R. Ingalls

Low Temperature Apparatus for High Pressure Mössbauer Studies

Relatively simple apparatus for lowering the temperature of a sample subjected to a very high pressure is presented. The pressure system is of the supported anvil type and can produce pressures in excess of 200 kilobars. Either a liquid nitrogen flow system or a liquid helium cryostat is used to cool the pressure cell. The pressure transmitting column is made of a special low conductive grade of fiberglass. Sample temperatures of 14 K at pressures up to 170 kilobars can be obtained.

Mössbauer studies of iron silicate spinel at high pressure

We have measured in situ Mössbauer transmission spectra of iron silicate spinel (γ-Fe2SiO4) in a diamond anvil cell at room temperature and pressures up to 16 GPa. The observed spectra show a doublet characteristic of the paramagnetic state. The isomer shift and quadrupole splitting at atmospheric pressure are 1.10 and 2.63 mm/s, respectively, which are smaller than those of fayalite (α-Fe2SiO4). Both the isomer shift and quadrupole splitting decrease linearly with pressure with slope of −0.003(1) and −0.020(1) mm/sec · GPa, respectively. This simple linear trend suggests that no electronic or polymorphic transitions occur under 16 GPa except for those due to the small and continuous changes of volume and local symmetry under pressure. On the basis of a crystalline field calculation, the negative pressure derivative of the quadrupole splitting is associated with a trend towards an ideal cubic symmetry of the oxygen sublattice.

Automated spectrometer for pressure measurement using ruby fluorescence

A scanning fluorescence spectrophotometer intended for measurement of the ruby R1 lines in a high‐pressure diamond anvil cell is described. A rigid framework supports the electronic and optical components and an XYZ translation stage to hold the pressure cell. Innovations include a digital monochromator scan controller, the automatic placement of tick marks on the recorder output at 0.1 nm intervals, and light detection with a photodiode. The unit is compact, weighs 50 kg, and requires only a chart recorder for operation as an automated high‐pressure measurement station. These features allow easy shipment and reassembly at remote experimental sites.

Modified apparatus for low temperature/high pressure Mössbauer absorber studies

A method is described in which a system designed to study Mossbauer sources as a function of pressure and temperature is modified to include absorbers as well. This is accomplished by coupling the Mossbauer source, mounted inside a cryostat, to an external transducer by means of a bellows. In addition, a simple modification has converted an existing helium Dewar to a dynamic gas flow cryostat. Temperature can be continuously varied from 300 to 20 K at pressures up to 200 kilobars.

Determining crystalline atomic positions using XAFS, a new addition to the UWXAFS analysis package

XAFS and x-ray diffraction (XRD) are complementary structure determination techniques. The combination of XAFS and XRD can be used to determine the complete crystal structure when diffraction can not be refined. This is often the case at high pressures or high temperatures where there is limited access to the samples and energy dispersive x-ray diffraction is used. A new method to determine the atomic positions within the unit cell using EXAFS data with the programs RUNFIT and MKFIT is described. These programs systematically produce and test models for the XAFS data that are consistent with the diffraction results. The programs were written to solve the structure of two intermediate high pressure phases of AgCl, and are distributed with a working example.