(Tetrathiotetracene)2(iodide)3, (TTT)2I3, is metallic and can be crystallized with varying amounts of disorder. The crystal structures of highly disordered, h.d., (TTT)2I3 and (TTT)2I3 with significant lower disorder, l.d., have been solved. For (TTT)2I3 (l.d.) the basic structure, defined primarily by the TTT molecules, was refined in the space group Cmca. The doubled cell, which includes data from the strongest diffuse layer line, was refined in Pmc21. All iodide sites in (TTT)2I3 (l.d.) have less than full occupancy, indicating that the contents of cells vary and suggesting the presence of species such as I3−, I2, and I−. The crystal structure of (TTT)2I3 (h.d.) was investigated at room temperature (∼294 °K), 164, 74, and 19 °K. At all four temperatures the symmetry of the basic structure, defined primarily by the TTT molecules, remains orthorhombic. The basic TTT structure at all four temperatures was successfully refined in the space group Cmca. During slow cooling the diffuse layer lines were carefully monitored. Even with slow cooling the iodide chains do not three dimensionally order, and no distortions occur in the basic TTT structure down to 19 °K. A model of the iodide chains is presented which explains the positions and intensities of the diffuse layer lines and the absence of three-dimensional ordering at low temperatures.

(TTF)Clx, x=0.67 and 0.70, is a quasi-one-dimensional organic conductor with a room temperature conductivity of ∼ 150 Ω−1 cm−1. At room temperature the structure is tetragonal and consists of chains of uniformly spaced, eclipsed TTF molecules surrounding channels occupied by chloride ions, which form a disordered structure. The chloride substructure becomes ordered and the TTF substructure undergoes a phase transition from tetragonal to monoclinic symmetry at ∼250 °K. The angle β of the monoclinic phase increases continuously as the temperature is decreased from 245 ° to 19 °K. The electrical conductivity shows a sharp decrease at the phase transition which is suggestive of the formation of commensurate charge density waves in the monoclinic phase.

A unique low-temperature, full-circle goniometer for single-crystal X-ray diffraction has been constructed with the novel adaptation of a closed-cycle refrigerator in such a manner that the cold finger remains stationary during the ϕ and χ rotations. This has been made possible through the development of a flexible heat-transfer link between the top of the ϕ shaft and the cold station. The new design concept incorporates several important features: (1) the vacuum shroud and radiation shields also remain stationary so as to reduce the X-ray windows to narrow slots; (2) the sample crystal is enclosed in a black, nearly isothermal cavity (Hohlraum); and (3) the ϕ shaft is mechanically decoupled from the thermal contraction of the cold finger and other parts. The new system has considerable advantages over presently existing low-temperature adaptations for X-ray diffraction: (1) the goniometer operates between 300 and 18 K without expenditure of liquid cryogen and with minimal attendance; (2) the system is capable of operating for long periods of time with minimal risk of interruption of the cooling cycle; (3) absorption of X-rays by the windows is minimal and independent of the Bragg angle; and (4) data collection at any temperature down to its present lowest limit of 18 K is virtually as easy as with any conventional room-temperature diffractometer. At the time of writing the instrument has been in use for over 10000 h and has measured over 100000 X-ray reflections from single crystals.

An X-ray focusing monochromator for small-angle diffraction studies was designed for use with the synchrotron radiation from the storage ring, SPEAR, at the Stanford Linear Accelerator Center. It incorporates a 7 cm long silicon crystal, cut at 8°30′ to the (111) planes and bent to a logarithmic spiral curvature for horizontal focusing and monochromatization. A 120 cm long elliptically curved float-glass mirror is used for vertical focusing, and provides means of eliminating higher-order harmonics of smaller wavelength. With SPEAR operating at 3.7 GeV, 20 mA, the two elements produce a 0.5 × 0.5 mm focused beam with an intensity of 6 × 108 photons s−1. The diffraction pattern of frog sciatic nerve myelin obtained with this system was compared with that obtained with a 300 W conventional microfocus X-ray source and a toroidal camera. The new system shows a 190-fold gain in the integrated intensity on photographic film. Synchrotron radiation provides a broad X-ray spectrum. The monochromator is tunable to any wavelength between 0.5 and 3 Å, with a total wavelength spread in the focused beam of about 0.01 at 1.74 Å. The broad spectrum allows wavelength selection for anomalous-scattering experiments.

A total-reflecting mirror of 120-cm length was designed and built to focus synchrotron radiation emanating from the electron–positron storage ring at the Stanford Linear Accelerator Center (SPEAR). The reflecting surface is of unpolished float glass. The bending and tilt mechanism allows very fine control of the curvature and selectability of the critical angle for wavelengths ranging from 0.5 to 3.0 Å. Elliptical curvature is used to minimize aberrations. The mirror is placed asymmetrically onto the ellipse so as to achieve a tenfold demagnification of the source. The bending mechanism reduces nonelastic deformation (flow) and minimizes strains and stresses in the glass despite its length. Special design features assure stability of the focused image. The mirror reduces the intensity of shorter wavelength harmonics by a factor of approximately 100.

This paper describes the kind and sequence of measurements that lead to an accurate assessment of all instrument parameters of an x-ray diffractometer that incorporates up to six axes of rotation. Included in this description are determinations of the zero settings of 2θ, ω, and χ with or without the use of x rays. Each parameter can be uniquely determined, one at a time, in a completely straightforward manner with the use of only two tools, the autocollimator and the dial indicator, and a small assortment of special fixtures. A few design features are outlined that facilitate alignment checks. Directions for the alignment of a number of components are given, and can be performed by the user, thereby eliminating the need for expensive machining. A brief discussion of some serious errors caused by defective counters is also included.