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

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 P3(1) 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. (C) 2015 Elsevier Masson SAS. All rights reserved.