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The structure of the sample as-synthesized was refined from the X-ray diffraction data using the Rietveld method 9 implemented in the FullProf 10 program. The background was taken by linear interpolation between selected points and pseudo-Voigt peak functions were used to describe the peak shapes. The background was refined with a 4 th order Chebychev polynomial and a Thompson-Cox-Hastings pseudo-Voigt 11 peak function was used to describe the peak profile.

In total 14 parameters were refined in TOPAS: 5 background coefficients, 6 peak shape parameters and the cell parameters a, b and c. As can be seen from Fig. No additional peaks were observed in the powder diffraction pattern showing that the sample is a single phase HEA. This was observed as a decrease in the hydrogen pressure in the reactor. All peaks could be indexed with the lower symmetry body centred tetragonal lattice. A neutron diffraction study is required to completely determine the crystal structure of the hydrogenated material but this was not possible due to the large neutron absorption cross-section of Hf.

Isothermal hydrogen absorption showed that TiVZrNbHf absorbs hydrogen readily with a plateau pressure of 0. The maximum measured storage capacity was 2. The observed hydrogen desorption appears to be a one-step reaction causing all the hydrogen to leave the HEA simultaneously.

However, the peaks in the XRD pattern after desorption are doublets indicating a phase separation into two different BCC phases with slightly different unit cell parameters during hydrogen desorption. A detailed investigation of the structural stability of the alloy during hydrogen absorption-desorption cycling is currently being performed but is outside the scope of this communication.

The results presented confirm our hypothesis that an HEA can have excellent hydrogen absorption properties. It is striking that the hydrogen content in this alloy is significantly higher than in any of the binary hydrides of the constituent elements 14 , 15 , 16 , 17 , In binary hydrides with a cubic close packed FCC structure such as TiH 2 and ZrH 2 , the hydrogen is placed in tetrahedral interstitial sites.

This behaviour is unique and has never been observed before in pure transition metal hydrides. The archetypical structural transition upon creating a hydride is the formation of a FCC lattice in the fully hydrogenated form MH 2. This type of lattice distortion, illustrated in Fig. The elongation is assumed to be caused by hydrogen initially occupying octahedral sites in the BCC structure followed by a transformation to a fully hydrogenated VH 2 phase with hydrogen in the tetrahedral sites.

Wang et al. They found a stronger Zr-Zr bond in the FCT lattice compared to the FCC lattice that is explained by a planar type crystal field splitting in the FCT lattice and occupancy of the degenerate d yz and d xz orbitals. In this case, the pure metal has a double hexagonal close packed dHCP structure without hydrogen. This tetragonal distortion has been shown by neutron diffraction on NdD 2.

In the case of CeH 2. An astonishing observation is that the TiVZrNbHf alloy based only on transition metals indicates that an HEA can exhibit a combination of these two routes. This requires that hydrogen can be placed in both octahedral and tetrahedral sites in the HEA in contrast to other transition metal hydrides. Similar intermediate structures as in the normal BCC-case with initial distortion of the BCC lattice has been observed during in situ hydrogenations and these results are currently being evaluated, the details of this is however outside the scope of this communication.

We suggest that this is due to the presence of strain in the lattice due to variations in atomic radii. Xin et al. Even at low hydrogen concentrations, a change from tetrahedral to octahedral occupancy is observed 4. The built in strain in an HEA could be the driving force to open up new interstitial sites for hydrogen.

Theoretical modelling using ab initio methods is needed to explain the influence of strain on the stability of hydride formation in these types of alloys. In summary, we have studied the hydrogenation of the high entropy alloy TiVZrNbHf and observed that extremely large amounts of hydrogen can be absorbed.

The formation of a distorted FCC or BCT structure in the fully hydrogenated alloy is also similar to the structure formed in rare-earth compounds. It is suggested that the unprecedented hydrogen storage capacity is an effect of the strain in the distorted HEA lattice, which favours hydrogen occupying both tetrahedral and octahedral sites. This is due to the high mass of Hf and Zr. It is likely that much higher storage capacity by weight percent can be achieved with other HEAs by replacing these elements to appropriate lighter ones.

Hence, we propose that HEAs can be used as a new class of alloy for hydrogen storage that does not involve any rare-earth metals. How to cite this article : Sahlberg, M. Superior hydrogen storage in high entropy alloys. Author Contributions M. All authors interpreted the data and wrote the manuscript. Sci Rep. Published online Nov Author information Article notes Copyright and License information Disclaimer. Received Aug 5; Accepted Oct This work is licensed under a Creative Commons Attribution 4.

This article has been cited by other articles in PMC. Abstract Metal hydrides MH x provide a promising solution for the requirement to store large amounts of hydrogen in a future hydrogen-based energy system. Results As can be seen from Fig. Open in a separate window. Figure 1. Figure 2. The line is added as guide to the eye.

Figure 3. We previously established that the volume of the whole cell is , and so if we know the lattice parameters a and c, and atomic radius r, we can calculate the atomic packing factor. The conventional BCC cell that I have shown you is a conventional unit cell, not a primitive unit cell.

This conventional cell has advantages because it is highly symmetric and easy for humans to understand. However, when dealing with mathematical descriptions of crystals, it may be easier to describe the unit cell in the smallest form possible. The smallest possible unit cell is called the primitive cell. If you are interested in primitive cells, you can read all about them in this article.

The body-centered tetragonal crystal structure closely resembles the body-centered cubic crystal structure, but with 1 axis distorted to be longer or shorter than the other two. If you know about martensite in steel, you may have heard that this phase is BCT. Remember, BCT can mean a crystal structure or a Bravais lattice. Martensite has a BCT Bravais lattice, but the crystal structure is more complex than the 1-atom-per-lattice point definition of the BCT crystal.

If you want to know more about the basics of crystallography, check out this article about crystals and grains. I also mentioned atomic packing factor APF earlier in this article. This is an important concept in your introductory materials science class, so if you want a full explanation of APF, check out this page.

If you want to learn about specific crystal structures, here is a list of my articles about Bravais lattices and some related crystal structures for pure elements. Body-Centered Tetragonal is one of these 14 Bravais lattices and also occurs as a crystal structure. Simple Cubic 2. Face-Centered Cubic 2a. Diamond Cubic 3. Body-Centered Cubic 4. Simple Hexagonal 4a. Hexagonal Close-Packed 4b. Double Hexagonal Close-Packed La-type 5. Rhombohedral 5a. Rhombohedral Close-Packed Sm-type 6. Simple Tetragonal 7.

Body-Centered Tetragonal 7a.

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Allotropes of carbon — Eight allotropes of carbon: a Diamond, b Graphite, c Lonsdaleite, d C60 Buckminsterfullerene or buckyball , e C, f C70, g Amorphous carbon, and h single walled carbon nanotube or buckytube. This is a list of the allotropes of carb … Wikipedia. Adamantane — Adamantane … Wikipedia. Steel — For other uses, see Steel disambiguation. The steel cable of a colliery winding tower … Wikipedia.

Martensite — For the transformation, see Diffusionless transformations. Subscripted numbers are printed next to the number being modified e. A bar above a number is entered with a minus sign. Occasionally there are variations in how space groups are referenced.

In Crystal Structures v. This sort of incongruity is unfortunate. If you cannot resolve the incongruity using this list, try using the Schoenflies notation. In the literature there is less variation in the application of these conventions. The Schoenflies convention is, in fact, less precise than the Hermann-Maguin in that the complete symmetry characteristics of the crystal are not encoded in the space group designation. Adaptations to the keyboard have been made here as well.

Spaces are not allowed in the keyboard designation. The underscore does not need to precede to superscript. Each of the space groups as designated by the Schoenflies notation is listed below in the same order as the listing of the Hermann-Maguin notation. The two conventions are equally supported in the code.

The atoms list in atoms. A unique site is one and only one of a group of equivalent positions. The equivalent positions are related to one another by the symmetry properties of the crystal. ATOMS determines the symmetry properties of the crystal from the name of the space group and applies those symmetry operations to each unique site to generate all of the equivalent positions. If you include more than one of a group of equivalent positions in the atom list, then a few odd things will happen.

A series of run-time messages will issued telling you that atom positions were found that were coincident in space. This is because each of the equivalent positions generated the same set of points in the unit cell. ATOMS removes these redundancies from the atom list. The atom list and the potentials list written to feff. However, the site tags and the indexing of the atoms will certainly make no sense.

Also the density of the crystal will be calculated incorrectly, thus the absorption calculation and the self-absorption correction will be calculated incorrectly as well. The McMaster correction is unaffected. For some common crystal types it is convenient to have a shorthand way of designating the space group.

These words may be used as the value of the keyword space and ATOMS will supply the correct space group. Note that several of the common crystal types are in the same space groups. For copper it will still be necessary to specify that an atom lies at 0,0,0 , but it isn't necessary to remember that the space group is F M 3 M. Listed here are the labeling conventions for the axes and angles in each Bravais lattice. In three dimensional space there is an ambiguity in choice of right handed coordinate systems.

Given a set of mutually orthogonal axes, there are six choices for how to label the positive x , y , and z directions. For some specific physical problem, the crystallographer might choose a non-standard setting for a crystal. The choice of standard setting is described in detail in The International Tables. The Hermann-Maguin symbol describes the symmetries of the space group relative to this choice of coordinate system. The symbols for triclinic crystals and for crystals of high symmetry are insensitive to choice of axes.

Monoclinic and orthorhombic notations reflect the choice of axes for those groups that possess a unique axis. Tetragonal crystals may be rotated by 45 degrees about the z axis to produce a unit cell of doubled volume and of a different Bravais type. Alternative symbols for those space groups that have them are listed in Appendix A.

This resolution of ambiguity in choice of coordinate system is one of the main advantages of the Hermann-Maguin notation system over that of Shoenflies. In a situation where a non-standard setting has been chosen in the literature, use of the Schoenflies notation will, for many space groups, result in unsatisfactory output from ATOMS.

Here is an example. In the literature, La 2 CuO 4 was given in the non-standard b m a b setting rather than the standard c m c a. As you can see from the axes and coordinates, these settings differ by a 90 degree rotation about the A axis. The coordination geometry of the output atom list will be the same with either of these input files, but the actual coordinates will reflect this 90 degree rotation.

There are seven rhombohedral space groups. Crystals in any of these space groups that may be represented as either monomolecular rhombohedral cells or as trimolecular hexagonal cells. These two representations are entirely equivalent.

The rhombohedral space groups are the ones beginning with the letter R in the Hermann-Maguin notation. ATOMS does not care which representation you use, but a simple convention must be maintained. If the hexagonal representation is used, then a and c must be specified in atoms. Atomic coordinates consistent with the choice of axes must be used. Some space groups in The International Tables are listed with two possible origins.

The difference is only in which symmetry point is placed at 0,0,0. This orientation places 0,0,0 at a point of highest crystallographic symmetry. Again Mn 3 O 4 is an example. Twenty one of the space groups are listed with two origins in The International Tables. ATOMS knows which groups these are and by how much the two origins are offset, but cannot know if you chose the correct one for your crystal.

If you use one of these groups, ATOMS will print a run-time message warning you of the potential problem and telling you by how much to shift the atomic coordinates in atoms. This warning will also be printed at the top of the feff. If you use one of these space groups, it usually isn't hard to know if you have used the incorrect orientation.

Because it is tedious to edit the atomic coordinates in the input file every time this problem is encountered and because forcing the user to do arithmetic invites trouble, there is a useful keyword called shift. For the Mn 3 O 4 example discussed above, simply insert this line in atoms. This vector will be added to all of the coordinates in the atom list after the input file is read. The above input file gives the same output as the following.

Here the shift keyword has been removed and the shift vector has been added to all of the fractional coordinates. These two input files give equivalent output. The following is my attempt to demystify the crazy symbolism used by the Hermann-Maguin and Schoenflies conventions. This is by no means an adequate explanation of the rich and beautiful field of crystallography. For that, I recommend a real crystallography text.

An important part of the demystification process is to define some of the important terms used to describe crystal symmetries. The words system , Bravais lattice , crystal class , and space group have well-defined meanings. The symbols used in each of the notation conventions specifically relate the various symmetries of crystals.

There are seven systems of crystals. The system refers to the shape of the undecorated unit cell. They are:. There are fourteen Bravais lattices. The Bravais lattices are constructed from the simplest translational symmetries applied to the seven crystal systems. A P lattice has decoration only at the corners of the unit cell. An I lattice has decoration at the body center of the cell as well as at the corners. An F lattice has decoration at the face centers as well as at the corners.

A C lattice has decoration at the center of the face as well as at the corners. Likewise A and B lattices have decoration at the centers of the and faces respectively. R lattices are a special type in the trigonal system which possess rhombohedral symmetry. All seven crystal systems have P lattices, but not all the classes have the other type of Bravais lattices. This is because there is degeneracy when all the Bravais lattice types are applied to all the crystal systems. Considering such degeneracies reduces the possible decorations of the seven systems to these 14 unique three dimensional lattices:.

For historic reasons, hexagonal cells are sometimes called C lattices. Modern literature usually uses the P designation. The decorations placed on the Bravais lattices come in 32 flavors called classes or point groups which represent the possible symmetries within the decorations.

Each type of symmetry is defined either by a reflection plane, a rotation axis, or a rotary inversion axis. A reflection plane can either be a simple mirror plane or a glide plane, which defines the symmetry operation of reflecting through a mirror followed by translating along a direction in the plane. A rotation axis can either define a simple rotation or a screw rotation, which is the symmetry operation of rotating about the axis followed by translating along that axis.

A rotary inversion axis defines the symmetry operation of reflecting through a plane followed by rotating about an axis in that plane. These three symmetry types, reflection plane, rotation axis, and rotary inversion axis, can be combined in 32 non-degenerate ways. It would seem that the 32 classes could decorate the 14 Bravais lattices in ways. In fact, the number might be larger as there are numerous types of screw axes and glide planes.

Again, considering degeneracies reduces the total number of combinations, leaving unique decorations of the Bravais lattices. These are called space groups. The space groups are a rigorously complete set of descriptions of crystal symmetries in three dimensional space. That is, there may be new crystals but there are no new space groups.

Here I am only considering space-filling crystals with translational periodicity. The Hermann-Maguin notation uses a set of two to four symbols to completely specify the symmetries of a space group. The first symbol is always a single letter specifying the Bravais lattice. The next three symbols specify the class of the space group. These three symbols are some combination of the following characters:. These are sufficient to completely specify the various planar and axial symmetries of the classes and sub-classes.

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