Single crystals of two new borates, KGd[B6O10(OH)2] and KHo[B6O10(OH)2] , 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.