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(Y0.9Dy0.1)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 4S3/2→4I15/2, and the luminescence line at 690 nm is the result of a 4F9/2→4I15/2 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.
Keywords: alkali rare-earth silicates; crystal structure; lanthanide silicates; photoluminescence; polymorphism.