Site-preference Rule Research Articles

Revealing the trend of structural, electronic, mechanical and vibrational properties in Co2VX (X= Si, Ge, Sn)

We report systematic first-principles calculations on the series of Cobalt based full-Heuslers Co2VX (X ​= ​Si, Ge, Sn) highlighting the role of X (X ​= ​Si, Ge, Sn) atom in determining the strength of hybridization of d-orbitals and ultimately influencing half-metallic character. The effect of Hubbard potential (U) for transition elements of Co and V has been included demonstrating half-metallic ferromagnetism. It has been found that three of them are likely to exhibit the Cu2MnAl phase instead of Hg2CuTi phase, well in accordance with Site Preference Rule. The computed elastic constants validate Born-Haung stability criteria for all of them, affirming the mechanical stability while the mechanical parameters uncover ductile, isotropic and non-directional metallic bonding nature. The obtained positive phonon frequencies establish dynamical stability for three of them. The electronic properties explain the hybridisation of d-orbitals responsible for half-metallicity with narrow band gaps of 1.162 ​eV, 0.545 ​eV and 0.493 ​eV for Co2VSi, Co2VGe and Co2VSn respectively in minority spin channel. The magnetic moments of Co and V are coupled with different strengths in parallel direction indicating ferromagnetic order with total magnetic moment per formula unit obeying Slater Pauling rule of 24. The current DFT based application would further create scope of experimental work to explore their technological applications.

Phase Transition and Electronic Structures of All-d-Metal Heusler-Type X2MnTi Compounds (X = Pd, Pt, Ag, Au, Cu, and Ni).

In this work, we investigated the phase transition and electronic structures of some newly designed all-d-metal Heusler compounds, X2MnTi (X = Pd, Pt, Ag, Au, Cu, and Ni), by means of the first principles. The competition between the XA and L21 structures of these materials was studied, and we found that X2MnTi favors to feature the L21-type structure, which is consistent with the well-known site-preference rule (SPR). Under the L21 structure, we have studied the most stable magnetic state of these materials, and we found that the ferromagnetic state is the most stable due to its lower energy. Through tetragonal deformation, we found that the L21 structure is no longer the most stable structure, and a more stable tetragonal L10 structure appeared. That is, under the tetragonal strain, the material enjoys a tetragonal phase transformation (i.e., from cubic L21 to tetragonal L10 structure). This mechanism of L21-L10 structure transition is discussed in detail based on the calculated density of states. Moreover, we found that the energy difference between the most stable phases of L10 and L21, defined as ΔEM (ΔEM = ECubic-ETetragonal), can be adjusted by the uniform strain. Finally, the phonon spectra of all tetragonal X2MnTi (X = Pd, Pt, Ag, Au, Cu, and Ni) phases are exhibited, which provides a powerful evidence for the stability of the tetragonal L10 state. We hope that our research can provide a theoretical guidance for future experimental investigations.

Site preference, magnetic and electronic properties of half-metallic Vanadium-based full Heusler alloys

We reported a new atomic site preference occupation for Vanadium-based X2YZ full Heusler alloys. It tells that alloys with less 24 valence electrons form the L21-type structure, while others form the XA-type structure. This fact means that the site preference rule for Vanadium-based alloys depends on the total number of valence electrons instead of the electropositive between X and Y atoms. The magnetic moments of XA-type alloys follow the Slater-Pauling rules, with two forms, Mt = NV − 24 and Mt = NV − 18 corresponding to different origins of the band gaps. XA-type V2YZ (Y = Cr, Mn and Fe; Z = Ga, Ge and As), V2CoGa, V2CoGe and V2NiGa alloys have half-metallic property. L21-type V2YZ (Y = Cu and Zn; Z = Ga, Ge and As) alloys show no spin polarization and no half-metallic property due to the structural symmetry and the non-magnetism of copper and zinc atoms.

Complete spin gapless semiconductivity in equiatomic quarternary Heusler material TiZrMnAl

A new equiatomic quarternary Heusler compound TiZrMnAl has been theoretically predicted. Two possible structural configurations are considered under the site preference rule and their equilibrium lattices have been derived. Electronic band structures identify TiZrMnAl compound as the spin gapless semiconductors in both two structure types. The total magnetic moments of TiZrMnAl are both equal to zero with the partial moments of Ti and Zr antiparallelly aligned to that of Mn. This integral value of total moment follows the Slater-Pauling rule in the form of Mt = Zt − 18, where Mt represents the total magnetic moment and Zt is the total number of valence electrons. The mechanical and dynamic stabilities of TiZrMnAl have also been validated and strong elastic anisotropy is further revealed with the calculated directional dependent Young’s modulus and shear modulus for both structures. Besides, the effect of random swap disorder between Ti and Zr atoms has been examined and it is found that through the whole swap range, TiZrMnAl maintains its fully compensated spin gapless semiconductivity, which is a very good property for possible applications in spintronic devices. Lastly, the uniform and tetragonal strains have been accessed and their effects on the electronic and magnetic properties have been studied. This systematic study can provide a comprehensive reference for the further development of Ti and Zr based quarternay Heusler compounds and even inspire other relative studies for the exploration of new spin gapless semiconductors.

Computational study of the electronic, magnetic, mechanical and thermodynamic properties of new equiatomic quaternary heusler compounds TiZrRuZ (Z = Al, Ga, In)

The electronic, magnetic, and mechanical properties of the new equiatomic quarternary Heusler compounds TiZrRuZ (Z = Al, Ga, In) have been studied with the first-principles calculations. Three types of different atomic configurations have been examined and results show the atomic site occupation follows the general site preference rule. The equilibrium lattice constants have been determined and also the total and atom-resolved magnetic moments have been calculated. These new equiatomic quarternary Heusler compounds TiZrRuZ (Z = Al, Ga, In) are ferromagnetic half metals at the ground states. In particular for the total magnetic moment (MT), an integral number of 1 µB is obtained with the main contribution from Ti atoms and it follows the general Slater-Pauling rule, i.e. MT = ZT -18, where ZT is the total valence electron number, 19 for TiZrRuZ. Furthermore, the derived mechanical properties have shown that these materials behave in a strong ductile manner with a high elastic anisotropy, which is further confirmed by the calculated 3D Young's modulus and shear modulus. Their mechanical and dynamic stabilities have also been verified with the generalized elastic criteria and phonon dispersion spectrum. Afterwards, the effects of the uniform and tetragonal strains have been studied. The variation of the conduction band minimum and the valence band maximum with both strains have been revealed and the materials show good robustness of the half metallicity. Also, the total magnetic moment maintains constant of 1 µB against both strains while the atomic magnetic moments vary. In the last, the thermodynamic properties have also been analyzed with the quasi-harmonic Debye model under pressure 0 to 10 GPa and temperature from 0 to 500 K. These materials can be good candidates for the future spintronic application and can even inspire other relative work for material research in this field.

Competition between cubic and tetragonal phases in all-d-metal Heusler alloys, X 2-x Mn1+x V (X = Pd, Ni, Pt, Ag, Au, Ir, Co; x = 1, 0): a new potential direction of the Heusler family.

In this work, a series of all-d-metal Heusler alloys, X 2 - x Mn1 + x V (X = Pd, Ni, Pt, Ag, Au, Ir, Co; x; = 1, 0), were predicted by first principles. The series can be roughly divided into two categories: XMn2V (Mn-rich type) and X 2MnV (Mn-poor type). Using optimized structural analysis, it is shown that the ground state of these all-d-metal Heusler alloys does not fully meet the site-preference rule for classic full-Heusler alloys. All the Mn-rich type alloys tend to form the L21 structure, where the two Mn atoms prefer to occupy the A (0, 0, 0) and C (0.5, 0.5, 0.5) Wyckoff sites, whereas for the Mn-poor-type alloys, some are stable with XA structures and some are not. The c/a ratio was also changed while maintaining the volume the same as in the cubic state to investigate the possible tetragonal transformation of these alloys. The Mn-rich Heusler alloys have strong cubic resistance; however, all the Mn-poor alloys prefer to have a tetragonal state instead of a cubic phase through tetragonal transformations. The origin of the tetragonal state and the competition between the cubic and tetragonal phases in Mn-poor alloys are discussed in detail. Results show that broader and shallower density-of-states structures at or in the vicinity of the Fermi level lower the total energy and stabilize the tetragonal phases of X 2MnV (X = Pd, Ni, Pt, Ag, Au, Ir, Co). Furthermore, the lack of virtual frequency in the phonon spectra confirms the stability of the tetragonal states of these Mn-poor all-d-metal Heusler alloys. This work provides relevant experimental guidance in the search for possible martensitic Heusler alloys in all-d-metal materials with less Mn and new spintronic and magnetic intelligent materials among all-d-metal Heusler alloys.

Site preference and tetragonal distortion in palladium-rich Heusler alloys.

In this work, two kinds of competition between different Heusler structure types are considered, one is the competition between XA and L21 structures based on the cubic system of full-Heusler alloys, Pd2 YZ (Y = Co, Fe, Mn; Z = B, Al, Ga, In, Tl, Si, Ge, Sn, Pb, P, As, Sb). Most alloys prefer the L21 structure; that is, Pd atoms tend to occupy the a (0, 0, 0) and c (0.5, 0.5, 0.5) Wyckoff sites, the Y atom is generally located at site b (0.25, 0.25, 0.25), and the main group element Z has a preference for site d (0.75, 0.75, 0.75), meeting the well known site-preference rule. The difference between these two cubic structures in terms of their magnetic and electronic properties is illustrated further by their phonon dispersion and density-of-states curves. The second type of competition that was subjected to systematic study was the competitive mechanism between the L21 cubic system and its L10 tetragonal system. A series of potential tetragonal distortions in cubic full-Heusler alloys (Pd2 YZ) have been predicted in this work. The valley-and-peak structure at, or in the vicinity of, the Fermi level in both spin channels is mainly attributed to the tetragonal ground states according to the density-of-states analysis. ΔE M is defined as the difference between the most stable energy values of the cubic and tetragonal states; the larger the value, the easier the occurrence of tetragonal distortion, and the corresponding tetragonal structure is stable. Compared with the ΔE M values of classic Mn2-based tetragonal Heusler alloys, the ΔE M values of most Pd2CoZ alloys in this study indicate that they can overcome the energy barriers between cubic and tetragonal states, and possess possible tetragonal transformations. The uniform strain has also been taken into consideration to further investigate the tetragonal distortion of these alloys in detail. This work aims to provide guidance for researchers to further explore and study new magnetic functional tetragonal materials among the full-Heusler alloys.

Site preference and tetragonal distortion of Heusler alloy Mn-Ni-V

Abstract Site preference, tetragonal distortion and phase stability of all d-metal Heusler alloys Mn2−xNi1.5+xV0.5 (x = 0, 0.5, 1), Mn2−xNi1+xV (x = 0, 0.25, 0.5, 0.75, 1) and Mn2−xNi0.5+xV1.5 (x = 0, 0.5, 1, 1.5) are investigated. It has been found that Ni2MnV is stable in Hg2CuTi-type Heusler structure, while Mn2NiV tends to crystallize into Cu2MnAl-type Heusler structure. Mn atoms in both Ni-rich and Mn-rich Mn-Ni-V alloys preferentially occupy A, C sites, suggesting that the site occupation in Mn-Ni-V ternary alloy is not following the conventional site preference rule. A potential tetragonal distortion is expected in cubic structure of Ni-rich alloys MnNi2V, Mn1.25Ni1.75V, MnNi2.5V0.5 and Mn0.5Ni2V1.5. It is found that the tetragonal distortion is mainly attributed to the decrease of intensity for DOS at the Fermi level in the minority spin direction which is caused by the splitting of peaks. This study will guide us to design new tetragonal Heusler materials for spintronic devices.

171 Scandium-based full Heusler compounds: A comprehensive study of competition between XA and L21 atomic ordering

The site-preference rule (SPR), which is widely used to design and study the properties of full-Heusler alloys X2YZ, is applied to determine the positions of different transition-metal elements with various numbers of valence electrons in Heusler alloys. The scandium-based full Heusler alloys should form an XA structure according to the SPR, because the transition-metal element Sc with fewer valence electrons tends to occupy the Wyckoff sites A(0, 0, 0) and B(0.25, 0.25, 0.25). As opposed to previous investigations, L21-type scandium-based full Heusler alloys, which do not follow the SPR, are also considered in this work. Theoretical computations are used to study the atomic-site preferences of 171 scandium-based full Heusler alloys, and the results indicate that most of these alloys form L21-type structures rather than XA-type structures. Two possible orderings are also studied in terms of their difference in charge density. Furthermore, the ground state, electronic structure, and magnetic characteristics of these alloys are also investigated in detail in both XA and L21 structures. The results reveal dramatic differences between XA- and L21-type alloys. This study thus presents a counterexample of the SPR rule and should provide significant guidance for the future design of full-Heusler alloys.

L2₁ and XA Ordering Competition in Hafnium-Based Full-Heusler Alloys Hf₂VZ (Z = Al, Ga, In, Tl, Si, Ge, Sn, Pb).

For theoretical designing of full-Heusler based spintroinc materials, people have long believed in the so-called Site Preference Rule (SPR). Very recently, according to the SPR, there are several studies on XA-type Hafnium-based Heusler alloys X2YZ, i.e., Hf2VAl, Hf2CoZ (Z = Ga, In) and Hf2CrZ (Z = Al, Ga, In). In this work, a series of Hf2-based Heusler alloys, Hf2VZ (Z = Al, Ga, In, Tl, Si, Ge, Sn, Pb), were selected as targets to study the site preferences of their atoms by first-principle calculations. It has been found that all of them are likely to exhibit the L21-type structure instead of the XA one. Furthermore, we reveal that the high values of spin-polarization of XA-type Hf2VZ (Z = Al, Ga, In, Tl, Si, Ge, Sn, Pb) alloys have dropped dramatically when they form the L21-type structure. Also, we prove that the electronic, magnetic, and physics nature of these alloys are quite different, depending on the L21-type or XA-type structures.

Theoretical and experimental study of the phase formation for Ti2YAl and Ti2Y′Ga (Y = Co, Fe; Y′ = Cr, Fe)

The site preference and phase formation of Ti2YAl and Ti2Y′Ga (Y = Co, Fe; Y′ = Cr, Fe) have been investigated by theoretical calculation and experimental study. It has been found that Ti2CoAl and Ti2FeAl are stable in Hg2CuTi-type structure, while Ti2CrGa and Ti2FeGa compounds are most likely to form Cu2MnAl-type structure. These results indicate that not all full-Heusler alloys follow the site preference rule, especially when the transition metals are low-valent metals. The differences of band structures and magnetic properties between Hg2CuTi-type and Cu2MnAl-type structures for the above compounds are also studied by first-principles calculation. It has been predicted that Ti2FeAl, Ti2FeGa and Ti2CoAl are half-metallic compounds in Hg2CuTi-type structure, while their half-metallic properties are completely destroyed as they formed Cu2MnAl-type structure.

Modeling of hydrogen storage materials by density-functional calculations

Theoretical investigations on the electronic structure, chemical bonding, and ground-state and high pressure properties of metal and complex hydrides are reported. Recently a series of metal hydrides with unusually short H–H separations has been synthesized in Norway. The origin of this unexpected short H–H separation has been explained and several compounds with even shorter H–H separations have been predicted. From a systematic study on around 100 ZrNiAl-type compounds we have demonstrated that the electron localization function is a powerful tool to predict hydrogen positions in metal, intermetallic and alloy matrices. Based on these observations we have devised a new empirical rule, termed as “site-preference rule”, for prediction of suitable sites for hydrogen accommodation. First principle total-energy calculations have been carried out for AH, EH 2, AEH 3, AXH 4, and A 3XH 6 (A = alkali metal, E = alkaline earth metal, X = B, Al or Ga) compounds to study structural phase stability at ambient and high pressures. We have reproduced crystal structures of known phases and predicted structural parameters for other compounds within these series. We have found that, if the cation (A + or E 2+) radius is relatively small, one can expect several pressure-induced structural transitions with huge volume collapse at the first phase-transition point. These materials have mixed chemical bonding character which we have characterized by charge density, charge transfer, electron localization function, site- and angular–momentum projected density of states, crystal orbital Hamilton population analysis, Born effective charges, and Mulliken population analyses.