G. R. Hearne

A multipurpose miniature piston-cylinder diamond-anvil cell for pressures beyond 100 GPa

A miniature piston-cylinder diamond-anvil cell (DAC) was constructed and tested for pressure operation at and beyond 100 GPa. Its advantages compared to other piston-cylinder DACs are its compactness (22-mm diam by 21-mm high), self-contained force generator, and simple way of operation. Tungsten carbide backing plates are used for supporting the anvils; one with a hemispherical shape allowing for parallelism alignment, and one with a flat circular shape allowing for lateral alignment of the anvils’ culets. The force is generated by six M3 Allen screws and is conveyed to the piston via force rings. Pressures to 130 GPa were achieved with beveled culets having 300-μm flats and Re gaskets. Design features, mode of operation, and performance are described. The latter has been demonstrated for the particular case of Mossbauer spectroscopy in La57FeO3.

57Fe Mössbauer spectroscopy in a diamond‐anvil cell at variable high pressures and cryogenic temperatures

A detailed description is given of a new facility for transmission 57Fe Mossbauer spectroscopy (MS) studies on absorbers in a miniature diamond‐anvil cell in the temperature range 10–300 K and pressure range 0–45 GPa. Spectra with an adequate signal‐to‐noise ratio for a reliable analysis to be made have each been measured in 20–30 h of data accumulation time. This has been accomplished in spite of the severe constraints imposed by both the minute sample area and the high attenuation of the 14.4 keV probing radiation by the diamond anvils compared with the experimental conditions of conventional MS experiments. A custom‐made Rh(57Co) ‘‘point’’ source with a high specific activity has been used in conjunction with absorbers that have an optimal enrichment (10%–20%) of 57Fe. The successful implementation of 57Fe Mossbauer spectroscopy at very high pressures and variable cryogenic temperatures has been accomplished by making relatively simple refinements to existing elements of high‐pressure MS methodology.

57Fe Mössbauer studies at high pressures and low temperatures

A brief description is given of a new facility that permits57Fe Mössbauer spectroscopy (MS) studies in transmission geometry to be made in the range 10–300 K on absorbers pressurized in a diamond-anvil cell. Examples are given of good quality spectra which have each been obtained in 20–30 hours by using a “point” source of ∼5mCi in conjunction with absorbers that have a comparatively low-enrichment (10–20%) of57Fe. The successful execution of57Fe MS at high pressures and variable cryogenic temperatures has been achieved by making simple refinements to existing elements of high-pressure MS methodology. The facility described herein finds important application in fields of geophysics and condensed matter physics.

High pressure metallization of Mott insulators; magnetic, structural and electronic properties

High pressure studies of the insulator‐metal transition in the (TM)I2 (TM=V, Fe, Co, and Ni) compounds are described. Those divalent transition‐metal iodides are structurally isomorphous and classified as Mott Insulators. Resistivity, X‐ray diffraction, and Mossbauer Spectroscopy were employed to investigate the electronic, structural, and magnetic properties as a function of pressure both on the highly correlated and on the metallic regimes.

Pressure evolution of the EFG and magnetic hyperfine field in the layered antiferromagnet FeI2

When FeI2 is subjected to pressures of up to 20 GPa, a change of approximately 20% occurs in the unit cell volume.57Fe Mössbauer spectroscopy (MS) in a diamondanvil cell has been used to monitor the pressure evolution of the hyperfine interaction parameters of this layered antiferromagnetic insulator. The pressure dependence of the quadrupole splittingQS at 296 K exhibits a maximum at ∼12 GPa and the saturation magnetic hyperfine fieldH0 increases from 7.4 T at ambient pressure to 12 T at 18 GPa. A qualitative analysis identifies the pressure evolution ofQS with changes in the trigonal component of the crystal field splitting. The pressure variation ofH0 is attributed to an increase in the average value of the 3d charge density distribution.

Pressure dependence of the electronic and magnetic properties of the layered antiferromagnetic insultaor FeI2

Nuclear hyperfine interaction parameters have been deduced from 57Fe Mossbauer spectra recorded in the range of 0–26 GPa and at temperatures of 10–296 K in anhydrous ferrous iodide. X‐ray diffraction data indicate that the compound is isostructral in the range 0–25 GPa. There is an abrupt change in the electronic state and spin dynamics of the transition‐metal (TM) atom, and in the magnetic behavior of the compound at 20 GPa. In the range 0–19 GPa, antiferromagnetic ordering occurs below well‐defined Neel temperatures TN. TN increases by more than ten‐fold and static internal fields of up to 12 Tesla are generated at the TM nuclear site. At pressures higher than 20 GPa hyperfine structure associated with atomic‐spin fluctuations are evident down to 10 K. An enhanced dynamic internal field of ∼30 Tesla — associated with the spin fluctuations — is sensed by the iron nucleus. No static magnetic ordering due to cooperative exchange interactions is observed above 20 GPa, in contrast to the magnetism that occur...