. Intense narrow lines in the emission spectrum are a result of the 4f-4f transition. The green emission line at 560 nm is the result of the Er³⁺ transition ⁴S_3/2→⁴I_15/2, and the luminescence line at 690 nm is the result of a ⁴F_9/2→⁴I_15/2 transition. The crystal morphologies of the two polymorphs are similar. Crystals of polymorph 1 are in the form of a hexagonal prism in combination with a hexagonal base, while crystals of polymorph 2 contain a dihexagonal prism in combination with a hexagonal base, although poorly developed faces of the dihexagonal pyramid can also be noticed.