Pauling's Rule Research Articles

Eu(2+)-Activated Alkaline-Earth Halophosphates, M5(PO4)3X:Eu(2+) (M = Ca, Sr, Ba; X = F, Cl, Br) for NUV-LEDs: Site-Selective Crystal Field Effect.

Eu(2+)-activated M5(PO4)3X (M = Ca, Sr, Ba; X = F, Cl, Br) compounds providing different alkaline-earth metal and halide ions were successfully synthesized and characterized. The emission peak maxima of the M5(PO4)3Cl:Eu(2+) (M = Ca, Sr, Ba) compounds were blue-shifted from Ca to Ba (454 nm for Ca, 444 nm for Sr, and 434 nm for Ba), and those of the Sr5(PO4)3X:Eu(2+) (X = F, Cl, Br) compounds were red-shifted along the series of halides, F → Cl → Br (437 nm for F, 444 nm for Cl, and 448 nm for Br). The site selectivity and occupancy of the activator ions (Eu(2+)) in the M5(PO4)3X:Eu(2+) (M = Ca, Sr, Ba; X = F, Cl, Br) crystal lattices were estimated based on theoretical calculation of the 5d → 4f transition energies of Eu(2+) using LCAO. In combination with the photoluminescence measurements and theoretical calculation, it was elucidated that the Eu(2+) ions preferably enter the fully oxygen-coordinated sites in the M5(PO4)3X:Eu(2+) (M = Ca, Sr, Ba; X = F, Cl, Br) compounds. This trend can be well explained by "Pauling's rules". These compounds may provide a platform for modeling a new phosphor and application in the solid-state lighting field.

A new method applicable to study solid compounds with multiple polyhedral structures.

A new direct summation method, named as polyhedron method, is proposed to calculate Madelung energy. This method calculates sums of electrostatic interactions over sets of neutral polyhedron unit pairs rather than conventional ion pairs; this gives Madelung constant in a matrix. With robustly rapid convergence, polyhedron method is generally applicable for complex compounds containing multiple polyhedral building-blocks and numerical polyhedral connection modes. The matrical analysis suggests face-sharing between octahedral pairs and edge-sharing between tetrahedral pairs can be electrostatically stable, against Pauling's third rule. Further, the matrical calculation of Madelung energies offers a unique advantage to evaluate enormous configurations of cation distributions in a given lattice in a high-throughput manner. That is applicable to study solid solution composites, polymorphism, and defect structures, including but not limited to intermediate phase of delithiated cathode compounds, charge order or antisite defects, and extensively magnetic order. © 2016 Wiley Periodicals, Inc.

Electronic structure, magnetism and stability of Co2CrX (X =Al, Ga, In) ab initio study

The structural, electronic as well as the magnetic properties of the Co2CrX (X[Formula: see text]Al, Ga and In) full-Heusler alloy have been studied using first-principles calculations performed in the framework of density functional theory (DFT) within the generalized gradient approximation (GGA). It was taken into account both possible L21 structures (i.e. Hg2CuTi- and Cu2MnAl-type). Basically, for all compounds, the Cu2MnAl-type structure is energetically more stable than Hg2CuTi-type structure at the equilibrium volume. The electronic structure calculations for Co2CrAl reveal that half-metallic (HM) character in Cu2MnAl-type structure, Co2CrGa show nearly HM behavior and Co2CrIn has a metallic character. The predicted total magnetic moment is [Formula: see text] for Co2CrX (X[Formula: see text]Al, Ga) which is in good convergence with the Slater–Pauling (SP) rule.

First-Principles Study of La Doping Effects on the Electronic Structures and Photocatalytic Properties of Anatase TiO2

The effects of doping concentration, position and oxygen vacancy defect on the stability, electronic and optical properties of La-doped anatase TiO2 have been investigated based on DFT+U method. The calculations indicated that the doping concentration and sites of La affected the stability and band gap of La-doped TiO2 significantly due to the lattice distortion, which obey the ionic Pauling's rules and crystal field theories; moreover, the simulated adsorption spectrum shows an obviously increase in the photocatalysis properties, which are in good agreement with recently experimental measurements. The oxygen vacancy defect will enhance the structural stability and the adsorption of visible light in La-doped TiO2 system, which is important in photocatalytic application.

The crystal chemistry of inorganic metal borohydrides and their relation to metal oxides

The crystal structures of inorganic homoleptic metal borohydrides are analysed with respect to their structural prototypes found amongst metal oxides in the inorganic databases such as Pearson's Crystal Data [Villars & Cenzual (2015). Pearson's Crystal Data. Crystal Structure Database for Inorganic Compounds, Release 2014/2015, ASM International, Materials Park, Ohio, USA]. The coordination polyhedra around the cations and the borohydride anion are determined, and constitute the basis of the structural systematics underlying metal borohydride chemistry in various frameworks and variants of ionic packing, including complex anions and the packing of neutral molecules in the crystal. Underlying nets are determined by topology analysis using the program TOPOS [Blatov (2006). IUCr CompComm. Newsl. 7, 4-38]. It is found that the Pauling rules for ionic crystals apply to all non-molecular borohydride crystal structures, and that the latter can often be derived by simple deformation of the close-packed anionic lattices c.c.p. and h.c.p., by partially removing anions and filling tetrahedral or octahedral sites. The deviation from an ideal close packing is facilitated in metal borohydrides with respect to the oxide due to geometrical and electronic considerations of the BH4(-) anion (tetrahedral shape, polarizability). This review on crystal chemistry of borohydrides and their similarity to oxides is a contribution which should serve materials engineers as a roadmap to design new materials, synthetic chemists in their search for promising compounds to be prepared, and materials scientists in understanding the properties of novel materials.

Influence of the Composition and Dispersion of Mineral Supplements on the Strength of Cement Paste

At the initial phase of hardening, it is the limestone component that plays a major role in the hardening process, which acts as the substrate for the crystallization of hydrate tumors due to its chemical affinity with the products of Portland cement hydration. After 7 days, the diopside supplement influences the processes more significantly. Diopside has a high modulus of elasticity compared to the cement paste. As a result, stresses are redistributed within the cement paste and the whole composition is hardened. An increase in the quantity of diopside in the compound supplement to more than 66.7% does not provide a substantial increase in the strength of the cement paste. As the hardness of diopside is higher than the hardness of limestone, much more energy is required to grind it down to a usable component. Therefore, a further increase in the quantity of diopside in the compound supplement is not economically feasible. An evaluation of the optimum quantity of input compound mineral supplements can be made based on the ideas of close packing of spherical particles and the Pauling rules. The optimum content of the supplement is 8-8.5 % provided that its dispersion and density are close to the dispersion and density of the binder. An increase in the dispersion of the supplement reduces its optimal quantity. During the addition of mineral supplements, thermo grams show a displacement of the endoeffects' temperatures to the area of higher temperatures. If there are mineral supplements in the cement paste, which may be substrates for the crystallization of tumors, the cement paste’s structure is hardened, which is shown by the integrated thermal analysis.

First-principles study on the structural, electronic and magnetic properties of the Ti2VZ (Z = Si, Ge, Sn) full-Heusler compounds

In the present work, we have investigated the structural, electronic and magnetic properties of Ti2VZ (Z = Si, Ge, Sn) alloys with Hg2CuTi-type structure in the framework of density functional theory with generalized gradient approximation (GGA). The calculated results show that Ti2VSi and Ti2VGe alloys belong to half-metallic compounds with a perfect 100% spin polarization at the Fermi level while Ti2VSn alloy is just a conventional ferrimagnetism compound. And the total magnetic moment of Ti2VSi and Ti2VGe obey the Slater–Pauling (SP) rule. In a moderate variation range of lattice distortion, Ti2VSi and Ti2VGe remain half-metallicity. We expect that our calculated results may trigger Ti2VZ (Z = Si, Ge, Sn) applying in the future spintronics field.

Search for fully compensated ferrimagnet in Co substituted Mn2VGa alloy

Crystallographic and magnetic properties of bulk (Mn1−xCox)2VGa alloys with 0≤x≤0.50 are reported in this work. All the alloys exhibit stable L21 structure. Unit cell volume of this series of alloys decreased from 207.5Å3 to 195.1Å3 as x was increased from 0 to 0.50. All the alloys shows ferrimagnetic behavior with Curie temperature decreasing from 763K to 367K with increase in x. Saturation magnetization (Ms) measured for the alloys with x=0, 0.25 and 0.50 are 1.84μB/f.u., 0.85μB/f.u. and 0.30μB/f.u., respectively, as compared to the values of 2.00μB/f.u., 1.00μB/f.u. and 0μB/f.u., predicted by the Slater–Pauling (S–P) rule. While explaining the deviations in the Ms from the values predicted by the S–P rule, a fully compensated ferrimagnet is expected in an alloy with total number of valance electrons of 24.1.

Effect of Fe substitution on the electronic structure, magnetic and thermoelectric properties of Co2FeSi full Heusler alloy: A first principle study

A large number of Heusler compounds have been discovered and their properties have been investigated both theoretically and experimentally. Among all the discovered Heusler compounds, Co2FeSi is focused for spintronic applications due to its high magnetic moment and high Curie temperature. In this aspect, the electronic structure, magnetic and thermoelectric properties of full Heusler alloy Co2−xFe1+xSi (x=0–1) have been investigated by using first principle, Full Potential Linear Augmented Plane Wave method. The density of states calculation shows half-metallicity at x=0 and 0.25 (i.e. Co2FeSi and Co1.75Fe1.25Si). The calculated magnetic moments were compared with experimental values measured at 4K and also with the magnetic moments calculated by Slater–Pauling rule. The thermoelectric properties of Co2FeSi and Fe2CoSi were also calculated over a wide range of temperatures and it was found that Fe2CoSi shows higher figure of merit than Co2FeSi.

Investigation of electronic structure, magnetic and transport properties of half-metallic Mn2CuSi and Mn2ZnSi Heusler alloys

The electronic and magnetic properties of Mn2CuSi and Mn2ZnSi Heusler alloys have been investigated using full-potential linearized augmented plane wave method. The optimized equilibrium lattice parameters in stable F-43m configuration are found to be 5.75Å for Mn2CuSi and 5.80Å for Mn2ZnSi. Spin-resolved calculations show that the Mn atoms at inequivalent Wyckoff positions have different contributions to the total magnetic moment in the unit cell. The anti-parallel magnetic moments of inequivalent Mn atoms sum to an integer with total magnetic moment per unit cell. The 100% spin-polarization at Fermi energy together with the total magnetic moment of 1.0µB for Mn2CuSi and 2.0µB for Mn2ZnSi per unit cell, predict that the materials follow MT=ZT – 28 Slater–Pauling rule. Both the materials under study exhibit half-metallicity with an energy gap in the spin-down channels. In the study, we predict a rather fine value of Seebeck coefficient. Further, the decreasing electrical conductivity with temperature shows a metallic character in spin-up configurations, while the electrical conductivity of spin-down states follows a semiconductor-like trend.

Phase equilibria and magnetic properties of Heusler-type ordered phases in the Co–Mn–Ga ternary system

Phase equilibria at 800, 900 and 1000°C, as well as order–disorder and magnetic transitions of Heusler-type ordered phase were investigated in the Co–Mn–Ga ternary system. The β phase with bcc structure including the Heusler-type ordered phase exists in a wide composition range of the ternary system. It was confirmed by STEM–HAADF observation that the crystal structure of the Heusler-type ordered alloy Mn2CoGa is L21b-type Fm3¯m (i.e., (Mn,Co)2MnGa), but not inverse-Heusler (Hg2CuTi) type F4¯3m. The L21/B2 order–disorder transition temperature shows a maximum at the stoichiometric composition of Co2MnGa and the Curie temperature of the L21 phase decreases with increasing Ga content. Magnetic moments converted from magnetization curves at 4.2K for CoxMn(76−x)Ga24 alloys well follow the generalized Slater–Pauling rule in the composition of 25⩽x⩽50, and magnetization curves indicate very small high-field magnetic susceptibility.

Structural, electronic and thermodynamic properties of half-metallic Co2CrZ(Z=Ga, Ge and As) alloys: First-principles calculations

Using first principles calculations, the structural, electronic, and magnetic properties of ferromagnetic half-metallic full-Heusler Co2CrZ(Z=Ga,GeandAs) alloy via the FP-LAPW method in the generalized gradient (GGA) and GGA+U approximations are compared with other experimental and theoretical results. The calculated equilibrium lattice constants were in qualitative agreement with the previous results. The existence of the energy gap in the minority spin (DOS and band structure) of Co2CrZ(Z=Ga,GeandAs) are an indication of being a potential half-metallic ferromagnetic HMF. The half-metallicity of the obtained material may prove useful for applications in spin-polarizers and spin-injectors of magnetic nanodevices. The calculated total spin magnetic moments are almost exactly that expected from Slater–Pauling rule. Thermal effects on some macroscopic properties of Co2CrZ(Z=Ga,GeandAs) are predicted using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variations of the primitive cell volume, volume expansion coefficient, bulk modulus, heat capacity and Debye temperature with pressure and temperature in the ranges of 0–20GPa and 0–3000K, respectively are obtained successfully.

The half-metallicity of LiMgPdSn-type quaternary Heusler alloys FeMnScZ (Z=Al, Ga, In): A first-principle study

Based on the first-principles calculations, quaternary Heusler alloys FeMnScZ (Z=Al, Ga, In) including its phase stability, band gap, the electronic structures and magnetic properties has been studied systematically. We have found that, in terms of the equilibrium lattice constants, FeMnScZ (Z=Al, Ga, In) are half-metallic ferrimagnets, which can sustain the high spin polarization under a very large amount of lattice distortions. The half-metallic band gap in FeMnScZ (Z=Al, Ga, In) alloys originates from the t1u-t2g splitting instead of the eu-t1u splitting. The total magnetic moments are 3μB per unit cell for FeMnScZ (Z=Al, Ga, In) alloys following the Slater–Pauling rule with the total number of valence electrons minus 18 rather than 24. According to the study, the conclusion can be drawn that all of these compounds which have a negative formation energy are possible to be synthesized experimentally.

A First-Principles Study on CrZrMnGa

In this paper, a new quaternary full Heusler compound CrZrMnGa is obtained and systematically studied by first-principles calculations. The calculations show the majority spin is strongly metallic character, while the minority spin has an insulating behavior in its band structure. The compound has a precisely total magnetic moment of 2.00 μB per formula by first-principles calculations. This result complies well with the generalized Slater–Pauling (SP) rule Mt = Zt − 18. All the results show that CrZrMnGa with Hg2CuTi-type structure is a new half-metal ferromagnet by first-principles calculations.

Electronic, structural, and magnetic properties of the quaternary Heusler alloy NiCoMnZ (Z=Al, Ge, and Sn)

The electronic, magnetic, and structural properties of the Heusler alloys NiCoMnZ (Z=Al, Ge, and Sn) have been investigated both theoretically and experimentally. NiCoMnGe and NiCoMnSn have ordered cubic Heusler structure (with a possible disorder between Ni and Co), while NiCoMnAl has a B2 type disordered Heusler structure with random occupancy between Mn and Al atom at their crystallographic sites. Electronic structure calculation shows that NiCoMnGe and NiCoMnSn are normal ferromagnets, whereas NiCoMnAl is nearly half metallic (~100% spin polarization) in nature with its magnetic moment close to an integer value following the Slater–Pauling rule. Ab-initio calculations show ~56% and ~60% spin polarization for NiCoMnGe and NiCoMnSn, respectively. Magnetization measurements show all the three compounds have a high Curie temperature (>583 K).

Structural and electronic properties of half-Heusler alloys PtXBi (with X=Mn, Fe, Co and Ni) calculated from first principles

Abstract First principles calculations with spin polarization based on density functional theory have been performed on half-Heusler alloys PtXBi, with X=Mn, Fe, Co and Ni, in three different atomic configurations (i.e. α, β, and γ phases). For each configuration, their optimized lattice constants are determined. Electronic and magnetic properties are also investigated. The differences reflect the atomic arrangements of the three phases and varied transition metal elements X. Meanwhile, the possibility of having the integer magnetic moment for each phase is explored. PtMnBi in α phase show half-metallic (HM) properties when its lattice constant is reduced from −3.0% to −11.2% with magnetic moment consistent with the values given by the modified Slater–Pauling rule. Additionally, we examined the effects of the spin–orbit (S–O) interaction on half-metal PtMnBi by comparing the relative shifts of the valence bands and the indirect semiconducting gap with respect to the spin polarized results.

Theoretical investigations of an influence of Ti on electronic structure and magnetic properties of half-metallic Fe2Mn1−xTixSi0.5Al0.5 alloys

Abstract Ab-initio electronic structure calculations are carried out for quinternary Fe 2 Mn 1− x Ti x Si 0.5 Al 0.5 alloys basing on the density functional theory. When x =0, the alloy is a half-metallic ferromagnet with magnetic moment following the Slater–Pauling rule. Main carrier of magnetism of the alloy is manganese with the magnetic moment of about 2.5 μ B . Replacement of Mn by Ti, changes its electronic and magnetic structure. Half-metallicity is present up to Ti concentration x =0.375. However, the further increase of Ti content leads to a strong decrease of electronic spin polarization. When the concentration of Ti increases, total magnetic moment strongly decreases. Fe magnetic moment, in the presence of titanium, changes its orientation into antiparallel in respect to the total magnetic moment and its absolute value increases with increasing Ti content. However, absolute value of Fe magnetic moment does not exceed 0.17 μ B . Ti exhibits very weak spin polarization with magnetic moment not higher than 0.05 μ B .

Ab-initio study of electronic structure and magnetic properties of half-metallic Fe2Mn1−xVxSi0.5Al0.5 alloys

Ab-initio electronic structure calculations are carried out for quinternary Fe2Mn1−xVxSi0.5Al0.5 alloys. When x=0 the alloy is half-metallic ferromagnet, with magnetic moment following the Slater–Pauling rule. Replacement of Mn by V, changes its electronic and magnetic structure. V-doped alloys exhibit half-metallic behavior for x≤0.25. However, even for higher V concentrations, electronic spin polarization is still very high, what makes the alloys interesting for spintronic applications.

Insights into the crystal chemistry of Earth materials rendered by electron density distributions: Pauling's rules revisited

Experimental and calculated electron density distributions determined for oxide and silicate crystals and siloxane molecules provide a new basis for addressing the classic foundation of the crystal chemistry of silicates, including atomic/ionic radii, the radius ratio rule and the nexus between the Pauling’s bond strength, resonance bond number, and bond length. The distributions indicate that the charge density of a bonded oxygen atom is highly distorted with its bonded radius decreasing systematically from 1.50 Å when bonded to highly electropositive atoms like sodium to ~ 0.65 Å when bonded to highly electronegative atoms like nitrogen. Rather than a single radius, the atom has as many bonded radii as it has bonded interactions. Bonded radii determined for the metal atoms match the Shannon effective ionic radii for the more electropositive atoms, but they depart and decrease systematically as the electronegativity of the M atoms increases. Pauling’s first rule is considered to be irrelevant given the asphericity and the range of the bonded radii displayed by the O atom. A power law regression expression is formulated between the average M-O bond lengths, <R(M-O)>, and the average value of the electron density, <ρ( r c )> = r[1.41/<R(M-O)>] 4.76 , at the bond critical point, r c , between pairs of bonded M-O atoms. The expression applies to a host of crystals and molecules comprising M-atoms for all rows, r, of the periodic table. The <ρ( r c )> values correlate with bond strength and resonance bond strength for the M-O bonded interactions on a one-to-one basis, demonstrating that the Pauling bond strength is a direct measure of the electron density involved in a bonded interaction and the accumulation of the electron density between the bonded pair. The widespread applications of the Brown-Shannon bond valence model in the Earth sciences and material science owes much of its success to the direct connection that exists between bond strength and the quantum mechanical observable, the electron density distribution. Compelling evidence is presented that supports the argument that the Si-O bonded interactions within siloxane molecules and silicate crystals are fundamentally the same, and that the local Si-O bonded interactions comprising molecules are, at the very core, equivalent to the Si-O bonded interactions observed in silicate crystals. Bond paths between the O atoms comprising shared polyhedral edges are consistent with Pauling’s third rule, the shorter the O-O shared edges, the greater the accumulation of the electron density between the O atoms, the greater the stabilization of the shared edges.

A First Principles Study of the Electronic Structures and Tetragonal Distortion of the Ti2NiGa Heusler Alloy

The electronic structures and tetragonal distortion for the full Heusler compound Ti2NiGa have been studied by first principles calculations. The results predict that Ti2NiGa presents half-metallicity within the lattice constants range of 5.70 to 6.50 A together with the integral magnetic moment 3.00μ B . The total magnetic moment (M t ) and the number of valence electrons (Z t ) of Ti2NiGa obey the Slater–Pauling (SP) rule of M t =Z t −18 very well. The results of tetragonal distortion indicate that the half-metallicity of Ti2NiGa alloy can be retained in the range of the c/a ratio from 0.85 to 1.20. Ti(A) atom has dominating contribution to the magnetism of Ti2NiGa even when the lattice constant and the structure experience large change. In the presence of vacancy defects, Ti2NiGa can still maintain half-metallicity.