45796-AC10
New Materials from High-Pressure, High-Temperature Synthesis
Significance: The rapidly increasing demand for materials with new or improved properties necessitates materials chemists to explore alternative preparation methods. This ACS-PRF sponsored research explores the versatility of multi-anvil high-pressure techniques for the synthesis of new metal hydride systems employing in-situ generated hydrogen from the decomposition of a suitable precursor and new thermoelectric materials from molten metal fluxes. Both types of materials are of high relevance to energy technology. Organization of research: The project consists of two components. (i) Optimization of multi-anvil techniques for handling large sample volumes (approaching the cm3 range) at still sizeable pressures (8 GPa and above). (ii) The actual synthesis part. While the technical development steps obliges a postdoctoral researcher (Dr. Emil Stoyanov) the synthesis part adjoins to the PhD projects of two graduate students (Johanna Nylen and Kati Puhakainen). During summer 2007 (May 15 – August 15) the project was enriched by a PRF-sponsored SUMR researcher (Craig Naseyowma) and an intern student from India (Subhadep Kal) who then became attracted by high pressure research and will join ASU as a graduate student next year.
Results: (1) As of August 2008 the optimization steps are basically concluded with the development and extensive testing of two new large volume assemblies that allow initial sample volumes of around 150 mm3 and 550 mm3 for pressures up to 10 GPa and 7.5 GPa, respectively. These new assemblies are not only important for the synthesis of materials targeted in this PRF project, but will generally advance high-pressure materials synthesis by allowing a much broader variety of chemical reactions than what has been previously possible. A manuscript on the development and testing of our large volume assemblies is currently in preparation.
(2) Subhadeep Kal’s intern project (summer 2007) was concerned with the high pressure structure and phase stability of CeCu2 type intermetallic compounds. Here we could establish a general transformation behavior leading the Laves phase structures. His work resulted in two publications.
(3) New thermoelectric materials: During his SUMR research Craig Naseyowma performed research with graduate student Johann Nylen on the thermoelectric properties of amorphous Zn-Sb materials which were obtained by subjecting the crystalline compounds ZnSb and b-Zn4Sb3 and mixtures of ZnSb with elemental Zn to high-pressure high-temperature conditions. A manuscript on this, and subsequent work, has been recently submitted to Chemistry of Materials (Y. Wu, J. Nylén, N. Newman, C. Naseyowma, F. J. Garcia-Garcia, U. Häussermann “Comparative study of the thermoelectric properties of amorphous Zn41Sb59 and crystalline Zn4Sb3”).
(4) New hydrogen rich materials: We identified ammonia borane (BH3NH3) as an ideal hydrogen source for multi-anvil hydrogenations. BH3NH3 has a high hydrogen capacity and decomposes cleanly into inert BN and hydrogen at elevated pressures. The pressure dependency of decomposition temperatures and decomposition products of ammonia borane have been established with diamond-anvil cell (DAC)-Raman experiments (Figs. below). Importantly, the fully deuterized derivative BD3ND3 is also easily accessible and can be employed for the preparation of deuterized samples for neutron diffraction studies. A manuscript on the decomposition of BH3NH3 under pressure and temperature is currently in preparation. For the application of ammonia borane as a hydrogen source in the multi-anvil experiment starting materials are sealed in NaCl containers to prevent hydrogen diffusion. As a preliminary result we report here the synthesis of Li2PtH6 - where Pt formally possesses the oxidation state IV - from a mixture of LiH and Pt at 7.7 GPa and 500 oC. With low pressure autoclave synthesis the elemental combination Li/Pt affords only Li5Pt2H9, where Pt remains in the oxidation state II.
Outlook: With the development of large volume assemblies and the identification of BH3NH3 as internal hydrogen source, multi-anvil hydrogenations at gigapascal pressures can now be performed routinely. In the remaining time of the project we will further explore the potential of s-block metal/transition metal and main group metal/semimetal combinations to produce hydrogen rich compounds and characterize their structural and physical properties.
