Egbert Keller

The crystal structure of Bi4O5I2 and its relation to the structure of Bi4O5Br2

Abstract The crystal structure of Bi4O5I2 has been solved from a 3:1 twinned crystal; accurate lattice parameters have been determined by a Rietveld refinement. Bi4O5I2 crystallizes with space group P21 (no. 4), Z=4, a=14.944(1), b=5.6983(3), c=11.263(1) Å, β= 99.81(1)°. Its structure is isotypic to the structure of the homologous Bi4O5Br2 for which more reliable lattice parameters have been determined by a Rietveld refinement. The effects of replacement of Br by I on the structure are discussed and compared with corresponding effects in the BiOX family (X = Cl, Br, I). A structural reason for the deviation of the twin supercells of the two compounds from orthogonality is discussed.

The crystal structure of α-Bi5O7I

Abstract The structure of α-Bi5O7I, synthesized by the addition of a diluted H[BiI4] solution to 5N KOH, has been determined. α-Bi5O7I crystallizes with space group Ibca, Z=8, a=16.265(2), b=5.3439(6), c=23.020(3) Å. It shows a close relationship to the structure of BiOI. The chemical reaction which leads to the synthesis of α-Bi5O7I is discussed in the light of this relationship.

Crystal structure of bismuth oxide iodide, β-Bi5O7I

/?-Bi507I crystallizes in space group C2/m with lattice parameters a = 18.387(4), b = 4.2497(9), c = 13.254(2) Α, β = 108.10(1)°, Ζ = 4. The structure was solved with 747 independent reflections and refined to R = 0.040 (R„ = 0.034); it consists of (Bi1 0O1 4)* + columns parallel [010] which are connected by double rows of iodine. Structural relationships to Sb 5 0 7 I exist.

Crystal structure and twinning of Bi4O5Br2

Abstract The monoclinic crystal structure of Bi4O5Br2 has been determined from 1:1 twinned crystals pretending orthorhombic symmetry. Bi4O5Br2 crystallizes with space group P21 (no. 4), Z=4, a=14.539(4), b=5.605(1), c=10.782(3) Å, β= 97.75(7)°. Its structure shows much closer relationships to the structure of α-Bi5O7I than to that one of the homologuous compound Bi4O5Cl2.

Structural changes within and between the two isotypic series ABiO2 (A = Na, K, Rb, Cs) and ASbO2 (A = K, Rb, Cs)

Abstract Structural changes within the isotypic series ABiO2 (A = Na, K, Rb, Cs) and ASbO2 (A = K, Rb, Cs) and between corresponding members of the two series are analyzed after re-determination of the structure of NaBiO2. Despite the low valence of A—O bonds as compared to E—O bonds (E = Sb, Bi), the A atoms of different sizes are largely able to satisfy their individual spatial requirements. Small deviations between expected and experimentally determined Na/K size differences in the ABiO2 series are explained by a change in the bonding scheme in the transition from NaBiO2 to KBiO2. The results of DFT calculations show that the unexpected increase of the lattice parameters b in the three transitions ABiO2 → ASbO2 is not caused – as assumed before – by the different stereochemical activities of the Sb and Bi lone electron pairs. Instead, this increase is caused by cation-cation repulsions owing to the observed decrease in c.

The Crystal Structure of Bi3Se4Br

Abstract The crystal structure of Bi3Se4Br has been determined. With respect to its (Bi,Se) substructure it is related to orthorhombic Bi2Se3 and to Bi19S27Br3

On the unit cell volume expansion in homogeneous series of isotypic structures of “ionic” inorganic compounds

Abstract Within a “homogeneous” series of (crystal-chemically) isotypic structures one and only one chemical element E of the sum formula is replaced successively by heavier elements of the same group of the periodic table. For corresponding series of “ionic” compounds, a (coarse) “expectation value” ΔVth for the unit cell volume expansion following the replacement of E(i) by E(j) (with i<j=row numbers in the periodic table) can be calculated by using the empirical formula ΔVth=56.5 (zE ΔρE(i) E(j)) – 0.80 (zE ΔρE(i) E(j))2, where zE is the number of atoms E per unit cell and ΔρE(i) E(j) is the linear statistical size (= radius) difference for the two elements E(i) and E(j). Two hundred and twenty-eight different series have contributed to the statistical evaluation of this formula. Series for which E are elements of low valence have been excluded a priori from the calculations if the residual atoms of higher valence by themselves form an infinite framework, as it was suspected, that in structures of this kind the “natural” volume expansion caused by the E(i) → E(j) size increase is hampered by the rigidity of this framework. Application of the above formula to structures of this kind clearly supports this suspicion only, if the framework is of dimensionality three. The results derived from all other investigated structures are qualitatively compatible with the “volume increments rule” published in the 1930s. This suggests that they should be applicable also to isotypic series of crystal structures defined in a less restricted manner.