Synthesis and crystal structure of commensurate polymorph of Ln ₄AlCu₂B₉O₂₃ (Ln=Lu,Ho) and refinement of Cu₂Al₆B₄O₁₇

Single crystals of new Cu,Lu(Ho)-alumoborate and known Cu,Al-borate were synthesized through reaction between CuB₂O₄ and LnBO ₃ on the Al₂O₃ surface by annealing at 1100°C. Structure of commensurate modification of Ln₄AlCu ₂B₉O₂₃, (Ln = Lu,Ho), sp. gr. P42₁m, was solved at room temperature. It was found that a low-temperature (110 K) modification possesses incommensurate modulations with modulation vector q =(0, 0, 0.132). The nonaborate block - [B₉O₂₃]^19- - 9[6T + 3Δ] forms an isolated unique dense closed anionic unit. This block is terminated by Al-tetrahedrons in the chessboard pattern, resulting in formation of complex alumoborate layer [AlB₉O₂₃] ^16-. Apical oxygen of central BO₃ triangle of the nonaborate block seems to be the source of modulations observed in low temperature polymorph. Cationic layers with the Ln and Cu atoms are alternating along c axis with anionic layers. The structure Cu₂Al ₆B₄O₁₇, previously studied by the Rietveld method, was corroborated by single crystal data and was compared with LiAl ₇B₄O₁₇.