Crystal Growth, Crystal Structure, Optical Properties, and Phase Transition of BaCaBO3F

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• 作者：Manfred Burianek ; Michael Teck ; Carolin Niekamp ; Johannes Birkenstock ; Iris Spieß ; Olaf Medenbach ; Lennart A. Fischer ; Paul E. Wolff ; Johannes Neumann ; Reinhard X. Fischer
• 刊名：Crystal Growth & Design
• 出版年：2016
• 出版时间：August 3, 2016
• 年：2016
• 卷：16
• 期：8
• 页码：4411-4420
• 全文大小：637K
• 年卷期：0
• ISSN：1528-7505

文摘

A crystal of BaCaBO₃F was grown by the Czochralski method. It shows a phase transition at 242.7 °C upon cooling where the hexagonal high-temperature modification is transformed to a monoclinic low-temperature form (space group C2, a = 17.8779(8) Å, b = 9.0596(4) Å, c = 12.9735(5) Å, β = 118.875(13)°, V = 1840.03(22) ų, Z = 18). The monoclinic phase has a symmetry lowered by an index of 18 relative to the hexagonal supergroup. This represents a higher symmetry than the rhombohedral model which has a symmetry of index 36 lower than the hexagonal form determined in a previous study (Li and Zeng, J. Cryst. Growth 2013, 382, 47). The monoclinic system is confirmed by optical investigations with a polarizing microscope showing biaxial negative character and refractive indices n_x = 1.615(1), n_y = 1.664(1), and n_z = 1.665(1), 2V_x = 10.0(2)° as measured on a microrefractometer spindle stage in immersion liquids. Electron microprobe analysis yielded a slight excess of Ba resulting in the composition Ba_1.04Ca_0.96BO₃F corresponding to ^[7]Ba_4.7^[8]Ba₆^[9]Ba₄^[10]Ba₄^[7]Ca_17.3^[3]B₁₈O₅₄F₁₈ in a crystal-chemical notation for the unit-cell content with the cations in their respective coordination. On the basis of this composition, the mean refractive index <n> can be calculated from individual electronic polarizabilities of the ions resulting in <n_calc> = 1.638 which is close to the observed value of <n_obs> within an error of 0.5%.