Ab initio study of mechanical stability, thermodynamic and elastic properties of Rh, RhH, and RhH 2 under high temperatu

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Chenghua Hu,a) P. Zhou, and Feng Wang Chongqing Jiaotong University, Chongqing 400074, China

Zhou Zheng Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China

Bo Liangb) School of Civil Engineering & Architecture, Chongqing Jiaotong University, Chongqing 400074, China (Received 22 February 2014; accepted 12 June 2014)

In this work, mechanical stability, thermodynamic and elastic properties of rhodium (Rh), rhodium monohydride (RhH), and the newly discovered rhodium dihydride (RhH2) under high temperature and pressure are studied by ab initio method together with quasiharmonic Debye model. Mechanical stability test indicates that RhH2 is no longer mechanically stable when pressure is higher than 22.7 GPa, which is quite less than the dynamically stable pressure (90 GPa). The heat capacity at constant volume (Cv) of Rh, RhH, or RhH2 increases proportional to T3 at low temperature, and tends to Dulong–Petit limit (about 241.67, 478.47, and 706.15 J/(kgK), respectively). The thermal expansion coefficient (a) of Rh, RhH, and RhH2 increases acutely when temperature is not more than 300 K. And then, the increase of a slows down. The a reduces with pressure transiently. H atom’s entering in fcc-Rh lattice would greatly change the electron density distribution, which would cause obvious difference in thermodynamic and elastic properties between Rh, RhH, and RhH2. I. INTRODUCTION

Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2014.141

octahedral or tetrahedral sites of the close-packed metal atoms. The chemical formula of MH (M is metal element) is common monohydrides, and the typical crystal structure has been drawn in Fig. 1(a). Hydrides of platinum group metals (PGM: ruthenium (Ru), rhodium (Rh), iridium (Ir), osmium (Os), palladium (Pd), and platinum (Pt)) are widely focused because of those favorable characteristics. For instance, the palladium hydride shows exceptional catalytic properties and kinetic reversibility of hydrogen.2,7,8 However, the shortcomings of PGM hydrides are their low gravimetric and volumetric H densities due to their high atomic mass and low n in MHn (usually, n # 1). Recently, B. Li and Y. Ding et al.9 discovered a new rhodium dihydride (RhH2) with high volumetric hydrogen density (163.7 g/L) by using high-pressure synchrotron x-ray diffraction technique and theoretical calculations. fcc-Rh metal absorbs hydrogen in the process of being compressed in fluid hydrogen at ambient temperature. fcc-Rh expands unit cell volume by two discrete steps to NaCl typed fcc rhodium monohydride (RhH), and then to fluorite-typed fcc RhH2 at 8 GPa. RhH2 is the first dihydride discovered in the PGM under high pressure. The newly discovered PGM dihydride, RhH2, may be prospectively applied in many scientific and engineering fields. However, before the new RhH2 is applied, many

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Ó Materials Research Society 2014

Hydrogen is always considered as an ideal, abundant, environmentally friendl

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