Carroll K. Johnson

Superstructure and modulation wave analysis for the unidimensional conductor hepta- (tetrathiafulvalene) pentaiodide

When acetonitrile solutions of tetrathiafulvalene [i.e., (C3S2H2)2; herein called TTF] and iodine, I2, are allowed to mix by diffusion, monoclinic crystals are formed with the approximate composition (TTF)7I5. These crystals have an electrical conductivity along the needle axis c of 300 ohm−1⋅cm−1 at room temperature. A complete crystal structure analysis based on an approximate supercell with a=48.016(16) A, b= 16.041(6) A, c=24.877(7) A, β=91.31(2) °, Z=12 (RRF)7I5 was carried out using space group P21/a with 225 nonhydrogen atoms in the asymmetric unit. The TTF molecules stack along c and form a TTF subcell a1=a/3, b1=b, c1=c/7, Z1=4 TTF and subcell symmetry C2/m. The iodine atoms are in chains along c and form an A‐centered sublattice complex (space group A2/m) with a2=8.213 A, b2= b, c2=5.023 A, β2=103.0°, and Z2=2 I. The supercell space group P21/a is the mathematical intersection of the two subcell space groups. The supercell is only approximate because 5c2=25.113(14) A which is about 1% longer tha...

A neutron diffraction study of the diaquohydronium ion in yttrium oxalate trihydrate

Five structures of yttrium oxalate trihydrate and its deuterated isomorphs have been determined from least‐squares refinement of neutron diffraction data. Three structures with different H–D isotopic ratios were determined from 25 °C data and two structures with the same isotopic ratios were determined from data collected at 60 and −155 °C. The cell dimensions for the structures are as follows: YHOX 25 °C, a0 = 8.6962(6) A, c0 = 12.824(3) A YDOX 25 °C, a0 = 8.6989(7) A, c0 = 12.818(3) A YHDOX 25 °C, a0 = 8.704(2) A, c0 = 12.832(2) A YHDOX 60 °C, a0 = 8.708(2) A, c0 = 6.435(3) A YHDOX −155 °C, a0 = 8.677(4) A, c0 = 12.751(7) A. In each of the structures a proton or deuteron is on a twofold symmetry axis and nearly centered between two H2O molecules forming an H5O2 ion. The O–H–O bond is symmetrical but not linear; the z parameters for H and O are slightly different. The O–H–O distance in the diaquo ion is 2.442(2) A, and the O–D–O distance is 2.454(2) A. The four other protons in the diaquo ion (paired by ...

A multi-anode proportional counter

Abstract A neutron detector has been designed and constructed which contains seven independent anodes inside a cylindrical envelope. The inside of the detector contains one anode element in the center and the other six elements arranged in a ring surrounding the center element. Each element acts as an independent proportional counter, but because all the elements share in a common gas volume the operating characteristics will always remain closely matched. The multi-anode detector was designed for use in a four-circle single-crystal diffractometer. The relatively low cost, compactness and simplicity of this “7 pixel area detector” make it attractive for applications where larger arrays would be prohibitive. The main purpose of the multicounter system is to map diffuse intensity in the six peripheral counters while the main Bragg peak is being measured in the center counter.

Crystallographic analysis of superstructure modulation in the organic conductor (TTF)7I5

Abstract The approximate supercell has P21/a symmetry with Z = 12(TTF)7I5 and contains 21 C-centered TTF subcells and 30 A-centered iodine subcells. A complete x-ray structure analysis, followed by Fourier analyses of the rigid motion displacements from the mean sublattice sites, yielded a 3-dimensional modulation wave spectrum. The major comcomponents of the TTF modulation spectrum are directly related to the iodine sublattice periodicity and the major components of the iodine modulation spectrum are related to the TTF sublattice periodicity. These dual sublattice perturbation harmonics apparently arise from the strictly geometrical aspects of the dual sublattice system. The results at 300 K do not require a charge density wave explanation.