BaCuSi₂ O₆:A new cyclosilicate with four-membered tetrahedral rings

Single-crystal X-ray diffractionand Raman spectroscopy were used tocharacterize the crystal structure and vibrational properties of BaCuSi₂O₆, a new silicate structure typewith isolated4-rings of tetrahedraand square-plane-coordinated Cu.This new phase, synthesized inadvertently in the search for high-temperature superconductors, possessesonly seven variablestructure parametersand thus is oneof the simplest of all known cyclosil-icates. A Raman peak at 506 cm⁻¹, which results froman in-plane breathing mode of the Si₄O₁₂ ring, provides a marker for 4-rings in other silicate crystals and glasses.