New isoformula borates with similar structures and different properties – Acentric nonlinear optical KGd[B6O10(OH)2] and centrosymmetric KHo[B6O10(OH)2]

•New borates KGdB6O10(OH)2 and KHoB6O10(OH)2 are synthesized hydrothermally at different pH.•Two similar structures with equal new layers of boron tetrahedra and triangles are determined.•Optical nonlinearity is in acentric P-62m Gd-borate in contrast to centrosymmetric P-31m Ho.•Ordered polar P31 model of K, Gd-structure is suggested in unit cell tripled along c-axis.

Single crystals of two new borates, KGd[B6O10(OH)2] and KHo[B6O10(OH)2], have been synthesized in multi-components hydrothermal solutions at controlled pH. Similar in unit cell, the structures differ in relation to the symmetry: K,Gd-structure is solved in acentric space group P-62m, while K,Ho-structure is centrosymmetric, P-31m. Polyborate anionic layers of new type from tetrahedra and triangles are identical; however multiplication of the layers by symmetry is differently produced either by mirror plane or the inversion center. The layers of oxygen–boron tetrahedra and triangles in K,Gd- and K,Ho-borates are connected by GdO6 prisms and HoO6 octahedra, correspondingly. Second-order nonlinearity is clearly manifested by intensive SHG effect in K,Gd-borate, and absent in K,Ho-borate. K- and B-triangles positions are statistically occupied in both structures. It is noticed that an ordered model of K,Gd-structure is possible with the unit cell tripled along c-axis, symmetry P31 and polar orientation of the B-triangles. In any model, absence of center of symmetry and following different symmetrical ordering in crystal structure both distinguish K,Gd-borate from its similar Ho-counterpart, and also explain their different properties.

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