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Point defects in semiconductors or insulators are a promising platform to realize quantum information science, composed of quantum computing, quantum communication, and quantum metrology. These so-called quantum defects are particularly appealing because they are fixed in a controlled solid-state environment, hold the promise of room temperature operation, and will benefit from mature semiconductor fabrication techniques for integration and scaling. First-principles calculations based on density functional theory have been indispensable for the study of point defects: such calculations provide crucial microscopic insight that may be inaccessible in experiments. In this dissertation, we develop and apply first-principles methodologies to treat quantum defects.Nonradiative transitions are integral to the control and operation of quantum defects. Indeed, nonradiative transitions dissipate energy through vibrations and thus can impact the quantum efficiency of a given quantum defect. We developed the Nonrad code, which implements a quantum-mechanical formalism to evaluate the nonradiative transition rate from first principles. We also put into effect several important modifications that are essential for attaining accurate rates.Identifying novel quantum defects is of vital importance for their widespread utilization in quantum information science. Boron nitride is an ultra-wide-band-gap material with excellent thermal and chemical stability, making it a promising host for quantum defects and for applications in electronic devices. Control over conductivity is essential to utilize boron nitride in the proposed applications. In cubic boron nitride, we assess potential dopants and their ability to produce n-type conductivity.In hexagonal boron nitride, bright single-photon emitters have been observed in the visible spectrum; however the microscopic origin of the emission has eluded researchers. Here we propose boron dangling bonds as the origin of the emission and provide a thorough characterization of their properties. We find that boron dangling bonds possess an optical transition with minimal coupling to phonons; we also calculate the magnetic-field dependence and show it to be in agreement with experiments. In a monolayer, we find that the boron dangling bond will behave similarly to when it is embedded in bulk material. Furthermore, we demonstrate the importance of out-of-plane distortions on the dangling bond, a result that has implications for other quantum defects in two-dimensional materials. Finally, in a fruitful collaboration with the experimental group of Prof. Lee Bassett at the University of Pennsylvania, we elucidated the optical dynamics of boron dangling bonds.In total, this work advances the study of quantum defects through the development and application of first-principles techniques.

The formation of the Ti substituted Mg2Ni alloys, a promising hydrogen storage material for various applications is studied in detail. Mg1.95Ti0.05Ni alloy and ribbons are successfully prepared by vacuum arc melting and melt spinning methods. The phases, microstructures, and thermal behavior of the alloys and ribbons are characterized by XRD, SEM, TEM, DTA/TG. Sievert-type apparatus is used to study hydrogen sorption properties. Apart from the dominant Mg2Ni phase, the formation of MgNi2, Mg, and Ni3Ti phases is seen in both Mg1·95Ti0·05Ni alloy and ribbons. During the initial three cycles, Mg1·95Ti0·05Ni ribbons showed 2 wt % hydrogen storage capacity. To explain the atomic-scale influence of Ti dopant in the studied alloys and hydrides, FP(L)APW + lo method based on Density Functional Theory (DFT) is applied to Mg2-xTixNi (x = 0.25 and 0.5) alloys and Mg2-xTixNiH4 (x = 0.25 and 0.5) hydrides. An increase in the Ti dopant on the Mg site leads to the hydrides destabilization. Bader's charge density topology analysis provides insight into the charge transfer and bonding between the constituent atoms.

In this thesis we use first-principles density functional theory (DFT) and related methods to predict and understand the properties of two categories of materials with particular promise for technological applications: topological semimetals, and antiferromagnets whose magnetic order can be controlled by electrical current.Topological semimetals (TSMs), a subset of topological materials which are the particular focus of the first part of this thesis, have robust band crossings in reciprocal space protected by crystalline symmetries and characterized by mathematical invariants. They exhibit a variety of exotic phenomena such as ultrahigh mobility of electrons, giant magnetoresistance, and chiral anomalies. Moreover, analogously to the better known topological insulators (TIs), the ``bulk-boundary correspondence", related to the change in topological invariant in going from material to vacuum, implies the existence of electronic states localized at the compound surface which can differ significantly from the semimetallic bulk states in TSMs. The development of group-theoretical methods to identify TSMs and TIs have revealed that TSMs are far more ubiquitous than initially hypothesized; to date over $10000$ candidates have been identified. While this might seem to imply that the goal of harnessing properties of TSMs for practical purposes is a solved problem, most candidate materials have one or more features which make experimental manipulation and detection of the topological properties difficult if not impossible. If the symmetry-protected band crossings occur at energies far from the Fermi level, or if they are obscured by other trivial bands at the same energy, the topological signatures will be obscured. Thus, the identification of specific materials, structural motifs, and possible tuning parameters through which one can realize ``functional" TSMs is highly desirable.\\\indent The first section of this thesis describes a set of studies focused on the interplay of symmetry, orbital character and magnetism in yielding electronic structures with controllable TSM features near the Fermi level and free from interfering trivial bands. First, we use a combination of DFT and tight-binding to examine the electronic structure of a previously synthesized compound, $\mathrm{TiRhAs}$. We find that $\mathrm{TiRhAs}$ hosts a topological nodal line protected by a mirror plane nearly exactly at the Fermi level, with no other energetically degenerate trivial bands. Next, in combination with experimental ARPES data which confirmed our DFT findings, we investigate the transition metal dichalcogenide (TMD) $\mathrm{NiTe_2}$, and find that is a Dirac semimetal with a bulk tilted Dirac cone and topological surface states. While previous isostructural $\mathrm{MX_2}$ ($\mathrm{M}=\mathrm{Pd}$, $\mathrm{Pt}$: $\mathrm{X}=\mathrm{Te}$, $\mathrm{Se}$) compounds have been shown to host similar ``ladders" of topologically protected bulk and surface states, the features of interest occur at large binding energies which render their topological properties irrelevant to transport. We show that the increased hybridization between $\mathrm{Ni}$ $\mathrm{d}$ and $\mathrm{Te}$ $\mathrm{p}$ states as compared to the other $\mathrm{MX_2}$ compounds is responsible for tuning the Dirac cone very close to the Fermi level; thus substitution of the transition metal element is an effective method for designing functional TSMs within this class of TMDs. Finally, we examine the possibility of realizing TSM features in compounds isostructural to the multiferroic hexagonal manganites. This was motivated by the numerous order parameters in multiferroic compounds which can be controlled by external fields; thus, a multiferroic compound with TSM features in a particular phase would provide an opportunity to switch from nontrivial to trivial topology by tuning of the ferroic order parameters. We find through our DFT calculations that by enforcing a metastable ferromagnetic order in the nonpolar centrosymmetric phase, hexagonal $\mathrm{YCrO_3}$ and $\mathrm{YVO_3}$ become topological nodal line semimetals, in contrast to the insulating band structures that occur with the ground state antiferromagnetic order of the transition metal ions.\\\indent The second section of this thesis focuses on first-principles characterization of ``functional materials" in which the feature to leverage for functionality is magnetic, rather than topological, order. We focus on antiferromagnetic (AFM) materials. There has been a recent surge of interest in using AFMs rather than their traditional ferromagnetic (FM) counterparts for spintronic devices whose magnetic order can be manipulated by an electrical current. The vanishing bulk magnetization of AFMs makes them particularly robust to magnetic field perturbations, and the limiting rate of spin dynamics (i.e. the rate at which spins can rotate) in AFMs is order $\sim \mathrm{THz}$ as opposed to $\sim \mathrm{GHz}$ for FMs.\\\indent Our studies focus on one example, the iron-intercalated TMD, $\mathrm{Fe_{1/3}NbS_2}$. The triangular lattice of $\mathrm{Fe}$ ions has an antiferromagnetic (AFM) order which can be manipulated with electrical pulses of very low current density. While numerous experimental characterizations have been performed on this compound, ambiguities regarding the magnetic ground state, spin exchange constants, and the specifics of the current-induced magnetization dynamics, remain. In one study, we calculate the nearest-neighbor Heisenberg exchange constants in $\mathrm{Fe_{1/3}NbS_2}$ and find that competition between strong nearest-neighbor interplanar and intraplanar $\mathrm{Fe}$ exchange constants is responsible for an observed half-magnetization plateau. In the second part of our first-principles characterization of $\mathrm{Fe_{1/3}NbS_2}$, we explore the working hypothesis that the current-induced manipulation of AFM order, which is detected by changes in electrical resistance, is due to a repopulation of three energetically equivalent AFM domains on the triangular lattice. Based on calculated conductivity tensors within a constant relaxation time approximation with experimentally proposed AFM magnetic orders, we verify that the transport parallel to the $\mathrm{Fe}$ layers is anisotropic, a necessary condition for the domain repopulation hypothesis. Finally, by comparing our ab-initio transport with experimental changes in resistance for specific pulse directions, we infer the likely current-domain response for $\mathrm{Fe_{1/3}NbS_2}$, that is, which domains are favored for a given current direction.

Thioether polymers are fundamental for a variety of applications. Their synthesis is, however, more challenging than that of other metal-catalyzed reactions due to the reported detachment of the S atom during thermal activation. In this study, it has been demonstrated unambiguously that thermal annealing results in the thioetherification of the 4-bromo-4-mercaptobiphenyl molecule (Br-MBP) adsorbed on the surface of Au(111). Through complementary techniques, such as scanning tunneling microscopy, spectroscopy, and first-principle calculations, we have identified four reaction steps, involving sulfhydryl or bromine molecular functional groups and leading to the formation of intermolecular C–S bonds. To form the thioether polymer and to overcome the competitive formation of C–C bonds, two reaction steps, the dehalogenation, and dissociation of the S–Au bond, must occur simultaneously. We detail the electronic properties of the phenyl–sulfur bond and the polymer as a function of the ligand length. This result suggests a wider perspective of this chemical synthesis., L.V. acknowledges the funding of Diputación Foral de Guipuzcoa (Red/Sarea 2021-CIEN-000032-01, Red/Sarea 2022-CIEN-000017-01), the Basque Government (Proyectos de Investigación Básica y/o Aplicada grant number PIBA-2021-0026, and Grupo consolidado IT1453-22. R.R. and N.L. acknowledge financial support from the Spanish State Research Agency Grant PID2021-127917NB-I00 funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe”; from the European Union project ESiM 101046364; and they are grateful for the computer resources at Finisterrae II, and the technical support provided by CESGA.

The methanol decomposition reaction has gained substantial attention due to the wide range of applications that its intermediates offer. In this work, methanol (CH3OH) decomposition on Copper-embedded graphene (CuG) surface has been investigated via density functional theory with Grimme-D2 dispersion correction. The charge density of the CuG surface has been analyzed and the redistribution of the electron density of the surface has been represented via the electron density difference (EDD) map. Moreover, the decomposition reaction mechanism of CH3OH on the CuG surface through the cleavage of C–H, O–H and C–O bonds has been investigated in detail. In the initial state, the C–O and O–H bonds of CH3OH have similar activation barriers, thereby the adsorption and degradation mechanism of the intermediate states arising through O–H bond cleavage on the CuG surface has been investigated. In addition, the charge density calculations of the transition state geometries have been conducted and examined with EDD maps. The results have revealed that the previously adsorbed oxygen molecule exhibited high catalytic activity towards O–H decomposition compared to the bare surface. The CuG surface has offered higher activity on the C–H bonds compared to the C–O bonds of the intermediate states generated by CH3OH decomposition. The results revealed that the proposed CuG structure can be utilized as an alternative electrode catalyst that can prevent the CO poisoning issue in direct methanol fuel cells. © 2021 Hydrogen Energy Publications LLC

Control of Néel vector in antiferromagnetic materials is one of the challenges preventing their use of as active device components. Several methods have been investigated such as exchange bias, electric field, and spin injection, but little is known about strain-meditated anisotropy. This study of antiferromagnetic L10-type MnX alloys MnIr, MnRh, MnNi, MnPd, and MnPt shows that a small amount of strain effectively rotates the direction of the Néel vector by 90⁰ for all of the materials. For MnIr, MnRh, MnNi, and MnPd, the Néel vector rotates within the basal plane. For MnPt, the Néel vector rotates from out-of-plane to in-plane under tensile strain. The effectiveness of strain control is quantified by a metric of efficiency and by direct calculation of the magnetostriction coefficients. The values of the magnetostriction coefficients are comparable with those from ferromagnetic materials. These results indicate that strain is a mechanism that can be exploited for control of the Néel vectors in this family of antiferromagnets. CrSb is a layered antiferromagnet (AFM) with perpendicular magnetic anisotropy, a high Néel temperature, and large spin-orbit coupling (SOC), which makes it interesting for AFM spintronic applications. To elucidate the various mechanisms of Néel vector control, the effects of strain, band filling, and electric field on the magnetic anisotropy energy (MAE) of bulk and thin-film CrSb are determined and analysed using density functional theory. The MAE of the bulk crystal is large (1.2 meV per unit cell). Due to the significant ionic nature of the Cr-Sb bond, finite slabs are strongly affected by end termination. Truncation of the bulk crystal to a thin film with one surface terminated with Cr and the other surface terminated with Sb breaks inversion symmetry, creates a large charge dipole and average electric field across the film, and breaks spin degeneracy, such that the thin film becomes a ferrimagnet. The MAE is reduced such that its sign can be switched with realistic strain, and the large SOC gives rise to an intrinsic voltage controlled magnetic anisotropy (VCMA). A slab terminated on both faces with Cr remains a compensated AFM, but with the compensation occurring nonlocally between mirror symmetric Cr pairs. In-plane alignment of the moments is preferred, the magnitude of the MAE remains large, similar to that of the bulk, and it is relatively insensitive to filling.

Chromic oxide (Cr2O3) monolayer is a promising alternative hydrogen evolution reaction (HER) catalyst compared with expensive platinum (Pt) due to its advantages such as low cost, large specific surface area, high reserves, and designability. In this study, the two practical strategies, strain engineering and transition metal (TM) doping (Mn, Fe, Zn, etc.), are proposed to activate the catalytic sites of Cr2O3 monolayer for the HER. The density functional theory (DFT) calculations demonstrate that the strained Cr2O3 monolayer can stimulate the HER activity with the Gibbs free energy of hydrogen adsorption (ΔGH∗) close to 0.09eV, which can be considered as a performable strategy to tune the HER catalytic behavior of Cr2O3 monolayer. For the TM doping, it also plays a role in the performance adjustment. These results provide a guideline to optimize the HER performance of Cr2O3 monolayer.

By substituting the A site in P21/c-A(CN)2 and varying the lattice parameters a, b, c, and the unit-cell angles, along with using crystal graph convolutional neural networks to calculate their cohesive energy, the candidate compounds, Al(CN)2 and Si(CN)2, were selected from the structure with the lowest cohesive energy. The two candidate structures were then optimized using first-principles calculations, and their phonon, electronic, and elastic properties were computed. As a result, two dynamically stable structures were found: Al(CN)2 with a space group of Cmcm and Si(CN)2 with a space group of R3¯m. Their phonon spectra exhibited no imaginary frequencies; thus, their elastic constants satisfied the mechanical stability criteria. Structurally, Si(CN)2 is similar to 6H-SiC and 15R-SiC. Its elastic constants indicated that it is harder than those SiC materials. Al(CN)2 exhibits metallic properties and the indirect wide-bandgap of Si(CN)2 was calculated by the generalized gradient approximation, the local density approximation, and the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06) is found to be 3.093, 3.048, and 4.589 eV, respectively. According to this wide bandgap, we can conclude that Si(CN)2 has the potential to be used in high-temperature and high-power environments, making it usable in a broad range of applications.

One of the biggest problems facing contemporary medicine is cancer. New approaches to therapy are required due to the difficult and prolonged treatment, the numerous adverse properties of the medications employed, and the developing confrontation of neoplastic cells to treatment. Ten 1,2,4-triazine sulfonamide derivatives (1–10) were chosen for the first time in the current work, and their chemical structures were examined by DFT studies. The in silico flexible docking analysis of the chosen receptors involved in cancer development and metastasis (3RHK, 5GTY, 6PL2, and 7JXH) revealed that the selected compounds are the most promising. The binding affinity of compounds 10, 2, 6, and 4 is much better than the standard drug, Erlotinib, whereas compounds 9, 3, 1, and 7 showed better affinities as compared to standard drugs Neratinib and Tepotinib in the case of 3RHK receptor. The binding affinity against the 5GTY receptor of compounds 10, 5, and 3 is much better than the standard drug Tepotinib, and compounds 7, 6, 2, 4, 1, 8, and 9 showed better than Erlonitib and Neratinib. The binding affinity against the 6PL2 receptor of compounds 8, 3, 5, 4, 9, and 1 is much better than the standard drug Tepotinib. Compounds 10, 6, 7, and 2 were better than Erlotinib and Neratinib. All selected drugs showed better binding affinities than the standard anti-cancer drug Neratinib in the case of the 7JXH receptor, whereas compounds 2, 10, 5, 9, and 8 are better than Erlotinib. In silico ADME experiments supported the identified compounds’ drug similarity. According to the MEP calculations, compounds 3 through 10 can interact non-covalently. The interactions might take the form of σ- and π-hole interactions. Softest compound 4 has the smallest energy gap, with an E-gap value of 3.25 Ev. Compound 4 has the largest energy gap at 3.41 eV. Compound 5 superior electron donor has the highest HOMO energy (6.5470 eV for HOMO). Compound 2 has the lowest LUMO energy, which suggests that it would be the best electron acceptor (ELUMO = 5.766364 eV).

We comprehensively study the ionic conductivity in lithium phosphides, promising materials for energy storage applications, by using a combination of first-principles computations and machine learning interatomic potentials. Using the quasiharminic approximation, we calculated convex hulls of the Li-P system at various temperatures and the temperature-composition phase diagram was obtained, delineating the stability regions of each phase. The ionic conductivity of stable (Li3P, LiP, Li3P7, Li3P11, LiP7) and metastable (Li4P3, Li5P4, LiP5) compounds was studied as a function of temperature. In some compounds we found have high ionic conductivity at room temperatures (10−3–10−2 S cm−1). Structures with the lowest ionic conductivity are LiP, Li3P11, and LiP7, in which diffusion is negligible in the whole temperature range 300–500 K. In Li3P, Li3P7, and Li4P3, LiP, there is the 3D diffusion of Li atoms, while in Li5P4 the 2D mechanism prevails, and in LiP5 and LiP7 the 1D mechanism was observed. This study may provide insights for the development of Li-P materials in lithium ion and lithium metal battery applications.

In the current study, the peculiar nonlinear optical (NLO) properties of KAgCh (Ch = S, Se) and their structural, electronic, and thermodynamic properties are computed utilizing the FP-LAPW (full-potential linearized augmented plane wave) approach as embedded in Wein2K code. The Perdew–Burke–Ernzerh of generalized gradient approximation (PBE-GGA) was considered for the structural optimization. The computed bandgaps are found to be 2.57 and 2.39 eV for KAgS and KAgSe, respectively. Besides the structural and electronic properties, we also computed the refractive indices n(ω), surface energy loss function (SELF), and nonlinear optical susceptibilities. The estimated refractive indices, energy band gap, and their frequency dependence for the investigated KAgCh (Ch = S, Se) compounds, along with the NLO coefficients, are found to be in good agreement with the earlier reports. These current findings suggest that KAgCh (Ch = S, Se) can be recommended for nonlinear optical applications in the near-infrared spectrum.

Magnesium is an essential element because of its many beneficial properties and advantages over other metals, including its lack of risk to people’s health and its reasonable cost. However, Mg has several disadvantages, one of which is its high corrosion rate. This work analysed magnesium alloy characteristics and quantum behaviour, including band structure, molecular orbital, and corrosion behaviour in the presence of water. Magnesium was characterised by density functional theory software using CASTEP and Dmol3. Results showed no Mg band structure displays a conductive Fermi level of 8.85 eV. Curvature studies revealed that Mg has strong curvature and electron mobility. The density of state (DOS) of Mg-Al-Zn changes with Al and Zn alloy atoms, and the electron density increases to −7.5 eV compared with pure Mg. HOMO–LUMO analysis elucidated that Mg-Al-Zn* has a large gap (0.419 eV), leading to its stability and low chemical reactivity. This study analysed the properties of Mg and then examines the effect of corrosion on Mg alloys using DFT at different element positions. Corrosion analysis indicated that Mg-Al-Zn has the highest activation energy, implying that its corrosion is less likely than that of other alloys.

In this paper, the ternary system La2O3-TiO2-Nb2O5 is studied to find new ternary phases with useful electrical properties. The solid solution La3−xTi5−3xNb10−2xO39.5−12.5x was recently identified, and this study focuses on the structural characterization of this solid solution with x = 0.04. The crystal structure, representing a new structural type, was determined from synchrotron and neutron powder diffraction data. The unit cell parameters are a = 7.332 Å, b = 7.421 Å, c = 10.673 Å, α = 84.15°, ß = 80.16°, γ = 60.37°, and space group P1¯. The titanium and niobium atoms are disordered in five different crystallographic sites coordinated octahedrally by oxygen atoms. The eight-coordinated La atoms are embedded in the octahedral framework. Ti and Nb preferentially occupy different sites, and this feature was studied using periodic density functional theory methods. Energies of possible Ti/Nb distributions were calculated and the results agree well with the site occupancies obtained by combined Rietveld refinement of synchrotron and neutron powder diffraction patterns. The geometries optimized by DFT also agree well with the structural parameters determined by diffraction. The general agreement between the theoretical calculations and the experimental data justifies the quantum chemical methods as reliable complementary tools for the structural investigation of ceramic materials.

The liquid structures of short straight chain aliphatic carboxylic acids (C1-C6) and halogenated acetic acids (trifluoro and trichloro) are analyzed with molecular dynamics simulations, three-dimensional reference interaction site model, and density functional theory calculations. The neat acids are found to exist in multimeric ordered form consisting of both open chain and cyclic oligomers. Introduction of water breaks the hydrogen bonding networks between adjacent acetic acid molecules by inserting water as a bridging molecule. The electronic structure calculations confirm the formation of chain and ring type structures in the liquid state.

The catalytic and photon-induced oxidation of NO2 on anatase TiO2 has been studied and compared with the surface nitrate species obtained after adsorption of HNO3. Using a combination of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), density functional theory (DFT), and temperature-programmed desorption (TPD), it is shown that identical products are obtained in all reaction systems but that their formation rates differ significantly. The surface reaction products are identified as combinations of surface–NO3– species, where NO2 bonds to the lattice oxygen, and freely adsorbed NO3– ions. These products can be obtained either by dissociative adsorption of HNO3 or by catalytic/photocatalytic oxidation of NO2, which is facilitated by UV light. A concerted reaction mechanism is unraveled that involves reorientation of bidentate nitrate that pushes out a neighboring protonated lattice oxygen to form a surface–NO3– species and a terminal OH group. The thermal stability of these surface species has been studied by means of TPD and simultaneous in situ DRIFTS measurements that reveal a main desorption peak (m/z = 46) at around 430 °C, which is attributed to concerted nitrate desorption through pentoxide (N2O5) formation. A weaker and broader TPD peak is found at about 185 °C and is attributed to desorption of nitrate species bonded in a compressed configuration. The experimental results can be explained by the changing stability of the identified nitrate products, which depends strongly on the surface chemical environment and the surface coverage. The DFT results show that the stabilization of intermediate NO2 adsorbates and the final nitrate reaction products occurs through a bifunctional charge exchange mechanism that is mediated by the TiO2 crystal. In particular, a stable surface–NO3– and NO3– ion pair configuration has been identified. This mechanism explains both the thermal and photoinduced oxidation of NO2 and their thermal stability and different formation rates, yielding high photoinduced oxidation reaction rates. Our results provide insights into the structure and chemical stability of nitrate surface products on TiO2 particles and their formation mechanism, which is important for understanding their catalyzed transformation into the harmful compounds HONO and N2O during continued UV light illumination. Title of manuscript version in list of papers for David Langhammer: Adsorption and Catalytic Oxidation of NO2 on Anatase TiO2: A Combined DFT and Operando DRIFT Spectroscopy StudyAuthors of manuscript version in list of papers for David Langhammer: David Langhammer, Lars Österlund

Calculations of AgSbTe2 thermoelectric material and 2d CdS/TiS2 and their heterostructures were carried out using Density Functional Theory in Cambridge Serial Total Energy Package code as implemented in Material Studio 2018 software. The work function, thermal transport, electronic and optical properties were calculated. The results revealed that the heterostructures are possible to be achieved with improved properties. The electronic and thermal transport properties were likened with the description of equations derived from Boltzmann transport theory and Mott expressed in the maximum achievable Figure merit. Orbital contributions from the electron movement show valence and conduction band atomic shells.

Na0.5(Bi3/4RE1/4)0.5TiO3 (RENBT, RE = Nd, Gd, Dy, and Ho) compounds were investigated in the framework of first-principles calculations using the full potential linearized augmented plane wave (FP-LAPW) method based on the spin-polarized density functional theory implemented in the WIEN2k code. Combined charge density distribution and Ti K-edge X-ray absorption spectra revealed that the RENBT compositions with high polarization values were accompanied by a higher TiO6 distortion, DyNBT, and NdNBT compounds. The effect of the rare-earth elements on the polarization were confirmed experimentally with the collection of the hysteresis loops. The investigation of the electronic properties of the compounds highlighted the emergence of a magnetization owing to the 4f orbital effect of the rare-earth elements. Besides, the investigation of the chemical ordering showed a short-range chemical ordering for the pure composition and an increased A-site disorder for dysprosium doped NBT system. The increased disorder may speak for increased relaxor properties in the RE doped compositions.

Magnetocaloric materials undergoing reversible phase transitions are highly desirable for magnetic refrigeration applications. (Mn,Fe)(2)(P,Si) alloys exhibit a giant magnetocaloric effect accompanied by a magnetoelastic transition, while the noticeable irreversibility causes drastic degradation of the magnetocaloric properties during consecutive cooling cycles. In the present work, we performed a comprehensive study on the magnetoelastic transition of the (Mn,Fe)(2)(P,Si) alloys by high-resolution transmission electron microscopy, in situ field- and temperature-dependent neutron powder diffraction as well as density functional theory calculations (DFT). We found a generalized relationship between the thermal hysteresis and the transition-induced elastic strain energy for the (Mn,Fe)(2)(P,Si) family. The thermal hysteresis was greatly reduced from 11 to 1 K by a mere 4 at.% substitution of Fe by Mo in the Mn1.15Fe0.80P0.45Si0.55 alloy. This reduction is found to be due to a strong reduction in the transition-induced elastic strain energy. The significantly enhanced reversibility of the magnetoelastic transition leads to a remarkable improvement of the reversible magnetocaloric properties, compared to the parent alloy. Based on the DFT calculations and the neutron diffraction experiments, we also elucidated the underlying mechanism of the tunable transition temperature for the (Mn,Fe)(2)(P,Si) family, which can essentially be attributed to the strong competition between the covalent bonding and the ferromagnetic exchange coupling. The present work provides not only a new strategy to improve the reversibility of a first-order magnetic transition but also essential insight into the electron-spin-lattice coupling in giant magnetocaloric materials. (C) 2022 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.