Atomic Occupancies Research Articles

Zero-field cooling exchange bias induced by spin metastable state in MnNiGa system

<p indent="0mm">The existence of the spin metastable state in magnetic materials often causes novel physics including magnetization jumps, metamagnetic transition, and skyrmions. In this paper, we design the occupancy of Mn atoms in the MnNiGa system, and based on first-principles calculations, we anticipate the existence of spin metastable state in Mn₂Ni_1.5Ga_0.5. Therefore, Mn₂Ni₂₋_{<italic>x</italic>}Ga_{<italic>x</italic>} (<italic>x</italic>=0.36–0.60) is selected as the research platform to carry out related research. The experimental results show that near the composition of Mn₂Ni_1.5Ga_0.5, the lattice constant, crystal axis ratio, DC and AC magnetic test all have abnormal changes, and the zero-field cooling exchange bias is observed, which indicates the dual effects of the spin metastable state on structure and magnetic structure, and proves the dominant effect of spin metastable state on zero-field cooling exchange bias. This work provides a new idea for the design of spin metastable states in magnetic materials, and a new method for the realization of a zero-field cold exchange bias system.

Transition behaviors of \u03b3\u2013\u03b20/\u03b2 in V-, Cr-, Mn-doped TiAl alloys

The behavior of γ–β/β0 phase transition in TiAl alloy doped with β stabilizers (V, Cr, Mn) are studied by using the first principles method. It is found that alloying addition as well as anharmonic lattice vibration and disordered atomic occupation contributes to enhance the stability of cubic structure and accordingly introduce the disordered β phase into the high-temperature microstructure. The formation of low-temperature β0 phase originates from not only the stabilization of cubic structure but also the destabilization of tetragonal structure. In particular, the latter is the main reason for the premature precipitation of the hard-brittle β0 phase in the room-temperature microstructure at low nominal doping concentrations. We also find a special doping region in which the γ and the β phases are stable, while the β0 phase is unstable. The existence of this region provides an opportunity for the regulation of the contents of β and β0 phases.

A first-principles study of hydrogen storage of high entropy alloy TiZrVMoNb

In this study, density functional theory calculations have been carried out to study the hydrogen storage properties of high entropy alloy (HEA) TiZrVMoNb. It reveals that a BCC→FCC phase transformation occurs when the hydrogen content reaches 1.5 wt% during hydrogenation process, and octahedral and tetrahedral interstitial sites are preferable for hydrogen occupation before and after phase transformation, respectively. Further energetic analyses show that different hydrogen occupations in HEAs play an important role in the thermal stability of hydrides. The maximum hydrogen storage capacity for TiZrVMoNb is predicted to be 2.65 wt%, which is comparable to the largest value of 2.7 wt% for TiZrVHfNb and larger than that of other reported HEA hydrogen storage materials reported in the literature. As compared with the previously reported HEA TiZrHfMoNb with the change of only one principal element, the TiZrVMoNb not only has much higher hydrogen storage capacity, but also has more moderate hydrogen desorption temperature. The difference in hydrogen storage properties between these two HEAs is mainly attributed to the atomic weight, site occupation, lattice distortion and chemical effect of metal elements. The present study thus suggests that the TiZrVMoNb HEA has great potential as hydrogen storage materials and proposes a strategy to enhance the hydrogen storage properties of HEAs.

DFT and HAADF-STEM Investigations of the Zn Effects on β″ Phase Structure in a Zn Added Al-Mg-Si-Cu Alloy

First-principles calculations were conducted to investigate the effects of Zn on the structure of β″ phase. The effects of Cu, which was often added in the alloy, were also taken into consideration. Firstly, single Zn or Cu atom was doped on different sites of the β″ phase. Then the formation enthalpies and lattice constants of doped β″ phases were calculated. The results showed that it was more energetically favorable for single Zn or Cu atom to occupy Si3/Al sites than other sites. Furthermore, different quantities of Zn or Cu atoms were doped on Si3/Al sites. With the amounts of doping atoms increasing, the formation enthalpies of β″ phases doped by Zn were lower than which doped by Cu, indicating that it was more preferential for Zn to enter the β″ phase when Zn content was higher than Cu. Additionally, the doping of Zn could reduce the formation enthalpies of the β″ phase, which promoted the formation of the β″ phases. As a result, the aging hardening response of the alloy was improved. High angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) characterization was also conducted on a peak-aging Zn added Al-Mg-Si-Cu alloy. The HAADF-STEM image of β″ phase showed that the occupancies of Zn atoms were just on the Si3/Al sites and substituted all the Al atoms, which was consistent with the results of first-principles calculations.

A new strategy for enhancing the cycling stability of superlattice hydrogen storage alloys

For the first time, a strategy to equalize the subunit volumes of [A 2 B 4 ] and [AB 5 ], and meanwhile increase the oxidation/corrosion resistance for superlattice hydrogen storage alloys is proposed. • A novel sublattice modification strategy for supperlattice HSAs is proposed. • V [A2B4] and V [AB5] are equalized in a superlattice alloy for the first time. • Oxidation/corrosion resistance is enhanced by adding Gd with high electronegativity. • Cycling stability is significantly improved without losing capacity or HRD. Superlattice hydrogen storage alloys are one of the most potential candidates for the negative electrode materials of nickel-metal hydride batteries. However, their crystal structure damage and oxidation/corrosion during battery cycling severely deteriorate their cycle life. In this work, we propose a new strategy based on the alloys’ structure characteristics analysis and First-principles calculation to enhance their cycling stability. The strategy equalizes the sublattice volumes of [A 2 B 4 ] and [AB 5 ] by atomic selective occupation and meanwhile increases the alloys’ electronegativity. The feasibility of the strategy is validated experimentally by designing a series of single-phase superlattice alloys with simple elemental compositions of La 0.75- x Gd x Mg 0.25 Ni 3.5 ( x = 0, 0.05, 0.1, 0.15). It is found that when appropriate amount of Gd is added, the sublattice volumes of [LaMgNi 4 ] and [LaNi 5 ] become equal, so that the alloy maintains a stable crystal structure in the long-term battery cycling. Moreover, the alloys’ oxidation/corrosion resistance is enhanced due to the high electronegativity of Gd. The alloys’ capacity retention after 100 cycles is improved from 82.1% (La 0.75 Mg 0.25 Ni 3.5 ) to 88.2% (La 0.60 Gd 0.15 Mg 0.25 Ni 3.5 ) without sacrificing the maximum discharge capacity, and meanwhile the high rate dischargeability is also increased. The design strategy provides new insights for overcoming the weakness in the cycling stability of superlattice hydrogen storage alloys, thus promoting their practical applications.

Electronic structure, magnetic properties and martensitic transformation of Ga2MnTM (TM = Sc, Y, Lu) Heusler alloys

The atomic site occupation, electronic structure, magnetism, martensitic transformation of Ga2-based Heusler alloys Ga2MnTM (TM = Sc, Y, Lu) have been investigated by first-principles calculations. For three alloys in cubic phase, the L21-type structure is more stable than XA-type structure, and the magnetic state is ferrimagnetic (FIM) at each equilibrium lattice constant. The total magnetic moments are mainly contributed by Mn atoms due to its strong exchange splitting. For Ga2MnY and Ga2MnLu, the magnetic states between ferromagnetic (FM) state and FIM state alternately appear with increasing crystal constant. Potential martensitic transformation can be gained due to the proper thermodynamic driving force (negative values of ΔE) and c/a, together with the analysis of phonon spectra, by considering the tetragonal distortions of cubic Ga2MnTM (TM = Sc, Y, Lu). The energy minima locate at c/a = 1.29, 1.21, 1.27, respectively. The three alloys in martensitic state show different variations of total magnetic moments and still maintain FIM state with changing c/a. The martensitic transformation and abundant changes in magnetic states motivate researchers to further explore new Z2-based Heusler alloys.

Tailoring the electronic structure and magnetic properties of pyrochlore Co2Ti1−x Ge x O4: a GGA + U ab initio study

We report the electronic structure and magnetic properties of Co2Ti1−x Ge x O4 (0 ⩽ x ⩽ 1) spinel by means of the first-principle methods of density functional theory involving generalized gradient approximation along with the on-site Coulomb interaction (U eff) in the exchange-correlation energy functional. Special emphasis has been given to explore the site occupancy of Ge atoms in the spinel lattice by introducing the cationic disorder parameter (y) which is done in such a way that one can tailor the pyrochlore geometry and determine the electronic/magnetic structure quantitatively. For all the compositions (x), the system exhibits weak tetragonal distortion (c/a ≠ 1) due to the non-degenerate and states (e g orbitals) of the B-site Co. We observe large exchange splitting (ΔEX ∼ 9 eV) between the up and down spin bands of t 2g and e g states, respectively, of tetrahedral and octahedral Co2+ (4A2(g)(F)) and moderate crystal-field splitting (ΔCF ∼ 4 eV) and the Jahn–Teller distortion (ΔJT ∼ 0.9 eV). These features indicate the strong intra-atomic interaction which is also responsible for the alteration of energy band-gap (1.7 eV ⩽ E g ⩽ 3.3 eV). The exchange interaction (J BB ∼ −4.8 meV, for (x, y) = (0.25, 0)) between the Co2+ dominates the overall antiferromagnetic behaviour of the system for all ‘x’ as compared to J AA (∼−2.2 meV, for (x, y) = (0.25, 0)) and J AB (∼−1.8 meV, for (x, y) = (0.25, 0)). For all the compositions without any disorderness in the system, the net ferrimagnetic moment (Δμ) remains constant, however, increases progressively with increasing x due to the imbalance of Co spins between the A- and B-sites.

Structural analyses of Gd3(Al,Ga)5O12 garnet solid solutions via X-ray and UV absorption spectroscopy experiments for Gd atoms

Local structures of Gd atoms in Gd3(Al, Ga)5O12 garnet solid solutions, grown using the Czochralski method, have been studied at low temperatures through X-ray and UV absorption spectroscopy experiments. From the Fourier transforms (FTs) of Gd K-edge extended X-ray absorption fine structure (EXAFS) oscillations, the preferential sites for the occupation of Ga and Al atoms were assigned to the 24d and 16a sites, respectively. The interatomic distances between Gd and neighboring atoms, and the Debye–Waller factors were determined through the fit to the inverse FTs. Positional fluctuations of neighboring O atoms buffered local distortions induced by the occupation of Ga and Al atoms at the same crystallographic sites. The change in the high-resolution UV absorption spectra due to Gd intra-4f transitions on the Ga content is explained by considering the crystal field and Coulomb repulsion on the Gd atom.

Interplay of Anionic Quasi-Atoms and Interstitial Point Defects in Electrides: Abnormal Interstice Occupation and Colossal Charge State of Point Defects in Dense fcc-Lithium.

Electrides are an emerging class of materials with highly localized electrons in the interstices of a crystal that behave as anions. The presence of these unusual interstitial quasi-atom (ISQ) electrons leads to interesting physical and chemical properties and wide potential applications for this new class of materials. Crystal defects often have a crucial influence on the properties of materials. Introducing impurities has been proved to be an effective approach to improve the properties of a material and to expand its applications. However, the interactions between the anionic ISQs and the crystal defects in electrides are yet unknown. Here, dense fcc-Li was employed as an archetype to explore the interplay between anionic ISQs and interstitial impurity atoms in this electride. This work reveals strong coupling among the interstitial impurity atoms, the ISQs, and the matrix Li atoms near to the defects. This complex interplay and interaction mainly manifest as the unexpected tetrahedral interstitial occupation of impurity atoms and the enhancement of electron localization in the interstices. Moreover, the Be impurity occupying the octahedral interstice shows the highest negative charge state (Be8-) discovered thus far. These results demonstrate the rich chemistry and physics of this emerging material and provide a new basis for enriching their variants for a wide range of applications.

Calculation of thermic and electric properties and valence electron structure for metallic electrodes of Na||Sb-Pb-Sn liquid metal battery

&lt;sec&gt;The valence electron structures and thermal and electric properties of Na||Sb-Pb-Sn liquid metal battery are systematically studies with solid and molecular empirical electron theory (EET). The theoretical studies show that the thermal and electric properties are strongly related to the valence electron structure of electrode. The cathodic alloys Na&lt;sub&gt;1–&lt;/sub&gt;&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;IA&lt;i&gt;&lt;sub&gt;x &lt;/sub&gt;&lt;/i&gt;(IA = K, Rb, Cs) are designed by doping IA group alkali metals (K, Rb, Cs) into Na electrode since the melting points of IA group metals (K, Rb, Cs) are all lower than that of sodium. The theoretical bond lengths and cohesive energy of cathodic alloys Na&lt;sub&gt;1–&lt;/sub&gt;&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;IA&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt; match the experimental ones well. The theoretical studies show the decreasing tendency of melting point, cohesive energy and electric potential with increasing doping content &lt;i&gt;x&lt;/i&gt; in Na&lt;sub&gt;1–&lt;/sub&gt;&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;IA&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt; alloys, which is due to the modulation of valence electron structure of IA group dopants. According to the analyses of valence structures, the number of lattice electrons decreases with the increasing of the doping content &lt;i&gt;x&lt;/i&gt; for the cathodic alloy and causes the melting point, electric potential and cohesive energy to decline. It reveals that the IA group dopant modulates the valence electron structure of cathodic alloy, and induces the electron transformation from lattice electron to covalent electron in s orbital. &lt;/sec&gt;&lt;sec&gt;The anode products such as NaSb&lt;sub&gt;3&lt;/sub&gt;, NaSn, Na&lt;sub&gt;15&lt;/sub&gt;Sn&lt;sub&gt;4&lt;/sub&gt; and NaPb are formed by transporting Na ions into the anode alloy Sb-Sn-Pb. The calculated bond-lengths and melting points fit the observed ones well for these anode products. Owing to their complex structures with various atomic occupations in unit cell, the thermal property or electric property is not only relative to lattice electron, but also depends on the covalent electron. The sublattice plays an important role in the forming of the four anode products. The lattice electrons are supplied by Na at 4&lt;i&gt;f&lt;/i&gt; sites in Na&lt;sub&gt;3&lt;/sub&gt;Sb, Na at 16&lt;i&gt;e&lt;/i&gt; and Sn at 32&lt;i&gt;g&lt;/i&gt; sites in NaSn, Sn at 16&lt;i&gt;c&lt;/i&gt; and Na at 48&lt;i&gt;e&lt;/i&gt; sites in Na&lt;sub&gt;15&lt;/sub&gt;Sn&lt;sub&gt;4&lt;/sub&gt;, and Na at 16&lt;i&gt;f&lt;/i&gt; and Pb at 32&lt;i&gt;g&lt;/i&gt; sites in NaPb, respectively. &lt;/sec&gt;&lt;sec&gt;The open-gate voltage is closely related to the lattice electrons and inversely proportional to the average number of lattice electrons per atom. The open-gate voltage of NaSb&lt;sub&gt;3&lt;/sub&gt; is the largest among the anode products, however, its averaged number of lattice electron per atom is the least. Since the lattice electron number of NaSn is the largest among the anode products, the open-gate voltage of NaSn is the least. It implies that the lattice electron plays a very important role in Na||Sb-Pb-Sn liquid metal battery, which can modulate the valence electron structures and thermal and electric properties. &lt;/sec&gt;

Electronic and Thermoelectric Properties of Li-Based Half-Heusler Alloys: A DFT Study

Abstract In this paper, we have studied the electronic, elastic and thermoelectric properties of the half-Heusler LiCrZ (Z = C, N, Si, and P) materials in Type II phase, in this structure the atomic occupations are X (1/2,1/2,1/2), Y (0,0,0) and Z(1/4,1/4,1/4). The ferromagnetic state of Type II structure was found to be the most stable phase for all studied alloys. After calculating the elastic constants, we found out that the conditions of mechanical stability were verified only for LiCrSi and LiCrP alloys in Type II phase, at both equilibrium a0 and half metallic ahm lattice constants, which indicates that these two compounds can be synthesized experimentally. We should also mention that the half metallic behavior in Type II structure, for LiCrSi and LiCrP compounds, was obtained by straining the equilibrium lattice constants by 2% and 6%, respectively. At ahm, these two systems were identified to be true half metals due to their complete spin polarization and integer value of total magnetic moment. These last ones have reached 3μB per unit cell when Z = Si, and 4μB when Z = P. Using the mean field approximation (MFA), the Curie temperatures of Type II structure were also determined, where the values are estimated to be 456.2 K and 302.8 K, respectively. Finally, the thermoelectric performance has been explored by the classical Boltzmann theory. At low temperatures, the figure of merit has reached 0.73 and 0.93 for LiCrSi and LiCrP, respectively. The considerable ZT values and all calculated physical properties make these two systems promising candidates for thermoelectric applications.

Dual-cone variational calculation of the two-electron reduced density matrix

The computation of strongly correlated quantum systems is challenging because of its potentially exponential scaling in the number of electron configurations. Variational calculation of the two-electron reduced density matrix (2-RDM) without the many-electron wave function exploits the pairwise nature of the electronic Coulomb interaction to compute a lower bound on the ground-state energy with polynomial computational scaling. Recently, a dual-cone formulation of the variational 2-RDM calculation was shown to generate the ground-state energy, albeit not the 2-RDM, at a substantially reduced computational cost, especially for higher $N$-representability conditions such as the T2 constraint. Here we generalize the dual-cone variational 2-RDM method to compute not only the ground-state energy but also the 2-RDM. The central result is that we can compute the 2-RDM from a generalization of the Hellmann-Feynman theorem. Specifically, we prove that in the Lagrangian formulation of the dual-cone optimization the 2-RDM is the Lagrange multiplier. We apply the method to computing the energies and properties of strongly correlated electrons -- including atomic charges, electron densities, dipole moments, and orbital occupations -- in an illustrative hydrogen chain and the nitrogen-fixation catalyst FeMoco. The dual variational computation of the 2-RDM with T2 or higher $N$-representability conditions provides a polynomially scaling approach to strongly correlated molecules and materials with significant applications in atomic and molecular and condensed-matter chemistry and physics.

Observation of gauge invariance in a 71-site Bose-Hubbard quantum simulator.

The modern description of elementary particles, as formulated in the standard model of particle physics, is built on gauge theories1. Gauge theories implement fundamental laws of physics by local symmetry constraints. For example, in quantum electrodynamics Gauss's law introduces an intrinsic local relation between charged matter and electromagnetic fields, which protects many salient physical properties, including massless photons and a long-ranged Coulomb law. Solving gauge theories using classical computers is an extremely arduous task2, which has stimulated an effort to simulate gauge-theory dynamics in microscopically engineered quantum devices3-6. Previous achievements implemented density-dependent Peierls phases without defining a local symmetry7,8, realized mappings onto effective models to integrate out either matter or electric fields9-12, or were limited to very small systems13-16. However, the essential gauge symmetry has not been observed experimentally. Here we reportthe quantum simulation of an extended U(1) lattice gauge theory, and experimentally quantify the gauge invariance in a many-body system comprising matter and gauge fields. Thesefields are realized in defect-free arrays of bosonic atoms in an optical superlattice of 71 sites. We demonstrate full tunability of the model parameters and benchmark the matter-gauge interactions by sweeping across a quantum phase transition. Using high-fidelity manipulation techniques, we measure the degree to which Gauss's law is violated by extracting probabilities of locally gauge-invariant states from correlated atom occupations. Our work provides a way to explore gauge symmetry in the interplay of fundamental particles using controllable large-scale quantum simulators.

High Pressure Effect on Structural and Electrochemical Properties of Anionic Redox-Based Lithium Transition Metal Oxides

Summary The richness of anionic redox chemistry in the solid state offers new opportunities and a possible paradigm shift in energy storage. The excess capacity that goes beyond conventional theoretical values is attributed to the anionic redox in Li-rich transition metal oxide cathodes for Li-ion batteries. Their electrochemical behavior is thermodynamically determined by structural evolution. To better understand the electrochemical dependence on structural factors, we have induced structural modifications in pristine and electrochemically activated Li1.144Ni0.136Co0.136Mn0.544O2 through high-pressure treatment. A unique cyclical change of structural reordering is observed in the anionic redox-activated material during operando pressure sweep, characterized by a periodic evolution of superlattice peak intensity in synchrotron X-ray diffraction patterns. During the structural reordering period, the bulk compressibility of the material decreases, even becoming negative. These insights elucidate the structural flexibility and metastability of anionic redox-based materials, which can undergo large compressions and structural modifications while delivering good electrochemical properties.

Improved thermoelectric properties of doped A0.5B0.5NiSn (A, B = Ti, Zr, Hf) with a special quasirandom structure

MNiSn (Ti, Hf, Zr) is used as the parent material in constructing special quasirandom structure-based half-Heusler compounds that experimentally mimic atomic occupation disorders. We randomly doped Ti, Hf, and Zr with an occupation probability of 50% to obtain Ti0.5Zr0.5NiSn, Ti0.5Hf0.5NiSn, and Zr0.5Hf0.5NiSn, respectively. We systematically studied the electronic structure and thermoelectric transport properties of these compounds by referring to density functional theory and the semiclassical Boltzmann transport equation. Compared with the parent compound MNiSn (M = Ti, Zr, Hf), the substituted compounds retain the characteristic of semiconductivity. The lattice thermal conductivity of the three compounds is about 2.5 Wm−1 K−1 at about 1000 K, which is lower than that of their corresponding parent materials (5.0–6.0 Wm−1 K−1). The maximum thermoelectric figures of merit for Ti0.5Zr0.5NiSn (0.57), Ti0.5Hf0.5NiSn (0.55), and Zr0.5Hf0.5NiSn (0.58) are higher than those of their parent compounds TiNiSn (0.35), ZrNiSn (0.496), and HfNiSn (0.473). The increase in thermoelectric figure of merit is due to low lattice thermal conductivity and high power factor. Therefore, the three predicted compounds can be considered candidate thermoelectric materials that can be used at a carrier concentration of 1020–1022 cm−3 and a temperature of 900–1200 K.

Anti-phase boundary energy of β series precipitates in Mg-Y-Nd system

Anti-phase boundary energy (APBE) is a vital contributing factor for the dislocation shearing mechanism which operates in Mg-RE alloys. To clarify the structure and composition dependence of APBE and provide the guidance for strength improvement of Mg alloys, the formation energy, misfit strain, and APBE of β series precipitates in Mg-Y-Nd system are investigated by first-principles calculations. The atomic occupations of ternary precipitates are determined based on misfit strains. The APBEs of βp″′ phases are higher than that of βs′, and precipitates in Mg-Nd system possess higher APBEs than their counterparts in Mg-Y system. The higher APBEs enable larger strength increments of precipitates in Mg-Nd system than Mg-Y system when the amount of Y and Nd are the same. However, when both elements reach their solubility limit, the large amount of Y makes up the low APBE and achieves superior strength increment than the precipitates in Mg-Nd system.

Design of half-heusler thermoelectric compound TiFe0.5Ni0.5Sb with special quasi-random structure using 18-electron rule

The double half-Heusler compound TiFe0.5Ni0.5Sb with special quasi-random structure (SQS) has been modeled by using 18-electron rule, which can simulate the inevitable atomic occupation disorder between two sublattices. We studied the electronic structure and thermoelectric (TE) properties of this compound based on first-principle calculations and semi-classical Boltzmann transport theory. The calculated band structure indicates that this compound belongs to a nonmagnetic indirect band gap semiconductor with a band gap of 0.45 eV. The lattice thermal conductivity κl at room temperature is 11.24 WK−1m−1, which was lower than 18.7 WK−1m−1 for the 18-electron TiCoSb, and decreases as the temperature increased. The reason is that the lattice vibration is enhanced while thermal conductivity is suppressed when the temperature rises. The dimensionless figure of merit ZT of p-type TiFe0.5Ni0.5Sb at 1390 K is evaluated to be 0.35, which is higher than 0.1 for its parent compound 17-electron TiFeSb. The theoretical prediction for TiFe0.5Ni0.5Sb can simulate corresponding experimental results because of the use of SQS and will increase the experimental research of other disordered alloys using 18-electron rule toward designing and improving TE performance.

Synthesis of the hollandite-like copper doped potassium titanate high-k ceramics

The hollandite-like copper doped potassium titanate was synthesized by two-step method, including the potassium polytitanate precursor modification with copper compounds in aqueous solution and the following calcination at 900 °C. The structure of material obtained was studied using the XRD and SEM analysis. The Rietveld method was used to estimate the crystalline unit cell parameters and calculate the exact stoichiometry of K1.53(Cu0.76Ti7.24)O16 by refining the atomic occupancies for chemical elements. High-k ceramic materials based on this shollandite-like solid solution were sintered at different temperatures varied in the range from 1000 to 1100 °C. The influence of sintering conditions on electric properties of the obtained ceramics was investigated in a wide frequency range. The contribution of different processes on permittivity was discussed using the electron-pinned defect-dipole (EPDD) model, and the internal barrier layer capacitance (IBLC) model. The optimal conditions to produce modified potassium polytitanate, used as intermediate, as well as optimal sintering conditions which allowed obtaining mono-phase hollandite-like ceramic materials were recognized. It was found that such ceramic materials had high permittivity (ε’ ≈ 0.9 × 104 at 10 Hz).

Synthesis, characterization, and antimicrobial properties of strontium-substituted hydroxyapatite

Ion-substituted hydroxyapatite (HAP) offers several advantages compared with pure HAP. Since the calcium amount is relatively lower compared with the stoichiometric ratio of pure HAP, the resulting calcium deficiency in ion-substituted HAP leads to higher degradation in the physiological environment. Therefore, ion-substituted HAP has promising applications in the biomedical industry, particularly in drug delivery systems. In the present work, strontium-substituted HAP (Sr-HAP) was prepared by wet chemical precipitation method. The powders were characterized by X-ray diffraction (XRD) method and scanning electron microscopy (SEM). The crystallite size of Sr-HAP was calculated as ≈ 9 nm. Furthermore, structural characterization of Sr-HAP was done by Rietveld refinement and the substituting atomic sites and occupancies were obtained. The atomic positions occupied by the substituted Sr atoms in the calcium (Ca) atomic positions in the hcp unit cell were identified. The unit cell parameters of synthesized Sr-HAP were precisely calculated. The synthesized Sr-HAP powders were compacted and the antimicrobial properties were assessed by using Escherichia coli (E. coli) and Sporosarcina pasteurii (S. pasteurii). From the antimicrobial studies, it was understood that Sr-HAP has shown lower viability of these strains compared with pure HAP which is an indication of antimicrobial properties of Sr-HAP.

Theoretical and Experimental Studies of the Structural, Phase Stability and Elastic Properties of AlCrTiFeNi Multi-Principle Element Alloy.

The fundamental challenge for creating the crystal structure model used in a multi-principle element design is the ideal combination of atom components, structural stability, and deformation behavior. However, most of the multi-principle element alloys contain expensive metallic and rare earth elements, which could limit their applicability. Here, a novel design of low-cost AlCrTiFeNi multi-principle element alloy is presented to study the relationship of structure, deformation behavior, and micro-mechanism. This structured prediction of single-phase AlCrTiFeNi by the atomic-size difference, mixing enthalpy and valence electron concentration (VEC), indicate that we can choose the bcc-structured solid solution to design the AlCrTiFeNi multi-principle element alloy. Structural stability prediction by density functional theory calculations (DFT) of single phases has verified that the most advantageous atom occupancy position is (FeCrNi)(AlFeTi). The experimental results showed that the structure of AlCrTiFeNi multi-principle element alloy is bcc1 + bcc2 + L12 phases, which we propose as the fundamental reason for the high strength. Our findings provide a new route by which to design and obtain multi-principle element alloys with targeted properties based on the theoretical predictions, first-principles calculations, and experimental verification.