Pressure-Induced Hydrogen-dominant metallic state in Aluminum Hydride HAGIHARA Toshiya Shimizu-group Igor Goncharenko et al., Phy. Rev. Lett. 100, (2008)

Contents Introduction theoretical prediction of hydrogen under high pressure why hydrides previous studies on AlH 3 Experimental method Result & discussion Summary

Hydrogen Interesting physical properties Metallic conductivity High-temperature superconductivity T c ~ room temperature Molecular hydrogenHydrogen dense state experimentally difficult. Theoretical prediction More than 400~500 GPa is required. under high pressure

hydrides Interstitial hydrides Covalent hydrides M: rare earth metalM: Al, Si…. The M-H bonding is covalent. Insulator at ambient pressure Hydrogen atoms are in the interstitial site of metal lattice. For the model of dense metallic hydrogen, hydrides (for example MH x ) are paid attention.

hydrides MH x ; YH 3, AlH 3 … Hydrogen molecular is contained 1.5 mol per 1.0 mol of M. Hydrogen in MH x is precompressed at ambient pressure. The partial hydrogen volume in MH x < That volume in H 2. At much lower pressure, hydrogen dense metallic state could be observed. 引き去ること Hydrogen atomic volume in MHx from subtraction of atomic volume of M.

AlH 3 R-3c structure Two AlH 6 -units share this H atom. AlH 6 octahedrons linked by Al-H-Al bridges The partial hydrogen volume per H atom in AlH 3 (5.5 A 3 ) is smaller than that in H 2 (7.5 A 3 ). Hydrogen in AlH 3 is precompressed ! P ~ ambient pressure H H Al-H distance: 1.7 A H-H distance: 2.5 A Volume per formula unit in AlH 3 : 33 A 3

Previous studies on AlH 3 P < 35 GPa, there is no structural transition. Insulator is stable at about 100 GPa. Band gap decreases by the pressure. X-ray diffraction measurement Electrical property Electrical density of states at 0, 50 and 100 GPa were calculated. Insulator to metal transition? (P > 100 GPa)

Purpose of this experiment the insulator to metal transition in AlH 3 To observe the crystal structure under high pressure ・ X-ray diffraction measurement ・ electrical resistance measurement were performed under high pressure.

Experimental method X-ray diffraction measurement ( at European Synchrotron Radiation Facility) ( P < 110 GPa, T = 300 K) Electrical resistance measurement ( P < 164 GPa, 4 K < T < 300 K) quasi-four-probe measurements pressure gauge : ruby fluorescence Raman ・ pressure medium : no use ・ pressure medium : hydrogen ・ wavelength is Å Sample : AlH 3 Pressure generator : DAC The image of DAC

Result (X-ray diffraction) X-ray diffraction patterns of AlH 3 The structure is simple. Aluminum atoms forms bcc. Structural transition occurs At 63 GPa, Phase  to Phase  (P1 structure) At 100 GPa, Phase  to Phase  Hydrogen atom ? Experimental diffraction pattern Pattern of P1

Discussion Phase  Crystal structure or P > 100 GPa Al-H distance: 1.72 Å H-H distance: 1.54 Å At 110 GPa, Calculated enthalpy in the R-3C, Im-3m, Pm-3n

Discussion Phase  Partial hydrogen volume per H atom in AlH 3 at 100 GPa Partial hydrogen volume per H atom in molecular hydrogen at 300 GPa. Phase  is a hydrogen dense phase ! Equal to

Result (electrical resistance) Sample becomes dark. Electrical resistance sharply drops. At 100 GPa At 120, 164 GPa, on cooling Electrical resistance shows typical metallic behavior. insulator to metal transition

Summary Crystal structure Electrical property R-3c insulatormetal P1P1Pm-3n 0 P (GPa) At over 100 GPa, Pressure-Induced Hydrogen- Dominant metallic state was observed in AlH 3. Hydrogen dense state

That’s all. Thank you for your attention.

Al-H distance: 1.7 Å H-H distance: 2.5 Å