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Project objectives: 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 (“hydrogen dominant materials”) employing in-situ generated hydrogen from the decomposition of a suitable precursor. Hydrogen dominant materials are metal or mixed metal/semimetal hydrides that have chemical compositions and/or crystal structures not accessible by conventional synthesis techniques. A major attraction of hydrogen dominant materials is their potential high temperature superconductor properties and their role as model systems towards a more fundamental understanding of metal/semimetal-hydrogen interactions, providing important input to applied research concerned with hydrogen technology.

 Organization of research: The project consists of three 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) Identification of suitable precursors for in-situ hydrogen generation. (iii) Synthesis of hydrogen dominant materials.

Results:

(1) Optimization of multi-anvil techniques for handling large sample volumes.

High pressure materials synthesis is generally hampered by small sample volumes which restrict seriously the arsenal of preparation processes. Hitherto it has been limited to transformations and decompositions of single component starting materials and simple solid-solid reactions. For materials synthesis/design larger reaction volumes are needed that allow the setup of more elaborated synthesis schemes and furthermore afford sample quantities that allow subsequent studies of quenchable high-pressure phases, such as detailed property measurements or their use as precursors for further reactions. Importantly, without the possibility of scaling up their preparation, high-pressure phases would be excluded as technological materials. We have developed and extensively tested two new multi-anvil assemblies specially designed for chemical synthesis at high pressures. They combine large initial sample volumes (around 150 mm³ and 350 mm³ for pressures up to 10 GPa and 7.5 GPa, respectively) with very small thermal gradients. (E. Stoyanov, U. Häussermann, K. Leinenweber “Large volume multianvil cells designed for chemical synthesis at high pressures” High Pressure Research, submitted for publication.)

(2) Identification of suitable precursors for in-situ hydrogen generation.

Ammonia borane, BH₃NH₃ has been extensively studied as a potential candidate for chemical hydrogen storage. We investigated the effect of high pressure on the thermal decomposition of BH₃NH₃ in situ by Raman spectroscopy. At ambient pressure the molecular solid decomposes irreversibly in three steps and the release of all the hydrogen atoms is only accomplished at 500 °C. At high pressures only two steps of decomposition take place. While the residual after the first decomposition, polymeric (BH₂NH₂)_x is also observed at ambient pressure, the residual after the second decomposition is unique to high pressure. Raman spectroscopy and powder x-ray diffraction suggest a close relationship of to h-BN, with B and N atoms arranged in macromolecular hexagon layers that are terminated by H atoms. Increasing pressure increases the temperature of both decomposition steps. Due to the increased first decomposition temperature it becomes possible to observe a high pressure, high temperature phase (or melting) of BH₃NH₃ which has been previously unknown. With its high hydrogen content and clean decomposition into an inert residual, ammonia borane represents an ideal hydrogen source for high pressure in-situ hydrogenations.

This study continues by further investigating the nature of the new high pressure, high temperature phase of BH₃NH₃ for which beamtime at the Argonne Advanced Photon Source (APS) has been awarded. Concomitantly, also the high pressure structural phase transitions of this compound at room temperature will be elucidated. From Raman spectroscopy several transitions have been suggested, but no structure characterization has been performed yet. BH₃NH₃ displays unconventional dihydrogen bonding in its ambient pressure molecular crystal structure, where intermolecular H^d+...H^d- distances approach 2 Å. For a more fundamental understanding of this kind of bonding it would be important to know how directional dihydrogen bonds are compared to classic hydrogen bonds, and how dihydrogen bonds will rearrange when applying external pressure.

(J. Nylén, T. Sato, E. Soignard, J. L. Yarger, E. Stoyanov, U. Häussermann “Thermal decomposition of ammonia borane at high pressures” Journal of Chemical Physics, 131 (2009), 104506)

(3) Synthesis of hydrogen dominant materials.
The combination of large volume multi-anvil assembly and ammonia borane as internal hydrogen source has been employed so far for the preparation of Li₂PtH₆ from a mixture of LiH and Pt at 7.7 GPa and 500 ^oC. This compound contains octahedral [PtH₆]^2- entities and thus Pt formally in the oxidation state IV. In contrast, by low pressure autoclave synthesis only Li₅Pt₂H₉ with Pt in the oxidation state II is accessible. BD₃ND₃, which can be prepared from the reaction of BD₃∙ THF with ammonia and subsequent proton exchange with D₂O, allows the synthesis of deuterides for neutron diffraction studies. Remarkably, with the new multi-anvil assemblies sample amounts suitable for neutron diffraction can be achieved in a single run.

(K. Puhakainen, E. Stoyanov , K. Leinenweber, U. Häussermann ”Li₂PtH₆ – a new ternary metal hydride from high-pressure hydrogenation“, manuscript in preparation).

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. Currently we are investigating if multi-anvil hydrogenations afford more new complex transition metal (T) hydrides containing homoleptic hydrido [TH_n]^m- anions. New hydrogen dominant materials may also be found for main group metal/semimetal systems. Such systems have been predicted as potential high-Tc superconductors. Furthermore, the new large volume multi-anvil assemblies together with a variety of sample capsules (BN, noble metals, Teflon, salt) represent a platform for a broad high pressure materials synthesis program in the future. Emerging capabilities lie especially in reactions involving a solvent/liquid phase (flux and hydrothermal reactions) and solid-molecular gas reactions. With this, the ACS-PRF sponsored project ”New Materials from High-Pressure, High-Temperature Synthesis” truly fulfilled its purpose to seed-fund new directions.