Volume 209, Issue 11

Pb 5 Al 3 F 19 undergoes three first-order phase transitions; on cooling from phase I (4/ m ) to phase II (2/ m ) at 365 K, from phase II to phase III (4/ m ) at 305 K, and from phase III to phase IV (4 mm ) at 120 K. The lengths of the a - and c -axes undergo continuous change at the highest temperature phase transition as the β -angle increases from 90[unk] below 365 K; no difference is detectable between the lengths of the a - and b -axes in monoclinic phase II, within an accuracy of 0.007 Å. The β -angle temperature dependence follows a power law with exponent that does not differ significantly from 1/2. At the 305 K first-order phase transition, the β -angle abruptly changes from its maximum value of 90.41° in phase II to 90[unk] in tetragonal phase III; the a - and c -axes, however, maintain their continuous change in length through this transition. The 120 K first-order phase transition is characterized by an abrupt 0.77% decrease in the length of the normalized a -axis, and an equally abrupt 1.89% increase in the c -axis length, on cooling from phase III to phase IV. The a - and c -axes in all four phases are characterized by linear thermal coefficients, with magnitudes presented below. The normalized a -axis lengths in phases I, II and III extrapolated to 0 K differ by less than 0.021 Å from 14.174 Å, the corresponding c -axis lengths at 0 K differ by less than 0.004 Å from 7.194 Å. The normalized unit cell volume thermal dependence of phases I, II and III is linear and continuous between 400 and 120 K but undergoes a discontinuity of 0.40% at the transition from phase III to phase IV.

Single crystals of arsenic v ditelluroidide semihydrate, AsTe 2 I · 0.5 H 2 O( Fw = 466.033), were prepared hydrothermally. The crystal structure was determined from the intensities of 501 independent reflections, measured on a computer-controlled single-crystal diffractometer (Philips PW 1100), with Mo K α radiation ( λ = 0.71069 Å). The cell constants at 293 K, refined by least-squares, are a = b = 7.221(1) Å, c = 21.478(7) Å, Z = 6, space group R 3. The density values are ϱ meas = 4.6 g · cm −3 and ϱ calc = 4.786 g · cm −3 . Solution of the structure was performed by Patterson methods combined with successive Fourier syntheses. The positional and anisotropic thermal parameters were refined by full-matrix least-squares calculations. All reflections were corrected for absorption ( μ = 177.365 cm −1 ) and values for anomalous dispersion used to all non hydrogen atoms. Refinement converged to R = 0.051 and R w = 0.050. The As atoms are surrounded by six [Te, I] atoms, three with shorter and three with longer distances, forming strongly distorted octahedra. Each of the As(1) octahedra is linked with three As(2) octahedra by common edges [Te, I](1)–[Te, I](2), and vice versa, forming an infinite band perpendicular to the c axis. In each unit cell are three parallel bands, an arrangement analogous to AsI 3 . The water molecules are enclosed between successive bands. The bands are connected by hydrogen bonds.

Series of structural studies of systematically selected local portions of the same crystal of ilvaite CaFe 2 2 + Fe 3 + OOHSi 2 O 7 with moderate size have revealed a trend that the monoclinic and apparently orthorhombic structures co-exist in the crystal regardless of their mode of occurrence. The diffraction-intensity profiles of high angle reflections such as 24,0,0 and 008 were then studied by means of step-scanning on a four-circle diffractometer, using graphite monochromatized Mo K α radiation: the former was scanned along c * and the latter along a *. The result has brought out that either reflection from the crystals of high monoclinicity ( β ≈ 90.2° ∼ 90.3°) splits into a double peak, suggesting that those crystals are most likely made up of two kinds of domains, each having a monoclinic structure with β = 90.45°, one twinned on (001) and the other twinned on (100). In the case of crystals of low monoclinicity ( β ≈ 90.0°), those reflections tend to exhibit single broad profiles with fine structure. A profile-fitting procedure has shown that they are in general composite of three peaks, suggesting that they consist of complex composites of fine monoclinic components twinned on (001) and those twinned on (100). The distribution of Mn in the crystals studied tends to increase towards their marginal portions. Crystal portions containing more than roughly 0.25 Mn per formula unit assume orthorhombic symmetry without exception. The coexistence of the different symmetries in the same crystal of ilvaite is thus originated in the variation in scale or mode of twinnings incorporated mainly with inhomogeneous distribution of Mn in the crystal. The occurrence of orthorhombic ilvaite is unlikely.

In the system Mg 1 − x Cu 2 + x O 3 the phase with x = 0, i.e. the compound MgCu 2 O 3 , does not exist. Neutron powder measurements and single crystal X-ray studies clearly show that the Mg lattice site is partially occupied by Cu 2+ ions with typical values of x = 0.1. This substitutional disorder, best denoted by the formula (Mg 1 − x Cu x )Cu 2 O 3 , is surprising, since the coordination polyhedron of the Mg site is a compressed oxygen octahedron, while the Jahn-Teller ion Cu 2+ usually prefers a tetragonally elongated oxygen octahedron, as it exists on the regular Cu site of the lattice. The local environment of the Cu 2+ ions on the Mg site was also checked by electron paramagnetic resonance (EPR) measurements.

The crystal structure of mercury oxalate, HgC 2 O 4 was solved from synchrotron X-ray and neutron powder diffraction patterns. The unit cell was obtained by auto-indexing of the X-ray powder pattern, and the positions of the atoms were found using the direct method program SIRPOW92. The structure was refined by Rietveld analysis of the neutron powder diffraction data. The unit cell is monoclinic, space group P 2 1 with lattice parameters from the synchrotron refinement: a = 5.033(1) Å, b = 5.237(1) Å, c = 6.453(1) Å, β = 108.74(1)°, and Z = 2. The mercury atoms are bounded to the oxalate ions by two short mercury-oxygen bonds Hg–O3 = 2.10 Å and Hg–O2′ = 2.12 Å, and six longer bonds in the range 2.63 Å to 2.83 Å. The mercury-oxygen coordination polyhedron is a slightly deformed box.

The crystal and molecular structure of Sesibiricin (C 20 H 24 O 4 ) has been determined from three dimensional X-ray diffraction data. The gross structure ( P 2 1 with a = 13.142(2) Å; b = 7.552(3) Å; c = 18.364(3) Å; β = 91.53(4)°, Z = 4, V = 1821.9(8) Å 3 , d 0 = 1.198 Mgm −3 ) was determined from 4477 observed reflections ( R -index = 0.063). The basic 2 H-[1]-benzopyran-2-one is planar. The side chains at C(5) and C(8) are nearly planar and are inclined to the flat ring system of the molecule. The two asymmetric molecules indicate different conformations in the two side chains. These are interconnected by a weak hydrogen bond ( d (C1A … O2B) = 3.48(1) Å). The closest contact between the symmetry related molecules is d (C12B … O2A) = 3.48 Å.

The crystal structure of trimethylsulfonium oxalatotriphenylstannate, C 23 H 24 O 4 SSn, synthesized from the metathetical reaction of trimethylsulfonium iodide, silver Oxalate and triphenyltin chloride in methanol, consists of [(CH 3 ) 3 S] + ions interspersed among the linear [(C 6 H 5 ) 3 SnO 2 CCO 2 ] n n − chains that are constructed from [O 2 CCO 2 ] 2− dianions bonded axially to planar [(C 6 H 5 ) 3 Sn] + (Sn–O = 2.220(3) Å, ΣC–Sn–C = 360.0(4)°) cations.

The crystal structures of the title compounds P , P -dicyclohexyl- N -methyl-phosphinylthioformamide, [ c Hex 2 P(O)C(S)N(H)Me], C 14 H 26 ONPS, P , P -dicyclohexyl- N -phenylphosphinylthioformamide, [ c Hex 2 P(O)C(S)N(H)Ph], C 19 H 28 ONPS, P , P -dicyclohexyl- N -methylthiophosphinylthioformamide, [ c Hex 2 P(S)C(S)N(H)Me], C 14 H 26 NPS 2 , and P , P -dicyclohexyl- N -phenyl-thiophosphinylthioformamide, [ c Hex 2 P(S)C(S)N(H)Ph], C 19 H 28 NPS 2 , have been determined at room temperature. The green crystals of [ c Hex 2 P(O)C(S)N(H)Me] are monoclinic, space group C 2/ c , with unit cell dimensions a = 10.247(2) Å, b = 16.025(4) Å, c = 19.358(4) Å, β = 97.75(2)°, Z = 8 and D x = 1.212 Mg m −3 . Crystals of [ c Hex 2 P(O)C(S)N(H)Ph] are also monoclinic, space group C 2/ c , with unit cell dimensions a = 14.856(4) Å, b = 11.666(4) Å, c = 22.039(3) Å, β = 95.59(1)°, Z = 8 and D x = 1.221 Mg m −3 . The colorless crystals of [ c Hex 2 P(S)C(S)N(H)Me] are orthorhombic, crystallizing in the Pnma space group with dimensions a = 10.2600(9) Å, b = 15.447(2) Å, c = 10.141(3) Å, Z = 4 and D x = 1.254 Mg m −3 . Yellow crystals of [ c Hex 2 P(S)C(S)N(H)Ph] are triclinic, space group P [unk], with dimensions a = 9.766(2) Å, b = 11.111(2) Å, c = 9.388(2) Å α = 100.54(2)°, β = 104.66(1)° and γ = 83.86(1)°, Z = 2 and D x = 1.249 Mg m −3 . The structures were solved by directmethods and each refined by a full-matrix least-squares procedure to final R = 0.036 (0.046; 0.040; and 0.038) using 1383 (1257; 1362; and 2758) reflections for each of the above [ c Hex 2 P(Y)C(S)N(H)R] (where Y = O or S and R = Me or Ph) compounds, respectively. The conformation about the C–N bond in each of [ c Hex 2 P(Y)C(S)N(H)R] is Z . Important parameters for Y = O, R = Me: P = O 1.480(2), P–C 1.814(4), C = S 1.630(3) and C–N 1.306(4) Å; Y = O, R = Ph: P = O 1.479(4), P–C 1.817(5), C = S 1.618(5) and C–N 1.327(6) Å; Y = S, R = Me: P = S 1.942(1), P–C 1.823(4), C = S 1.644(4) and C–N 1.290(5) Å; and Y = S, R = Ph: P = S 1.9495(9), P–C 1.846(3), C = S 1.624(2) and C–N 1.317(3) Å. Symmetry related molecules of [ c Hex 2 P(O)C(S)N(H)R] associate in the lattice via hydrogen bonding contacts. The P–C(1) bond distances in the [ c Hex 2 P(Y)C(S)N(H)R] compounds are shown to be significantly shorter than those found in the related [Ph 2 P(Y)C(S)N(H)R] compounds.

Two organically templated vanadium arsenate phases have been synthesized and characterized by single X-ray diffraction and thermogravimetric analyses. Both are comprised of clusters of vanadium arsenate anions that are held together by hydrogen bonding with the included species. The V 4 As 6 O 30 H 6 · 2 N 2 C 6 H 14 · 4 H 2 O [VAs-Dab] ( M r = 1439.76) crystal is monoclinic, space group P 2 1 / c (No. 14), with a = 9.247(3) Å, b = 13.932(3) Å, c = 16.105(8) Å, β = 106.31(4)°, V = 1991.26 Å 3 and Z = 2. Final residuals of R = 3.71% and R w = 4.17% were obtained for 3044 observed reflections with I > 3 σ ( I ). The crystalline compounds is comprised of clusters H-bonded to the organic and water molecules. The V 4 As 6 O 30 H 8 · 2 N(CH 3 ) 4 · 5.33 H 2 O [VAs-TMA]( M r = 346.43) crystal is orthorhombic, space group Cmca (No. 64), with a = 18.742(5) Å, b = 12.208(3) Å, c = 17.689(6) Å, V = 4047.24 Å 3 and Z = 16. Final residuals of R = 3.89% and R w = 4.13% were obtained for 1756 observed reflections with I > 3 σ ( I ). The crystalline compound contains one dimensional channels, surrounded by the H-bonded clusters, containing the water molecules and tetramethyl ammonium cations.