Volume 161, Issue 3-4

The crystal structure of a new sulfosalt, TlHgAs 3 S 6 , has been determined. The crystal is monoclinic with space group P 2 1 / n . Four formula units are contained in a unit cell of dimensions a 0 = 5.948(2) Å, b 0 = 11.404(6) Å, c 0 = 15.979(5) Å, β = 90.15(1)°. The structure was determined by direct methods and refined to a final R -value of 0.05 for 2462 observed reflections. The AsS 3 pyramids form infinite AsS 2 -chains parallel [101]. The Hg atom has a deformed octahedral coordination by 3 + 3 S atoms. The octahedra form a double chain parallel to [100]. The Tl atom is coordinated by 7 + 2S atoms. The coordination polyhedra form layers parallel to (001). The mean Tl – S, Hg – S and As – S distances are 3.43, 2.86, 2.27 Å, respectively.

The distribution of Fe 3+ and Ga 3+ between the octahedral and tetrahedral sites in two garnets with the compositions Y 3 FeGa 4 O 12 and Y 3 Fe 3 Ga 2 O 12 was studied after heating at temperatures between 1000°C and 1500°C using Mössbauer and electronic absorption spectroscopy. Fe 3+ shows preference for the octahedral sites and Ga 3+ for the tetrahedral sites. At elevated temperatures, a more disordered distribution is observed. The standard Gibbs free energy change ΔG ° per exchange pair according to the reaction Fe 3+ (oct) + Ga 3+ (tet)⇄ Ga 3+ (oct) + Fe 3+ (tet) is 4.7 – 5.3kcal and the enthalpy change ΔH ° is of the same magnitude. ΔH ° may be correlated with the d -electron site preference for the tetrahedral coordination, which is considerably larger for d 10 than for d 5 configurated cations. The rather slow kinetics of the cation exchange between octahedral and tetrahedral sites at temperatures below 1200°C is probably due to the cornersharing topology of the coordination polyhedra.

The slipping scheme of double crankshaft structure in tectosilicates is investigated. Starting with 23 unique structures, all the possible slipping ones are derived and classified into 7 groups in terms of their slipping relations. Some mineralogical implications are discussed.

The author describes a new method by which it is possible to calculate easily and rapidly the three lattice parameters from the angular position of each diffracted line. Also described is a procedure whereby it is possible to eliminate partially the influence of systematic errors in the square of the axial ratios used for these parameter determinations. This method and procedure is illustrated by an actual example.

Solid solutions of Me 3 (PO 4 ) 2 in Fe 3 (PO 4 ) 2 , with the graftonitetype structure, have been prepared and equilibrated at 1070 K ( Me = Mg, Ca, Co, Ni, Zn, or Cd). The (Fe 1 – z Me z ) 3 (PO 4 ) 2 phases are isomorphous with Fe 3 (PO 4 ) 2 as well as with the mineral graftonite. Accurate unit cell dimensions ( P 2 1 / c ) have been obtained from Guinier-Hägg photographic data. The unit cell volume is strongly correlated with the amount and size of the incorporated Me 2+ cation. Mössbauer spectroscopy ( 57 Fe) has been used to determine the cation distribution of Fe 2+ and Me 2+ between the three distinct cationic environments in the structure: one octahedron and two five-coordinated polyhedra, all distorted. The order of preference for five- over six-coordination may be given as Zn 2+ > Fe 2+ , Mg 2+ , Co 2+ , Ni 2+ > Ca 2+ > Mn 2+ > Cd 2+ .

Tl 8 Cd 3 (N 3 ) 14 was prepared by the reaction of aqueous solutions of cadmium azide with thallium azide. The lattice parameters are at 298 K (100K): a = 8.603 (8.552), b = 10.532 (10.487), c = 11.767 (11.614) Å, α = 99.92 (100.31), β = 107.68 (107.50), γ = 113.51 (113.47)°, space group P [unk], Z = 1. The crystal structure was determined by the Patterson method using 6198 independent low temperature Mo Kα diffractometer data and refined to R = 0.087. The structure consists of Cd(N 3 ) 6 -octahedra which share two nitrogen corners. The thallium ions are surrounded by eight azide groups. The azide groups are linear with N – N-distances of 1.17(5) Å.

The crystal structure of 6-bromo-2-cyano-4′-diethylamino-4-nitro-6′-propionamidoazobenzene has been determined by single-crystal X-ray analysis. The crystals are monoclinic, a = 29.015(6), b = 10.867(2), c = 24.945(5) Å, β = 147.80(3)°, space group C 2/ c , Z = 8. The structure was refined to R = 0.091 and R w = 0.065 by block-diagonal least-squares methods, using 2364 independent reflections measured with a single-crystal diffractometer. The conformation found has the propionamide group trans to the Br atom, and is apparently stabilized by intramolecular hydrogen bonding.

The crystal structure of the title compound ( 1 ) has been determined [space group P 2 1 2 1 2 1 , a = 5.592(3), b = 14.724(8), c = 23.966(10) Å, Z = 4] and refined to R 0.046 for 2965 unique reflections. The trend towards longer exocyclic bonds at the acetal centre in compounds with strongly electronegative aglycones is maintained in ( 1 ).

The crystal structure of the title compound ( 1 ) has been determined [space group P 2 1 2 1 2 1 , a = 5.747(3), b = 14.182(5), c = 24.480(5) Å, Z = 4] and refined to R 0.087 for 2059 unique observed reflections. The limited accuracy (typical bond length e.s.d.'s 0.008 Å) is associated with a disordered acetyl group at C(6). The endocyclic acetal bond is longer than the exocyclic by 0.018 Å, reversing the pattern observed in the α -isomer (preceding paper).

The crystal structure of the title compound ( 1 ) has been determined [space group P 2 1 / n , a = 7.940(2), b = 7.911(2), c = 19.455(4) Å, β = 98.34(2)°, Z = 4] and refined to R 0.050 for 1542 unique observed reflections. The exocyclic acetal bond length is within half an e.s.d. of that predicted by a previously published bond length/reactivity correlation (Jones and Kirby, 1979).

In the calculation of the electrostatic part of the lattice energy of polar crystals by slice-wise summation, as occurs in the Madelung method, an extra energy term, E corr , is needed. This term is due to a double limit to infinity, one parallel to the slices, the other perpendicular to them. It was found that E corr = – 2 πμ 2 / V , where μ is the dipole moment of the slice and V the unit cell volume. The method is applied to BeO.

Ni(SCN) 2 (NH 3 ) 2 and Ni(SCN) 2 are stable intermediates in the course of the thermal decomposition reaction Ni(SCN) 2 (NH 3 ) 4 → NiS 2 . Ni(SCN) 2 (NH 3 ) 2 crystallizes in the monoclinic space group C 2/ m with a = 8.698(5), b = 7.991(4), c = 5.619(3) Å, β = 105.91(9)°, Z = 2, D x = 1.85g cm −3 . The crystal data for Ni(SCN) 2 are: space group C 2/ m , a = 10.476(7), b = 3.628(2), c = 6.165(5) Å, β = 106.89(9)°, Z = 2, D X = 2.59g cm −3 . The structures were determined from X-ray intensities measured on a four-circle diffractometer and were refined to R w = 0.016 for 524 independent observed reflections [Ni(SCN) 2 (NH 3 ) 2 ] and to R w = 0.037 for 611 independent observed reflections [Ni(SCN) 2 ]. Ni(SCN) 2 (NH 3 ) 2 exhibits a structure with infinite [unk] chains along [001]. The resulting approximate square planar coordination of nickel by two S and two N atoms is completed to a distorted octahedron by two NH 3 ligands. The infinite chains [unk] are preserved in Ni(SCN) 2 in the course of the thermal decomposition of Ni(SCN) 2 (NH 3 ) 2 . By additional linking of neighbouring chains through [unk] bridges a layer structure (001) results for Ni(SCN) 2 .

The recently described strengthened translation functions (translation functions after application of direct methods to difference structure factors) were used in the structure determination of a small molecule, which resisted solution by one direct method. Detailed results for various correct and incorrect orientations of the molecule (obtained by a Patterson orientation function) are discussed and criteria for the selection of possibly correct orientations are given. The molecule, C 14 H 14 N 2 O 2 , crystallizes in the orthorhombic centrosymmetric space group Pbca , with 8 molecules in the unit cell: a = 8.925(2), b = 14.077(4) and c = 20.237(4) Å. The structure was solved using orientation search methods in vector space and strengthened translation functions. The structure was refined by full-matrix least-squares methods to a conventional R -value of 0.032. The failure of the a priori direct method is discussed in an appendix.

The structure of synthetic K 2 Mg(CO 3 ) 2 is trigonal with space group R [unk]m, a 0 = 5.150(1), c 0 = 17.290(3) Å, Z = 3 and D x = 2.79g cm −3 , and was refined to R (unweighted) = 0.057 and R (weighted) = 0.036 using 280 non-equivalent reflections. The compound is isostructural with buetschliite, K 2 Ca(CO 3 ) 2 . The structure consists of isolated [CO 3 ] 2 − polyhedra (C – O distance 1.283 Å), [MgO 6 ] octahedra (Mg – O distance 2.093 Å) and [KO 9 ] polyhedra (K – O mean distance 2.801 Å). The [CO 3 ] 2− group is non-planar and the distance of the carbon atom from the plane of the three oxygen atoms is 0.014(3) Å.

The temperature derivative ∂s 123 / ∂T of the electrogyration co-efficient s 123 is negative in α - and positive in β -alums. In ammonium and ammonium substituted alums this change of sign is masked by a large positive temperature effect of the ammonium ion. The observed effect at room temperature, essentially of dynamic origin, results in all alums from a partial cancellation of different contributions.

The two independent electro-optic coefficients of α -quartz were redetermined at constant stress employing a sensitive dynamic method of optical interferometry. The results at room temperature and λ = 633 nm are: r σ 111 = – 0,481(8), r σ 231 = – 0.235(10).10 −12 m/V.

Large single crystals of optical quality of isotypic Rb 2 ( d -C 4 H 4 O 6 ) and Cs 2 (( d -C 4 H 4 O 6 ) (space group P 3 1,2 21) with dimensions up to 5cm have been grown from water solutions by isothermal evaporation at 20°C. The linear electrooptical tensor [ r ijk ] and the tensor of the linear electrostriction [ d ijk ] have been determined for both crystals: r 111 = −1.26(3), r 231 = −0.26(4), d 111 = +3.60(4), d 123 = −0.26(2) [Rb 2 ( d -C 4 H 4 O 6 )]; r 111 = −1.24(2), r 231 = −0.30(3), d 111 = +4.43(2), d 123 = −0.34(2) [Cs 2 ( d -C 4 H 4 O 6 )] (Unit: 10 −12 m/V, T = 295 K, λ = 633 nm).

KCd(N 3 ) 3 · H 2 O crystallizes in space group Pnma with a = 11.991(2), b = 3.712(1), c = 17.292(2) Å and Z = 4. K 2 Cd(N 3 ) 4 crystallizes in space group C 2/ m with a = 14.272(2), b = 3.787(1), c = 8.887(2), β = 92.83(2)° and Z = 2. The structures have been determined from single crystal X-ray data. The final R -values are 0.074 and 0.063 respectively. In both structures cadmium is surrounded octahedrally by six azide groups; the Cd(N 3 ) 6 -octahedra share edges. Potassium is coordinated by seven azide groups and two water molecules in KCd(N 3 ) 3 · H 2 O and by eight azide groups in K 2 Cd(N 3 ) 4 . In both structures all azide groups are approximately parallel, with mean N – N = 1.17(5) Å.