Erika Michela Dematteis
email: erikamichela.dematteis@unito.it

Jussara Barale

Giovanni Capurso

Stefano Deledda

Magnus H Sørby

Fermin Cuevas

Michel Latroche

Marcello Baricco

In-situ neutron diffraction during reversible deuterium

Keywords: hydrogen storage, intermetallic compound, TiFe, substitution, in-situ neutron diffraction

Introduction

The increase of pollution and global temperatures reveals how urgent and essential is to look for alternatives to fossil fuels. The efficient storage of renewable energy would enable the transition towards CO2-free energy. Hydrogen can be produced from water electrolysis using renewable or nuclear energy sources and, as an efficient energy carrier, it can be stored for a long period. Metal hydrides are promising and safe materials for solid-state hydrogen storage, allowing storing hydrogen in mild conditions (i.e. ambient pressure and close to room temperature -RT) with high volumetric densities. [START_REF] Kharbachi | Metal Hydrides and Related Materials. Energy Carriers for Novel Hydrogen and Electrochemical Storage[END_REF][START_REF] Bannenberg | Metal (boro-) hydrides for high energy density storage and relevant emerging technologies[END_REF][START_REF] Züchner | Hydrogen in Intermetallic Compounds I[END_REF][START_REF] Pasquini | Magnesium-and intermetallic alloys-based hydrides for energy storage: modelling, synthesis and properties[END_REF] TiFe intermetallic is a low-cost and promising compound for H2 storage. [START_REF] Sujan | An overview on TiFe intermetallic for solid-state hydrogen storage: microstructure, hydrogenation and fabrication processes[END_REF] TiFe exhibits, however, drawbacks and particular features as hydrogen storage material. Firstly, it is difficult to activate towards hydrogen absorption, i.e., to promote hydrogen uptake for the first time. A thermal cycling between RT and 400 °C is necessary for activation and the full capacity is only attained after tens of hydrogen sorption cycles. [START_REF] Reilly | Formation and properties of iron titanium hydride[END_REF] Secondly, differing from the paradigmatic LaNi5 alloy, Pressure-Composition-Isotherms (PCI) are characterized by two plateau pressures, instead of showing a single one. [START_REF] Reilly | Formation and properties of iron titanium hydride[END_REF] Thirdly, TiFe was reported to have significant isotope effect. [START_REF] Burzo | Hydrogen Storage Materials[END_REF][START_REF] Sicking | Isotope effects in metal-hydrogen systems[END_REF] Near RT, plateau pressures of deuterides have been reported significantly higher than those of the hydrides, which makes it difficult to achieve a full deuteration of TiFe alloy for pressures below 10 MPa. [START_REF] Burzo | Hydrogen Storage Materials[END_REF] All these properties are at the origin of controversial results concerning the structure of TiFe hydrides/deuterides. TiFe alloy shows a cubic CsCl-type (B2, space group S.G. Pm-3m; 1:1 atomic composition, a = 2.975 Å) [START_REF] Somenkov | Structure of Hydrides[END_REF][START_REF] Busch | Hydrogen storage in FeTi: Surface segregation and its catalytic effect on hydrogenation and structural studies by means of neutron diffraction[END_REF] crystal structure, as called α phase (Table 1). [START_REF] Busch | Hydrogen storage in FeTi: Surface segregation and its catalytic effect on hydrogenation and structural studies by means of neutron diffraction[END_REF][START_REF] Mohammedi | Ab-initio structural and electronic properties of the intermetallic compound TiFeH2[END_REF][START_REF] Fischer | Deuterium storage in FeTi. Measurement of desorption isotherms and structural studies by means of neutron diffraction[END_REF][START_REF] Schober | Transmission electron microscopy and neutron diffraction studies of Feti-H(D)[END_REF][START_REF] Reilly | Lattice expansion as a measure of surface segregation and the solubility of hydrogen in α-FeTiHx[END_REF][START_REF] Thompson | The application of the Rietveld method to a highly strained material with microtwins: TiFeD1.9[END_REF][START_REF] Kinaci | Ab initio investigation of FeTi-H system[END_REF][START_REF] Thompson | Neutron diffraction study of α-iron titanium deuteride[END_REF][START_REF] Schäfer | Investigation of TiFe Deuteride Structures by Means of Neutron Powder Diffraction and the Mössbauer Effect[END_REF][START_REF] Fruchart | Structural and activation process studies of FeTi-like hydride compounds[END_REF] During hydrogenation, the α phase can accommodate up to 0.10 hydrogen atoms per formula unit (H/f.u.) as solid solution. [START_REF] Reilly | Formation and properties of iron titanium hydride[END_REF] The lattice expansion in the solid solution region was described by Reilly et al. back in 1980, defining a α/β phase boundary at TiFeH0.04. [START_REF] Reilly | Lattice expansion as a measure of surface segregation and the solubility of hydrogen in α-FeTiHx[END_REF] The dissolution of hydrogen in the α phase is observed as an increasing branch in PCI curves at low pressures, followed by a low pressure plateau that characterises the formation of the β phase, a monohydride with formula TiFeH. [START_REF] Fischer | Deuterium storage in FeTi. Measurement of desorption isotherms and structural studies by means of neutron diffraction[END_REF] Through the low-pressure plateau, the orthorhombic β phase forms (S.G. P2221), [START_REF] Busch | Hydrogen storage in FeTi: Surface segregation and its catalytic effect on hydrogenation and structural studies by means of neutron diffraction[END_REF][START_REF] Mohammedi | Ab-initio structural and electronic properties of the intermetallic compound TiFeH2[END_REF][START_REF] Fischer | Deuterium storage in FeTi. Measurement of desorption isotherms and structural studies by means of neutron diffraction[END_REF][START_REF] Kinaci | Ab initio investigation of FeTi-H system[END_REF][START_REF] Thompson | Neutron diffraction study of β iron titanium deuteride[END_REF][START_REF] Schefer | Structural phase transitions of FeTideuterides[END_REF] even if discrepancies concerning the TiFe hydride crystal structures are still present in the literature (Table 1). [START_REF] Schober | Transmission electron microscopy and neutron diffraction studies of Feti-H(D)[END_REF][START_REF] Schäfer | Investigation of TiFe Deuteride Structures by Means of Neutron Powder Diffraction and the Mössbauer Effect[END_REF][START_REF] Fruchart | Structural and activation process studies of FeTi-like hydride compounds[END_REF] Firstly, in 1974, Reilly et al. reported a tetragonal structure (TiFeH1.06, a = 3.18 Å, c = 8.73 Å) determined by XRD. [START_REF] Reilly | Formation and properties of iron titanium hydride[END_REF] Furthermore, in the literature, two different β phases have been reported, namely β1 and β2, related to the TiFeH and TiFeH1.4 compositions, respectively. [START_REF] Schefer | Structural phase transitions of FeTideuterides[END_REF][START_REF] Reidinger | An X-ray diffraction examination of the FeTi-H2 system[END_REF][START_REF] Cantrell | Comparison of structures and electronic properties between TiCoHx and TiFeHx[END_REF] With further proceeding of the hydrogenation reaction, at higher hydrogen pressure, a second plateau is observed in PCI curves, related to the full hydrogenation of the material up to a TiFeH2 composition (γ phase). As it can be observed from Table 1, structural studies of the γ phase suggested different possible structures, i.e. cubic (TiFeH1.93, a = 6.61 Å), [START_REF] Reilly | Formation and properties of iron titanium hydride[END_REF] orthorhombic (S.G.

Cmmm) [START_REF] Mohammedi | Ab-initio structural and electronic properties of the intermetallic compound TiFeH2[END_REF][START_REF] Thompson | The application of the Rietveld method to a highly strained material with microtwins: TiFeD1.9[END_REF][START_REF] Kinaci | Ab initio investigation of FeTi-H system[END_REF][START_REF] Schäfer | Investigation of TiFe Deuteride Structures by Means of Neutron Powder Diffraction and the Mössbauer Effect[END_REF][START_REF] Fischer | Orthorhombic structure of γ-TiFeD≈2[END_REF][START_REF] Nambu | Electronic structure and hydriding property of titanium compounds with CsCl-type structure[END_REF] and monoclinic structure (S.G. P2/m). [START_REF] Schober | Transmission electron microscopy and neutron diffraction studies of Feti-H(D)[END_REF][START_REF] Thompson | The application of the Rietveld method to a highly strained material with microtwins: TiFeD1.9[END_REF][START_REF] Fruchart | Structural and activation process studies of FeTi-like hydride compounds[END_REF][START_REF] Schefer | Structural phase transitions of FeTideuterides[END_REF][START_REF] Fischer | Orthorhombic structure of γ-TiFeD≈2[END_REF][START_REF] Thompson | Neutron diffraction study of gamma iron titanium deuteride[END_REF][START_REF] Schäfer | Neutron and electron diffraction of the FeTi -D(H) -γ -phase[END_REF] In the monoclinic structure, deuterium is localised in four special positions with octahedral coordination. [START_REF] Schäfer | Neutron and electron diffraction of the FeTi -D(H) -γ -phase[END_REF] The structural evolution of TiFe upon deuterium loading was described by Schafer et al. back in 1980. [START_REF] Schäfer | Neutron and electron diffraction of the FeTi -D(H) -γ -phase[END_REF] From the disordered α phase the D-ordering on octahedral sites is taking place with an orthorhombic distortion to form the ordered β phase, then the distortion further proceeds to form the γ phase, with two additional deuterium positions close to Fe atoms towards the monoclinic structure. [START_REF] Schäfer | Neutron and electron diffraction of the FeTi -D(H) -γ -phase[END_REF] Indeed, distortions of peak shape have been evidenced in neutron diffraction studies, correlated to the formation of possible defects like dislocations, vacancies, disorder, strain and distortions. [START_REF] Züchner | Hydrogen in Intermetallic Compounds I[END_REF][START_REF] Fischer | Deuterium storage in FeTi. Measurement of desorption isotherms and structural studies by means of neutron diffraction[END_REF][START_REF] Bo | Thermodynamic re-assessment of Fe-Ti binary system[END_REF] Substitutional effects on hydrogenation properties in TiFe have been recently reviewed by Dematteis et al. [START_REF] Dematteis | Substitutional effects in TiFe for hydrogen storage: a comprehensive review[END_REF], TiFe can accommodate by substitution different transition metals and p-block elements on either Ti or Fe sites. [START_REF] Züchner | Hydrogen in Intermetallic Compounds I[END_REF][START_REF] Bo | Thermodynamic re-assessment of Fe-Ti binary system[END_REF] Mn-substituted TiFe materials are good candidate for H2 storage with the di-hydride having a gravimetric capacity of 1.9 wt.% H2, mild activation conditions and enhanced hydrogenation properties. [START_REF] Kharbachi | Metal Hydrides and Related Materials. Energy Carriers for Novel Hydrogen and Electrochemical Storage[END_REF][START_REF] Bannenberg | Metal (boro-) hydrides for high energy density storage and relevant emerging technologies[END_REF] Multi-substituted alloys can display enhanced activation properties and the possibility of tuning plateau pressure as a function of substituent concentration in the formulated alloy, as in the case of TiFe0.90-x-yMnxCuy-substituted materials, that enable hydrogen storage in milder conditions of pressure and temperature. [START_REF] Dematteis | Hydrogen storage properties of Mn and Cu for Fe substitution in TiFe0.9 intermetallic compound[END_REF] When Mn is introduced in the alloy, it substitutes Fe enlarging the cell parameter, as reported by Fruchart et al. using neutron diffraction for the TiFe0.90Mn0.10 composition (a = 2.985 Å). [START_REF] Fruchart | Structural and activation process studies of FeTi-like hydride compounds[END_REF] Upon hydrogenation, Mn-substituted alloy is reported to crystallize in an orthorhombic structure, that was assigned to S.G. P2cm or Pmc21 for TiFe0.90Mn0.10H1.25. [START_REF] Fruchart | Structural and activation process studies of FeTi-like hydride compounds[END_REF] for the γ phase, a monoclinic structure has been reported in Mn-substituted materials [START_REF] Fruchart | Structural and activation process studies of FeTi-like hydride compounds[END_REF][START_REF] Cantrell | Comparison of structures and electronic properties between TiCoHx and TiFeHx[END_REF] and it has been recently studied by in-situ synchrotron radiation studies on TiFe0.80Mn0.20. [START_REF] Endo | Hydrogenation of a TiFe-based alloy at high pressures and temperatures[END_REF] In real applications, the selection of a proper alloy with adequate thermodynamics in the operative temperature and pressure ranges is a milestone. Costs, use of non-critical raw materials and high hydrogen content should be also considered. Furthermore, the alloy should have an easy activation procedure and fast hydrogen sorption kinetics to be integrated in the plant. [START_REF] Dornheim | Research and development of hydrogen carrier based solutions for hydrogen compression and storage[END_REF] Mn-substituted TiFe alloys fulfil these criteria. As an example of real applications, the EU-funded HyCARE project aims at realizing an innovative large-scale hydrogen storage tank based on TiFe-based solid-state carrier. [START_REF]HyCARE project[END_REF][START_REF]HyCARE focuses on large-scale, solid-state hydrogen storage[END_REF] One of the goals is to investigate and select the composition and structure of an optimum alloy for large-scale hydrogen storage at the solid-state. [START_REF] Dematteis | HyCARE Deliverable D2.1 -Optimized alloy composition[END_REF][START_REF] Dematteis | HyCARE Deliverable D2.2 -Selected alloy characterisation[END_REF][START_REF] Dematteis | HyCARE Deliverable D2.3 -Processing parameters[END_REF][START_REF] Dematteis | HyCARE Deliverable D3.2 -Characterisation of powder and pellets[END_REF] The selected material will be scaled up to approx. 5 tons and used in an efficient integrated energy storage system, where about 40 kg of hydrogen will be safely stored. The study of the industrially produced alloy has been recently published discussing its hydrogenation properties and activation procedure compared with the labscaled one. [START_REF] Barale | TiFe0.85Mn0.05 alloy produced at industrial level for a hydrogen storage plant[END_REF] The present investigation, carried out in the framework of the HyCARE project, is an in-situ neutron diffraction study of the structural evolution of three different Mn-substituted TiFe alloys to better understand the evolution of their crystal structures upon hydrogenation.

Using deuterium instead of hydrogen will guarantee the avoidance of relatively high H incoherent scattering, while the combination of neutron diffraction as well as in-lab x-ray diffraction will allow to determining crystal structures and composition, as well as Ti and Mn substitutions since Ti/Fe contrast in ND-experiments is high (bound coherent scattering length, bc = -3.44 and 9.45 fm for Ti and Fe, respectively. However, care should be taken to analyse simultaneous occupation of Ti and Mn atoms on same site as they have similar scattering lengths (bc (Mn) = -3.73 fm).

The determination of structural volumetric expansion during the process supports the scale-up and modelling of the hydrogen storage tank from an engineering point of view.

Finally, thet aim of the paper is to analyse the influence of stoichiometry and Mn for Fe substitution in TiFe-type alloys on structural properties during reversible deuterium loading, which is a topic still unsolved and seldom explored. The TiFe(0.90-x)Mnx alloys (x = 0, 0.05 and 0.10) have been selected within a promising composition range for an integrated, renewable, large scale stationary hydrogen storage tank [START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn[END_REF].

Experimental

Sample preparation

In the frame of the HyCARE project, TiFe-type alloys have been synthesized in an induction furnace, annealed at 1000 °C for 1 week and subsequently activated and deuterated up to the γ phase, nominally TiFeD2. [START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn[END_REF] Three of these alloys (Ti-rich samples s2, s6, s9 in ref. [START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn[END_REF], now referred to as A: TiFe0.90, B: TiFe0.85Mn0.05, and C: TiFe0.80Mn0.10, respectively) were selected, owing to improved hydrogenation properties, to be structurally characterised through in-situ neutron diffraction (ND), and to determine their phase composition, structural evolution during sorption of deuterium at RT and the influence of Mn content on structural and deuteration properties. For each TiFe(0.90-x)Mnx sample (x = 0, 0.05, 0.10) the formula unit (f.u.) is expressed as Ti1.05Fe(0.95-x)Mnx that, considering the occupancy of crystallographic sites, corresponds to

[1a][1b]:[Ti0.05Fe(0.95-x)Mnx][Ti].
Argon-filled glove boxes, with a circulation purifier, were used to prepare, store, and manipulate samples. O2 and H2O levels were lower than 1 ppm to minimize oxidation and degradation of the materials.

Characterizations

Previous characterizations evidenced that samples at the Ti-rich side have good storage reversible capacities and, after synthesis and annealing, they present a microstructure with a TiFe-substituted matrix and secondary phases precipitates at grain boundaries (assigned to Ti-rich β-Ti and Ti4Fe2Otype phases). [START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn[END_REF] The compositional balance determined by EPMA (electron probe microanalysis) analysis suggests that both Ti and Mn partially substitute Fe. The formation of a limited amount of secondary phases, reactive to hydrogen, allows first hydrogenation in mild conditions of temperature and pressure. Finally, Mn-substitution has been evidenced to enlarge TiFe cell volume, reduce secondary phases amount, reduce equilibrium pressure, reduce hysteresis and increase reversible capacity in mild conditions. [START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn[END_REF]

Deuteration properties

Activation, kinetic studies and PCI curves were monitored with a home-made Sieverts-type apparatus under hydrogen, as reported in ref. [START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn[END_REF].

After the activation, the entire synthetized sample (approximately 8-10 g) was loaded into a custom stainless steel sample cell. PCI curves were first measured in lab under deuterium gas, both in absorption and desorption at 25 °C by the manometric Sieverts' method, using calibrated and thermalized volumes. A water thermostatic bath was used for maintaining a constant temperature during PCI recording.

In-situ neutron diffraction

Powder ND-patterns were obtained at RT at Institut Laue-Langevin (ILL) on beamline D1B (λ=1.286 Å, Cycle 20201, Proposal CRG 5-22-771 [START_REF] Cuevas | In-situ neutron diffraction during reversible deuterium loading in under-stoichiometric and Mn-substituted Ti(Fe,Mn)0.9 alloys[END_REF]) in a 2θ-range of 10-128°, step size 0.1° and equipped with 3He/CF4 position sensitive detector composed of a system of multi electrodes with 1280 wires.

Samples were deuterated at ca. 8-9 MPa and transferred to the neutron facility for in-situ neutron diffraction studies already inside the stainless-steel cell, which were placed in the beam and connected to a manual Sieverts-type volumetric rig. ND-patterns were recorded in the as-mounted state as a function of deuterium pressure. At the beginning of experiments, a high deuterium pressure was applied (ca. up to 9 MPa) to guarantee a complete deuteration to the γ phase. The deuterium uptake was determined using a pressure gauge. ND patterns were recorded every 10 minutes during the deuteration process. Several patterns were then collected and summed up, once thermodynamic equilibrium was attained. [START_REF] Cuevas | In-situ neutron diffraction during reversible deuterium loading in under-stoichiometric and Mn-substituted Ti(Fe,Mn)0.9 alloys[END_REF] By stepwise lowering the deuterium pressure, ND patterns were collected during acquisition of PCI desorption isotherm at RT, followed by the same analysis on absorption increasing the deuterium pressure. In the present study, fully deuterium-free samples were not analysed, since the lower pressure used was 0.02 MPa and only evacuation at RT was performed; thus, the α phase always contains a small amount of deuterium as solid solution. D1B experiment allowed to characterize various phase transitions. To fully solve the structure of the γ-dihydride, a single scan at high resolution beamline D2B has been performed (λ=1.594 Å, Proposal 83988 EASY-593), in a 2θ-range of 10-160° with step size 0.05°. [START_REF][END_REF] All neutron diffraction patterns obtained at ILL were refined by the Rietveld method, [START_REF] Young | The Rietveld Method[END_REF] using the FullProf package. [START_REF] Rodriguez-Carvajal | FULLPROF: a program for Rietveld refinement and pattern matching analysis[END_REF] The pseudo-Voigt profile function was used to model the shape of diffraction peaks and the instrumental function was obtained by analysing a Si standard.

In addition, ND patterns were also obtained at RT at the Science and Technology Facilities Council (STFC) ISIS Neutron and Muon Source on GEM beamline (Experiment number RB192055). [START_REF] Dematteis | In-situ neutron diffraction during reversible deuterium loading in under-stoichiometric and Mn,Cu-substituted Ti[END_REF] ISIS samples were desorbed under dynamic vacuum at 200 °C for 30 minutes and transferred to the neutron facility for in-situ neutron diffraction studies. The un-deuterated sample was loaded at ISIS using an Ar-filled glovebox into an Al cylindrical cell that can reach 300 bar of pressure and 100 °C (A2 SI-4105-409).

The samples were placed in the beam and connected to a manual Sieverts-type volumetric rig, where volumes were calibrated using a reference volume and helium. ND-patterns of the samples were recorded in the as-mounted state and the beam delimiter was optimised to 30x10 mm (HxW). A long acquisition on as-mounted sample was performed before absorption.

Then, the buffer volume was filled with increasing deuterium pressure, short acquisitions were started and the valve to the cell was opened during the first acquisition. Deuterium loading was monitored until the equilibrium conditions were reached. When changes were no longer observed, a long acquisition was recorded.

ND patterns were recorded every 10 minutes during the deuteration process, while 60 minutes-long acquisition were recorded at the equilibrium, both in absorption and desorption. Pressure and PCI curves were continually monitored.

All data sets obtained at ISIS from long acquisitions were analysed with the Rietveld method [START_REF] Young | The Rietveld Method[END_REF], using the software TOPAS Academic version 6. [START_REF] Coelho | TOPAS and TOPAS-Academic : an optimization program integrating computer algebra and crystallographic objects written in C++[END_REF] Data from all six detector banks were used simultaneously in each refinement. The instrumental function was given by macros developed for GEM based on a pseudo-Voigt function convoluted with the Ikeda-Carpenter function. [START_REF] Ikeda | Wide-energy-range, high-resolution measurements of neutron pulse shapes of polyethylene moderators[END_REF]

Results and Discussion

Pressure Composition Isotherms and isotopic effects

PCI curves under deuterium were obtained at RT at ILL, and then compared with those under hydrogen previously determined in lab at 25 °C. [START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn[END_REF][START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn -Dataset related to publication[END_REF] As it can be observed from Figure 1, lab PCIs under deuterium show similar capacities and hysteresis, as well as similar slopes, compared to those monitored under hydrogen. While significant isotopic effects were previously reported in the literature for TiFe, [START_REF] Schefer | Structural phase transitions of FeTideuterides[END_REF] the present investigation for Ti-rich Ti(Fe,Mn) alloys evidences minor isotopic effects for the low-pressure plateau. This might tentatively be attributed to their different Ti-contents.

However, the high-pressure plateau is shifted towards higher equilibrium pressures for deuterium, as compared to hydrogen for all three samples characterized. In-situ evaluations of PCI curves at RT during deuterium loading at ILL are reproducing well the deuterated PCI curves at 25 °C monitored in lab, considering differences in temperature and possible experimental faults due to different setups.

In-situ neutron diffraction experiments

Phases evolution and transition

The in-situ diffraction analyses performed at ILL during deuterium absorption and desorption are outlined for samples A, B and C in Figure 2, together with the monitored PCI curves. Phase formation and their transition can be appreciated following the peak intensity lines in the two-dimensional contour plot displayed on left in Figure 2.

The analyses allowed the phase identification and evolution of their crystal structure (solid solution, two-phase reaction domains and single-phase deuterated phases) during reversible deuterium sorption.

As it can be observed Figure 2-A, the single γ phase upon absorption for the TiFe0.90 sample has been obtained at high pressure (approx. 9 MPa D2), with a final deuterium capacity of 1.65 D/f.u. according to PCI data. During desorption, the single-phase β is detected at 1.00-1.07 D/f.u., while it is detected between 1.07-1.23 D/f.u. during absorption. Single α phase was detected below 0.06 D/f.u. deuterium capacity. The recorded capacities reported in PCI are expected to be higher than deuterium occupancy obtained during the in-situ analysis by Rietveld refinement, as it can be observed in Figure S1, since deuteride-forming secondary phases such as β-Ti and Ti4Fe2O are detected in the samples.

Nevertheless, owing to the low amount of secondary phases, differences between the deuterium content determined by PCI or refinement are within experimental errors. The formation of the single γ phase upon absorption occurs at high pressures (approx. 9 MPa D2) with a 1.89 D/f.u. final deuterium capacity, according to PCI data. During desorption, a single-phase β is detected at 1.11 D/f.u., while the capacity related to single-phase β is equal to 1.28 D/f.u. during absorption. Single α phase was detected below 0.18 D/f.u. deuterium content.

Finally, Figure 2-C evidences the formation of the single γ phase for the TiFe0.80Mn0.10 sample upon absorption at high pressure (approx. 9 MPa D2) with a final 1.99 D/f.u. deuterium content. The singlephase β was recorded at 1.12-1.27 D/f.u. during desorption. Single α phase was detected below 0.24 D/f.u. deuterium content.

Figure 3 shows the phase evolution during an absorption and desorption cycle for Sample C measured at the GEM time-of-flight instrument at ISIS. The same phase transitions observed during the measurement at ILL (Figure 2) were observed. However, some notable differences can be pointed out. The sorption cycle started from the deuterium-free alloy (α phase) and proceeded with absorption up to 9.8 MPa. As expected, deuteride phases are formed upon increasing the pressure. First, the β phase was observed above 0.2 MPa, followed by the formation of the γ phase above 0.3 MPa, in agreement with PCI data shown in Figure 1. It is worth noting that a complete transformation from α to β and from β to γ was not observed and still 8.9 wt.% of α and 3.7 wt.% of β phases were still detected at 9.8 MPa (as quantitatively determined by Rietveld refinement). Accordingly, the deuterium capacity reported in Figure 3 (D/f.u. = 1.7 at 9.8 MPa), which refers to the overall deuterium content of the material, is lower than that measured at ILL (D/f.u. = 1.99 at 8.37 MPa, Figure 2). This could be due to an incomplete activation of sample C prior to the measurement at ISIS, which limited deuterium absorption. As the pressure was decreased, desorption set in. The γ phase was no longer detected below 0.3 MPa and only the α phase with a residual D/f.u. = 0.03 was observed below 0.05 MPa.

Rietveld refinement

The Rietveld refinement was performed on the experimental patterns to determine the crystal structures and phases composition (and related deuterium content in the crystal structure, Table 2).

Ti/Fe contrast in ND-experiments, where the bound coherent scattering length is bc = -3.44 and 9.45 fm for Ti and Fe, respectively, is rather high. However, care should be taken to analyze the simultaneous occupancy of Ti and Mn atoms at the same site, as they have similar scattering lengths (bc (Mn) = -3.73 fm). For this reason, the site occupancy factors (SOF) of Ti, Fe and Mn have been fixed equal to that determined by EPMA analysis performed in ref. [START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn[END_REF] and reported in Table 3, while deuterium occupancies at different sites were refined through the sequential in-situ analysis. [START_REF] Dematteis | In-situ neutron diffraction during reversible deuterium loading in Ti-rich and Mn-substituted Ti(Fe,Mn)0.90 alloys -Dataset related to publication[END_REF] Since the compositions of analyzed samples are located on the Ti-rich side of the Ti(Fe,Mn) phase field, results confirm the Ti substitution on Fe site. Challet et al. already demonstrated that the combination of in-lab XRD, EPMA chemical analysis and density measurements can allow to establish location of possible vacancies, as well as Ti and Mn substitutions. [START_REF] Challet | Hydrogen Storage in TiFe(0.70+x)Mn(0.20-x)[END_REF] In a recent review on TiFe-substituted compounds, [START_REF] Dematteis | Substitutional effects in TiFe for hydrogen storage: a comprehensive review[END_REF][29] the authors collected lattice parameters and Mn content for different alloys in the literature. Sometimes it is not possible to directly compare the lattice parameters with the real amount of Mn in the TiFe-phase, however, the data determined in this study are in line with literature determinations, supporting the declared concentration of Mn in the sample. Atomic positions and isotropic displacement factors were refined and assessed for single-phase Ti(Fe,Mn)-D patterns and then fixed during the sequential refinement, because of the limited resolution of D1B beamline. Isotropic displacement factors were observed to vary between 0.1-0.5 Å 2 for metals and 1.8-2.0 Å 2 for deuterium. The shape factor of the pseudo-Voigt function was refined through the sequential refinement, since it can change because of strain generated during absorption and desorption. The asymmetry factor at low angles was refined on single-phase α patterns and then maintained fixed for all refinements. It is worth noting that during the in-situ analysis, anisotropic peak broadening effects have been observed: in fact, in the β phase, asymmetric profiles are present on (011) and (100) peaks, while for the γ phase, asymmetry in ( 200) and (020) peaks in opposite angle directions was recorded, as it can be observed in published research datasets and refinements (see the Supplementary data section and ref. [START_REF] Dematteis | In-situ neutron diffraction during reversible deuterium loading in Ti-rich and Mn-substituted Ti(Fe,Mn)0.90 alloys -Dataset related to publication[END_REF]), and, as an example, in the zoom on Rietveld refinement reported in Figure S2. This result reflects a high strain and the presence of micro-twins and dislocations in the above-mentioned samples during sorption, as already reported in previous literature studies. [START_REF] Fischer | Deuterium storage in FeTi. Measurement of desorption isotherms and structural studies by means of neutron diffraction[END_REF] These effects can be solved by implementing different strain models, but this was considered to be outside of the scope of this work. As a matter of fact, the non-implementation of these asymmetries results in higher Rietveld R-factor for the β and γ phases compared to the α phase, as it can be observed in Table 2.

All samples contain a small amount of fluorite-type TiH2 phase, in which, according to the ternary Ti-Fe-Mn diagram, [START_REF] Dew-Hughes | The addition of Mn and Al to the hydriding compound FeTi: Range of homogeneity and lattice parameters[END_REF] Ti is partially substituted by Fe and Mn. The precipitation of this phase results from the fact that the Ti-content of the alloys (52.5 at.%) is at the limit of the homogeneity domain of the CsCl-type Ti(Fe,Mn) phase. In addition, only at low deuterium pressures, the presence of an oxide Ti4Fe2O-type phase could be detected and refined, while it could not be distinguished at high deuterium pressures, due to severe peak overlapping. Secondary phases have been reported to have a major role in improving activation at milder conditions of temperature and pressure with respect to the pure compound, since they can interact with hydrogen and behave as hydrogenation gates during the first hydrogenation of the alloy. [START_REF] Dematteis | Hydrogen storage properties of Mn and Cu for Fe substitution in TiFe0.9 intermetallic compound[END_REF][START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn[END_REF] In the present work, the crystal structure of secondary phases were refined on single-phase -Ti(Fe,Mn)-D patterns and then it has been fixed during the sequential refinement apart from their abundance. The determined amount of TiH2-type phase variated between 1 and 3 wt.% in all samples, while the oxide-type phase ranges between 1 and 4 wt.%, in good agreement with previous XRD determinations, as reported in Table 4. [START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn[END_REF] Good agreement is also observed in the cell parameter of the α phase (Table 4), confirming the role of manganese in increasing the cell size of the pristine alloy, thus lowering the pressure plateaus of PCI curves. [START_REF] Dematteis | Substitutional effects in TiFe for hydrogen storage: a comprehensive review[END_REF] Table 2 gathers the results of the Rietveld refinement performed on single-phase patterns collected at ILL in absorption for the γ phase (orthorhombic, S.G. Cmmm), during desorption and absorption for the β phase (orthorhombic, S.G. P2221) and during desorption for the α phase (cubic, S.G. Pm-3m).

Errors are reported from the Rietveld analysis by using the Berar formula and so considering a multiply factor on standard deviation of 2.75. [START_REF] Rodriguez-Carvajal | FULLPROF: a program for Rietveld refinement and pattern matching analysis[END_REF] Graphical output of Rietveld refinements (including prf files) and the obtained refinement quality factors (Rb for each phase and Rwp for the whole pattern) are available at ref. [START_REF] Dematteis | In-situ neutron diffraction during reversible deuterium loading in Ti-rich and Mn-substituted Ti(Fe,Mn)0.90 alloys -Dataset related to publication[END_REF].

Absorption and desorption effects on the crystal structure of the H2-carrier material were determined and therefore the values for volume expansion are listed in Table 2. Volume expansions do not vary significantly among samples, and they can be defined as 11-15% for the β phase and equal to 17% for the γ phase, with respect to the dehydrogenated α phase. [START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn[END_REF] For the β phase, the volume variation is higher on absorption (13-15%) than on desorption (11-12%), a fact that concurs with the higher deuterium content during the absorption step with respect to the desorption one, in agreement with previous findings for the TiFe-D system. [START_REF] Reilly | Formation and properties of iron titanium hydride[END_REF][START_REF] Schäfer | Investigation of TiFe Deuteride Structures by Means of Neutron Powder Diffraction and the Mössbauer Effect[END_REF] Result of Rietveld refinements using ISIS data, which are all multi-phase patterns besides that initial and final states, largely confirmed the results discussed above for data from ILL. It is worth noting that similar anisotropic broadening in the peak shapes of the β phase were observed in the ISIS data.

This confirms that they are due to microstructural features of the phase, rather than instrumental effects. The secondary phases observed in the time-of-flight measurements were also the same. The weight fractions of various phases and corresponding cell parameters are summarized in Table 4.

They largely coincide with the results obtained with ILL data. Most importantly, the amount of secondary phases is not greater than those reported in Table 4, which supports that surface oxidation and/or incomplete activation are the likely cause for incomplete deuterium absorption (see Figure 3).

Structural analysis

Figure 4 gathers, for samples A (left) and B (right), the cell volume as well as the total deuterium content (Dtot/f.u.) and its distribution per site (Dsite/f.u.) for α, β and γ phases, as determined by the Rietveld refinements on patterns collected at ILL, as a function of the scan number. In addition, TiH2type and Ti4Fe2O-type phase amounts are also displayed.

The α phase is properly refined in the cubic structure (S.G. Pm-3m), its cell volume evolution during deuteration and desorption for different Mn contents can be followed comparing Figure 4-(a In principle, cell volumes should remain constant during all phase transformations, but the β phase does not observe this rule (Figure 4-(a)). During phase transitions on desorption (→→) the cell volume of the β phase gradually decreases. This variation is less pronounced on increasing the Mn content in the alloy (Figure 5). On absorption (→→), and irrespectively on the Mn content, the β cell volume strongly increases. The total deuterium content and individual contributions from the two different D-sites in the  phase (2a and 2d) both on absorption and desorption can be observed in Figure 4-(b) and Figure 5-(b). Deuterium content varies more significantly over the 2d site, especially on absorption. As introduced, different β phases were reported in the literature with two β phases observed on desorption and one on absorption. [START_REF] Schäfer | Investigation of TiFe Deuteride Structures by Means of Neutron Powder Diffraction and the Mössbauer Effect[END_REF] However the present analysis offers a good description of phase transitions using a unique β phase both in absorption and desorption with orthorhombic structure (S.G. P2221). Nonetheless, the occurrence of several β phases with different but close deuterium contents cannot be ruled out, explaining the observed variations of the β cell volume during phase transitions.

The γ phase has been refined in an orthorhombic structure (S.G. Cmmm). A good refinement is obtained during cycling, showing almost constant cell parameters and volume for all Mn contents (Figure 4-(a) and Figure 5-(a)). However, a variation in the total deuterium content can be observed after the second absorption at high scan numbers (Figure 4-(b)), which may be connected with either a solid solubilization of deuterium or to residual stress and generated dislocations as discussed in previous neutron studies in ref. [START_REF] Züchner | Hydrogen in Intermetallic Compounds I[END_REF][START_REF] Fischer | Deuterium storage in FeTi. Measurement of desorption isotherms and structural studies by means of neutron diffraction[END_REF][START_REF] Bo | Thermodynamic re-assessment of Fe-Ti binary system[END_REF].

The γ phase neutron diffraction pattern of sample B (TiFe0.85Mn0.05) was also recorded at the high-resolution beamline D2B at ILL (Figure S3). Results from D2B are in good agreement with the D1B determination and support the quality of the in-situ refinement (Figure 4 Results on structural evolution highlight that the deuterium content in the γ phase at a given pressure increases with Mn substitution. Higher Mn contents in the alloy allow a higher degree of deuteration, linked to a lower equilibrium pressure, as observed comparing results obtained for sample A (TiFe0.90) with those related to sample C (TiFe0.80Mn0.10). Hysteresis effects are also observed to diminish upon increasing the Mn content. [START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn[END_REF] In the β phase, a large variation of cell volume during desorption is observed for Mn-free sample A.

Upon increasing the Mn content, this variation is almost suppressed. In contrast, on absorption, the cell volume of the β phase gradually increases irrespectively of Mn-content.

Finally, in the α phase, the maximum deuterium content (0.06 D/f.u.) does not change with Mn content, while the cell volume increases with Mn content, due to Fe by Mn substitution.

Figure 5 shows the phase abundance, deuterium occupancy and cell volume for sample C measured at ISIS. The changes in the phase abundance are markedly different from those observed at ILL, due to the different initial and final states of the sample and to the incomplete hydrogen absorption reaction. On the other hand, the deuterium occupancies and cell volumes provide similar values for various phases, confirming the consistency of the Rietveld refinement results from different data sets.

In conclusion, it has been evidenced how the inclusion of Mn enhances the overall capacity and leads to decreased hysteresis compared to the parent TiFe material. This positive substitutional effect is mostly related to the structural modification of the TiFe cell, causing an expansion and enabling easier hydrogen diffusion into the structure to nucleate and form the hydride. A possible mechanism for this behaviour can suppose that both structural and electronicelectronic-based effects are induced by Mnsubstitution, both by expanding the cell volume and interacting with hydrogen guaranteeing a stronger binding energy, thus lowering the enthalpy and the equilibrium pressure at which hydrogen can be absorbed. Since lower hydrogenation pressures are required after Mn substitution, higher capacities can be reached by forming the di-hydride (full hydrogenation) at lower pressure. Similarly, the effect of Mn substitution on hysteresis can be supposed to be connected with its chemistry and effect on mechanical properties of the alloy, guaranteeing a reversible hydrogenation in a narrower pressure range between absorption and desorption. Especially, it can be related to an effect of Mn in reducing plastic deformation during hydrogen absorption.

Conclusions

Reversible deuterium sorption and phase transformations have been successfully characterized in three different TiFe-type alloys by in situ neutron diffraction, while recording the Pressure-Composition Isotherms at RT.

Neutron diffraction analysis demonstrates that different contents of Mn do not significantly change the volumetric expansion during phase transitions. Mn substitution affects the deuterium content for the γ phase and the cell evolution for the β phase. This is linked with the lowering of the high-plateau pressure with Mn content.

Analyses of Mn content on structural and hydrogenation properties proved that, increasing the Mn content, a higher deuterium capacity can be obtained, while structural features are mostly retained.

Especially regarding the β phase, Mn substitution favors a stable deuterium occupancy over the two available 2a and 2d sites, compared to Mn-free sample where a large variation in deuterium occupancy is evidenced over 2d site, particularly during desorption.

The study confirms that the deuterated structures of the γ phase upon absorption, β and α phase upon desorption, correspond to S.G. Cmmm, P2221 and Pm-3m, respectively.

In addition to the determination of structural and composition evolution, the fundamental knowledge of hydrogenation and dehydrogenation mechanisms of TiFe0.90-xMnx alloys is relevant for the scientific community and helps in the final design of the material, since the determined volume expansion and reduction of the alloy during H2 uptake and release must be considered during tank design. [START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn[END_REF] and datasets from [START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn -Dataset related to publication[END_REF]). Solid lines in absorption and dashed lines in desorption are a guide for the reader, errors are within data points.

Figure 2 -

 2 Figure 2-B displays the formation of phases and their transitions as a function of pressure for the

  ) for sample A, B and Figure 5-(a) for sample C, evidencing an increased cell volume when Mn is introduced. Figure 4-(b) shows that the deuterium content keeps below 0.10 D/f.u., in good agreement with the literature.

  -B, scan number 25). The lower deuterium occupancy obtained from the refinement and visible in scan 25 on Figure 4-B-(b) can be justified because of a lower pressure reached in the vial due to a deuterium leak over time between the measurements.

Figure 1 -

 1 Figure 1 -Absorption (abs, full and half-full symbols) and desorption (des, empty and crossed

Figure 2 -

 2 Figure 2 -In-situ neutron diffraction patterns (counter plot, λ=1.286 Å) and PCI curve at RT

Figure 3 -

 3 Figure 3 -In-situ neutron diffraction patterns (counter plot) and PCI curve during deuterium

Figure 4 -

 4 Figure 4 -Rietveld refinement results as a function of the scan number of the in-situ neutron

Figure 5 -

 5 Figure 5 -Rietveld refinement results as a function of the scan number of the in-situ neutron

ESIFigure S1 -

 S1 Figure S1 -PCI determined capacity at ILL and total deuterium content determined by Rietveld refinement as a function of scan number for sample A, B, and C. Yellow area is evidencing desorption determinations, while white area refers to absorption. Scan numbers correspond to PCI points reported in the manuscript.

Figure S2 -

 S2 Figure S2 -Zoomed region of the Rietveld refinement on β (down) and γ (up) phase from scan 7and 2, respectively. Asymmetric peak profiles can be appreciated.

Figure S3 -

 S3 Figure S3 -Neutron diffraction pattern and Rietveld refinement of the γ phase of sample B (scan number 25, TiFe0.85Mn0.05) recorded at ILL beamline D2B at RT.

Table 1 -

 1 Literature review of TiFe and its hydrides structures and structural parameters.

	Structure	S.G.	a (Å)	b (Å)	c (Å)	β/γ (°)	Atomic positions	Formula	Ref.
						α phase
	Cubic	Pm-3m	2.959					TiFe	[11]
	Cubic	Pm-3m	2.975					TiFe	[10]
	Cubic	Pm-3m	2.976					TiFe	[12-14]
	Cubic	Pm-3m	2.979					TiFe	[15]
	Cubic	Pm-3m	3.011					TiFe	[16]
	Cubic	Pm-3m	2.978					TiFeH0.02	[52]
	Cubic	Pm-3m	2.977					TiFeH0.04	[14]
	Cubic	Pm-3m	2.980					TiFeH0.057	[17,18]
	Cubic	Pm-3m	2.979					TiFeH0.06	[22]
	Cubic	Pm-3m	2.985					TiFe0.885Mn0.115	[19]
	Cubic	Pm-3m	2.986					TiFe0.80Mn0.20	[31]
	Cubic	Pm-3m	2.983					TiFe0.94Ce0.06H0.03	[52]
						β phase
	Tetragonal		3.180		8.730			TiFeH1.06	[6]
	Orthorhombi		4.500	4.300	2.980			TiFeH0.67-1.02	[18]
	c
	Orthorhombi	Pmcm	2.954	4.538	4.381			TiFeH0.94	[22]
	c
	Orthorhombi	Pmcm	2.991	4.556	4.398			Ti1.04Fe0.96H1.15	[23]
	c
	Orthorhombi	Pmcm	3.094	4.513	4.391			TiFeH1.4	[22]
	c
	Orthorhombi	Pmcm	2.968	4.535	4.382			Ti0.98Fe1.02H1.03	[23]
	c
	Orthorhombi	Pmcm	2.973	4.561	4.407			Ti1.02Fe0.98H1.03	[23]
	c
	Orthorhombi	P2221	2.956	4.543	4.388		yFe, 2c = 0.206	TiFeH1.0±0.04	[20]
	c	Pmc21					xD1, 2a = 0.058
		Pmcm					yD1, 2a = 0.537
		P2cm
	Orthorhombi	P2221	2.966	4.522	4.370		yTi, 2d = 0.757	TiFeH1.0	[10,12,21]
	c	Pmc21					yFe, 2c = 0.294
		P2/c					yD2, 2d = 0.300
	Orthorhombi	P2221	2.978	4.549	4.417			TiFeH1.0	[16]
	c
	Orthorhombi	P2221	2.967	4.522	4.371		yTi, 2d = 0.757	TiFeH1.0	[12]
	c						yFe, 2c = 0.294
	Orthorhombi	P2221	2.932	4.515	4.276			TiFeH1.0	[11]
	c
	Orthorhombi	P2221	3.088	4.515	4.391		yTi, 2d = 0.750	TiFeH1.37	[21]
	c						yFe, 2c = 0.303
							yD2, 2d = 0.297
	Orthorhombi	Pb21m	4.475	4.319	2.925			TiFeH1.0	[13]
	c	P2212
	Orthorhombi	Pb21m	4.543	4.369	2.972			TiFeH1.0	[13]
	c	P2212
	Orthorhombi	Pmc21	2.995	4.531	4.397		yTi, 2d = 0.249	TiFe0.90Mn0.10H1.25	[19]
	c	P2cm					yFe,Mn, 2c = 0.694
							xD1, 2a = 0.037
							yD2, 2d = 0.555
							yD2, 2d = 0.737
						γ phase

Table 4 -

 4 Secondary phase amount and lattice parameters as determined from neutron diffraction (ND) at ILL and ISIS reported in this work. X-ray diffraction (XRD) analysis performed in ref.[START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn[END_REF][START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn -Dataset related to publication[END_REF][START_REF] Dematteis | Fundamental hydrogen storage properties of TiFe-alloy with partial substitution of Fe by Ti and Mn -Raw Dataset related to publication[END_REF] are reported for comparison. Range of values are reported (minimum and maximum values) when multiple in-situ determinations were performed.

			ND	XRD	ND	ND	XRD	ND	ND	XRD
	Sample		TiFe	TiFe	TiD2	TiD2	TiH2	Ti4Fe2O	Ti4Fe2O	Ti4Fe2O
			a (Å)	a (Å)	a (Å)	wt.%	wt.%	a (Å)	wt.%	wt.%
	A -ILL A -ISIS	TiFe0.90 TiFe0.90	2.978-2.984 2.985(1)	2.982(7)	4.407 4.377	1.1-1.8 0.6	5.0±0.5	11.841 11.924	1.8-3.7 0.9	4.0±0.5
	B -ILL B -ISIS	TiFe0.85Mn0.05 TiFe0.85Mn0.05	2.980-2.985 2.988(1)	2.985(6)	4.402 4.393	1.3-1.7 0.1	2.8±0.5	11.855 11.780	0.7-2.1 0.3	2.4±0.5
	C -ILL C -ISIS	TiFe0.80Mn0.10 TiFe0.80Mn0.10	2.981-2.987 2.987-2.994	2.987(0)	4.372 4.389-4.415	2.3-2.7 0.7-2.3	2.8±0.5	11.865 11.574-11.928	1.3-2.6 0.05-3.03	1.7±0.5

Table S1 -

 S1 Structural details (atom coordinates, thermal factor, occupancies) of single-phase Ti(Fe,Mn)-D phases for samples A, B, and C as determined in this work from in-situ neutron diffraction during deuterium absorption and desorption (at ILL and ISIS). Errors in the last digit are given between brackets.

			-γ (1 st absorption, S.G. Cmmm) -
	A) TiFe0.90
	Atom	X	Y	Z	Biso	Occ	Capacity from PCI [Pressure (MPa)]
	Ti4h	0.211(2) 0	0.5 0.1	4.000
	Fe4i	0	0.285(1) 0	0.1	3.856
	Ti4i	0	0.285(1) 0	0.1	0.144
	D14e	0.25	0.25	0	2.0	3.7(1)
	D12c	0.5	0	0.5 2.0	1.3(1)
	D12a	0	0	0	2.0	1.6(1)
	Dtotal/4					1.7(1)	1.7(1) [8.47]
	B) TiFe0.85Mn0.05
	Atom	X	Y	Z	Biso	Occ	Capacity from PCI [Pressure (MPa)]
	Ti4h	0.213(1) 0	0.5 0.4	4.000
	Fe4i	0	0.286(1) 0	0.4	3.648
	Ti4i	0	0.286(1) 0	0.4	0.152
	Mn4i	0	0.286(1) 0	0.4	0.200
	D14e	0.25	0.25	0	2.0	4.1(1)
	D12c	0.5	0	0.5 2.0	1.6(1)
	D12a	0	0	0	2.0	1.9(1)
	Dtotal/4					1.9(1)	1.9(1) [8.77]
	C) TiFe0.80Mn0.10
	Atom	X	Y	Z	Biso	Occ	Capacity from PCI [Pressure (MPa)]
	Ti4h	0.213(1) 0	0.5 0.1	4.000
	Fe4i	0	0.285(1) 0	0.1	3.508
	Ti4i	0	0.285(1) 0	0.1	0.136
	Mn4i	0	0.285(1) 0	0.1	0.360
	D14e	0.25	0.25	0	2	4.2(1)
	D12c	0.5	0	0.5 2	1.7(1)
	D12a	0	0	0	2	2.0(1)
	Dtotal/4					2.0(1)	2.0(1) [8.37]

Acknowledgements

HyCARE project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking (now

The authors wish to thank Laetitia Laversenne at ILL, and Ivan da Silva Gonzalez and Mark Kibble at ISIS for their help and support during beamtimes and in-situ experimental setup. This manuscript honours the memory of our colleague Michel Latroche, who significantly contributed to the work presented herein.

Electronic Supporting Information Description

Supplementary data

Supplementary data to this article can be found online at https://dx.doi.org/10.5291/ILL-DATA.5-22-771 and https://doi.org/10.5286/ISIS.E.RB1920559, which contains raw data recorded at ILL and ISIS respectively. [START_REF] Cuevas | In-situ neutron diffraction during reversible deuterium loading in under-stoichiometric and Mn-substituted Ti(Fe,Mn)0.9 alloys[END_REF][START_REF] Dematteis | In-situ neutron diffraction during reversible deuterium loading in under-stoichiometric and Mn,Cu-substituted Ti[END_REF] Origin resume file with all determination and graphs, and Rietveld refinements outputs are available at https://doi.org/10.5281/zenodo.6620531. [START_REF] Dematteis | In-situ neutron diffraction during reversible deuterium loading in Ti-rich and Mn-substituted Ti(Fe,Mn)0.90 alloys -Dataset related to publication[END_REF] From the cited dataset, Rietveld refinements graphs for all patterns analysed can be downloaded, ".out" refinement files can also be found and downloaded for further refinement details and related Rp, Rexp, GoF factors of each pattern which are consistent with the ones reported of pure phases.