Synthesis and Characterization of Nitrides of Iridium and Palladium
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Synthesis and Characterization of Nitrides of Iridium and Palladium Jonathan Crowhurst1, Alexander Goncharov2, Babak Sadigh1, Joseph Zaug1, Yue Meng3, and Vitali Prakapenka4 1 Lawrence Livermore National Laboratory, 7000 E Avenue, Livermore, CA, 94551 2 Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, 20015 3 HPCAT, Advanced Photon Source, Argonne National Laboratory, Argonne, IL, 60439 4 GSECARS, Advanced Photon Source, Argonne National Laboratory, Argonne, IL, 60439
ABSTRACT We describe the synthesis of nitrides of iridium and palladium using the laser-heated diamond anvil cell. We have used the in-situ techniques of x-ray powder diffraction and Raman scattering to characterize these compounds and have compared our experimental findings where possible to the results of first-principles theoretical calculations. We suggest that palladium nitride is isostructural with pyrite while iridium nitride has a monoclinic symmetry and may be isostructural with baddeleyite. INTRODUCTION Metallic and ceramic nitrides are of great technological and fundamental importance [16]. Traditional applications have taken advantage of the hard and refractory nature of many of these compounds, but numerous more recent applications are based on their optical, electronic, and magnetic properties. Of the transition metals most are known to form compounds with nitrogen but it is only recently that solid "bulk" nitrides (as apposed to small individual molecules) of Au [7], Pt [1,8,9], Ir [9-11], Os [11], and Pd (present work) have been reported. Synthesis techniques have included nitrogen ion irradiation (Au [7]), reactive laser ablation (Pt [8]), and the use of the laser-heated diamond anvil cell (Pt [1,9], Ir [1,9,11], Os [11], Pd). The anvil cell has also been used to synthesize novel phases of known nitrides e.g. silicon, zirconium and hafnium [12-13]. EXPERIMENT AND THEORY The relevant metal was placed into the DAC cavity either in the form of thin squares (approximately 30 X 10 microns) or fine powder. Raman scattering was used to determine when a reaction had occurred. Our combined laser-heating and Raman system which we used to synthesize all of our samples has been described elsewhere [14]. In the case of iridium, nitrogen was loaded into the DAC cryogenically for all synthesis experiments. For palladium, several cryogenic loadings were also performed but for one synthesis, nitrogen was loaded under high pressure at room temperature. It is well known that the latter method permits much higher reactant purity [15] while "open vessel" cryogenic loading may well be expected to result in impurities such as oxygen and water. However, our Raman spectra for iridium nitride was
essentially identical to that reported by Young et al. [11] who we believe employed highpressure gas loading in their investigations. Since palladium nitride had not been previously reported we thought it necessary to demonstrate at least that the Raman spectra were the same for both loading procedures. For the XRD measuremen
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