Solid State Communications 149 (2009) 1944–1946 Contents lists available at ScienceDirect Solid State Communications journal homepage: www.elsevier.com/locate/ssc Structure and electronic properties of BaH 2 at high pressure John S. Tse a,∗ , Zhe Song a , Yansun Yao a , Jesse S. Smith b , Serge Desgreniers b , Dennis D. Klug c a Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5E2 b Laboratorie de physique des solides denses, Department of Physics, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5 c Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6 article info Article history: Received 27 April 2009 Received in revised form 9 July 2009 Accepted 25 July 2009 by S. Das Sarma Available online 30 July 2009 PACS: 61.10.-I 61.50.Ah 63.20.Kr 74.25.K Keywords: A. Superconductors C. x-ray scattering D. Electron–phonon interaction E. High pressure abstract The structure of BaH 2 at high pressure up to 58 GPa has been investigated with synchrotron powder x- ray diffraction. A structural phase transition from the low pressure Ni 2 In structure to a simple hexagonal structure, which started at 40 GPa and was completed at approximately 45 GPa, was confirmed. A loss of Raman signal in this pressure range suggests that an insulator to metal transition has occurred. This observation is corroborated with first-principles calculations. The investigation of the electron–phonon coupling shows that the metallic phase is a very weak superconductor with an estimated superconducting critical temperature T c in the range of mK. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Simple hydrides are suggested to exhibit superconducting behavior under pressure by recent extensive theoretical and experimental investigations [1]. So far, these studies have been focused mainly on group III and IV hydrides. An optimistic prediction of the superconducting critical temperature (T c ), in excess of 200 K [2], was first made for silane, although more refined calculations showed that the T c [3,4] is somewhat lower. Experimentally, silane was indeed found to metallize at around 50 GPa [5] and becomes a superconductor with a maximum T c of 17 K at 120 GPa [6]. In contrast, aluminum hydride [7] was found to be metallic at 100 GPa, but not superconducting. Alkaline earth metal hydrides form another class of possible superconductors. It has been shown that CaH 2 [8], SrH 2 [9], and BaH 2 [10,11] transform to a hexagonal Ni 2 In (P 6 3 /mmc ) close-packed structure under pressure. In the Ni 2 In structure, alkaline earth metal hydrides are found to be stable over a large pressure range, except for BaH 2 , which transformed into a simple hexagonal (SH) structure at 50 GPa. The structural transition in BaH 2 is accompanied by a 15% ∗ Corresponding author. Tel.: +1 306 966 610; fax: +1 306 966 6400. E-mail address: John.Tse@usask.ca (J.S. Tse). volume reduction, indicating substantial changes in the bonding. A theoretical calculation of the electronic properties of BaF 2 [12] in the Ni 2 In phase suggested that it is metallic. In comparison, BaH 2 in the Ni 2 In structure is not metallic, however, it is not unreasonable to speculate that higher pressure SH polymorph of BaH 2 may be metallic. Anticipating that very low frequency vibrations due to the Ba atoms, that may enhance electron–phonon coupling (EPC) [13] and the high frequency H vibrations [1] in favour of a large Debye temperature, the metallic phase may be a superconductor as well. In a previous study [10], the Ni 2 In → SH transition in BaH 2 is clearly identified, but the property of the SH, in particular, whether it is a metal, has not been established. The objectives of this investigation are to characterize the electronic state of BaH 2 in the high pressure SH structure and to explore the possibility of a superconducting state. For this purpose, the high pressure structure and properties of BaH 2 are studied with synchrotron radiation powder x-ray diffraction, Raman spectroscopy measurements, and first-principles calculations. 2. Experimental and theoretical details Barium hydride powder (99.5% metals analysis, Strem Chem- icals) was loaded into gasketed diamond anvil cells (DAC) un- der an inert gas (argon) atmosphere. No pressure-transmitting 0038-1098/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2009.07.044