C. E. Weir

Crystal Structure of Benzene II at 25 Kilobars

Crystals of a high-pressure form of benzene (benzene 11) were grown in the diamond-anvil pressure cell at elevated temperature and pressure from the transition of solid I to solid II. X-ray precession data were obtained from a single-crystal in the high-pressure cell. At 21�C and about 25 kilobars, benzene II crystallizes in the monoclinic system with a = 5.417 � 0.005 angstroms (S.D.), b = 5.376 � 0.019 angstroms, c = 7.532 � 0.007 angstroms, β = 110.00� � 0.08�, space group P21/ c, Pc= 1.26 grams per cubic centimeter. The crystal structure was solved by generating all possible molecular packing configurations and calculating structure factors, reliability factors, and packing energies for each configuration. This procedure produced a unique solution for the molecular packing of benzene II.

Polymorphism in benzene, naphthalene, and anthracene at high pressure.

Optical observations, in which a microscope was used with the diamond-anvil pressure cell, were carried out on benzene, naphthalene, and anthracene up to temperatures of about 600�C and pressures of approximately 40 kilobars. New high-pressure phases of benzene (benzene III) and anthracene (anthracene II) were observed, and the existence of the high-pressure polymorph, naphthalene II, was verified. All three materials decompose initially to a reddish-orange liquid, and ultimately to amorphous carbon. The decomposition temperatures decrease with increasing molecular size.

Crystallography of Some High‐Pressure Forms of C6H6, CS2, Br2, CCl4, and KNO3

From single crystal x‐ray diffraction at high pressure and room temperature unit‐cell and space‐group data were obtained for the following materials: C6H6, I—orthorhombic, a = 7.17, b = 9.28, c = 6.65, Pbca, CS2—orthorhombic, a = 6.16, b = 5.38, c = 8.53, Cmca, Br2—orthorhombic, a = 8.54, b = 6.75, c = 8.63, Cmca, CCl4, I—rhombohedral, a = 14.27, α = 90°, CCl4, II—monoclinic, a = 22.10, b = 11.05, c = 25.0, β = 114°, Cc or C2 / c, CCl4, III—orthorhombic, a = 11.16, b = 14.32, c = 5.74, C2221, KNO3, III—rhombohedral, a = 4.31, α = 78°54′, KNO3, IV(?)—orthorhombic, a = 5.58, b = 7.52, c = 6.58, P2lnb or Pmnb. All unit‐cell dimensions are given in angstroms with estimated uncertainties of ± 2 in the last decimal place given and uncertainties of ± 0.5 deg in angles.

Compressibility of Inorganic Azides

The compressibility of α lead azide, β lead azide, barium azide, potassium azide, sodium azide, and thallium azide have been measured by single‐crystal x‐ray diffraction techniques for the first time in a new application of the diamond anvil pressure cell. Both the anisotropic and volume compressibilities are reported. The pressures were determined by measurements at the known freezing points of n‐hexane and ethanol. A phase transition occurs in thallium azide at a pressure between the freezing points of chloroform (5390 bar) and n‐decane (2990 bar). Pressure–temperature observations in the diamond cell of lead azide were carried out to 300°C and approximately 30 kbar. No phase transitions were observed. Radiation damage to azide crystals under high pressures is reduced significantly.

Allotropy in Some Rare-Earth Metals at High Pressures

Allotropes of lanthanum, cerium, praseodymium, and neodymiumc have been observed at elevated pressures with an x-ray diffraction camera which incorporates a diamondanvil, high-pressure cell. In each case a high-pressure modification was observed which has a face-centered cubic structure. At room temperature the unit cell dimensions (αo) for the high-pressure face-centered cubic structures and approximate pressures at which they were determined are as follows: La, 5.17 A(23 kb); Ce, 4.82 A(15 kb); Pr, 4.88 A(40 kb); and Nd, 4.80 A(50 kb). The unit cell dimensions for the high-pressure forms of La, Pr, and Nd apparently have never been reported.

Studies of infrared absorption spectra of solids at high pressures

Abstract Infrared spectra of solids were studied with a diamond pressure cell in the wavelength range 5–15 μ at pressures between 1 atm and 50,000 atm. The calibration of the cell at the 14,000 atm transition of NaNO2 is described. Spectra were studied for aromatic organic compounds, inorganic hydrates, and ammonium halides. In general, band shifts produced by pressure were to higher frequencies and at most 10 cm −1 10,000 atm . Many bands exhibited large changes in intensity. Occasionally bands increased in intensity or were unaffected but in general a decrease in intensity was observed at elevated pressure. Representative spectra are given, one to a pressure of 160,000 atm. Suggestions for the causes of the frequency shifts are given.

Instrumentation for Single Crystal X‐Ray Diffraction at High Pressures

A diamond anvil high pressure cell made of beryllium, a special goniometer head, and a Buerger‐type precession camera designed for x‐ray diffraction by single crystals at high pressures are described in some detail. The most important problems still to be solved are discussed.

Low temperature infra-red spectra of crystalline inorganic compounds containing tetrahedral anions☆

Abstract Infra-red absorption spectra of K 2 SO 4 , BaSO 4 , SrSO 4 , PbSO 4 , K 2 CrO 4 , KClO 4 and KMnO 4 have been obtained on single crystals at room temperature, under liquid nitrogen refrigeration and under liquid helium refrigeration. The diffused absorption observed at room temperature was reduced upon cooling by the removal of a large number of bands, internal fundamentals—lattice modes, from the spectra. The band envelopes present in the low temperature spectra are interpreted as fundamentals and as overtones or combination tones of these fundamentals. The structure in the fundamental band envelopes have been assigned as either librational modes + fundamental combination tones or acoustic modes + fundamental combination tones.

Lattice Frequencies and Rotational Barriers for Inorganic Carbonates and Nitrates

Infrared absorption spectra of inorganic nitrates and carbonates were obtained on single crystals at temperatures between room temperature and liquid- helium temperature. Band limits observed in the spectra are interpreted as representing rotational energy barriers. The barrier heights are of the order of 200 cm/sup -1/ and depend on the crystal struFture and the ions involved. Almost all of the bands are interpreted as summation bands of fundamental frequencies with successive levels of a liberating oscillator. The liberation is considered to represent a planar torsional oscillation of the anion about the trigonal axis. M.C.G.I