Decomposition of single-source precursors under high-temperature high-pressure to access osmium–platinum refractory alloys

https://doi.org/10.1016/j.jallcom.2019.152121Get rights and content

Highlights

  • Os–Pt binary alloys were prepared from (NH4)2[OsxPt1-xCl6] as single-source precursors.

  • Thermal decomposition of (NH4)2[OsxPt1-xCl6] can be performed under ambient pressure as well as under high-pressure.

  • Miscibility gap between fcc- and hcp-structured alloys does not change with pressure up to at least 50 GPa.

Abstract

Thermal decomposition of (NH4)2[OsxPt1-xCl6] as single-source precursors for Os–Pt binary alloys has been investigated under ambient and high pressure up to 40 GPa. Thermal decomposition of mixed-metal (NH4)2[OsxPt1-xCl6] precursors in hydrogen atmosphere (reductive environment) under ambient pressure results in formation of β-trans-[Pt(NH3)2Cl2] and α-trans-[Pt(NH3)2Cl2] crystalline intermediates as well as single and two-phase Os–Pt binary alloys. For the first time, direct thermal decomposition of coordination compound under pressure has been investigated. A formation of pure metallic alloys from single-source precursors under pressure has been shown. Miscibility between fcc- and hcp-structured alloys has been probed up to 50 GPa by in situ high-pressure X-ray diffraction. Miscibility gap between fcc- and hcp-structured alloys does not change its positions with pressure up to at least 50 GPa.

Introduction

Refractory alloys based on platinum group metals have been proposed as materials for extreme environments such as high mechanical impact, oxidative stress as well as high-temperature high-pressure conditions. Phase diagrams, mechanical and functional properties of refractory platinum group alloys were investigated in many details by heating up to their melting temperatures (i.e. above 1500–3000 °C). However, investigations under high hydrostatic pressure were performed only for a limited number of binary systems, mainly for Fe–Ru, Ir—Hf, Ir–Os, and Ir—Re [[1], [2], [3], [4], [5]]. Recent investigations suggested high phase stability of refractory metals and their alloys up to extremely high-pressures reaching 1 TPa [6,7]. Such finding makes refractory alloys as possible models for investigation and development of ultra-incompressible materials [8]. Investigation of refractory binaries with hexagonal close packed (hcp: Os, Re, Ru) and face centred cubic (fcc: Ir, Pt, Rh) refractory metals allowed one to formulate general trend in their phase stability under pressure as follows: along compression the miscibility gap between hcp and fcc alloys shifts towards the metal with larger atomic volume [9]. Previously we have confirmed this tendency for Ir–Os and Ir—Re alloys, where metals have close atomic volumes and significantly different compressibility (Ir–Os binary alloys) or different atomic volumes and nearly identical compressibility (Ir—Re binary alloys).

Os–Pt binary alloys have relevance not only as refractory constructional system but also as functional materials with high catalytic activity in CO oxidation [[10], [11], [12]]. It should be mentioned that Os–Pt binaries for catalytic and mechanical tests were originally prepared using arc-melting which requires relatively large amount of materials as well as cannot be considered as an effective tool for preparation of supported metallic particles with high porosity. Alternatively, materials for catalytic tests were prepared from water solutions using NaBH4 as reducing agent [13].

In our previous studies, an alternative strategy to access nanoporous refractory alloys from solid or supported single-source precursors has been reported [[3], [4], [5]]. So, Os–Pt alloys can be prepared by reducing a solid single-source precursor in a hydrogen flow according to the following chemical reaction:(NH4)2[OsxPt1-xCl6] + 2H2 —600-800 °C→ OsxPt1-x + 4HCl + 2NH4Cl;or through thermal decomposition in inert atmosphere (He, Ar, or N2):(NH4)2[OsxPt1-xCl6] —600-900 °C→ OsxPt1-x + 2/3N2 + 16/3HCl + 2/3NH4Cl.

Current strategy allows us to prepare Os–Pt alloys in the whole range of concentrations using relatively low temperatures and further probe their phase stability under high-pressure high-temperature conditions.

In the current study, we report an investigation of OsxPt1-x alloys prepared from (NH4)2[OsxPt1-xCl6] coordination compounds as single-source precursors. Thermal decomposition of the precursors in inert and reductive atmospheres at ambient and high pressure up to 40 GPa has been investigated using in situ X-ray diffraction. Phase stability and phase separation of OsxPt1-x alloys have been investigated under extreme conditions up to 50 GPa and 3000 °C. We report behaviour of Os–Pt binary alloys under high-pressure high-temperature conditions to extend our knowledge with a system where both atomic volumes and compressibilities for fcc- and hcp-structured metals are significantly different. Thermal decomposition of (NH4)2[OsxPt1-xCl6] compounds as single-source precursors under high-pressure in inert and reductive environments gave us a possibility to extend our knowledge about behaviour of coordination compounds under extreme conditions as well as to show a possibility to decompose coordination salts under high-pressure without formation of parasitic binary compounds.

Section snippets

Experimental section

Single-source precursors, (NH4)2[OsxPt1-xCl6], were crystallized by adding an excess of saturated water solution of NH4Cl to a mixture of hot concentrated water solutions of (NH4)2[OsCl6] and (NH4)2[PtCl6] (obtained from Acros Organics) [14,15]. Salts were filtered, washed with diluted room temperature water solution of NH4Cl, absolute ethanol and dried in air. Elemental compositions were confirmed in 10 points using a Hitachi S-4800 Field Emission scanning electron microscope (SEM) equipped

Results and discussion

Os–Pt alloys under ambient pressure. A peritectic Os–Pt ambient pressure binary phase diagram was investigated using arc-melted samples [25] (Fig. 1). Peritectic temperature was estimated at around 1955 °C. The maximal solubilities of Pt in hcp-structured (10 at. %) and Os in fcc-structured (20 at. %) alloys were obtained by Rudman [26] using high-temperature sintering of fine metallic powders (Table 1). Later, more concentrated alloys were prepared using thermal decomposition of single source

Conclusions

Os–Pt binary alloys have been investigated in the whole range of compositions. Existing experimental data suggest that published Os–Pt binary phase diagram should be corrected to be able to explain all experimental data. Thermal decomposition of (NH4)2[OsxPt1-xCl6] under ambient and high pressure in inert and reductive atmosphere results in a formation of single and two-phase Os–Pt binary alloys. Alloys recovered from annealing correspond to equilibrium phase diagram and can be used for mapping

Acknowledgements

The authors thank the ID11 and ID06-LVP beamlines at the ESRF, and P02.2 beamline at the PETRA III for providing us measurement time and technical support. We also thank Dr. Michael Hanfland (ESRF) for the help with measurements of recovered samples at the beamline ID15B, and Dr. Harald Müller (ESRF) for his kind assistance with the Chemistry Laboratory facilities at ESRF. The work was partly carried out in accordance with the state assignment for the Institute of Solid State Chemistry of the

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