Defect studies of molecular and rare-gas crystals down to 50 mK and up to 150 MPa

Conference · Thu Jan 01 04:00:00 EST 1981

OSTI ID:6128433

Simmons, R O

Techniques have been developed to prepare single crystals of condensed gases of mm or larger size, to ascertain their degree of crystalline perfection and fix their orientation, to measure Bragg spacings, changes in Bragg spacings and intensities. For precise work at higher temperatures and pressures a Be specimen cell attached to a heat exchanger is used at the focus of a large orientable back-reflection camera. Phase transitions arising from changes in molecular rotational ordering in solid CH/sub 4/ and CD/sub 4/ and lattice parameters during sublimation and melting of solid Ar, Kr, and Xe have thus been studied. For lower temperatures, the x-ray scattering specimen cell is fixed to a helium dilution refrigerator; it can be taken down to 50 mK and internally pressurized to 20 MPa. The sealed-off x-ray source is orientable about three orthogonal axes. Scattered x-rays are collected in an orientable position-sensitive detector. Examples of investigations by coherent scattering include 1) determination of the symmetries of the different phases below 1 K of the system solid ortho- and para- H/sub 2/, and 2) microscopic study of isotopic phase separation in solid /sup 3/He-/sup 4/He mixtures below 0.4 K. In case 1), the known fcc form of solid hydrogen, stable below about 2 K for ortho-rich material, was determined to transform to hcp form at lower concentrations and temperatures. The transformation temperatures and compositions could be followed, despite internal specimen heating due to spontaneous ortho- to paraconverision, below 200 mK. In case 2), x-ray studies of isotopic phase separation in /sup 3/He-/sup 4/He bcc solids were carried out by B. A. Fraass. Direct determination of Bragg spacings of the daughter phases and their perfection and orientation with respect to the matrix could be made for the first time, and new characteristics of the phase separation phenomena discovered as specimens were cooled and reheated in a range below 400 mK.

• Research Organization: Illinois Univ., Urbana (USA). Dept. of Physics
• Sponsoring Organization: USDOE
• DOE Contract Number: AC02-76ER01198
• OSTI ID: 6128433
• Report Number(s): DOE/ER/01198--135; CONF-810386--1; ON: DE81026617
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360602* -- Other Materials-- Structure & Phase Studies
656000 -- Condensed Matter Physics
75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
ALKANES
ARGON
COHERENT SCATTERING
CRYSTAL DEFECTS
CRYSTAL STRUCTURE
CRYSTAL-PHASE TRANSFORMATIONS
CRYSTALS
DEUTERIUM COMPOUNDS
DIFFRACTION
ELEMENTS
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
FLUIDS
GASES
GRAIN ORIENTATION
HELIUM 3
HELIUM 4
HELIUM ISOTOPES
HYDROCARBONS
HYDROGEN
HYDROGEN COMPOUNDS
ISOTOPES
KRYPTON
LATTICE PARAMETERS
LIGHT NUCLEI
METHANE
MICROSTRUCTURE
MOLECULAR CRYSTALS
MONOCRYSTALS
NONMETALS
NUCLEI
ORGANIC COMPOUNDS
ORIENTATION
PHASE TRANSFORMATIONS
RARE GASES
SCATTERING
STABLE ISOTOPES
ULTRALOW TEMPERATURE
VERY HIGH PRESSURE
X-RAY DIFFRACTION
XENON