At room and lower temperatures, hydrogen atoms occupy octahedral (O) interstitial sites in all known monohydrides of d-metals with close-packed metal lattices (fcc, hcp or double hcp) (see [1] and references therein). On the other hand, the ground-state energy of the H atom sitting on an alternative tetrahedral (T) interstitial site is not much higher. This opens the possibility of a partial Boltzmann occupancy of the Tsites at elevated temperatures. A considerable T-occupancy reaching about 1/3 of all D atoms has for the first time been found in fcc deuterides of palladium by in situ neutron diffraction at 310 °C and deuterium pressures up to 9 MPa [2, 3]. A few years later, another in situ ND investigation showed that about 1/6 of D atoms are likely to occupy the T-sites in fcc deuteride of iron at T = 715 °C and P = 6.3 GPa [4]. The subtle difference between the profile fits using the O-model and O+T model was not however sufficient to establish the presence of D atoms on the T-sites with certainty. At the same time, modeling this pattern assuming that D atoms could fill only the O-sites and allowing them to occupy both Oand T-sites gave noticeably different total D/Fe ratios of 0.47 and 0.64, respectively [4]. In order to examine which of the two predicted D/Fe values better agrees with experiment, we constructed an isobar of deuterium solubility in iron at P = 6.3 GPa and temperatures from 100 to 800 °C using a quenching technique [5]. The point (6.3 GPa, 715 °C) chosen for the experiment in Ref. [4] and the temperature interval of the isobar constructed in the present work are shown on the T-P diagram of the Fe-H system (Fig. 1) copied from Ref. [6]. The isobar is presented in Fig. 2. As one can see from Fig. 2, our experiment confirms the value of D/Fe = 0.64(3) resulting from the O+T model [4]. Such a value corresponds to the deuteriuminduced volume expansion of fcc iron dVa/dx = 2.21(4) Ǻ3/atom D [4], and therefore this estimate of Ref. [4] is also confirmed. The isobar in Fig. 2 also demonstrates a step-wise decrease in the deuterium solubility in iron from D/Fe =1 to D/Fe ≈ 0.9 at T0 ≈ 260 °C due to the transition from the low-temperature stoichiometric dhcp (ε′) FeD phase to the high-temperature fcc (γ) Fe-D phase with a variable composition. Since the direct synthesis of single-phase samples of ε′-FeD from α-Fe at a pressure of 6.3 GPa is not possible for kinetic reasons, the points shown by the solid blue triangles were obtained with the samples first transformed to γ-FeDx at 500 °C. Figure 1. T-P diagram of the Fe-H system [4]. α – dilute H solutions in bcc Fe; γ – hydrogen solutions in fcc Fe with the H/Fe ratio varying from x = 0 to x ≈ 1 depending on the temperature and pressure; ε ́ a stoichiometrc hydride FeH with a double hcp metal lattice.

Samples of the clathrate Na x Si 136 were saturated with hydrogen to 100 atm at 25°C in a Sievertstype apparatus and at pressures of 6 and 28 kbar in lentil-type high-pressure apparatuses at 100 and 250°C. X-ray powder diffraction analysis and Raman spectroscopy of the samples quenched after the saturation with hydrogen showed that the phase composition of the clathrates did not change. Heating of the quenched samples to room temperature in a thermal desorption setup produced not hydrogen, but hydrogen-containing gases, as we assumed, silanes. Heating to 650°C leads to decomposition of these compounds to form hydrogen.