The nonlinear optical properties of the XYB2O6 family of compounds

Structure of KNbOB2O5 – a commensurately modulated structure

“…The possible three-dimensional supercell space-group symmetries for KNbOB 2 O 5 are given by Pbn2 1 [for = (2n/16), n an integer], Pbn2 1 with a different origin choice [for = (2n + 1)/16] and Pb11 (otherwise). These possible space groups are different to those for RbNbOB 2 O 5 (Schmid et al, 2000) and are not consistent with the results of Nicholls et al (1993Nicholls et al ( , 1997Nicholls et al ( , 2000Nicholls et al ( , 2001.…”

“…It is readily seen that the apical O atoms of the niobium oxygen octahedra are strongly displaced from the average position along a, leading to a meander arrangement of the chain along b. The boron ions are mainly displaced along the b direction, which is in stark contrast to Nicholls et al (2001), who describe an almost planar network of (B 2 O 5 ) 4À groups. This projection also shows the large channels, which are formed by the niobium oxygen octahedra connected via pyroborate groups, and accommodate the large A cations.…”

“…The intensity distribution in reciprocal space does not support such lowering of symmetry and a conventional refinement must therefore suffer from severe correlation problems as well as the neglect of some of the structural features. It is worth noting that the space group reported by Nicholls et al (2001) indeed had monoclinic symmetry. Refinement as a commensurately modulated structure will preserve the higher symmetry and even if that is slightly broken, it will still give the better model since it avoids the high-frequency noise usually present in such long-period superstructure refinements.…”

“…The synthesis of KNbOB 2 O 5 was first reported by Becker in 1991(Becker et al, 1991, including the observation from Buerger precession photographs that an eightfold superstructure was formed. A few years later that compound was reported again (Nicholls et al, 1993) and over the next five years three more publications from the same group appeared (Nicholls et al, 1997(Nicholls et al, , 2000(Nicholls et al, , 2001. While their interest was mainly on the optical properties and applications in lasers and non-linear optical devices, they reported a structure for KNbOB 2 O 5 .…”

“…One example for such a case is the family of non-centrosymmetric oxo pyroborates, AMOB 2 O 5 (A = K, Rb, Cs, Tl; M = Nb, Ta; Gasperin, 1974;Baucher & Gasperin, 1975;Baucher et al, 1976;Akella & Keszler, 1995;Becker et al, 1995;Gopalakrishnan et al, 1999;Nicholls et al, 2001). Some of these compounds attracted considerable interest owing to their potential use as non-linear optical materials.…”