Metal Sublattice Research Articles

The specific features of low-temperature thermal properties of the heterovalent (BaF2)0.59(TmF3)0.41 solid solution

ABSTRACT The temperature dependences of the heat capacity (Cp(T)) and crystal lattice parameters (a(T) and Vu(T)) of barium difluoride, BaF2, as well as the heterovalent (BaF2)0.59(TmF3)0.41 solid solution, in the temperature range of 2–300 K are experimentally studied. Anomalous deviations of the dependences of Cp(T), a(T) and Vu(T) of the solid solution at low temperatures (2–20 K) both in the direction of higher and lower values, due to the rearrangement of the crystal field, as well as the appearance of two-level systems in the metal sublattice of the solution, are revealed. At elevated temperatures (150–300 K), an anomalous feature of the studied characteristics with a deviation towards lower values is also observed, which we attribute to the increased influence of the anharmonicity of the vibrations of the solution lattice. It is established that the occurrence of the mechanisms of the revealed anomalies may be attributed to the structural disorder of the solid solution due to the random nature of the placement of Tm3+ ions when replacing Ba2+ ions in the metal sublattice, as well as the appearance of F− ions in the interstitial spaces of the barium difluoride structure.

Influence of alloying elements and the state of order on the formation of antiphase boundaries in B2 phases

Alloys with single phase B2 structure exhibit interesting physical properties, such as stability at high temperatures and oxidation resistance, but have low fracture toughness and limited ductility. The presence of antiphase boundaries (APBs) has been proposed as one of the factors improving the ductility of the B2 phases. An example of novel B2/A2 dual-phase alloy, which comprises such defects, is the AlCoCrFeNi multicomponent alloy (also known as high-entropy alloy (HEA)). Presence of APBs positively affected the performance of this alloy; therefore, control over the appearance of APBs is important. Current paper presents a first step in this direction, by studying the energy of APBs formation in binary B2 structures forming in the Al–Co–Cr–Fe–Ni system as a function of doping by ternary element from this system. Calculations, based on Density Functional Theory, took into account atom occupancy of the additives (i.e. in Al or in transition metal's sub-lattice) and position (at the APBs or at grains' interior). Following the presented here calculations, casting and characterization of the alloy, where APBs should be promoted, was made, proving the outcomes of this work. AlFe binary system was found to be the most probable (out of all possible binary prototypes) to form APBs. Furthermore, alloying by Cr (which substitutes Al) and Ni (which substitutes Fe) promote even more APBs formation. Taking into account anti-site defects, frequently forming in the AlFe B2 structure, the degree of disorder was related to the temperature – proposing an interesting aspect of promoting the formation of APBs.

Composition modulation in the GaxIni-xPyAs1-y - InP heterostructure during spinodal decomposition under the conditions of internal energy resonance

The Cahn-Hilliard concepts are generalized and used to the description of the spinodal decomposition of A3B5 quaternary semiconductor solid solutions, when the mixing of components occurs simultaneously in the metallic and metalloid sublattices of the sphalerite structure. The resulting system of differential equations for material decomposition was used to describe the effect of composition modulation observed in the synthesis of GaxIn1-xPyAs1-y - InP heterostructures. Numerical simulation of the spinodal decomposition of the GaxIm-xPyAsuy solid solution is carried out. The intervals of the thermodynamic parameters of the technological process of the synthesis of structures, in which the effect of modulation of the composition should be manifested, are found.

Low-temperature anomalies of thermodynamic properties of lanthanum trifluoride LaF3 and (SrF2)0.5 (LaF3)0.5 multivalent solid solution

Temperature dependences of the heat capacity Cp(T) (2–300 K) and lattice parameter a(T) (5–300 K) of LaF3 and (SrF2)0.5(LaF3)0.5, in addition to the earlier studied SrF2, are experimentally investigated for the first time. The experimental dependences of Cp(T) and the unit cell volume V(T) of the studied fluorides are analysed using the Debye–Einstein model considering the influence of the disorder of their crystal structures. The parameters of the model are determined. An anomalous arrangement of the Cp(T) and V(T) curves of the solution with respect to the corresponding curves of the components is revealed. At low temperatures (T < 150 K), the values of Cp(T) and V(T) of the solution exceed the values calculated by the additivity rule. At higher temperatures (T > 150 K), deviations towards smaller values are observed. We have established that the disorder in solid-solution crystal structure is the common reason of anomalies revealed. It is found that at low temperatures, the cause of these anomalies is a decrease in the vibrational frequencies in the solid-solution metal sublattice upon the replacement of Sr with La, which has a large size and mass. Such a replacement leads to distortions of the original structure and an increase in its disorder. At elevated temperatures, the deviations are caused by an increase in the vibrational frequencies in the F- sublattice due to distortion of the crystal structure, as well as the increased influence of the anharmonicity of the disordered crystal lattice vibration.

Phase stability and mechanical properties of carbide solid solutions with 2–5 principal metals

The phase stability and mechanical properties of N-metallic carbides (N = 2–5) are assessed using entropy forming ability (EFA), mixing enthalpy, and Mazhnik-Oganov ab initio model of mechanical properties. EFA quantifies configurational disorder of solid solutions and is currently used to predict the phase stability and assess the hardness of high-entropy carbides with 5 transitional metals on the metallic sublattice. In this work, we use the concept of EFA for the 13 N-metallic carbides (N = 2–5), four of which are investigated for the first time. Additionally, we propose a semi-empiric version of the Mazhnik-Oganov model and perform micro-indentation testing of the investigated compositions to verify the model’s capabilities. The proposed approach allows for a fast modeling and microindentation-based screening of the most promising carbide solid solutions for more detailed characterization.

Large Magnetocrystalline Anisotropy and Giant Coercivity in the Ferrimagnetic Double Perovskite Lu2NiIrO6

We discover that large uniaxial magnetocrystalline anisotropy driven by the simultaneous presence of spin-orbit coupling and structural distortions is the origin of the giant coercivity observed experimentally in the double perovskite Lu2NiIrO6. The magnetic easy axis turns out to be the monoclinic b-axis with an anisotropy constant as high as 1.9 × 108 erg/cm3. The predicted coercive field of 50 kOe and Curie temperature of 220 K agree with the experimentally observed values and point to potential of Lu2NiIrO6 in spintronics applications. We find that the spin-orbit coupling induces a rare Ir4+ Jeff = 1/2 Mott insulating state, suggesting that Lu2NiIrO6 provides a playground to study the interplay between spin-orbit coupling and electronic correlations in a 5d transition metal oxide. The spin-orbit coupling also results in a direct band gap with the valence and conduction states localized on different transition metal sublattices, i.e., efficient electron-hole separation upon photoexcitation and low electron-hole recombination.

LaBH8 : Towards high- Tc low-pressure superconductivity in ternary superhydrides

In the last five years a large number of new high-temperature superconductors have been predicted and experimentally discovered among hydrogen-rich crystals, at pressures, which are way too high to meet any practical application. In this paper, we report the computational prediction of a hydride superconductor, $\mathrm{La}{\mathrm{BH}}_{8}$, with a ${T}_{\text{c}}$ of 126 K at a pressure of 50 GPa, thermodynamically stable above 100 GPa, and dynamically stable down to 40 GPa, an unprecedentedly low pressure for high-${T}_{\text{c}}$ hydrides. $\mathrm{La}{\mathrm{BH}}_{8}$ can be seen as a ternary sodalite-like hydride, in which a metallic hydrogen sublattice is stabilized by the chemical pressure exerted by the La-B scaffolding, which achieves a more efficient packing of atoms than in binary sodalite hydrides thanks to the combination of elements with very different sizes. The proposed aufbau principle may be exploited to design high-${T}_{\text{c}}$ hydrides that survive at even lower pressure, through a careful choice of the elements.

Enhanced thermal stability of (Ti,Al)N coatings by oxygen incorporation

Thermal stability of protective coatings is one of the performance-defining properties for advanced cutting and forming applications as well as for energy conversion. To investigate the effect of oxygen incorporation on the high-temperature behavior of (Ti,Al)N, metastable cubic (Ti,Al)N and (Ti,Al)(OxN1-x) coatings are synthesized using reactive arc evaporation. X-ray diffraction of (Ti,Al)N and (Ti,Al)(OxN1-x) coatings reveals that spinodal decomposition is initiated at approximately 800°C, while the subsequent formation of wurtzite solid solution is clearly delayed from 1000°C to 1300°C for (Ti,Al)(OxN1-x) compared to (Ti,Al)N. This thermal stability enhancement can be rationalized based on calculated vacancy formation energies in combination with spatially-resolved composition analysis and calorimetric data: Energy dispersive X-ray spectroscopy and atom probe tomography data indicate a lower O solubility in wurtzite solid solution compared to cubic (Ti,Al)(O,N). Hence, it is evident that for the growth of the wurtzite, AlN-rich phase in (Ti,Al)N, only mobility of Ti and Al is required, while for (Ti,Al)(O,N), in addition to mobile metal atoms, also non-metal mobility is required. Prerequisite for mobility on the non-metal sublattice is the formation of non-metal vacancies which require larger temperatures than for the metal sublattice due to significantly larger magnitudes of formation energies for the non-metal vacancies compared to the metal vacancies. This notion is consistent with calorimetry data which indicate that the combined energy necessary to form and grow the wurtzite phase is larger by a factor of approximately two in (Ti,Al)(O,N) than in (Ti,Al)N, causing the here reported thermal stability increase.

Detailed total scattering analysis of disorder in ZIF-8

This work investigates the X-ray scattering signatures of disorder in the zeolitic imidazolate framework ZIF-8. Two layer disorder models are examined in reciprocal space and compared with conventional Rietveld analysis. Stacking faults along the [001] direction of the cubic lattice are in poor agreement with experimental powder diffraction data, consistent with previously reported density functional theory studies showing that these defects are energetically unfavorable compared with amorphization. Meanwhile, fluctuation of layer position along the [110] direction of the cubic lattice shows a significant agreement with experimental data. This result is interpreted analogously to an anisotropic strain mechanism, suggesting links between elastic anisotropy and crystallographic imperfections found in metal–organic framework materials. In direct space, it is demonstrated that models accounting for the static position disorder amongst the linker and metal sublattices are required to fit the experimental pair distribution function data.

Peculiarities of Phonon Spectra of Crystals with Cuprite Lattice Due to Sublattice Structure

The different translational symmetry of the cuprite crystal sublattices is taken into account by unfolding the phonon spectrum from the Brillouin zone of the crystal into the Brillouin zones of the anion and metal. Such a representation of the spectrum is reasonable because 6 optical branches are essentially caused by the vibrations of anions and 12 acoustic branches are due to the vibrations of metal owing to the great value difference between their masses. The volumes of the Brillouin zones of the anion and metal sublattices are two and four times, respectively, as large as those of the crystal resulting in the qualitative differences in the corresponding unfolded spectra because of the metal tetrahedron vibrations of the distortion type in the acoustic part of the spectrum. These vibrations located on the Х′(1,0,0)–W′(1,0,1/2) and Х′(0,1,0)– W′(0,1,1/2) lines of the metal Brillouin zone are responsible for the negative thermal expansion. The unfolded spectra have only three branches in the corresponding Brillouin zones and this simplifies the selection rules for the physical processes with the participation of the sublattice particle vibrations.

Magnetocaloric and Mössbauer effects studies of the multicomponent Tb-Dy-Ho-Co-Fe-H compounds with a Laves phase structure near the Curie temperature

In this work, the effects of substitutional elements on the magnetism of complex rare-earth-transition metal compounds RT2 with the Laves phase structure are studied with the aim to find new materials with high magnetocaloric effect (MCE) above room temperature. We determine the main regularities in the MCE magnitude variation when varying composition of the Tb-Dy-Ho-Co-Fe alloys and also after hydrogenation. The crystal structure of the compounds is characterized by X-ray diffraction in a wide temperature range while information on the magnetic properties of the 3d-transition metal sublattice is accessed by means of the 57Fe Mössbauer effect studies. We design a series of Tbx(Dy0.5Ho0.5)1-xCo1.75Fe0.25 samples with constant MCE values, suitable for the use as cooling elements of devices operating at elevated temperatures.

Strain-attenuated spin frustration in double perovskite Sr2FeOsO6

Using density functional theory together with Monte Carlo simulations, we demonstrate that epitaxial strain, both compressive and tensile, attenuates the spin frustration of double perovskite ${\mathrm{Sr}}_{2}\mathrm{Fe}\mathrm{Os}{\mathrm{O}}_{6}$ to significantly enhance the critical temperature to 310 K, enabling room-temperature applications. We discover under tensile strain a tetragonal ($I4/m$)-to-monoclinic ($P{2}_{1}/n$) structural transition concomitant with an antiferromagnetic-to-ferrimagnetic transition. Furthermore, an indirect-to-direct band gap transition is observed with the valence and conduction states localized on different transition metal sublattices, opening a route to efficient electron-hole separation upon photoexcitation.

Dependence of characteristics of Hf(M)SiBCN (M = Y, Ho, Ta, Mo) thin films on the M choice: Ab-initio and experimental study

Amorphous HfMSiBCN materials (M = Y, Ho, Ta, Mo or an enhanced Hf content instead of any other M) are investigated by ab-initio calculations and magnetron sputtering. We focus on combining the high-temperature stability and oxidation resistance of these materials with optimised mechanical, optical and electrical properties. First, we predict the corresponding trends by calculating the effect of the M choice and fraction on formation energy (Eform) and mechanical properties of MN and HfxM1–xN crystals. We discuss the dependence of Eform(HfxM1–xN) on the crystal structure and the distribution of Hf and M in the metal sublattice. The mechanical properties calculated for MN correlate with those measured for HfMSiBCN. The driving force towards N incorporation, decreasing with the periodic-table group number of M according to the calculated Eform(MN), correlates with the measured increasing electrical conductivity and extinction coefficient of HfMSiBCN. Second, we model the amorphous HfMSiBCN materials themselves by ab-initio molecular dynamics. The calculated band gap, localisation of electronic states and bonding preferences of M also correspond to the increasing metallicity with respect to the periodic-table group number of M and confirm the possibility of predicting the trends in characteristics of HfMSiBCN using those of MN. Third, we study the measured HfMSiBCN properties as functions of each other and identify sputter target compositions leading to hard films with high electrical conductivity at a relatively low extinction coefficient. The results are important for the design of hard, conductive and/or transparent high-temperature coatings.

Phase Stability and Mechanical Properties of N-Metallic Carbides

The phase stability and mechanical properties of N-metallic carbides are assessed using entropy forming ability (EFA), mixing enthalpy, and Mazhnik-Oganov ab initio model of mechanical properties. EFA quantifies configurational disorder of solid solutions and is currently used to predict the phase stability and assess the hardness of high-entropy carbides with 5 transitional metals on the metallic sublattice. In this work, we use the concept of EFA for the 13 N-metallic carbides (N=2-5), four of which are investigated for the first time. Additionally, we propose a semi-empiric version of the Mazhnik-Oganov model and perform micro-indentation testing of the investigated compositions to verify the model’s capabilities. The proposed approach allows for a fast modeling and nanoindentation-based screening of the most promising carbide solid solutions for more detailed characterization.

Магнитные свойства сплавов Pb-=SUB=-1-y-=/SUB=-Sс-=SUB=-y-=/SUB=-Te

The magnetic field dependences of the magnetization (B &lt; 9 Т, T = 2.0 - 70 K) of the samples from a single crystal Pb1-yScyTe (y=0.01) ingot, synthesized by the Bridgman method, are studied. It is established that, in accordance with the generally accepted model of the rearrangement of the electronic structure of alloys during doping, there is no paramagnetic contribution of single scandium ions located in the nodes of the metal sublattice in the studied samples. The magnetization of the samples contains several contributions: superparamagnetism of scandium clusters, linear in field diamagnetism of the crystal lattice and the paramagnetism of free electrons, as well as the oscillating contribution of the de Haas – van Alphen effect. The field dependences of the main contribution of scandium clusters are successfully approximated using the Langevin function. The average concentration, magnetic moment and the total magnetic moment of the clusters per unit volume of the sample were determined with an increase in the impurity concentration along the ingot.

Two-orbital degeneracy lifted local precursor to a metal-insulator transition in MgTi2O4

The recent discovery of a local fluctuating t2g orbital-degeneracy-lifted (ODL) state in CuIr2S4 as a high temperature precursor to the metal-insulator transition (MIT) opens the door to a possible widespread presence of precursor states in scarcely studied high-temperature regimes of transition metal based quantum materials. Although in CuIr2S4 the ODL state comprises one orbital per Ir, there is no fundamental reason to exclude multi-orbital ODL states in general. The MgTi2O4 spinel exhibits a MIT on cooling at Ts ~250 K, accompanied by Ti t2g orbital ordering (OO) and spin dimerization with the average symmetry reducing to tetragonal. It shares with CuIr2S4 the pyrochlore transition metal sublattice with active t2g orbitals. This, together with its different orbital filling (t2g1 vs t2g5.5) make it a candidate for hosting a multi-orbital ODL precursor state. By combining x-ray and neutron pair distribution function analyses to track the evolution of the local atomic structure across the MIT we find that local tetragonality already exists in the metallic globally cubic phase at high temperature. Local distortions exist up to at least 500 K. Significantly, the high temperature local state is not continuously connected to the OO band insulator ground state, and so the transition cannot be characterized as a trivial order-disorder type. The shortest Ti-Ti spin singlet dimer bonds expand abruptly on warming across the transition but remain shorter than those seen in the cubic structure. These seemingly contradictory observations can be understood within the model of a local fluctuating two-orbital t2g ODL precursor state. The ODL state in MgTi2O4 has a correlation length of about 1 nm at high temperature. We discuss that this extended character of the local distortions is consistent with the two-orbital nature of the ODL state imposed by the charge filling and the bond charge repulsion.

Extremely hard and tough high entropy nitride ceramics

Simultaneously hard and tough nitride ceramics open new venues for a variety of advanced applications. To produce such materials, attention is focused on the development of high-entropy ceramics, containing four or more metallic components distributed homogeneously in the metallic sublattice. While the fabrication of bulk high-entropy carbides and borides is well established, high-entropy nitrides have only been produced as thin films. Herein, we report on a newel three-step process to fabricate bulk high-entropy nitrides. The high-entropy nitride phase was obtained by exothermic combustion of mechanically-activated nanostructured metallic precursors in nitrogen and consolidated by spark plasma sintering. The fabricated bulk high-entropy nitride (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)N demonstrates outstanding hardness (up to 33 GPa) and fracture toughness (up to 5.2 MPa∙m1/2), significantly surpassing expected values from mixture rules, as well as all other reported binary and high-entropy ceramics and can be used for super-hard coatings, structural materials, optics, and others. The obtained results illustrate the scalable method to produce bulk high-entropy nitrides with the new benchmark properties.

Tailoring Electronic and Optical Properties of MXenes through Forming Solid Solutions.

Alloying is a long-established strategy to tailor properties of metals for specific applications, thus retaining or enhancing the principal elemental characteristics while offering additional functionality from the added elements. We propose a similar approach to the control of properties of two-dimensional transition metal carbides known as MXenes. MXenes (Mn+1Xn) have two sites for compositional variation: elemental substitution on both the metal (M) and carbon/nitrogen (X) sites presents promising routes for tailoring the chemical, optical, electronic, or mechanical properties of MXenes. Herein, we systematically investigated three interrelated binary solid-solution MXene systems based on Ti, Nb, and/or V at the M-site in a M2XTx structure (Ti2-yNbyCTx, Ti2-yVyCTx, and V2-yNbyCTx, where Tx stands for surface terminations) showing the evolution of electronic and optical properties as a function of composition. All three MXene systems show unlimited solubility and random distribution of metal elements in the metal sublattice. Optically, the MXene systems are tailorable in a nonlinear fashion, with absorption peaks from ultraviolet to near-infrared wavelength. The macroscopic electrical conductivity of solid solution MXenes can be controllably varied over 3 orders of magnitude at room temperature and 6 orders of magnitude from 10 to 300 K. This work greatly increases the number of nonstoichiometric MXenes reported to date and opens avenues for controlling physical properties of different MXenes with a limitless number of compositions possible through M-site solid solutions.

Novel Multicomponent B2-Ordered Aluminides: Compositional Design, Synthesis, Characterization, and Thermal Stability

For the first time, multicomponent alloys belonging to a B2-ordered single phase were designed and fabricated by melting route. The design concept of high entropy alloys is applied to engineering the transition metal sublattice of binary B2 aluminide. The equiatomic substitution of transition metal elements in the Ni sublattice of binary AlNi followed to produce Al(CoNi), Al(FeNi), Al(CoFe), Al(CoFeNi), Al(CoFeMnNi), and Al(CoCuFeMnNi) multicomponent alloys. CALculation of PHAse Diagrams (CALPHAD) approach was used to predict the phases in these alloys. X-ray diffraction and transmission electron microscopy were used to confirm the B2 ordering in the alloys. Thermal stability of the B2 phase in these alloys was demonstrated by prolonged heat treatments at 1373 K and 1073 K up to 200 h.

A two-sublattice model for extracting rare-earth anisotropy constants from measurements on (Nd,Ce)2(Fe,Co)14B single crystals

Anisotropy constants are obtained from an analysis of single crystal magnetization curves measured up to high fields. The anisotropy of the 3d transition metal sublattice is considered, as well as molecular exchange field coupling between the rare-earth (R) and transition metal sublattices (M). This procedure allows for non colinear R and M magnetic moments, meaning that their angles with respect to the easy axis are independent variables. With this approach we obtain anisotropy constants that are larger than those reported in the literature, which reflects the anisotropy of the isolated R sublattice. Results for Co and/or Ce doped Nd2Fe14B single crystals are presented, showing the influence of such substitutions on the magnetocrystalline anisotropy. These results indicate that the enhanced performance of NdFeB-based magnets co-doped with Ce and Co can be ascribed to an improvement in intrinsic properties.