ИСТИНА

Hydrogen is known to form wide ranges of continuous solid solutions with fcc Pd and fcc Pd-based alloys. X-ray diffraction studies showed that a characteristic feature of most such solutions is a significant decrease in the slope of the concentration dependences of their hydrogen-induced volume expansion ΔV(x) at H/Me atomic ratios of x > 0.6–0.8 (see [1,2] for a compilation of experi mental results and references). This strong effect has been debated for a few decades and various explanations have been proposed, such as filling of the d band [3], formation of superabundant vacancies in the metal sublattice [4] and even metallization of the hydrogen sublattice [5]. However, none of the explanations has had adequate experimental grounds so far. As will be shown in the present paper, the effect was mainly due to the stretched state of the outer layers of the samples examined by X-rays, and this stretched state was caused by the un-relaxed elastic stresses of the order of the yield l stress, which remained after the plastic flow of the material in the process of hydrogenation. In our experiments, the samples in the form of foils 0.15 mm thick and 5 mm across made of Pd and fcc Pd-Au alloys with 6 and 20 at.% Au were loaded with hydr ogen at pressures from 0.6 to 7.4 GPa and a temperature of 600 °C, quenched to the liquid N2 temperature, recovered to ambient pressure and examined by X-ray diffraction at 85 K. The synthesis temperature 600 °C was suffici ent to anneal the stresses and strains arising on hydrogenation. An X-ray investigation of the obtained homogeneous and stress-free samples showed that their ΔV(x) values as a function of x all fall onto one straight line with the slope dV/dx = 2.41 Å3 per metal atom. This line agrees with the earlier experimental ΔV(x) values for the Pd-H solutions with 0.83 < x < 0.97 [5,6], but passes considerably lower than the points for the Pd-H solutions with x ~ 0.6 [6] and Pd-Au-H solutionswith x < 0.55 [5]. We also prepared a few samples by two-side and one-side hydrogenation at a lower T = 300 °C. The samples still had a homogeneous distribution of the H atoms over the volume, but their ΔV(x) values deviated from the linear dependence, and the deviations were similar to those of the data taken from the literature. Simple estimates show that an elastic deformation of this very order of magnitude should have been produced in the outer layers of the samples by the stresses generated in side the samples at the boundary of the growing layer of the hydride due to its large specific volume. In most works, the Pd-Me samples were earlier hydrogenated at room temperature and were in the form of a wire or powder that always resulted in the stretched outer layers. Another interesting feature of the quenched high-pressure Pd-H and Pd-Au-H solutions was their two-stage thermal decomposition in vacuum, which will also be discussed in the paper. [1] Y. Fukai, The Metal-Hydrogen System, Springer, 2005. [2] N. Armanet and M. Bonnard, in preparation. [3] T. Tsuchida, J. Phys. Soc. Japan 18 (1963) 1016. [4] Y. Fukai and N. Ōkuma, Jpn. J. Appl. Phys. 32 (1993) L1256. [5] B. Baranowski et al., Fiz. Nizk. Temp. 1 (1975) 616. [6] J.E. Schirber and B. Morosin, Phys. Rev. B 12 (1975) 117.