Transition-metal-hydrogen systems at extreme conditions
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Date
01/07/2013Author
Scheler, Thomas Herbert
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Abstract
The application of extreme conditions offers a general route for the synthesis
of materials under equilibrium conditions. By finely tuning the thermodynamic
variables of pressure and temperature one can manipulate matter on an atomic
scale, creating novel compounds or changing the properties of existing materials.
In particular, the study of hydrogen and hydrogen compounds has attracted
the attention of researchers in the past. Although hydrogen readily reacts
with many elements at ambient conditions, there is a significant “hydride gap”
covering the d-metals between the Cr-group and Cu-group elements. At elevated
pressures however, the chemical potential of hydrogen rises steeply. At sufficient
pressures, hydrogen overcomes the dissociation barrier at the metal surface and
atomic hydrogen diffuses into the metal, usually occupying interstitial sites in the
host matrix. These interstitial hydrogen alloys can exhibit interesting physical
properties, such as modified crystalline structures, different compressibility,
altered microstructure (nanocrystallinity), hydrogen mediated superconductivity
or potential hydrogen storage capabilities. Furthermore, theory predicts that
hydrogen confined in a host matrix might undergo the elusive transition to a
metallic groundstate at considerably lower pressures than pure hydrogen. Most
d-metals have been found to exhibit hydride phases at extended conditions of
pressure and temperature. However, besides rhenium, the 6th row metals between
tungsten and gold, as well as silver, have not or only very recently been found to
form bulk hydrides. In the course of this PhD-thesis, several of the missing metalhydrides
were successfully synthesized in the diamond anvil cell and characterized
by in-situ x-ray diffraction using synchrotron radiation.