K₂Ca₄[(UO₂)(Si₂O₇)₂]: A Uranyl Silicate with a One-Dimensional Chain Structure

A new uranyl silicate, K₂Ca₄[(UO₂)­(Si₂O₇)₂], with a 1D chain structure has been synthesized from a solution of mixed alkali- and alkaline-earth-metal cations under hydrothermal conditions at 550 °C and 1400 bar and characterized by single-crystal X-ray diffraction and photoluminescence spectroscopy. It crystallizes in the triclinic space group P1̅ (No. 2) with a = 6.6354(2) Å, b = 6.6791(2) Å, c = 9.6987(3) Å, α = 98.324(2)°, β = 93.624(2)°, γ = 112.310(2)°, and Z = 1. Its crystal structure consists of a 1D chain of uranyl disilicate formed of corner-sharing UO₆ tetragonal bipyramids and Si₂O₇ double groups. The adjacent chains are separated by K⁺ and Ca²⁺ cations. It is the first example of uranyl silicate with a 1D chain structure.