Structural changes in PdDx in the temperature region of the 50 K anomaly

Abstract

Diffuse neutron scattering in fl-PdD x (for x = 0.710, 0.742, 0.754 and 0.780) was observed around the 1 1 0X* ,~, j point in reciprocal space in the temperature range between 50 and 150 K. The detailed shape of the diffuse pattern is dependent on the deuterium concentration for x between 0.71 and 0.742 and nearly independent for higher concentrations. An increase of the diffuse intensities was found when cooling down the samples to 50 K and was associated with the anomaly of the resistivity. During an annealing treatment in PdD0.780 at 70 and 75 K, superlattice reflections appeared at tt47,7,2 ,,Jn~* and equivalent positions whose intensity after 53 h was 2% of the intensity of the 200 Bragg reflection of the sublattice.

The ordered structure of at 70 - 75 K

“…At a higher concentration, D/M = 0.77, the growth of superlattice lines at (h± 2 5 h± 4 5 0) and equivalent positions was observed during annealing at 70 K [2]. The ordered superstructure was confirmed to be of the tetragonal MoNi 4 (I 4/m) type, in agreement with previous investigations using single-crystal samples [6,7,8,9]. Owing to the sluggishness of the transition at 70 K, the superlattice did not develop completely and we were unable to uncover much structural detail from the analysis of the collected data [2].…”

“…Owing to the sluggishness of the transition at 70 K, the superlattice did not develop completely and we were unable to uncover much structural detail from the analysis of the collected data [2]. A survey of the literature showed that detailed structural information such as accurate D locations and occupancies for this type of ordering was not obtained from the single-crystal studies [6,7,8,9] either.…”

Hydrogen diffusion in bulk and nanocrystalline palladium: A quasielastic neutron scattering study

“…The stable O-sites and metastable T-sites in the Pd lattice are also predicted by ab initio calculations [30][31][32]. Below T ≈ 50 K, at which tem-perature an anomaly is observed in the resistivity and heat capacity, additional diffraction peaks appear gradually with time, indicating a slow positional ordering of the hydrogen atoms [3,[5][6][7][8][9][10]. A recent heat capacity study attributed the anomaly to a glass transition corresponding to the freezing of the configurational motion of hydrogen, which occurs above a hypothetical orderdisorder transition temperature [33].…”

“…Neutron diffraction (ND) experiments have revealed that hydrogen atoms are accommodated at interstitial octahedral (O) sites (1/2, 1/2, 1/2) in the face-centered cubic (fcc) lattice of Pd (see Fig. 1) at ambient temperature [2][3][4][5][6][7][8][9][10][11][12]. At temperatures as high as 300 • C, hydrogen atoms can partially occupy tetrahedral (T) sites (1/4, 1/4, 1/4) [9][10][11][12].…”

Vibrational states of atomic hydrogen in bulk and nanocrystalline palladium studied by neutron spectroscopy

“…Bulk PdH x undergoes a phase transition from a hydrogen dilute α phase to a concentrated, lattice expanded β phase. In both phases, the H atoms occupy the interstitial octahedral (O) sites (1/2, 1/2, 1/2) in the face-centered cubic (fcc) Pd lattice [27][28][29][30][31][32][33][34][35][36][37] (see Fig. 1).…”