Transition-metal-hydrogen systems at extreme conditions

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.