ИСТИНА

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). The ground-state energy of the H atom sitting on an alternative tetrahedral (T) interstitial site is not much higher and this opens the possibility of a partial Boltzmann occupancy of the T sites 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]. 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 [3]. The quality of the collected ND pattern 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 O- and T-sites gave noticeably different total D/Fe ratios of 0.47 and 0.64, respectively [3]. In order to examine which of the two predicted D/Fe values better agrees with experiment, an isobar of deuterium solubility in iron at P = 6.3 GPa was constructed in the present paper using a quenching technique [4]. Our experiment confirmed the value of D/Fe = 0.64(3) resulting from the O+T model [3]. Such a value corresponds to the deuterium-induced volume expansion of fcc iron equal to 2.21(4) Ǻ3/atom D [3], and therefore this estimate is also confirmed. Additionally, the isobar demonstrated 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. The ε′→γ transition showed no hysteresis within the experimental accuracy of ±15 °C, and the temperature of the transition turned out to be approximately 100 °C lower than in the Fe-H system at the same pressure of 6.3 GPa [5]. The work was partly supported by the Russian Foundation for Basic Research under Grant No. 17-02-01142. [1] V.E. Antonov et al., J. Alloys Compd. 430 (2007) 22. [2] K.G. McLennan et al., Phys. Rev. B 78 (2008) 014104. [3] A. Machida et al., Nature Commun. 5 (2014) 5063. [4] S.N. Abramov et al., J. Alloys Compd. 672 (2016) 623. [5] V.E. Antonov et al., J. Phys.: Condens. Matter 14 (2002) 6427.