Nonequivalent Crystallographic Positions Research Articles

A combined approach for comprehensive explanation of defects' thermodynamics in highly non-stoichiometric oxides: Case of RBaCo2O6-δ family (R = La, Nd, Pr, Sm, Gd, Eu, Ho and Y)

The thermodynamics of defect formation in layered perovskite-like cobaltites RBaCo2O6-δ (R = La, Nd, Pr, Sm, Gd, Eu, Ho and Y) was investigated via two distinct methods – (I) processing experimental data on oxygen non-stoichiometry using a quasichemical model and (II) employing a combined approach within the framework of density functional theory (DFT) under general gradient (GGA) and quasiharmonic Debye (QHDA) approximations. The outcomes of the first-principles calculations exhibit a plausible agreement with the fitting results, thereby confirming the validity of the developed model. The study reveals that key thermodynamic parameters governing defect formation, such as the enthalpy and entropy of oxygen exchange, as well as the enthalpy of oxygen disordering over nonequivalent crystallographic positions, exhibit correlations with the size of the rare-earth element R. In contrast, the enthalpy of charge disproportionation is demonstrated to be independent of the nature of the R atom. In Addition, it is suggested that when R = Eu, partial reduction of the latter may be observed in the lattice of EuBaCo2O6-δ. The findings indicate that, at a first approximation, the parameters of defect formation in RBaCo2O6-δ remain relatively independent on temperature and oxygen non-stoichiometry δ. The employed combined approach has been demonstrated to be reliable for assessing the thermodynamic functions of defect formation at elevated temperatures.

Structure and magnetodielectric properties of titanium substituted barium hexaferrites

BaFe12-xTixO19 barium hexaferrites up to x = 2.00 were synthesized by the usual ceramic technology. Crystal structure refinement was realized by Rietveld method of powder XRD at room temperature. The unit cell parameters change non-monotonically with titanium concentration. The minimum volume of ~698.75 Å3 was determined for the x = 1.00. The mechanism of nonequivalent crystallographic positions occupation with titanium cations is established. SEM investigation was shown that the obtained samples are ceramics with a tightly compacted polycrystals (>95%) and average crystallite size of ~5 μm. The Mössbauer investigation confirms such localization of Ti4+ cations. The field dependence of magnetization at 5 K and 300 K was measured and the main magnetic parameters were determined using the law of approach to saturation. These values decrease almost monotonically with increase in titanium concentration and temperature. The minimum values of these quantities of ~23.4 emu/g, ~1.9 emu/g, 1.2*106 Erg/g and ~105 Oe were fixed for the x = 2.00 at 300 K. With an increase in the doping concentration, both the magnitude of the electrical ac-resistivity and the temperature of transition to the activation type of conductivity increase non-monotonically. At x = 0.50, the lowest electrical resistivity of ρ ~5.1*103 Ohm*cm is observed at room temperature with the electrical transition temperature is Tel ~406.5 K. With an increase in frequency the ac-resistivity decreases, as the value of the band gap. The real part of the permittivity increases constantly with increasing temperature and decreases with increasing frequency for all the compositions. The temperature peak of the tg(d) loss tangent with the doping concentration changes nonmonotonically in magnitude and position. An interpretation of the magnetic and electric states of the substituted BaFe12-xTixO19 barium hexaferrite is given taking into account the mechanism of occupation nonequivalent crystallographic positions with titanium cations.

Lattice dynamics of cobalt orthoborate Co3(BO3)2 with kotoite structure

Transition metal oxyborates M3(BO3)2 (M = Mn, Co, Ni) belong to a group of materials with the kotoite structure and many of them demonstrate complex magnetic behavior as a result of competing interactions between magnetic ions in nonequivalent crystallographic positions. In this paper, we report results of the detailed infrared and Raman spectroscopic studies of a Co3(BO3)2 single crystal. Most of the phonons are observed and identified. Unusually large phonon shifts, in comparison to isostructural compound Ni3(BO3)2, were registered in the low- and mid-infrared frequency range (100–700 cm−1) of the spectra. The experimental findings are supported by the ab initio simulations of the lattice dynamics, which allow us to propose the normal-mode assignments. We argue that the large phonon shift between cobalt and nickel compounds are associated with changes of force constants induced by the changes of M–O bond lengths within the two types of octahedral MO6 groups. In addition to the first-order Raman scattering, intense second-order Raman scattering was detected due to the resonant enhancement regime. Furthermore, we performed the single crystal XRD experiments which did not show any detected evidences of magneto-structural phase transition previously observed in Ni3(BO3)2.

Effect of titanium substitution and temperature variation on structure and magnetic state of barium hexaferrites

A number of solid solutions based on BaFe12−xTixO19 M-type barium hexaferrite doped with titanium cations up to x = 2.00 were obtained using conventional ceramic technology. The phase composition, crystal structure and unit cell parameters were refined by the Rietveld method using powder X-ray diffraction data up to T = 900 K. It was found that all the compositions have a magnetoplumbite structure satisfactorily described by P63/mmc space group (No. 194). With increasing temperature and doping concentration, the unit cell parameters increase almost monotonically. The minimum volume of V ~ 696.72 Å3 was determined for the composition with x = 1.00 at T = 100 K, while the maximum value of V ~ 714.00 Å3 is observed for the composition with x = 2.00 at T = 900 K. The mechanism of occupation nonequivalent crystallographic positions with titanium cations is established. The spin-glass component of the magnetic phase state is fixed. The Tdif temperature of the difference between the ZFC-FC curves decreases with an increase in the concentration of titanium cations and the magnetic field from ~237.2 K to ~ 44.5 K, while the Tinf inflection temperature of the ZFC curve increases from ~21.0 K to ~23.8 K. With an increase in the doping concentration, both the Dav average and Dmax maximum clusters grow up to ~ 100 nm. As the magnetic field increases above the critical value, the spin-glass component disappears. For compositions with x > 1.00, the magnetization is not saturated in fields up to 6 T. Along with the formation of the spin-glass component, doping with titanium cations for barium hexaferrite lowers the TC Curie temperature down to T ~600 K. The Ms spontaneous and Mr remanent magnetizations, as well as the Bc coercivity, decrease with increasing doping concentration almost monotonically, while the latter has an inflection point at x = 1.00. The minimum values of spontaneous and remanent magnetization, as well as coercivity, are observed for the composition with x = 2.00 and amount to Ms ~17.7 emu/g, Mr ~1.9 emu/g, and Bc ~3.9 × 10−3 T, respectively. An interpretation of the magnetic state of the doped BaFe12−xTixO19 barium hexaferrite is given taking into account the mechanism of occupation nonequivalent crystallographic positions with titanium cations.

HoFeTi2O7: Synthesis, Peculiarities of the Crystal Structure, and Magnetic Properties

Polycrystalline samples of HoFeTi2O7 were obtained by solid-phase synthesis and investigated using X-ray diffraction, gamma resonance, and SQUID measurements. Characteristics of the structural properties are presented, which give an evidence of the distribution of iron atoms among nonequivalent crystallographic positions and of the nonuniform populating of positions mixed with titanium by the iron atoms. Spin disorder caused by mixing of magnetic and nonmagnetic ions, spatial nonuniformity of the interactions defining the magnetic structure in the crystal lead to formation of concurrent magnetic exchange interactions between the nearest neighbors, frustration of magnetic couplings, and loss of the long-range magnetic ordering. The temperature dependence of the magnetic susceptibility at low temperatures was shown to have some peculiarities typical of the magnetic state of spin glass.

Соединение HoFeTi-=SUB=-2-=/SUB=-O-=SUB=-7-=/SUB=-: cинтез, особенности кристаллической структуры и магнитные свойства

Polycrystalline HoFeTi2O7 samples were obtained by solid-phase synthesis and studied by X-ray diffraction, gamma resonance, and SQUID methods. The characteristics of structural properties are presented, indicating the distribution of iron atoms over nonequivalent crystallographic positions and their uneven filling in the positions mixed with titanium. Spin disorder formed by mixing magnetic and nonmagnetic ions, spatial inhomogeneity of interactions that determine the magnetic structure in the crystal leads to the formation of competing exchange magnetic interactions between the nearest neighbors, frustration of magnetic bonds and the absence of long-range magnetic order. It has been shown that the temperature dependence of the static magnetic susceptibility at low temperatures has features characteristic of the spin glass magnetic state.

The Magnetic Structure of NiFe2O4 Nanoparticles

The Mossbauer studies of the magnetic structure of nickel ferrite (NiFe2O4) nanoparticles (NPs) are presented. It is shown that the redistribution of ions over nonequivalent crystallographic positions occurs upon a decrease in the size of NiFe2O4 crystallites to nanovalues, thus leading to the structural transformation of NiFe2O4 nanoparticles from inverse spinel to mixed spinel. It has been established that the magnetic moments of Fe ions in the volume of NiFe2O4 nanoparticles are ferromagnetically (collinearly) ordered, being noncollinear (or oblique) in the near-surface layer probably due to the frustration of moments in the near-surface layer of nanoparticles.

Structural, thermal and optical properties of elpasolite-like (NH4)2KZrF7

New fluoride compound (NH4)2KZrF7 with the elpasolite structure (sp. gr. Fm-3m) was synthesized by partial cationic substitution in (NH4)3ZrF7 cryolite. Due to small difference in ionic radii of K+ and NH4+, non-equivalent crystallographic positions 8c and 4b are occupied by both cations. Investigations of thermal, optical and structural properties in a wide range of temperature revealed two phase transitions Fm-3m ↔ P42/ncm ↔ P42/nmc of the first order. The results obtained are discussed in comparison with the data for the previously studied related cryolite (NH4)3ZrF7

Electronic Structure and Quadrupole Interactions in Promising Cathode Materials NaxMy(MoO4)3, M = Mn, Fe, Co, and Ni

The electronic structure and the magnetic properties of molybdates NaxMy(MoO4)3 (M = Mn, Fe, Co, and Ni) which are promising materials for sodium batteries have been studied in the framework of the density functional theory with the GGA and GGA+U approximations for the first time. The calculations show that all the compounds are insulators. An important role of the correlation effects, provided by the on-site Coulomb interactions, was established in the formation of the band gap in these compounds. The quadrupole constants of 23Na nuclei are calculated in the nonmagnetic and ferromagnetic states within the GGA and GGA+U approaches. It is shown that the quadrupole frequencies for nonequivalent crystallographic positions of sodium are in different frequency ranges, which allows to study the diffusion of sodium in these compounds by the Nuclear Magnetic Resonance method.

Электронная структура и квадрупольные взаимодействия в перспективных катодных материалах Na-=SUB=-x-=/SUB=-M-=SUB=-y-=/SUB=-(MoO-=SUB=-4-=/SUB=-)-=SUB=-3-=/SUB=-, M=Mn, Fe, Co и Ni

AbstractThe electronic structure and the magnetic properties of molybdates Na_ x M_ y (MoO_4)_3 (M = Mn, Fe, Co, and Ni) which are promising materials for sodium batteries have been studied in the framework of the density functional theory with the GGA and GGA+ U approximations for the first time. The calculations show that all the compounds are insulators. An important role of the correlation effects, provided by the on-site Coulomb interactions, was established in the formation of the band gap in these compounds. The quadrupole constants of ^23Na nuclei are calculated in the nonmagnetic and ferromagnetic states within the GGA and GGA+ U approaches. It is shown that the quadrupole frequencies for nonequivalent crystallographic positions of sodium are in different frequency ranges, which allows to study the diffusion of sodium in these compounds by the Nuclear Magnetic Resonance method.

Магнитная структура наночастиц NiFe-=SUB=-2-=/SUB=-O-=SUB=-4-=/SUB=-

AbstractThe Mössbauer studies of the magnetic structure of nickel ferrite (NiFe_2O_4) nanoparticles (NPs) are presented. It is shown that the redistribution of ions over nonequivalent crystallographic positions occurs upon a decrease in the size of NiFe_2O_4 crystallites to nanovalues, thus leading to the structural transformation of NiFe_2O_4 nanoparticles from inverse spinel to mixed spinel. It has been established that the magnetic moments of Fe ions in the volume of NiFe_2O_4 nanoparticles are ferromagnetically (collinearly) ordered, being noncollinear (or oblique) in the near-surface layer probably due to the frustration of moments in the near-surface layer of nanoparticles.

Structure and Luminescent Characteristics of Ba3Tb(PO4)3 Doped with Eu3+ Ions

It was shown that, unlike the series of phosphates with the compositionM3Ln(PO4)3 (M2+ = Ca, Sr, Ba; Ln3+ = Eu and others) with structures of the eulitine type, characterized by cationic disordering, the luminescence spectra of Ba3Tb(PO4)3 doped with Eu3+ ions depend significantly on the excitation wavelength. This arises from localization of the Eu3+ ions in the Ba3Tb(PO4)3 matrix at two nonequivalent crystallographic positions (Tb, Ba). From study of the excitation spectra and the kinetics of luminescence quenching it was concluded that energy is transferred effectively from the Tb3+ to the Eu3+ during excitation in the UV and visible regions of the spectrum.

Study of the crystalline and magnetic structures of BaFe11.4Al0.6O19 in a wide temperature range

The structural study of barium hexaferrite (BaFe12O19) whose iron ions are partly replaced by diamagnetic aluminum ions (x = 0.6) is carried out with the help of the neutron-diffraction method. Experimental data are acquired in a wide temperature range of 4–730 K via a high-resolution diffractometer, which makes it possible to obtain both precise information on changes in crystalline and magnetic structures and behavioral data on the sample microstructure. BaFe11.4Al0.6O19 retains the magnetoplumbite structure in the whole temperature range and exhibits the “invar effect,” according to which its bulk thermal-expansion coefficient is close to zero at low temperatures (<150 K). The magnetic structure is defined by the Gorter model with magnetic moments oriented along the hexagonal axis. For all nonequivalent crystallographic positions, the magnetic moment of F3+ is close to 4 μB at 4 K. The total magnetic moment per formula unit is 15.6 μB, i.e., less than the nominal value of 20 μB. This is caused by the fact that diamagnetic Al ions are included in the composition. Crystallite microstrains increase insignificantly with temperature due to increasing influence of the magnetic subsystem.

Magnetic and transport properties of Gd7−xYxPd3 (x=0–6) system

Abstract The single crystals of Gd 7− x Y x Pd 3 ( x = 0–6) system were grown by the Czochralski method. All samples crystallized in Th 7 Fe 3 -type of crystal structure. The Gd ions in this crystal structure are located in the triangular configurations in three nonequivalent crystallographic positions. The XRD and XPS examinations allow to determine which positions of gadolinium are occupied by nonmagnetic yttrium for each x value. This preferential occupation of gadolinium positions by the yttrium modifies not only the RKKY interactions between Gd ions but also magnetic frustration and lattice disorder as well. Moreover, it influences the hybridization between d -bands: Gd5 d , Y4 d, and Pd4 d, which determines the density of states at the Fermi level and the magnetic coupling. All above factors influence magnetic, transport, and magnetocaloric properties.

Structure and magnetism of copper-substituted cobalt ludwigite Co3O2BO3

Single crystals of Co3O2BO3 with partial (4%) substitution of Co by Cu ions were synthesized by the flux technique. X-ray diffraction and magnetic studies were carried out. The x-ray diffraction measurements show clearly that Cu preferentially occupies only one of the four nonequivalent crystallographic positions, the one with the smallest electric field gradient. The ferrimagnetic ordering near TN = 43 K and the high magnetic hardness in this magnetic phase are similar to those of Co3O2BO3. Copper substitution causes a small reduction in the effective magnetic moment.

Evolution of the mössbauer spectra of ludwigite Co3 − x Fe x O2BO3 with substitution of iron for cobalt

A concentration series of single crystals of iron-cobalt ludwigites Co3 − x Fe x O2BO3 (x = 0.0125, 0.025, 0.050, 0.10, 1.0) has been synthesized. The structure has been studied using X-ray diffraction and Mossbauer effect. A preferred occupation of nonequivalent crystallographic positions by iron in the ludwigite structure has been revealed. It has been found that the valence of substituting iron ions is three. It has been revealed that the structure of the γ-resonance spectrum of Co2FeO2BO3 is complicated due to a composition disorder in the system.

The luminescence microspectroscopy of Pr3+-doped LiBaAlF6 and Ba3Al2F12 crystals

Abstract The combined luminescence and Raman-scattering microspectroscopy technique allows separate spectroscopic studies on several crystal phases in mixed compounds. A mixed sample of LiBaAlF6 and Ba3Al2F12 doped with Pr was studied. The Pr3+ concentration was found to be much higher in the Ba3Al2F12 phase. Several non-equivalent Pr3+ centers are present in both compounds, however, only in Ba3Al2F12 there are several non-equivalent crystallographic positions of Ba. In LiBaAlF6 non-equivalent Pr3+ centers appear because of complicated scheme for charge compensation of ( Pr Ba 3 + ) + . Possible reasons of Ba3Al2F12 formation in the process of Pr3+:LiBaAlF6 growth are discussed. An excess of LiF in melt is assumed to produce ( Li Ba + ) − antisite defects as a charge compensation for ( Pr Ba 3 + ) + substitution.

Effect of the diamagnetic dilution on the magnetic ordering and electrical conductivity in the Co3O2BO3: Ga ludwigite

Single crystals of cobalt ludwigite Co3O2BO3 with diamagnetic substitution of Ga3+ ions for a part of the cobalt ions have been grown by the flux method. A detailed investigation of the crystal structure and magnetic properties of the compound has been carried out. A preferred character of the occupation of nonequivalent crystallographic positions by gallium has been revealed. It has been found that the effective magnetic moment and the magnetic ordering temperature are decreased compared to those in the original crystal of the Co3O2BO3 ludwigite. It has been noted that the pronounced magnetic anisotropy observed in the crystallographic ab plane of the original material of the Co3O2BO3 composition disappears in the presence of gallium.

Crystal structure and magnetic anisotropy of ludwigite Co2FeO2BO3

Co3O2BO3 and Co2FeO2BO3 single crystals with a ludwigite structure are fabricated, and their crystal structure and magnetic properties are studied in detail. Substituted ludwigite Co2FeO2BO3 undergoes two-stage magnetic ordering at the temperatures characteristic of Fe3O2BO3 (T N1 ≈ 110 K, T N2 ≈ 70 K) rather than Co3O2BO3 (T N = 42 K). This effect is explained in terms of preferred occupation of nonequivalent crystallographic positions by iron, which was detected by X-ray diffraction. Both materials exhibit a pronounced uniaxial magnetic anisotropy. Crystallographic direction b is an easy magnetization axis. Upon iron substitution, the cobalt ludwigite acquires a very high magnetic hardness.

Phase transitions and caloric effects in ferroelectric solid solutions of ammonium and rubidium hydrosulfates

Structural, calorimetric, dielectric, and electrocaloric investigations of Rbx(NH4)1 − xHSO4 ferroelectric solid solutions have been performed. It has been found that rubidium atoms inhomogeneously occupy nonequivalent crystallographic positions in the structure P21/c. The influence of the rubidium concentration on the sequence of phase transitions in the NH4HSO4 compound has been established. It has been revealed that the consequences of the Landau theory can be applied to the description of the temperature dependences of the anomalous heat capacity and the electrocaloric effect over a wide range of temperatures. Comparative evaluations of the electrocaloric and barocaloric effects in hydrosulfate and triglycine sulfate crystals have been carried out.