By combining experimental and computational studies, the orthorhombic stannide CeMgSn with a TiNiSi-type structure has been characterized as a potential hydrogen storage material. Experimental studies of the formed monohydride CeMgSnH including hydrogen absorption-desorption, thermal desorption spectroscopy, synchrotron and neutron powder diffraction (298 and 2 K), magnetization, and ¹¹⁹Sn Mössbauer spectroscopic measurements are discussed in parallel with ab initio electronic structure calculations. A small, 1.27 vol %, expansion of the unit cell of CeMgSn during its transformation into a thermally stable CeMgSnH monohydride is caused by an ordered insertion of H atoms into half of the available Ce₃Mg tetrahedral interstices leaving the CeMg₃ tetrahedra unoccupied. The bonding in CeMgSnH is dominated by strong Ce-Sn and Mg-Sn interactions which are almost not altered by hydrogenation, whereas the H atoms carry a small negative charge and show bonding interactions with Ce and Mg. Hydrogenation causes a conversion of the antiferromagnetic CeMgSn into ferromagnetic CeMgSnH with the Ce moments aligned along [001] with a magnetic moment of 1.4(3) μ_B. The ¹¹⁹Sn isomer shifts and the values of quadrupole splitting in the Mössbauer spectra suggest a similar s-electron density distribution for the Ce- and La-containing REMgSnH monohydrides.

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The work was aimed on reaching a better understanding of the effect of magnesium as a component of the hydride-forming LaMgSn intermetallic compound crystallising with the orthorhombic TiNiSi type of structure on the hydrogenation behaviours, crystal structure and bonding interactions with hydrogen. The LaMgSn structure is significantly expanded as compared to the earlier studied isotypic LaNiSn H storage material (volume expansion of 23%), as a result of a substitution of the smaller Ni atoms by much larger Mg atoms. This significantly affects the chemistry of the interaction of the intermetallic compound with hydrogen because a transition metal, Ni, in replaced by an active hydride-forming metal, Mg. The work involved computational studies of the electronic structure of the intermetallic compound and its hydride, and experimental studies of the hydrogenation behaviour and thermal stability of the formed hydride LaMgSnH, its structural characterisation by SR XRD and neutron powder diffraction, and Mössbauer spectroscopic studies of the stannide and its hydride. These studies showed that in the system LaMgSn-H₂ a monohydride LaMgSnH is a thermodynamically favourable hydride composition. PDOS levels show that hydrogen and all constituting elemental metals, La, Mg and Sn, have peaks of electron density in the range between − 6 and − 4 eV indicating their hybridisation. The results show the hybridization of H atoms not only with bonded La and Mg atoms forming H-filled tetrahedra La₃Mg, but also with Sn despite its atoms do not have bonding interactions with H. This explains the high stability of the metal substructure which does not disproportionate into the binary hydrides of La and Mg even when heated to 200 °C @ 20 bar H₂, but instead forms an insertion type hydride. Formation of the monohydride LaMgSnH (Sp.gr. Pnma; a=8.1628(4); b= 4.5555(3); c= 9.2391(5) Å; V= 343.56(5) ų) causes a small (1.26%) expansion of the unit cell volume compared to LaMgSn, and mainly proceeds along the [100] direction. Hydrogen absorption-desorption cycle results in a reversible formation of the initial compound LaMgSn, with the peak of hydrogen release occurring in vacuum at 355 °C, which is intermediate between the temperatures for the vacuum decomposition of the dihydrides MgH₂ and LaH₂. From the combined refinements of the Synchrotron (SR) XRD and Neutron Powder Diffraction (NPD) data, deuterium atoms completely and in an ordered way fill a half of the available La₃Mg interstitial sites with metal-H/D distances of Mg-D= 2.026 Å; La-D= 2.381 and 2.502 Å. The occupied La₃Mg sites are smaller in size than the vacant Mg₃La tetrahedra. Sn and D exhibit a nonbonding interaction with the closest Sn-D separation of 3.033 Å. ¹¹⁹Sn Mössbauer spectra of LaMgSn and LaMgSnH show isomer shifts of 1.98(2) and 1.99(1) mm/s which are typical for the chemically similar stannides.

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A combined experimental and modelling study into the Pb-Mo-O system has been conducted in view of the safety analysis for lead-cooled nuclear systems. The thermal expansion and low-temperature heat capacity of the ternary compounds PbMoO4 and Pb2MoO5 have been determined experimentally, as well as the melting enthalpy of PbMoO4. Moreover, XANES measurements have confirmed the hexavalent oxidation state of Mo in PbMoO4 and Pb2MoO5.

A thermodynamic model of the ternary system including the ternary phases PbMoO4, Pb2MoO5 and Pb5MoO8 has also been developed in this work based on the CALPHAD methodology. For the first time, an ionic two-sublattice model is used for the liquid phase, while the compound energy formalism is used for the solid phases.@en

Structural, magnetic and magnetocaloric properties of Mn₃Sn_1-xZn_xC antiperovskite carbides have been studied. With increasing Zn content the first-order magnetic transition (FOMT) is weakened. The Curie temperature (T_C) reduces first from 273 to 197 K and when x > 0.3, T_C increases, reaching its maximum of 430 K for x = 1.0. An increase in T_C is accompanied by pronounced changes in magnetic behaviour and a significant rise in magnetization from 21.82(4) to 76.2(2) Am²kg⁻¹ for x = 0.8 in the maximum applied magnetic field of 5 T. Neutron powder diffraction (NPD) was employed to study the magnetic structure of Mn₃Sn_1-xZn_xC compounds. The refinement of the NPD data for x = 0.3 revealed a magnetic structure with propagation vector k = (½,½,0) with a decrease in the canted antiferromagnetic (AFM) moment, which results in a reduction of the negative volume change at the magnetic transition and a decrease in the magnetocaloric effect (MCE). For x = 0.4, the magnetic structure is described by a propagation vector k = (½,½,½) for the AFM moment which dominates at low temperature, with the presence of a minor ferromagnetic (FM) component with a k = (0, 0, 0) propagation vector, which confirms the presence of the ferrimagnetic (FiM) state. For a higher Zn content (x = 0.6), the magnetic moment originates mainly from the FM component found on three independent Mn positions and an additional AFM moment oriented in the a-b plane. The results presented confirm the presence of competing AFM-FM interactions in Mn₃Sn_1-xZn_xC antiperovskite carbides.

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The quaternary compound Cs₂Pb(MoO₄)₂ was synthesized and its structure was characterized using X-ray and neutron diffraction from 298 to 773 K, while thermal expansion was studied from 298 to 723 K. The crystal structure of the high-temperature phase β-Cs₂Pb(MoO₄)₂ was elucidated, and it was found to crystallize in the space group R3̅m (No. 166), i.e., with a palmierite structure. In addition, the oxidation state of Mo in the low-temperature phase α-Cs₂Pb(MoO₄)₂ was studied using X-ray absorption near-edge structure spectroscopy. Phase diagram equilibrium measurements in the Cs₂MoO₄-PbMoO₄ system were performed, revisiting a previously reported phase diagram. The equilibrium phase diagram proposed here includes a different composition of the intermediate compound in this system. The obtained data can serve as relevant information for thermodynamic modeling in view of the safety assessment of next-generation lead-cooled fast reactors.

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