MS13-P02 New borates with similar structures and different properties – acentric nonlinear optical KGd[B6O10(OH)2] and centrosymmetric KHo[B6O10(OH)2] Anastasiia Topnikova (Department of Crystallography, Geological Faculty, Lomonosov Moscow State University, Moscow, Russia) Elena Belokoneva (Department of Crystallography, Geological Faculty, Lomonosov Moscow State University, Moscow, Russia) Olga Dimitrova (Department of Crystallography, Geological Faculty, Lomonosov Moscow State University, Moscow, Russia) Anatoliy Volkov (Department of Crystallography, Geological Faculty, Lomonosov Moscow State University, Moscow, Russia) Sergey Stefanovich (Department of Chemical Technology and New Materials, Chemical Faculty, Lomonosov Moscow State University, Moscow, Russia)email: nastya_zorina@rambler.ru
Single crystals of two new borates, KGd[B6O10(OH)2] and KHo[B6O10(OH)2] [1], have been synthesized under hydrothermal conditions  in complicate borosilicate and borate systems at different pH-values. The syntheses were performed at the temperature 270-290 °C under pressure of 70-100 atm. Both structures are similar in unit cell but differ in symmetry. The first borate is found to be acentric (sp.gr. P-62m), the second is centrosymmetric (sp.gr. P-31m) (Fig.1). The same anionic radical of new type in both borates is characterized as polyborate layer composed of tetrahedrons (T) in a form of mica-like layer added by triangles (∆). Crystal chemical formula of new layer is [B6O10(OH)2]4-∞∞ because B2-triangles hold statistically only two positions attributable to the four tetrahedra, notation is 6:[4T + 2∆]∞∞, or three on a ring of six tetrahedra, [6T + 3∆]∞∞. Multiplication of the layers by symmetry is differently produced in the structures:  by mirror plane in K, Gd-borate or the inversion center in K, Ho-borate. Two polar layers are attached to top and bottom of GdO6 trigonal prisms and HoO6 octahedrons, correspondingly. K- and B-triangles positions are statistically occupied in both structures. Disorder along c* in acentric KGd[B6O10(OH)2] and overlapping of K- and B2-positions in its interlayer space may be eliminated in larger cell with tripled period along c-axis and resulting polar structure of P31 symmetry. In centrosymmetric KHo[B6O10(OH)2], which also has disorder along c* axes and tripling of cell, K- and B2-positions holds the inversion center symmetry. Despite of the almost complete structural identity, new acentric K, Gd-borate clearly demonstrates second-order nonlinear optical activity (SHG response up to 45 SiO2 units) in contrast to centrosymmetric K, Ho-borate. We suppose that key of properties explanation is in the acentric structure of K, Gd-borate and polar ordering in this crystal.
The reported study was funded by RFBR according to the research project
№ 18-35-00645.
 
References:

[1] Belokoneva, E. L., Topnikova, A. P., Stefanovich, S. Yu, Dobretsova, E. A., Volkov, A. S., Dimitrova, O. V. (2015). Solid State Scien., 46, 43-48.
Keywords: structures, borates, symmetry