Shimizu-group M1 Yasuhiro Asakura Phase Diagram of Palladium- Hydrogen System H. Araki, M. Nakamura, S. Harada, T. Obata, N. Mikhin, V. Syvokon, M. Kubota, Journal of Low Temperature Physics, 134 (2004) 1145

・ Introduction ・ Experiment ・ Result and Discussion ・ Summary 1) Palladium-Hydrogen system 2) Previous work － The 55K anomaly － Superconductivity

Introduction Hydrogen-storage alloy ・ Hydrogen has high chemical energy and can emit as clean water when using the energy. ・ The formulation of “hydrogen energy system” lead the solution of the environmental and energy problems on a global scale. ・ Hydrogen storing alloy is the material can use hydrogen energy in safety. ・ First palladium (Pd) was proposed as the metal can heavily absorb hydrogen.

Introduction Palladium-Hydrogen system fcc structure palladium ・ face-centered cubic (fcc) structure ・ dissociate hydrogen molecules (H 2 /D 2 ) and absorb large amount of H(D) atoms up to H(D)/Pd=1 The spaces occupied by hydrogen are the interstitial octahedral (O) sites of palladium. PdH(D)x (x:hydrogen concentration H(D)/Pd)

Introduction Previous work in PdH(D)x -The 55 K anomaly- These anomalies were caused by an order-disorder transition of hydrogen in palladium. Journal of the Less-Common Metals, 100 (1984) Anomaly in the resistivity of PdD Phase Diagram of PdDx Long-range-ordered structures I4 1 /amd I4/m

Introduction Previous work in PdH -Superconductivity- R.W. Standly, M. Steinback, C.B. Satterthwaite, Solid State Commun. 31 (1979) 801. ・ In 1972, discovered by Skoskiewicz 1 (Palladium didn’t become the superconductive phenomenon. ) ・ In 1979, it was reported that the superconducting transition temperature (T c ) increases with increasing concentration of hydrogen. [ 1]T.Skoskiewicz, Phys. Solidi, all K123 (1972) Relation of T c and concentration x T c = 150.8(x-x 0 ) ( X 0 = for PdHx, for PdDx ) T c vs concentration x PdDx PdHx Above expression was estimated using this figure.

Introduction Palladium-Hydrogen system palladium hydride ・ Can hold hydrogen with higher density than liquid hydrogen It is expected that the property of more compressed hydrogen can be investigated under high pressure. ・ become superconductors ・ H(D) atoms in Palladium at low temperature has a large diffusion coefficient due to quantum tunneling. Peculiar temperature dependence (for example,The 55 K anomaly)

Introduction Motivation Like previous figure, the phase diagram of the Pd-H(D) system around 55 K was rather complicated because of the long-range-ordering of hydrogen in Pd with a long relaxation time. The specific heat of PdHx at various concentrations has been measured in order to clarify the anomaly of that caused by the phase transition and in order to check the migration of hydrogen in palladium.

Experiment Sample : Palladium rod ( % pure, 3.0 mmφ × 25 mm) － outgassed and annealed under vacuum (above 873 K for 24 hours) ・ Hydrogen was loaded into the palladium by electrolytic method. － at 213 K with electrolytes of 5.25 mol/ℓ H 2 SO 4 in H 2 O. (current density of about 0.2 A/cm 2 for 48 hours) Concentration x ⇒ the change of the mass of the sample before and after hydrogen loading. PdHx : x =0 and to P. Tripodi et al. / Physics Letters A 276 (2000) 122–126 H 2 SO 4

Result and Discussion The specific heat for PdHx was measured by the quasi- adiabatic heat pulse method ( 準断熱ヒートパルス法 ). Between 0.5 K to 100K C PdHx, C Pd measured specific heat. C electronPdHx,C electronPd were evaluate by past measurement data. C excess caused by the hydrogen atoms dissolved in the palladium. (Hydrogen in palladium)

Result and Discussion The specific heat capacity vs Temperature Measurement raw data for PdHx Measurement raw data for pure Pd The excess specific heat due to hydrogenation Electronic specific heat for pure Pd Electronic specific heat for PdHx Hydrogen Einstein mode specific heat ・ ・ Above 75 K is due to optical mode vibrations of the hydrogen

Result and Discussion The excess specific heat converted for each hydrogen concentration. C excess vs Temperatue PdHx In the upper concentration range from x = to ・ The height of the peak decreases with increasing x ―because of vacancies of hydrogen, 1-x At the same time ・ The hydrogen concentration x dependence of the sharp peak temperature exist. New phase, only β-phase !!

Result and Discussion This phase diagram show the peak of the specific heat by previous data and past data. The 55 K anomaly had been considered to be caused by an order-disorder transition. In β-phase, the sharp peak temperature moves to higher temperature linearly with increasing x. Extra anomalies of specific heat, however, were not recognized in this experiment. It was described that the phase diagram different of previous work. This simple phase diagram suggests several possibilities caused by hydrogen atoms and/or their vacancies in the sublattice. but

Summary ・ The specific heat of PdHx over temperature range (0.5 K ～ 100 K) in the high and large concentration region (0≤x≤0.8753) was measured. ・ Therefore, the new phase diagram was described. Thank you for your attention !! It is interesting that the pressure dependence of the anomaly temperature.