Synthesis and Crystal Structure of Commensurate Polymorph of Ln 4 AlCu 2 B 9 O 23 ( Ln =Lu,Ho) and Refinement of Cu 2 Al 6 B 4 O 17

Single crystals of new Cu,Lu(Ho)–alumoborate and known Cu,Al–borate were synthesized through reaction between CuB 2 O 4 and Ln BO 3 on the Al 2 O 3 surface by annealing at 1100 °C. Structure of commensurate modification of Ln 4 AlCu 2 B 9 O 23 , ( Ln = Lu,Ho), sp. gr. $P{\bar 4}2_{1}m$ , 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 – [B 9 O 23 ] 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 [AlB 9 O 23 ] 16– . Apical oxygen of central BO 3 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 Cu 2 Al 6 B 4 O 17 , previously studied by the Rietveld method, was corroborated by single crystal data and was compared with LiAl 7 B 4 O 17 .