Preferential Site Occupation Research Articles

Density functional theory study of aluminium and chromium doped Yttrium ion garnet

Density functional theory has been performed to investigate the atomic structure, electronic and magnetic properties of Al3+, Cr3+ and Al3+–Cr3+ co-doped cubic Y3Fe5O12 (YIG) garnet crystal. Results show that Al3+ and Cr3+ ions are energetically substitution of Fe3+ ions at the tetrahedral and octahedral sites in YIG, respectively. The doping concentration do not affect the preferential site occupancy for separate individual substitution. The magnetic moment increases and decreases for the single Cr and Al element doping, respectively. The magnetic moment and lattice constant decreases with the increase of Al–Cr concentrations. The magnetic moment decrease from 5.0 to 3.875 μB linearly with x increases from 0 to 0.4 in Y3AlxCrxFe5−xO12. These results indicate that doping can be adopted for the modification of magnetic properties of YIG.

Structures of quaternary chromium silicides revealed by a combination of resonant X-ray diffraction and ab initio calculations

The energetic stability and oxidation behaviour of quaternary chromium silicides mainly depends on their chromium content. In this work, the pseudolamellar crystal structures of six representatives of the X3T2(T1-xCrx)2Si6 compounds (X = Nb or Ti and T = Fe, Co or Ni) are determined by resonant X-ray diffraction using the synchrotron facility, combined with calculations based on the density functional theory. Experimentally, the transition metal positions are determined using the anomalous scattering effect close to the energy absorption threshold. First principles calculations carried out on the as-determined structures show that the preferential site occupations are explained by thermodynamic arguments. Theoretical calculations highlight the influence of Cr atoms on the lattice parameters, and reveal that the experimental Cr content corresponds to the amount required to reach the equilibrium between X3T2(T1-xCrx)2Si6 and X4T4Si7.

Half-metallic compensated ferrimagnetism in the Mn-Co-V-Al Heusler alloys

Abstract Detailed theoretical and experimental investigations on the electronic and magnetic properties of the Mn2Co1−xVxAl Heusler compounds are presented. The magnetization vs. temperature measurements on the Mn2Co1−xVxAl Heusler compounds show a decrease of the Curie temperature with V content, between 745 K (x = 0) and 671 K (x = 0.5). Ferrimagnetic near fully compensated behaviour with the saturation magnetization of 0.29 μB/f.u. has been obtained experimentally for Mn2Co0.5V0.5Al alloy, in slight disagreement with Slater-Pauling rule. The electronic band structure calculations performed using the Korringa-Kohn-Rostoker (KKR) Green’s function method for the Mn2Co0.5V0.5Al alloy show half-metallic almost compensated ferrimagnetic behaviour with total spin moment of 0.02 μB/f.u.. Accounting for the preferential site occupation determined by total energy calculations results an increase of the calculated spin moment, in agreement with the magnetic measurements. The fully compensated ferrimagnetic behaviour is predicted theoretically for the compounds derived from Mn2Co0.5V0.5Al by slight variation on composition. The half-metallic character deduced from density of states (DOS) calculations is expected to be preserved for the stoichiometric and off-stoichiometric alloys.

Structural, Magnetic, and Mössbauer Studies of Transition Metal-Doped Gd2Fe16Ga0.5TM0.5 Intermetallic Compounds (TM = Cr, Mn, Co, Ni, Cu, and Zn)

The effect of transition metal substitution for Fe and the structural and magnetic properties of Gd2Fe16Ga0.5TM0.5 (TM = Cr, Mn, Co, Ni, Cu, and Zn) compounds were investigated in this study. Rietveld analysis of X-ray data indicates that all the samples crystallize in the hexagonal Th2Ni17 structure. The lattice parameters a, c, and the unit cell volume show TM ionic radii dependence. Both Ga and TM atoms show preferred site occupancy for 12j and 12k sites. The saturation magnetization at room temperature was observed for Co, Ni, and Cu of 69, 73, and 77 emu/g, respectively, while a minimum value was observed for Zn (62 emu/g) doping in Gd2Fe16Ga0.5TM0.5. The highest Curie temperature of 590 K was observed for Cu doping which is 15 and 5% higher than Gd2Fe17 and Gd2Fe16Ga compounds, respectively. The hyperfine parameters viz. hyperfine field and isomer shift show systematic dependence on the TM atomic number. The observed magnetic and Curie temperature behavior in Gd2Fe16Ga0.5TM0.5 is explained on the basis of Fe(3d)-TM(3d) hybridization. The superior Curie temperature and magnetization value of Co-, Ni-, and Cu-doped Gd2Fe16Ga0.5TM0.5 compounds as compared to pure Gd2Fe17 or Gd2Fe16Ga makes Gd2Fe16Ga0.5TM0.5 a potential candidate for high-temperature industrial magnet applications.

Influence of atomic mixing and atomic order on molar volume of the binary sigma phase

The present work systematically clarifies the influence of the atomic mixing and the atomic order (i.e. atomic constituent distribution or site occupancy preference on inequivalent sites of a crystal) on the molar volume of the binary sigma phase. We calculated the molar volumes of 21 binary sigma compounds with different atomic order by using EMTO-CPA (Exact Muffin-Tin Orbitals - Coherent Potential Approximation) method. The calculation results follow a similar trend as the experimental data. Moreover, the calculated molar volumes with respect to the composition of the sigma phase in disordered states show that the sigma compounds do not comply with the linear volume-composition relationship. This is due to the influence of the atomic mixing and the deviation can be either negative or positive depending on the tendency of electron loss or gain of the two constituent elements. Additionally, we compared the molar volume difference (ΔVm = Vdisorder-Vorder) between the hypothetically disordered and ordered states of the binary sigma compounds. The results indicate that the effect of the atomic order on the molar volume of the sigma phase depends on the size difference and the electron configurations of the two constituent elements.

Identification of local structures around Ce3+ centers in potassium magnesium fluoride

Several Ce3+ centers were experimentally found in Ce3+-doped potassium magnesium fluoride (KMgF3) crystal by electron-spin resonance (ESR) spectra, but there was no definite conclusion about the charge-compensating mechanism. In the present work, we carried out a comprehensive investigation on local structures around Ce3+ centers in the host, based on a combination of first-principles calculations and phenomenological analyses. The site occupation preference, crystal-field energy levels and spectral parameters (e.g. crystal-field parameters, and g factors) for Ce3+ ions in the host were derived, which are compatible with previously measured excitation and ESR spectra. Accordingly, four Ce3+ centers (at K+ sites) observed in ESR spectra are assigned successfully. In particular, the Ce3+ center previously thought to be associated with a Mg2+ site is reassigned to a K+ site, in agreement with the results of defect formation energy calculations. The presented calculations together with optical and ESR spectroscopy techniques may offer cross check for reliable determination of impurity sites in luminescent materials.

First attempt to identify site occupation preference coefficients of a quaternary alloy: The InAsxPySb1-x-y system

The characterization of quaternary alloys is a particularly complex task. However, EXAFS is a powerful method, suitable to obtain information about the local atomic structure of a sample and to analyze the ion distributions within any defined configuration. In the truly quaternary InAsPSb alloy we collected EXAFS spectra at different atomic edges and their Fourier Transform were analyzed using the multivariate Partial Least Square Regression method. The InAsPSb structure and its ion distributions within the twelve multinary configurations were determined together with site occupation preferences. The sphalerite structured InAsPSb alloy is characterized in the metastable range, by preferences with a significantly attenuated value, a condition that enhances the populations of its three symmetrical binary configurations. The attenuations, respect to Bernoulli distributions, for the metastable growth region and beyond this region were also evaluated for the first time.

Influencing factors of atomic order in the binary sigma phase

The sigma phase can serve as a prototype of intermetallic phases when investigating the atomic order (i.e. atomic constituent distribution or site occupancy preference on inequivalent sites of a crystal structure), as the sigma phase bears a broad homogeneity range and there are numerous experimental site occupancy data available for the sigma phase. The present work brings new insights into the influencing factors of atomic order in the binary sigma phase. We calculated the atomic volumes and atomic charges of the constituent elements of 32 stoichiometric end-member compounds (i.e. the complete set of ordered configurations) of the binary sigma phase by using first-principles calculations. The calculation results show that besides the size factor and the number of valence electrons, the total number of electron shells is found as the third factor that affects the atomic order of the sigma phase. Moreover, the calculation results indicate that between the two constituent elements, the one with larger atomic size, smaller number of valence electrons or smaller total number of electron shells prefers occupying large coordination number (CN) sites (i.e. 4f, 8i1 and 8j) and the other prefers occupying small CN sites (i.e. 2a and 8i2). Furthermore, we dissociated the effects of individual influencing factors on atomic order of the sigma phase combining with the measured site occupancies from the literature.

Lithium uptake in strontium chlorapatite – A combined experimental and first principles study

Preferential site occupancy of lithium in the phosphate based mineral strontium chlorapatite has been studied using solid state NMR spectroscopy and first principles calculations. 31P Magic Angle Spinning (MAS) NMR shows evidence for strontium being substituted by lithium. The results are discussed on the basis of incommensurate lattice and lattice distortion caused by Li substitution. Detailed analysis of the 7Li MAS NMR spectra as well as ab-initio calculations show preferential occupation of lithium in eight coordinated cation site (M(II)) rather than the nine coordinated M(I) site and also distribution of the local environment around the cation ion site.

Bond energy, preferential occupancy and spontaneous reduction ability of Eu3+ doped in CaAl2Si2O8

A bond-energy method is used to determine the site occupancy preference of Eu3+ or Eu2+ and further firstly extended to study the spontaneous reduction of Eu3+ doped CaAl2Si2O8 based on the bond valence model. The phosphor CaAl2Si2O8: Eu was synthesized under air condition. The co-existence of Eu3+ and Eu2+ cations was detected through photoluminescence measurement. The occupancy of Eu3+ or Eu2+ in CaAl2Si2O8 can be determined by comparing the deviation of its bond energy in different locations at Ca2+, Si4+ and Al3+ sites. Theoretical calculation result indicates that the Eu3+ and Eu2+ would preferentially occupy the smaller energy variation sites Ca4, which can be assigned to Eu4 site. The more important result is that the energy variation of Eu2+ at Eu4 sites is smaller than that of Eu3+ at the same sites, which indicates that Eu2+ is more stable than Eu3+ at Eu4 sites, so the spontaneous Eu3+→Eu2+ reduction can be found in the CaAl2Si2O8. The same bond energy method can be extended to explain the reason why the Sm3+ doped CaAl2Si2O8 can't be reduced into Sm2+ under reduction condition. The theoretical result is consistent with the experimental phenomenon. This should help us to have a further understanding of spontaneous Eu3+→Eu2+ reduction in the inorganic material.

Influence of atomic order on the enthalpy of formation and bulk modulus of the sigma phase

We have calculated the enthalpy of formation and bulk modulus for 19 typical binary sigma compounds with different atomic order (i.e. atomic constituent distribution or site occupancy preference on inequivalent sites of a crystal structure) based on the experimental site occupancy as well as completely ordered and hypothetically disordered states by using the EMTO-CPA (Exact Muffin-Tin Orbitals - Coherent Potential Approximation) method. The calculation results show that at 0 K the sigma phase in ordered state bears a lower enthalpy of formation and a larger bulk modulus than the ones in less ordered state.

Effects of Ga substitution for Al and cation vacancy on microwave dielectric properties of spinel-structured Co1−3z(Al1−xGax)2+zO4 ceramics

The microwave dielectric properties and cation distribution of spinel-structured Co1−3z(Al1−xGax)2+zO4 (z = 0–0.25) ceramics, where z represents the concentration of the cation vacancy, were characterized in this study. Ga substitution for Al in the Co(Al1−xGax)2O4 ceramics was effective in increasing the quality factor (Q · f) values from 27,000 to 103,000 GHz, making the Co2+, Al3+, and Ga3+ cations randomly occupy both the tetrahedral and octahedral sites to form an intermediate spinel. A similar result was also obtained by the introduction of the cation vacancy for the Co1−3zAl2+zO4 (x = 0) ceramic and the highest Q · f value of 120,000 GHz was obtained for z = 0.25 at which the degree of inversion, y, was estimated to be 0.5 from 27Al NMR spectra. Further improvement in Q · f value was achieved by the introduction of cation vacancy and Ga substitution for Al in Co0.25(Al1−xGax)2.5O4 ceramics. 71Ga NMR spectra showed that Ga3+ cations occupied both the tetrahedral and octahedral sites, while a single peak was observed from 27Al NMR spectra, implying that Al3+ cations occupied only the octahedral site. The preferential site occupancy of Al in the an octahedral site led to the formation of ordered spinel structure with the space group P4332 and the Q · f value of the Co0.25(Al0.4Ga0.6)2.5O4 (z = 0.25 and x = 0.6) ceramic was 170,000 GHz.

Substitutional Ta-doping in Y2O3 semiconductor by sol-gel synthesis: experimental and theoretical studies

The Pechini sol-gel method has been employed for the synthesis of pure and (181Hf→)181Ta-doped Y2O3 nanopowders. We performed a structural characterization from the micro to the subnanoscopic scale by means of scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction, and time-differential perturbed γ–γ angular correlation (PAC) spectroscopy. The results show the formation of the cubic bixbyite structure after a thermal treatment at 1473 K. For the synthesized 181Ta-doped Y2O3, the PAC experiments demonstrate that the impurities are mainly located at both substitutional cationic sites of the bixbyite structure. The experimental investigation was complemented by performing first-principles electronic structure calculations for Ta atoms localized at the two cationic sites of the Y2O3 semiconductor structure, which allow the study of the structural and electronic modifications induced in the host system when the impurities are introduced. These calculations confirm that the measured electric-field gradients for the synthesized 181Ta-doped Y2O3 correspond to double-ionized impurities located at substitutional defect-free cationic sites of the bixbyite host structure and indicate the site occupancy preference for 181Hf(→181Ta) doping. The behaviour of the site preference of 181Ta impurities with temperature is also discussed. In addition to an extensive structural and electronic characterization of pure and Ta-doped Y2O3 semiconductor, our results demonstrate that the Pechini sol-gel process is an affordable and effective way to successfully synthesize these PAC substitutional doped samples.

High-temperature site preference and atomic short-range ordering characteristics of ternary alloying elements in γ’-Ni3Al intermetallics

Remarkable high-temperature mechanical properties of nickel-based superalloys are correlated with the arrangement of ternary alloying elements in L12-type-ordered γ′-Ni3Al intermetallics. In the current study, therefore, high-temperature site occupancy preference and energetic-structural characteristics of atomic short-range ordering (SRO) of ternary alloying X elements (X = Mo, W, Ta, Hf, Re, Ru, Pt or Co) in Ni75Al21.875X3.125 alloy systems have been studied by combining the statistico-thermodynamical theory of ordering and electronic theory of alloys in the pseudopotential approximation. Temperature dependence of site occupancy tendencies of alloying X element atoms has been predicted by calculating partial ordering energies and SRO parameters of Ni-Al, Ni-X and Al-X atomic pairs. It is shown that, all ternary alloying element atoms (except Pt) tend to occupy Al, whereas Pt atoms prefer to substitute for Ni sub-lattice sites of Ni3Al intermetallics. However, in contrast to other X elements, sub-lattice site occupancy characteristics of Re atoms appear to be both temperature- and composition-dependent. Theoretical calculations reveal that site occupancy preference of Re atoms switches from Al to both Ni and Al sites at critical temperatures, Tc, for Re > 2.35 at%. Distribution of Re atoms at both Ni and Al sub-lattice sites above Tc may lead to localised supersaturation of the parent Ni3Al phase and makes possible the formation of topologically close-packed (TCP) phases. The results of the current theoretical and simulation study are consistent with other theoretical and experimental investigations published in the literature.

Mössbauer Analysis and Cation Distribution of Zn Substituted BaFe12O19 Hexaferrites

Barium hexaferrite is a well-known hard magnetic material. Doping using nonmagnetic cation such as Zn2+ were found to enhance magnetization owing to preferential tetrahedral site (4 f 1) occupancy of the zinc. However, the distribution of cations in hexaferrites depends on many factors such as the method of preparation, nature of the cation, and chemical composition. Here, Zn-doped barium hexaferrites (Ba1−xZnxFe12O19) were synthesized by sol-gel method. In this study, we summarized the magnetic properties of Ba1−xZnxFe12O19 (x = 0, 0.1, 0.2, 0.3) BaM, investigated by Mossbauer spectroscopy. Moreover, cation distribution was also calculated for all the products. Mossbauer parameters were determined from 57Fe Mossbauer spectroscopy and according to it, the replacement of Ba-Zn affects all parameters such as isomer shift, the variation in line width, hyperfine magnetic field, and quadrupole splitting. Cation distribution revealed the relative area of undoped BaM, 12k, 2a, and 4 f 2 positions which are close to theoretical values.

Site Occupancy Preference and Antithermal Quenching of the Bi2+ Deep Red Emission in β-Ca2P2O7:Bi2+

The resistance to thermal quenching is an essential factor in evaluating the performance of luminescent materials for application in white light emitting diodes (WLEDs). In this work, we studied the site occupancy preference and thermal quenching of luminescence in β-Ca2P2O7:Bi2+ red phosphor at low (10-300 K) and high temperatures (303-573 K). In β-Ca2P2O7, the host lattice has four different calcium sites, at which Bi2+dopant can be located. After comparing the change of bond energy when the Bi2+ ions are incorporated into the four calcium sites, we found out that Bi2+ would preferentially occupy the smaller energy variation sites Ci(2) and Ci(1) in this compound, which can be assigned to Bi(2) and Bi(1), respectively. Surprisingly, we noticed that the variation of emission intensity is different under different excitations when the temperature changes from 10 to 300 K. When exciting into the typical absorption of Bi(1) sites at 419 nm, the emission intensity at 300 K remains only 38% as compared to that at 10 K, while exciting into typical Bi(2) absorption at 460 nm, the emission intensity increases to 110%. When further increasing the temperature from 303 to 573 K, we observed a similar phenomenon, and the emission at 460 nm excitation starts to quench at 453 K. The emission intensity at 573 K still remains 86.1% of that at 303 K. This might be attributed to the Bi(2) → Bi(1) energy transfer. It is also evidenced by the time-resolved emission spectra and lifetime values. This work gives new insights into better understanding luminescent behaviors of Bi2+-doped materials with multiple cation sites. This should be helpful in the future when designing the bismuth doped phosphor for WLEDs with better resistance to thermal quenching.

Dipole – dipole and dipole – quadrupole interactions between Sm3+ ions in K4BaSi3O9

K4BaSi3O9 (KBSO) doped with Sm3+ was synthesized for the first time by conventional solid-state method. Results of XRD measurements confirm that the obtained powder is K4BaSi3O9 single phase. The new material exhibits red luminescence with maximum at 641nm under 407nm excitation. At 300K the luminescence intensity is quenched for Sm3+ concentration above 0.5%. The decay profiles revealed the non-radiative energy transfer between the Sm3+ ions. The interaction has dipole – dipole character for low Sm3+ concentrations (up to 1%) then dipole – quadrupole for higher dopant concentration (over 1%). The critical distances are 8.8Å and 9.5Å for dipole – dipole and the latter interactions, respectively. This phenomenon is explained by analyzing the probability of occupation of all possible sites available for Sm3+ in KBSO taking into account local charge compensation. The site occupation preference by Sm3+ ions depends on the distances between available cation sites.

Clarification of copper species over Cu-SAPO-34 catalyst by DRIFTS and DFT study of CO adsorption

In this work, the nature, location and evolution of Cu+ ions in Cu-SAPO-34 are investigated by diffuse reflectance infrared Fourier transform spectrum (DRIFTS) of CO adsorption and density functional theory (DFT) calculation. By combination with DFT results, characteristic Cu+–CO bands located at 2154 and 2136 cm−1 are attributed to CO adsorbed on Cu+ ions located at sites I (in the plane of six-membered ring connected to the large cages) and site II (in the eight-membered ring cages near the tilted four membered ring) in the framework of H-SAPO-34 zeolite. Subsequently, both the influences of Cu loading and preparation method are considered and discussed. By varying the Cu loading, the site-occupation preference of Cu+ ions on site I is confirmed, especially at low Cu loadings. Through elevating the desorption temperature, migration of Cu+ ions is revealed because of the adsorption-induced effect. Furthermore, a facile and more efficient approach to introduce Cu+ ions into CHA zeolite, compared with solid-state ion exchange with CuCl and conventional ion exchange in aqueous solution, and the different preparation methods also result in different occupations of Cu+ ions.

Engineering the Lattice Site Occupancy of Apatite-Structure Phosphors for Effective Broad-Band Emission through Cation Pairing.

A series of britholite compounds were synthesized by simultaneous introduction of trivalent La3+ and Si4+ ions into an apatite structure. The variations in the average structure, electronic band structure, and microstructural properties resulting from the introduction of cation pairs were analyzed as a function of their concentration. The effects of the structural variance and microstructural properties on the broad-band-emitting activator ions were studied by introducing Eu2+ ions as activators. For the resulting compound, which had dual emission bands in the blue and yellow regions of the spectrum, the emission peak position and strength were dependent upon the concentration of La3+-Si4+ pairs. By engineering the relative sizes of the two possible activator sites in the structure, 4f and 6h, through the introduction of a combination of trivalent La3+ and a polyanion, the preferential site occupancy of the activator ions was favorably altered. Additionally, the activator ions responsible for the lower-Stokes-shifted blue component of the emission functioned as a sensitizer of the larger-Stokes-shifted yellow-emitting activators, and predominantly yellow-emitting phosphors were achieved. The feasibility of developing a white light-emitting solid-state device using the developed phosphor was also demonstrated.

Effect of Cu and Cation Redistribution on Structural and Magnetic Properties of Co-Mg Nanoferrite

The structural and magnetic properties of Cu-substituted, Co-Mg mixed nanoferrite synthesized by a sol-gel auto-combustion method are presented. Significant modifications in crystallite size, density, and grain size are observed with increasing Cu concentration. Comparable values of theoretical and experimental lattice parameters were obtained for all the Cu-substituted samples. Cation distribution estimated from X-ray intensity calculations shows that Cu influences the preferential site occupancy of Mg ions. It is found that Cu and Mg simultaneously occupy tetrahedral (A) and octahedral (B) sites with different ratios. The particle size calculated using TEM for undoped Co-Mg ferrite is about 80 nm. Saturation magnetization and permeability increase initially and reach a maximum value (x = 0.3) and then decrease. The observed variation is explained on the basis of redistribution of cations (Cu 2+ and Mg 2+) among the tetrahedral (A) and octahedral (B) sites. The initial permeability shows a good stability with increasing frequency.