Formation Energies Of Antisite Defects Research Articles

Point defect properties in high entropy MAX phases from first-principles calculations

MAX phase materials exhibit both metal and ceramic properties due to their unique laminated atomic structures, making them promising candidates in advanced nuclear energy systems. Recently, high entropy MAX (HE-MAX) phases have been developed and attracted much attention due to their unique properties. However, the role of chemical disorder in HE-MAX phases in their point defects properties is still not clear. In this work, we investigated the point defect properties in (TiVNb)2SnC, (TiZrHf)2SnC, (TiVNbZrHf)2SnC, and five corresponding single-component M2SnC phases (M=Ti, V, Nb, Zr, Hf) using first-principles calculations. The average vacancy (VM, VSn, VC) formation energies in the HE-MAX phases are (TiZrHf)2SnC > (TiVNbZrHf)2SnC > (TiVNb)2SnC. With the addition of Zr and Hf atoms, the charge transfer between atoms in the HE-MAX phase increases, hindering the formation of these vacancies. Meanwhile, the obtained migration energies show that the migration barrier of VM through Ti is lower than that of V, Nb, Zr, or Hf, while Zr and Hf atoms increase the VC migration barrier due to their large atomic sizes. Additionally, the formation energies of antisite defects in all three HE-MAX phases are lower than the single-component M2SnC phases, indicating that the HE-MAX phases are more resistant to radiation-induced amorphization. This work provides a fundamental understanding of the effect of chemical disorder on point defect properties in MAX phases and proposes a new strategy for designing novel HE-MAX phases with better performance in nuclear applications.

Defects in Li-rich manganese-based layered oxide: A first-principles study

Lithium-rich manganese-based layered oxides are of great interest as cathode materials for lithium ion batteries due to their high energy density. The voltage decay and capacity fading during prolonged charge/discharge cycling are the key obstacles for their practical usage. In this work, using density functional theory, we investigated the origin of the Ni surface segregation by calculating the defect formation energies of antisite defects, including Ni cation substituting a Li cation [Formula: see text] and pairs of Ni cation swapping with Li cation ([Formula: see text]–[Formula: see text]) in [Formula: see text], [Formula: see text] and [Formula: see text] to represent the Mn-rich, Mn–Ni equivalence and Ni-rich conditions, respectively. Results show that [Formula: see text]–[Formula: see text] defect is of energy favorable in Li-rich and Mn-rich layered oxide cathode. Al-doping is beneficial for improving the structure stability and preventing the surface segregation, thus Al-doping can improve cycle ability and rate capability of Li-rich and Mn-rich layered cathodes. This work provides deep insights for the design of layered cathode materials with both high capacity and voltage stability during cycling.

First principle calculation of helium in La2Zr2O7: Effects on structural, electronic properties and radiation tolerance

First principle calculations based on density functional theory have been employed to study structural effects of trapping helium in La2Zr2O7 pyrochlore. Lattice swelling and the distortion of unit cell have been found in He-La2Zr2O7 systems. By analyzing the electronic structures and chemical bonding of He-La2Zr2O7 systems, weak repulsive and attractive chemical interactions of helium in La2Zr2O7 pyrochlore have been observed. The formation energies have been calculated to assess the relative stability of various helium interstitial configurations and the results show that the octahedral interstitial site is the most stable structure. The cation antisite defect formation energies and the x positional parameter for 48f-site oxygen are calculated to predict the radiation resistance of He-La2Zr2O7 systems. The results indicate that the presence of low concentration of He interstitials may increase the radiation resistance of La2Zr2O7 pyrochlore.

First-principles study of defect formation in the photovoltaic semiconductors Cu2GeS3 and Cu2ZnGeS4 for comparison with Cu2SnS3, Cu2ZnSnS4, and CuInSe2

The formation energies of neutral Cu, Ge, and S vacancies in monoclinic Cu2GeS3 and those of neutral Cu, Zn, Ge, and S vacancies in kesterite-type Cu2ZnGeS4 were evaluated by first-principles pseudopotential calculations using plane-wave basis functions. The calculations were performed at typical points in a schematic ternary phase diagram of a Cu–Ge–S system for Cu2GeS3 and in Cu–(Zn1/2Ge1/2)–S and Cu29S16–ZnS–GeS2 pseudoternary phase diagrams for Cu2ZnGeS4. The results have been compared with those for Cu2SnS3, Cu2ZnSnS4, and CuInSe2 calculated with the same version of the CASTEP program code. The results indicate that Cu vacancies are easily formed in Cu2GeS3 and Cu2ZnGeS4 under the Cu-poor condition as in the cases of Cu2SnS3, Cu2ZnSnS4, and CuInSe2, suggesting that Cu2GeS3 and Cu2ZnGeS4 are also preferable p-type absorber materials for thin-film solar cells. Desirable preparation conditions of these thin films for photovoltaic application are discussed using the calculated formation energies of antisite defects.

Synthesis and radiation tolerance of Lu2−xCexTi2O7 pyrochlores

As a nonradioactive surrogate for Pu, Ce is selected to study the solubility and radiation tolerance in pyrochlore matrixes. In this paper, we synthesized a series of Lu2−xCexTi2O7 (x = 0–0.7) samples. X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy were used to study the structure of Lu2−xCexTi2O7 (x = 0–0.7) with different Ce3+ content x. The results show that the maximum solubility of Ce3+ in Lu2Ti2O7 is 32 mol%. In order to study the radiation tolerance of these compounds, 400 keV Ne2+ was used to perform the irradiation experiments. The grazing incident X-ray diffraction patterns revealed that the radiation tolerance of the Lu2−xCexTi2O7 decreases with increasing Ce-content. This correlation between radiation tolerance and Ce-content was interpreted in terms of the antisite defect formation energies of Lu2−xCexTi2O7 (x = 0, 0.5, 1) based on the density functional theory.

Effect of vanadium doping on the magnetic properties of LiMnPO4

Vanadium‐substituted LiMnPO4 has been synthesized by the solid state reaction route. To obtain tetravalent state of vanadium, the samples have been additionally annealed under hydrogen atmosphere. Comparative magnetic and neutron powder diffraction investigations of the two solid‐solution lines were performed as a function of V content. The divergence between static zero‐field‐cooled (ZFC) and field‐cooled (FC) magnetizations in the fields of H < 1 T and the magnetization hysteresis indicating the presence of a ferromagnetic component were observed for all vanadium concentrations. The ferromagnetic component in the reduced V‐doped LiMnPO4 is more intensive than in the as‐prepared samples. To gain insight into the reason for this effect, the formation energies of antisite defects in V‐doped and reduced LiMnPO4 were calculated within the density functional theory. The appearance of the ferromagnetic clusters above the Néel temperature is discussed.

First-principles Study of Antisite Defects in Orthorhombic PbZrO3

First-principles calculations based on density functional theory (DFT) within local density approximation were employed to investigate the antisite defects, including PbZr and ZrPb, in orthorhombic PbZrO3 by determining their defect formation energies. The formation energies of antisite defects were then compared with those of other dominant defects, i.e., lead Pb, zirconium Zr, and oxygen O vacancies to examine the likelihood of their existence. Our results revealed that PbZr defect in neutral charge state is the most dominant defect under O-rich or oxidizing condition in agreement with the previous work. In addition, there is a little structural relaxation when the Zr atom is replaced by Pb atom to form PbZr defect in neutral charge state. In opposite, under O-poor or reducing condition, the formation energies of antisite defects are quite high and higher than those of vacancy defects. This suggests that antisite defects are unlikely to form under reducing condition.

The structural transitions of Ti3AlC2 induced by ion irradiation

The structural transitions of Ti3AlC2 induced by ion irradiation were investigated over a wide fluence range by transmission electron microscopy. No amorphization occurs even at the highest dose of 31dpa, indicating a great tolerance to irradiation-induced amorphization. Dynamic electron diffraction simulations and high-resolution observations indicate that the nanolamellar structure of Ti3AlC2 is readily destroyed through the formation of antisite defects and a phase transformation from α-Ti3AlC2 to β-Ti3AlC2 occurs at 2.61dpa. A great number of stacking faults in basal planes are formed with increasing fluence, leading to the formation of nano Ti3AlC2 grains with different stacking sequences at 10.45dpa. Serious structural damage and polygonization are observed at the highest dose of 31dpa. Due to the similar structural transition process with some complex oxides (pyrochlore and murataite), it is assumed that the great irradiation tolerance of Ti3AlC2 results from the low formation energy of antisite defects. These findings first clarify the structural transition mechanism of Ti3AlC2 under ion irradiation and its relationship with irradiation tolerance, which is of vital importance in understanding the irradiation response of MAX phases and provides a clue in searching for materials with higher irradiation tolerance from MAX phases.

The effect of native point defect thermodynamics on off-stoichiometry in β-Mg17Al12

The mechanical strength of Mg–Al–Zn alloys can be affected by a fine spatial dispersion of β-Mg17Al12 precipitates in the Mg matrix. In an effort to understand the phase stability and the unusual asymmetric off-stoichiometry observed in β-Mg17Al12, we have performed a series of first-principles density functional theory (DFT) calculations of bulk and defect properties of Mg17Al12. Specifically, we consider native point defects (i.e. vacancies and anti-sites) in all four sublattices of Mg17Al12, i.e. 2a, 8c, 24g (Mg) and 24g (Al). The T=0K static energies of defect Mg17Al12 supercells indicate that anti-site defects are energetically favored over vacancies, and the lowest anti-site defect formation energies are in 24g sites for both AlMg and MgAl. These Al-rich and Mg-rich anti-site defect formation energies are similar in magnitude, and thus do not explain the asymmetric off-stoichiometry of Mg17Al12. We also investigate the effect of atomic vibrations via DFT phonon calculations on native point defect free energies of Mg17Al12 and combine these entropic contributions with the point defect formation energies to evaluate the thermodynamics of off-stoichiometry in this phase. We find that the formation of the AlMg anti-site is not strongly stabilized by vibrational entropy. Thus, we conclude that the observed asymmetry in the off-stoichiometry of the β-Mg17Al12 phase in the Mg–Al phase diagram is not explained by simple native point defect thermodynamics, and must involve a more complicated defect formation mechanism, such as multi-defect clustering.

First principles study of point defects in titanium oxycarbide

We have performed first principles density functional theory calculations to study the formation energy of point defects in TiC, TiO and TiCO compounds. The formation energy of isolated vacancies was obtained for different equilibrium conditions. For binary compounds, we have also calculated the formation energy of antisite defects. It was found that the defect formation energies strongly depend on the chemical environment. Our results show that C vacancies are easily formed in TiC and TiCO. For the TiO compound, Ti vacancies are highly probable to occur and O vacancies are also easily formed under titanium rich conditions.

Formation energies of antisite defects in Y3Al5O12: A first-principles study

We perform the first-principles calculations for the formation energies of cation antisite defects in Y3Al5O12. This method provides precise values of formation energy and thus allows us to estimate the defect concentration. The calculations show that YAl,16a is the most predominant antisite defects at high temperature for the single crystal growth and its concentration significantly decreases at low temperature for the single-crystalline film preparation. The calculated defect concentrations are quantitatively accord with the experimental estimation. AlY has high formation energy even with excess Al2O3, which indicates AlY is energetically unfavorable and the defect process is not intrinsic but nonstoichiometry.

Constitutional and thermal defects in B8 2–SnTi 2

Vacancy and antisite defect formation energies in B8 2–SnTi 2 are calculated by an ab initio approach. Three sublattices are introduced to account for the B8 2 structure. A statistical model based on a mean-field approximation is developed in the canonical ensemble. The defect concentrations are calculated as function of temperature and deviation from stoichiometry. For stoichiometric B8 2–SnTi 2 alloys, the dominant thermal defects are composed of one antisite Ti atom and three Ti vacancies. In the Sn-rich B8 2–SnTi 2, the constitutional defects are Ti vacancies; the thermal defect below 1000 K is an interbranch where Ti vacancies are replaced by Sn antisites; at high temperatures, it is a four point-defect comprising one Ti antisite and three Ti vacancies. In the Ti-rich B8 2–SnTi 2, the constitutional defects are antisite Ti atoms and the thermal defect is a four point defect comprising one Ti antisite and three Ti vacancies. The effective defect formation enthalpies are derived at low temperature. The Gibbs energy as well as the Sn and Ti chemical potentials in B8 2–SnTi 2 phase are obtained as function of composition for various temperatures. The extension of the one-phase domain of B8 2–SnTi 2 in the Sn–Ti phase diagram is discussed.

Constitutional and thermal defects in D0 19-SnTi 3

Vacancy and antisite defect formation energies in D0 19-SnTi 3 are calculated by an ab initio approach. Based on a statistical-thermodynamic model, the defect concentrations are calculated as function of temperature and deviation from stoichiometry. The constitutional defects are antisite Sn atoms and antisite Ti atoms in Sn-rich and Ti-rich SnTi 3, respectively. The dominant thermal defects are composed of two antisites for stoichiometric alloys. For Sn-rich D0 19-SnTi 3 alloys, the thermal defect is an interbranch where one antisite Sn atom is replaced by four Ti vacancies. For Ti-rich D0 19-SnTi 3 alloys, the thermal defect is a five point defect comprising one antisite Ti and four Ti vacancies. The effective defect formation enthalpies are derived for different concentration regions of D0 19-SnTi 3. The Gibbs energy as well as the Sn and Ti chemical potentials in the D0 19-SnTi 3 are obtained as function of composition for various temperatures. The extension of the one phase domain of D0 19-SnTi 3 in the Sn–Ti phase diagram is discussed.

Phase stability and ordering in diluted magnetic III–V semiconductors

We study the energetics of diluted ferromagnetic III–V semiconductors on an ab initio level using the tight-binding linear muffin-tin orbital–coherent potential approximation method and treating magnetic disorder within the disordered local moment model. Based on calculated total energies, we examine the stability of these alloys with respect to segregation and estimate formation energies of antisite defects and substitutional and interstitial Mn atoms. By using the generalized perturbation method, we calculate the parameters of an effective alloy Ising Hamiltonian including long-range Coulomb interactions. We investigate possible types of ordering, that is correlations of spatial positions of defects using the linearized concentration wave method and Warren–Cowley short-range-order parameters. The theory is illustrated for a GaAs alloy system.

Energetics of point defect formation in Ni 3Al

Vacancy and anti-site defect formation energies in L1 2-ordered Ni 3Al are calculated by ab initio approaches. Based on a statistical-thermodynamic model the thermodynamic activities of both components are derived as a function of temperature and composition. Comparison with measured activities demonstrates the high reliability of the ab initio approach to defect formation.

Monte Carlo and Molecular Dynamics Validation of an N-Body Potential for Cu3Au

ABSTRACTWe present the results of Monte Carlo and Molecular Dynamics simulations of some thermodynamical properties of Cu3Au. The calculations rely on an empirical n-body potential that reproduces satisfactorily the critical temperature, Tc, the temperature dependence of the lattice constant and atomic vibrational amplitudes as well as Cowley's short-range order parameters above Tc. Our results show that relaxation effects decrease considerably the formation energy of antisite defects and therefore should explicitly be considered for a realistic description of the transition.

Energy formation of antisite defects in doped Sb 2Te 3 and Bi 2Te 3 crystals

The free carrier concentration of the Sb 2−xIn xTe 3, Bi 2−xIn xTe 3 and Bi 2Te 3−xS x crystals has been determined from the values of the Hall constants and the free carrier concentration of the Sb 2−xTl xTe 3 has been calculated from the plasma resonance frequency; with increasing value of x, the hole concentration decreases. As the incorporation of the elements In, Tl and S into the lattice Sb 2Te 3 or Bi 2Te 3, respectively, gives rise to the uncharged defects In x Sb, Tl x Sb, In x Bi and S x Te, the x causes the decrease of the antisite defects concentration. The proven effect is explained in the following way: the antisite defects can be created only in crystals whose atoms are bound by weakly polarized bonds. The incorporation of In, Tl and S atoms into the crystal lattice of Sb 2Te 3 or Bi 2Te 3 increases the bond polarity, the ionicity of ternary crystals increases. This unfavorably affects the increase of antisite defects whose concentration decreases. The change of the bond polarity is considered from the changes discovered in the formation energy of antisite defects of the above mentioned ternary crystals.