Towards a unified description of the AMOB2O5 (A = K, Rb, Cs, Tl; M = Nb, Ta) family of compounds

Schmid, Siegbert; Withers, Raymond; Corker, D. L.; Baules, P.

doi:10.1107/s0108768100002871

Cited by 14 publications

(17 citation statements)

References 7 publications

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“…As mentioned in x3, the satellite re¯ections were strongly affected by overlapping. In the program Jana2000 the option for treating overlaps in the re®nement has already been introduced and used in four-dimensional cases (Petricek & Dusek, 2000;Schmid et al, 2000), but for ®ve and six dimensions this simple approach was not applicable. Therefore, we modi®ed the program in such a way that it automatically checks during the re®nement if in the de®ned neighbourhood of the calculated re¯ection another one exists which could also contribute to the integrated intensity.…”

Section: Refinement Of the Modulated Structurementioning

confidence: 99%

Five-dimensional structure refinement of natural melilite, (Ca_1.89Sr_0.01Na_0.08K_0.02)(Mg_0.92Al_0.08)(Si_1.98Al_0.02)O₇

Bindi

Bonazzi

Dusek

et al. 2001

Acta Crystallogr Sect B

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The structure of a crystal of natural melilite from San Venanzo, Umbria (Italy) of the general formula X 2 T1(T2)2O7, where X = Ca0.945Sr0.005Na0.04K0.01, T1 = Mg0.92Al0.08 and T2 = Si0.99Al0.01, has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P\bar 421 m:p4mg, cell parameters a = 7.860 (1), c = 5.024 (1) Å, modulation vectors q 1 = 0.2815 (3)(a* + b*), q 2 = 0.2815 (3)(−a* + b*). The data collection was performed on a KumaCCD diffractometer. The structure was refined from 7606 reflections to final R = 0.0481. A special modification of the refinement program Jana2000 was necessary to take into account overlapping of satellite reflections m × n = ±1, which could not be properly separated in the integration procedure. The final model includes modulations of the atomic positions as well as modulations of the thermal parameters. The latter are induced by strong differences in the neighbourhood of the actual modulated positions. The occupational modulation was neither significant for X nor for T1 sites and the sites were supposed to be occupied only by Ca and Mg, respectively. As a consequence of the Ca and O positional modulations six-, seven- and eightfold Ca coordination occur throughout the structure and the thermal ellipsoid changes its shape correspondingly. The positional modulation of the atoms causes variations in the interatomic distances which, however, do not affect bond-valence sums considerably, but induce flattening and rotation in T1 and T2 tetrahedra, respectively.

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Section: Refinement Of the Modulated Structurementioning

confidence: 99%

Five-dimensional structure refinement of natural melilite, (Ca_1.89Sr_0.01Na_0.08K_0.02)(Mg_0.92Al_0.08)(Si_1.98Al_0.02)O₇

Bindi

Bonazzi

Dusek

et al. 2001

Acta Crystallogr Sect B

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“…The University of Sydney, School of Chemistry, The University of Sydney, Sydney, NSW, 2006, Australia, E-mail s.schmid@chem.usyd.edu.au KNbOB2O5 and RbNbOB2O5 are both members of a family of noncentrosymmetric oxo pyroborates, AMOB2O5 (A = K, Rb, Cs, Tl; M = Nb, Ta) [1][2][3]. These have attracted considerable interest owing to their potential use as non-linear optical materials.…”

Section: Siegbert Schmidmentioning

confidence: 99%

Temperature dependence of the modulations in KNbOB₂O₅and RbNbOB₂O₅

Schmid¹

2008

Acta Cryst Sect A

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The diffraction patterns from single crystals from Stistaite are dominated by a set of strong reflections that indicate a rhombohedral distortion of a simple cubic lattice. Closer inspection reveals a set of satellites that indicate an incommensurate modulation. The structure of elemental Sb is a simple cubic pattern trigonally distorted by the formation of alternating long and short distances between layers along a cubic <111> direction to yield three long and three short bonds for each Sb atom. The unit cell of elemental Sb is doubled along the trigonal c-axis because of the alternation of interplanar distances. Formally, this may be interpreted as a q-vector of (0 0 1.5) in the rhombohedral unit cell (hexagonal setting). For stistaite, the range of the q-vector is 1.38-1.27 for the composition range 35-55% Sn. The alternating layers of elemental Sb can be interpreted as a sawtooth like modulation, and for stistaites with a low Sn content, this is largely retained, although the discontinuous portion of the atomic m o d u l a t i o n f u n c t i o n i s smoothened into a sinusoidal region.

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“…Much of the discussion on symmetry for KNbOB 2 O 5 is analogous to the equivalent discusion for RbNbOB 2 O 5 (Schmid et al, 2000). Consequently, the following contains Reconstructed image of the [1kl] zone axis of KNbOB 2 O 5 .…”

Section: Symmetry Considerationsmentioning

confidence: 99%

“…Recently, a modulated structure approach was used to rerefine the structure of RbNbOB 2 O 5 (Schmid et al, 2000). Starting values for the displacive modulation wave amplitudes used in the refinement were derived from the previously reported superstructure refinement.…”

Section: Introductionmentioning

confidence: 99%

Structure of KNbOB₂O₅ – a commensurately modulated structure

Schmid¹,

Wagner²

2005

Acta Crystallogr Sect B

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Members of the AMOB2O5 (A = K, Rb, Cs, Tl; M = Nb, Ta) family of compounds can be described as modulated structures with a single superspace group and very similar modulation functions. Single-crystal X-ray diffraction data (Mo Kalpha radiation) are used to solve and refine the structure of KNbOB2O5 in these terms for the first time. The average structure is solved and refined in the space group Pmn2(1). Subsequently, the atomic modulation functions are determined using JANA2000 and superspace-group symmetry Pmn2(1)(0,0.375,0)s. The commensurately modulated structure is finally refined as a superstructure in the space group Pbn2(1) using SHELXS97 converging to R(1) = 0.024.

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Towards a unified description of the AMOB₂O₅ (A = K, Rb, Cs, Tl; M = Nb, Ta) family of compounds

Cited by 14 publications

References 7 publications

Five-dimensional structure refinement of natural melilite, (Ca_1.89Sr_0.01Na_0.08K_0.02)(Mg_0.92Al_0.08)(Si_1.98Al_0.02)O₇

Five-dimensional structure refinement of natural melilite, (Ca_1.89Sr_0.01Na_0.08K_0.02)(Mg_0.92Al_0.08)(Si_1.98Al_0.02)O₇

Temperature dependence of the modulations in KNbOB₂O₅and RbNbOB₂O₅

Structure of KNbOB₂O₅ – a commensurately modulated structure

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Towards a unified description of the AMOB2O5 (A = K, Rb, Cs, Tl; M = Nb, Ta) family of compounds

Cited by 14 publications

References 7 publications

Five-dimensional structure refinement of natural melilite, (Ca1.89Sr0.01Na0.08K0.02)(Mg0.92Al0.08)(Si1.98Al0.02)O7

Five-dimensional structure refinement of natural melilite, (Ca1.89Sr0.01Na0.08K0.02)(Mg0.92Al0.08)(Si1.98Al0.02)O7

Temperature dependence of the modulations in KNbOB2O5and RbNbOB2O5

Structure of KNbOB2O5 – a commensurately modulated structure

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Towards a unified description of the AMOB₂O₅ (A = K, Rb, Cs, Tl; M = Nb, Ta) family of compounds

Five-dimensional structure refinement of natural melilite, (Ca_1.89Sr_0.01Na_0.08K_0.02)(Mg_0.92Al_0.08)(Si_1.98Al_0.02)O₇

Five-dimensional structure refinement of natural melilite, (Ca_1.89Sr_0.01Na_0.08K_0.02)(Mg_0.92Al_0.08)(Si_1.98Al_0.02)O₇

Temperature dependence of the modulations in KNbOB₂O₅and RbNbOB₂O₅

Structure of KNbOB₂O₅ – a commensurately modulated structure