Volume 215, Issue 7

An accuracy of 0.01 Å or better of experimentally determined bond lengths suggests that calculated bond valences higher than 0.06 v.u. are to be considered as representing chemical bonds, and that bond-valence sums are a measure of the effective valence of an atom. Consequently, bond valences and effective atom valences, which can easily be calculated on small personal computers, give information on the electronic structure from which conduction paths can be derived even for chemically and structurally complicated materials. Application of the method to eight semiconducting cetineite-type phases, A 6 [Sb 12 O 18 ][SbX 3 ] 2 [D x Y 6-y ], with A = Na + , K + , Rb + , Sr 2+ , Ba 2+ ; X = S 2- , Se 2- ; D = Na + , Sb 3+ , C 4+ ; Y = H 2 O, OH - , O 2- , reveals conduction paths in agreement with experimental observations and sophisticated ab initio calculations.

In our previous paper on prediction of crystal structures of two monosaccharides we pointed to a contrast between rather unrealistic potential energy values paired with acceptable R wp factors. By comparing the energy values E(potential) to E(HFSCF) and by the refinements combining trial structures and |F 0 | from a single crystal it was found that R wp could be a misleading indicator in structure prediction based on powder data.

14 families of minimal surfaces with straight self-intersections have been derived which subdivide R 3 into infinitely many congruent, two-periodic ‘flat labyrinths’. For eleven families, all flat labyrinths are parallel to each other. Two sets of mutual perpendicular flat lab-yrinths have been found three times. All these minimal surfaces are non-orientable. Their Euler characteristics vary between -3 and -13.

A new member of the boracite-family of compounds M 3 B 7 O 13 X was synthesized under hydrothermal conditions. This cadmium hydroxy boracite, Cd 3 B 7 O 13 (OH), crystallizes in the trigonal crystal system (space group R3c, No. 161; a = 8.8109(1) Å, c = 21.4572(4) Å) and is isotypic to the iron-rich boracite congolite, (Fe,Mg) 3 B 7 O 13 Cl. The crystal structure has been refined by Rietveld analysis of X-ray powder diffraction data. The boracite structure consists of a three-dimensional framework of vertex-linked BO 4 -tetrahedra and BO 3 -triangles, the large interstices of which contain the five-coordinated cation M and the anion X.

NaRbSi 2 O 5 , M r 244.63, λ(MoK α )=0.71073 Å, 150K, monoclinic, P2 1 /c, a=4.8570(10)°, b=13.5403(10)°, c=7.7328(10) Å, β=90.908(10)°, V=508.49(13) Å 3 , μ(MoK α )=10.23 mm -1 , Z=4, F(000)=464, D x =3.1954(8) g/cm 3 , R 1 =0.0899 for 743 reflections with I>2.0σ(I), wR 2 =0.2425 for all 2534 reflections. NaCsSi 2 O 5 , M r 584.13, λ(MoK α )=0.71073 Å, 150K, orthorhombic, Pna2 1 , a=17.0743(10)°, b=4.9011(10)°, c=13.3391(10) Å, V=1116.3(3) Å 3 , μ(MoK α )=7.08 mm -1 , Z=8, F(000)=1072, D x =3.4757(9)g/cm 3 , R 1 =0.0199 for 1103 reflections with I>2.0σ(I), wR 2 =0.0465 for all 2668 reflections. The crystal structures of NaRbSi 2 O 5 and NaCsSi 2 O 5 have been determined. Though the nominal composition of both phases is that of a phyllosilicate, their anion topology is similar and consists of double chains of [SiO 4 ] tetrahedra forming 4-membered T rings, i.e. rings consisting of 4 tetrahedra in chair conformation and built from Q 3 units. The alkali-oxygen distances in the Na 2 O-Cs 2 O-SiO 2 system tend to support the notion that in mixed alkali systems the alkali-oxygen distance of the smaller cation decreases at the expense of that of the larger one. The observed very high hygroscopicity of both crystalline phases might be rationalized by substantial underbonding of some of the oxygen atoms. 29-Si spin lattice relaxation times, T 1 , vary largely between 535 and 493 sec for NaRbSi 2 O 5 and 2440 and 2329 sec for NaCs-Si 2 O 5 .

Neutron powder diffraction data collection has been carried out at BENSC (Berlin, D), on synthetic Mg 0.94 Al 2.04 O 4 spinel, in the temperature range 300-1700 K, to investigate the intracrystalline cation and vacancy order-disorder (OD) reaction, which begins to be detectable at 944 K. The Rietveld method has been used in combination with advanced minimization techniques to determine the occupancy coefficients of magnesium, aluminum, and vacancies on the tetrahedral and octohedral sites. The temperature dependence of the cell parameter, of the T-O and M-O bond lengths, and of the oxygen u-coordinate are discussed in the light of the OD-reactions. The cation site partitioning has been interpreted as a function of temperature using the thermodynamic model of O´Neill and Navrotsky, whose parameters [α=30(2) and β=1(3) kJ/mol]are compared with earlier determinations from literature.

The preparation of the sodalite Na 8 [AlSiO 4 ] 6 ·(HCO 3 ) 1 (CO 3 ) 0.5 (H 2 O) 2 and cancrinite Na 7.6 [AlSiO 4 ] 6 ·(HCO 3 ) 1.2 (CO 3 ) 0.2 (H 2 O) 2 were carried out in a cold-sealed autoclave using a preheated mixture of γ-Al 2 O 3 and precipitated silica together with NaHCO 3 and NaOH. The material was encapsulated in a small silver tube and heated for 48 hours at a pressure of 0.1 GPa. Two different types of crystals remained during this process. From these crystals the structure of both compounds were determined from single crystal diffractometer data. The sodalite Na 8 [AlSiO 4 ] 6 (HCO 3 ) 1 (CO 3 ) 0.5 (H 2 O) 2 : P-43n, a = 903.3(1) pm, V = 0.7370(1) nm 3 , Z = 2, R = 0.013, wR 2 = 0.027 for 242 structure factors and 63 variable positional parameters; the cancrinite Na 7.6 [AlSiO 4 ] 6 (HCO 3 ) 1.2 (CO 3 ) 0.2 (H 2 O) 2 : P6 3 , a = 1272.5(2) pm, c = 517.7(1) pm, V = 0.7260(1) nm 3 , Z = 2, R = 0.034, wR2 = 0.066 for 845 F’s and 99 variables. Spectroscopic properties were measured with an IR microscope in transmission mode using single crystals polished to a thickness of 30 μm. Both structures are build by corner shared six rings of AlO 4 and SiO 4 tetrahedra stacked cubic close packed ABCA’B’C’ for the sodalite and hexagonal close packed ABA’B’ for the cancrinite. The mean Al-O (174.3 pm) and Si-O (161.4 pm) distances for the sodalite and 173.7 pm (Al-O) and 161.8 pm (Si-O) are in good agreement with former structure description of related materials. Hydrogencarbonate and carbonate was found statistically distributed in the sodalite β-cages and in the channels of the cancrinite structure. Whereas for the sodium atoms in the sodalite four different positions are observed, the sodium atoms in the cancrinite structure where found near the threefold axes in the cancrinite ε-cage and on one position in the channel.

Molecular orbital calculations using descrete-variational Xα method were carried out in order to investigate the bond nature of SiO 2 , GeO 2 and SnO 2 . The bond overlap populations and the d-orbital populations of cations were decreased with increasing the atomic number. The deformation density maps of electronic charge show the accumulation of electron in interatomic region which is most dominant in SiO 2 . The calculated density of states indicates that the bond character of SiO 2 is more covalent than that of GeO 2 and SnO 2 . From the excited d-electron partial density of states, it is inferred that the Si excited-3d has some connection with the bond nature of SiO 2 covalency.

The charge density and bond character of the rutile-type structures of SiO 2 , GeO 2 and SnO 2 were studied by X-ray diffraction using monopole refinement. The refinement of the charge density was performed by least-squares refinement including the Κ-parameter and the electron population in the X-ray atomic scattering parameters. The oxygen′s Κ-parameters of three oxides are 0.91 (SiO 2 ), 1.06 (GeO 2 ), 1.14 (SnO 2 ). The oxygen valence electron of each sample is 1.06(8)e for SiO 2 , 1.10(16)e for GeO 2 and 1.13(15)e for SnO 2 . These values indicate that the electron distributions are more localized with an increase in the cation atomic number. The difference Fourier map of each sample shows the deformation density of the valence electrons of cations. The overlapping electron orbitals cause the deformation of MO 6 octahedra of rutile-type structures. The ratio between O-O shared and unshared edge and the ratio between the apical and equatorial M-O interatomic distances decrease with the cation ionicity and the repulsive force between the two cations in adjacent octahedra.

Metallic radii and ionic radii are correlated by two linear relationships one of which comprises the main group elements and the transition elements of groups 3 up to 7. The other one refers to the elements of groups 8 up to 12. The valences (oxidation states) of the ions involved equal the group numbers of the main group elements of groups 1 up to 4, and 3 in the case of P, As, Sb, and Bi. The apparent valences W of the transition metals obey the rule W = 7 − |G−7| with G = group number. The valences of 19 metals only out of the 30 d -block elements seem to equal these apparent valences.

[N(C 4 H 9 ) 4 ][NbCl 4 OAlCl 3 ] 1 crystallized after two months in form of yellow needles in a Schlenk-tube from a CH 2 Cl 2 /n-hexane solution containing equimolar amounts of [N(C 4 H 9 ) 4 ]Cl, NbCl 5 and AlCl 3 . Yellow crystals of [N(C 3 H 7 ) 4 ][NbCl 6 ] 2 yielded from the reaction of equimolar amounts of [N(C 3 H 7 ) 4 ]Cl and NbCl 5 in the same solvent. The oxygen in 1 indicates a reaction of the educts with traces of water. Both compounds crystallize in the space group P2 1 /n. ( 1 : a=10.664(1)Å, b=14.177(1)Å, c=19.429(3)Å, β=90.01(1)°, Z=4; 2 : a=11.678(2)Å, b=15.581(2)Å, c=11.769(1)Å, β=91.51(2)°, Z=4). 1 is characterized by [N(C 4 H 9 ) 4 ] + cations and [NbCl 4 OAlCl 3 ] units which are linked by chlorine bridges to dimeric anions. [NbCl 4 OAlCl 3 ] - has a nearly linear Nb-O-Al bridge (bond angles 178.9(3)°) with a Nb-O bond length of 1.769(4) Å and a Al-O bond length of 1.771(5) Å. The structure of 2 contains [N(C 3 H 7 ) 4 ] + cations and [NbCl 6 ] - anions.