Observation of Metal-insulator and Metal-Metal Transitions in Hydrogen Iodide under Pressure

Abstract

Hydrogen iodide has been studied up to pressures of 70 GPa at low temperatures in a diamondanvil cell. Electrical-conductivity and optical measurements have been used to observe pressureinduced phase transitions which are interpreted as follows: First a metal-insulator transition takes place in the molecular solid; with increased pressure a metal-metal molecular-to-atomic transition occurs. An additional transition due to intrinsic impurities has been observed. Implications for metallic hydrogen are discussed. PACS numbers: 71.30. + H, 62.50. + p, 64.70.Kb Metal-insulator (MI) transitions in solids are currently of great experimental and theoretical interest. H 2 is the prototype for molecular solids and is predicted to have transitions due to band-gap closure from molecular insulator to molecular metal to atomic metal 1 at very high pressure; such transitions have been observed in I 2 . 2 A second type of MI transition occurs in heterogeneous systems such as doped semiconductors when the density is such that the electrons in effective Bohr orbits of the impurity atoms overlap. 3 Our observations on HI under pressures up to 70 GPa indicate two transitions at pressures of 42.5 and 51 GPa. The first we interpret as a molecular-insulator to molecular-metal transition; the second as a molecularmetal to atomic-metal transition. In addition, light, pressure, and elevated temperatures stimulate the formation of intrinsic impurities that we believe are If anions in acceptor states. We have also observed MI transitions in doped HI at sufficiently high pressure or impurity density. Our experimental techniques have included optical and uv transmission, Raman scattering, four-terminal electrical conductivity, and photoconductivity in a diamond-anvil cell at temperatures down to 5 K.HI is an aggressive toxic gas at room temperature, freezing as a clear solid at 222 K in an fee structure 4 with a zero-pressure molar volume of 34.9 cmVmole at r=5 K. 5 Two structural phase transitions take place at lower pressures and temperatures; although the structures are not known, they are probably similar to the other hydrogen halides. 6 The molar volume at which a MI transition takes place can be estimated by a simple model. In the Herzfeld approach 7 the dielectric constant becomes infinite at the MI transition, or, by use of the Lorentz-Lorenz relationships, (/7 2 -l)/(/1 2 -+-2)=(€~l)/(€+2)=i?/K,which is equal to 1 at the MI transition. If the molar refraction, i?, is independent of pressure then R = V m , the metallization volume. The index of refraction of liquid HI is 1.466 8 at an estimated volume of 49.76 cmVmole, 9 which yields V m = 13.76 cmVmole.Although the equation of state of HI has not been measured, Stewart 10 has studied HC1 and HBr to 20 kbar. Assuming the equation of state of HI to be similar, with a zero-pressure volume V 0 of 34.9 cmVmole, 5 we find 11 for the metallization pressure corresponding to V m , P m = 51.4 GPa. We can check the result for V m by considering HI as one of a family of metal hydrides. 12 In ...

Synthesis and stability of hydrogen iodide at high pressures

“…Raman spectroscopy was used to test the purity of the prepared sample, showing trace amounts of both the constituent elements. Trace amounts of I 2 and H 2 , were also observed in previous studies where high purity HI was loaded directly [6,17].…”

“…Other conductivity measurements presented in Ref. [6] clearly correspond to the metallization of I 2 occurring at 16 GPa [30,31]. Both diagnostic techniques used in these early studies do not probe either the crystal structure or molecular behavior at high pressures, meaning that dissociation-a simpler explanation of the experimental results-was overlooked.…”

“…Despite the sensitivity to decomposition it was previously assumed that HI would remain stable up to very high pressures and enter a metallic state at low temperature [6,7]. Metallization was initially inferred by the extrapolation of absorption measurements conducted at pressures below 25.5 GPa, i.e., below the conditions at which we report dissociation.…”

“…On the other hand, investigations into the pressurization of hydrogenbearing species have received a significant impetus by the recent claim of high-temperature superconductivity in hydrogen sulfide (H 2 S) at unprecedented high temperatures [5]. Hydrogen iodide (HI) has been proposed to exhibit both these phenomena at high pressures and low temperatures, with a claimed insulator-to-metallic transition at 50 GPa [6,7]. Additionally, compounds of HI-H 2 have been recently predicted to be stable above 50 GPa, with superconducting phases emerging above 100 GPa with a T c 17.5 K [8,9].…”

Superconductivity in Hydrides Doped with Main Group Elements Under Pressure

“…Experiments have shown that HI undergoes a pressure induced insulator to metal transition below 50 GPa [63]. At this pressure the PBE band gap of H 5 I was calculated to be ∼0.5 eV.…”