Showing papers in "Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry in 1977"

LaFe4P12 with filled CoAs3‐type structure and isotypic lanthanoid–transition metal polyphosphides

Abstract: The new compound LaFe4Pl2 was prepared by reaction of LaP and Fe powders with red P in evacuated silica tubes. Single crystals of LaFe4P~2 were obtained by reaction of the elemental components in molten Sn. They are cubic, space group Im3, a = 7.832 A, Z = 2. The structure was determined from single-crystal counter data by Patterson and Fourier methods and refined to R = 0.028 for 193 unique structure factors. It represents a new structural type which can be derived from the CoA% and WAlj2-type structures by filling the (somewhat distorted) icosahedral and octahedral voids with La and Fe atoms respectively. The P atoms are coordinated by two Fe and two P atoms forming a distorted tetrahedron augmented by a La atom outside one face of that tetrahedron. In the polyanionic IFe4Pj2 ]3framework the P P bonding distances are somewhat expanded, to accommodate the large La 3÷ cation, which in turn has shorter La-P distances than would be expected from the La -P distances in LaP, LaP 2, and LaP 5. The new compounds LnFe4P~2 (Ln = Ce, Pr, Nd, Sm, Eu), LnRu4P~2 (Ln = La, Ce, Pr, Nd, Eu), and LnOs4P.2 (Ln = La, Ce, Pr, Nd) are isotypic with LaFe4P~2. Their lattice constants indicate valencies IV and II for Ce and Eu, respectively, in the compounds. In CeFe4P~2 the polyanion I FeP 31is isoelectronic and isostructural with CoP 3.

Structural phase transition in polyphenyls. IV. Double‐well potential in the disordered phase of p‐terphenyl from neutron (200 K) and X‐ray (room‐temperature) diffraction data

Abstract: KHOTSYANOVA, T. L., KITAIGORODSKY, A. I. & STRUCHKOV, Yu. T. (1953). Zh. Fiz. Khim. SSSR, 27, 1330-1343. KxpPs, M. (1973). MSc Dissertation, Univ. o f Surrey. KITAIGORODSKY, A. I. (1961). Organic Chemical Crystallography, pp. 165-168, 252-253, London: Heywood. KITAIGORODSKY, A. I., KHOTSYANOVA, T. L. & STRUCHKOV, YU. T. (1953). Zh. Fiz. Khim. SSSR, 27, 1490-1502. RIETVELD, H. M. (1969a). Reactor Centrum Nederland, Res. Rep. RCN-104. RIE'rVELD, H. M. (1969b). J. Appl. Cryst. 2, 65-71. STRAND, T. G. (1967). Acta Chem. Scand. 21, 2111-2118. THACKERAY, D. P. C., SHmLE',', R., ORALRATMANEE, C., KIPPS, M. & STACE, B. C. (1974). J. Mol. Struct. 20, 293-299. WILSON, E. B. (1952). An Introduction to Scientific Research, p. 237. New York: McGraw-Hill.

Longueurs de liaison et transfert de charge dans les sels du tétracyanoquinodiméthane (TCNQ)

Abstract: An examination of most of the known crystal structures of TCNQ salts is made in connexion with deter mining the amount of charge transfer A method based on the bond lengths is shown to provide a sensitive test for determining this important quantity' The charge distribution between different TCNQ molecules of the same compound can also be deduced The results obtained are used to correlate the degree of charge with the occurrence of a hydrogen bond the rotation of C(CN)z groups and the experimental values of dipolar splittings D and E

Predicting bond lengths in inorganic crystals

Abstract: Bond lengths in inorganic crystals can be predicted by solving a model based on a network of chemical bonds. The results agree with observed bond lengths to within a few hundredths of an hngstrrm for most bonds except for those to alkali metals. An earlier method of predicting bond lengths [Baur (1970), Trans. Amer. Cryst. Assoc. 6, 129-155] gives predictions of comparable overall accuracy but the difference in the approaches leads to significant differences in the predictions for individual bonds. The two methods each have their own strengths and should be considered as complementary. The model has possible extensions to amorphous materials and suggests, for example, that the lone pair of valence electrons is not responsible for the distorted environments found in TeCI 4 and T e I 4 in the solid state. 1305

The geometry of the nonaaqualanthanoid(3+) complex in the solid bromates and ethyl sulphates

Abstract: The structures of [Ln(H20)9I(BrO3) 3 and ILn(H20)gI(CzHsSO4)3, Ln = Pr and Yb, have been determined by X-ray diffraction. The structures were refined to R = 0.025--0.040. Accurate cell dimensions for the bromates and ethyl sulphates of all the trivalent Ln ions except Pm were determined at 23°C with Guinier-H~igg powder photographs. The bromates crystallize in the space group P63/mmc with Z = 2, a = 11.8395 (11), c = 6.8012 (9)/~ for Pr and a = 1 I. 7056 (13), c = 6-6474 (9) A for Yb. The ethyl sulphates crystallize in space group P6ffm with Z = 2, a -14.0454 (8), c = 7. 1207 (6)/~ for Pr and a = 13.8991 (8), c -7.0247 (6) ,~ for Yb. Tricapped trigonal prisms of water O atoms with symmetry D3h in the bromates and C3h in the ethyl sulphates surround the nine-coordinated Ln ions. The complex ions form columns about the 6 axes. The anions are hydrogen-bonded to the complex ions and form columns about the 63 axes. The decrease in the prismatic Ln-O bonds between the Pr and Yb structures is what is expected from the lanthanoid contraction, 0.155 ,/~, but van der Waals repulsions between prismatic and equatorial O atoms make the decrease in the equatorial Ln--O bonds about hal f this value. The variation in cell dimensions through the Ln series for both types of compounds has been related to this repulsion and to the hydrogen-bond systems.

A reinvestigation of tolane

Abstract: Cl4Hl0, monoclinic, P2Ja, a = 12.778 (2), b = 5.764 (1), c = 15.508 (4) A, f l = 113.39 (2 ) ° ,Z = 4, R w = 0.036 for 1305 counter reflections. The asymmetric unit consists of two crystallographically independent half molecules with similar bond distances and angles. The molecules are planar. Semi-empirical CNDO/2 calculations predict a quasi free rotation of the phenyl ring around the C C single bond with a rotation barrier of 0.65 kcal molIntroduction. The structure of 1,2-diphenylacetylene was examined by Robertson & Woodward (1938). Their experimental data were limited to 106 reflections in the hOl plane, and the results are not very accurate. We have re-examined tolane counter data, to obtain better structural parameters. Tolane was synthesized by a standard method (Fieser & Fieser, 1967). Crystals were obtained by cooling (24 h) a solution in methanol. A suitable crystal, 0.6 x 0.6 x 0.4 mm, covered with glue to prevent sublimation, was used for data collection. The absences hOl: h = 2n, and 0k0: k = 2n and the monoclinic symmetry verified the previously reported space group (Robertson & Woodward, 1938). However, during the structure determination it was realized that the present unit cell (I) is related to that of Robertson & Woodward (II) by: a(I) = a(II), b(I) = -b(II ) , e(I) = [a ( I I ) + e(II)]. The density measured by flotation in CaC12 solution is 1.136 g cm -3 at 23°C and the calculated density based on four molecules per unit cell is 1.129 g cm -3 at 25°C. Diffraction data were collected at an average temperature of 24 .5°C with a Picker FACS-I automatic diffractometer using Zr-filtered Mo K¢~ radiation (/t = 0-69 cm-~). The lattice parameters were obtained by least-squares refinement of the 20, o~, g and tp angles of 12 reflections in the range 35 < 20 < 39 °, for which the ttl~t2 doublet of Mo K~t radiation was resolved. Intensities of reflections were measured by the 0-20 scan method with a scan speed of 1 °(20) min -~ and a 10 s background count at the start and end of each scan. The intensities of three standard reflections (232, 22(], 218) measured every 50 reflections were used to monitor X-ray damage and alignment of the crystal. Of 2062 unique reflections in the range 20 < 50 °, 209 were systematically absent, and 548 with I < a(I) were considered unobserved. The intensities were corrected for Lorentz and polarization factors, and for decay based on two different slopes: up to sin 0/2 = 0.54 A -I a maximum linear correction of 9% was applied and to the rest of the data (0.54 < sin 0/2 < 0.60 A -1) a maximum of 20%. Standard deviations for the structure factors were obtained from counting statistics (Wei & Ward, 1976). No corrections for absorption or extinction were made. Attenuation filters were used to keep the intensities within the linearity range of the counter. Approximate scale and isotropic temperature factors were determined by Wilson's (1942) method. The structure was solved from the model of Robertson & Woodward (1938). The coordinates of seven C atoms at y ~ 0 were used to synthesize a Fourier map. This revealed the positions of 13 of the 14 C atoms. Successive difference syntheses improved the coordinates of all the C atoms and gave R = Z [ i F o l IFcli/E IFot = 0.308. At this point a full-matrix leastsquares refinement with the program ORFLS (Busing, Martin & Levy, 1962) and unit weights lowered R to 0.141. A difference map revealed the positions of all H atoms. They were assigned isotropic temperature factors 1.25 /~2 greater than those of the C atoms to which they were bonded. The resultant structure factor calculation had an R of 0.128. A least-squares cycle on the C atom coordinates and the scale factor lowered R to 0-I12. Anisotropic thermal parameters were introduced, and two cycles of refinement, one on anisotropic thermal parameters and one on the positional parameters of the C atoms, reduced R to 0.085. The coordinates of the H atoms were improved by another difference synthesis and R dropped to 0.072. The refinement was continued by weighted [w = 1/a(F)] least squares until the parameter shifts were insignificant compared with the estimated standard deviations.