Disorder In Sublattice Research Articles

Thermodynamic properties of LaF3 superionic crystals near their phase transition

We examine the behavior of the entropy (S d), enthalpy (H), and concentration of disordered fluoride ions (n) in LaF3 in a broad temperature range around its critical temperature using a model for structural lattice disordering. We analyze the relationship between the free energy of a crystal and LaF3 lattice disordering parameters: entropy, S d; anion sublattice disordering energy, E a; and the concentration of vacancy-interstitial defect complexes, n. H(T), n(T), and S d(T) data are obtained for the phase transition region.

Ionic mobility in heptafluorozirconantes with a mixed cationic sublattice as probed by 1H and 19F NMR

The ionic mobility in heptafluorozirconates (NH4)2.7Rb0.3ZrF7 and (NH4)2.75Cs0.25ZrF7 has been studied by 1H and 19F NMR in the temperature range 150–430 K. The types of ion motion were determined and their activation energies were estimated. A phase transition results in a modification in which diffusion in the ammonium sublattice and orientational disorder of ZrF73− anions are observed. Owing to diffusion of ammonium ions, the compounds have relatively high ionic conductivity (σ ≥ 5 × 10−5 S/cm at 420 K).

Photoluminescence, FTIR and X-ray diffraction studies on undoped and Al-doped ZnO thin films grown on polycrystalline α-alumina substrates by ultrasonic spray pyrolysis

Undoped and aluminum-doped zinc oxide (ZnO) thin films have been grown on polycrystalline α-alumina substrates by ultrasonic spray pyrolysis (USP) technique using zinc acetate dihydrate and aluminum chloride hexahydrate (Al source) dissolved in methanol, ethanol and deionized water. A number of techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and photoluminescence (PL) were used to characterize the obtained ZnO thin films. It was seen that the orientation changed with increase in substrate temperature. During the ZnO deposition Zn source reacted with polycrystalline α-Al 2O 3 substrate to form an intermediate ZnAl 2O 4 spinel layer. It has been interestingly found that the intensity of green emission at 2.48 eV remarkably increased when the obtained ZnO:Al films were deposited at 380 °C. The FTIR absorbance intensity of spectroscopic band at 447±6 cm −1 is very sensitive to oxygen sublattice disorder resulting from non-stoichiometry, which is consistent with the result of PL characterization.

Origin of Colossal Ionic Conductivity in Oxide Multilayers: Interface Induced Sublattice Disorder

Oxide ionic conductors typically operate at high temperatures, which limits their usefulness. Colossal room-temperature ionic conductivity was recently discovered in multilayers of yttria-stabilized zirconia (YSZ) and SrTiO3. Here we report density-functional calculations that trace the origin of the effect to a combination of lattice-mismatch strain and O-sublattice incompatibility. Strain alone in bulk YSZ enhances O mobility at high temperatures by inducing extreme O disorder. In multilayer structures, O-sublattice incompatibility causes the same extreme disorder at room temperature.

Synthesis, structural and magnetic disordering in the IrSr2RECu2O8+x family of metalo-cuprates by HP+HT oxidation

In this work, we report the preparation of both ordered and disordered phases in the IrSr2RECu2O8 family by high pressure and high temperature oxidation. This chemical oxidation process is accompanied by a disordering of the metal sublattices of the triple perovskite cell (i.e. IrSr2RECu2O8) to an AA′ and BB′ disordered simple perovskite (Sr2RE)(Cu2Ir)O9−δ. It is especially relevant that both phases have exactly the same cation stoichiometry but different oxygen content. The disordering induced in the cation sublattices has dramatic consequences in the magnetic properties of these compounds.

Magnetic properties and magnetocaloric effect in Gd(Ni 1− xCo x) 3 intermetallic compounds

In this paper, we present magnetic properties of polycrystalline Gd(Ni 1− x Co x ) 3 intermetallic compounds. The partial replacement of Ni by Co atoms causes the increase in Curie temperature T C. As it was also shown saturation magnetization of the examined samples monotonically decreases with increasing Ni/Co substitution. The magnetocaloric effect (MCE) for several samples with T C values around the room temperature was estimated from magnetic isotherms. The decrease in magnetic entropy connected with the increase in magnetic disorder in 3d magnetic sublattice has been observed.

High-resolution optical spectroscopy ofTm3+ions inLiYF4: Crystal-field energies, hyperfine and deformation splittings, and the isotopic structure

High-resolution optical Fourier spectroscopy was used to study the energies and widths of the crystal-field (CF) levels, hyperfine and deformation level splittings, and isotopic effects in the ${\text{LiYF}}_{4}:{\text{Tm}}^{3+}$ single crystals. We present the corrected sets of the CF levels and CF parameters for the ${\text{Tm}}^{3+}$ ion in ${\text{LiYF}}_{4}$. The observed fine structure of spectral lines is shown to be caused by the hyperfine interaction, random lattice deformations, and isotopic disorder in the lithium sublattice. From a comparison between the observed and calculated fine structures we determine the characteristics of the random lattice deformations in highly diluted activated crystals and obtain an estimate of the fluorine displacements $(\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\text{ }\text{nm})$ in the nearest surrounding of the mass defect at the lithium site.

High temperature antistructure disorder in undoped ZnS

High temperature electrical conductivity (HTEC) under defined component vapor pressure was investigated in undoped ZnS. The appearance of different slopes on HTEC isotherms can be explained by a two-carrier conduction mechanism involving electrons and holes. The experimental data at sulphur vapor pressure can be explained by the inclusion of abnormal site occupation i.e. by antistructural disorder in Zn sublattice. The native defects for compensation of these defects may be two-fold ionized zinc vacancies. Antistructure disorder disappears with increasing of Zn vapor pressure. The method for solving the system of equations of quasichemical reactions without approximation was used because slopes of experimentally measured HTEC isotherms varied with component vapor pressure. High temperature defect equilibrium (HTDE) preliminary model containing antistructure disorder is proposed and compared with HTDE models for CdS and for CdTe.

Order-Disorder Transition and Phase Separation in the MgB2 Metallic Sublattice Induced by Al Doping.

MgB2 is a superconductor constituted by alternating Mg and B planar layers: doping of both the sublattices has been observed experimentally to destroy the outstanding superconductive properties of this simple material. In this study we present the investigation by first principles methods at atomistic scale of the phase separation induced by aluminum doping in the MgB2 lattice. The calculations were performed by Density Functional Theory in generalized gradient approximation and pseudopotentials. Orthorhombic oP36 supercells derived by the primitive hR3 MgB2 cell were built in order to simulate the aluminum-magnesium substitution in the 0-50% composition range. The computational results explained the occurrence of a phase separation in the Mg1-xAlxB2 system. The miscibility gap is predicted to be induced by an order-disorder transition in the metallic sublattice at high Al concentration. Indeed at 1000 K aluminum substitution takes place on random Mg sites for concentration up to 17% of the total metallic sites, whereas at Al content larger than 31% the substitution is energetically more favorable on alternated metallic layers (Mg undoped planes alternate with Mg-Al layers). The formation of this Al-rich phase lead at 50% doping to the formation of the double omega Mg1/2Al1/2B2 ordered lattice. From 17 to 31% the two phases, the disordered Mg1-xAlxB2 (x < 0.17) and the ordered Mg1/2+yAl1/2-yB2 (y < 0.19) lattices, coexist. This phase separation is driven by the balance of the enthalpy and entropy contributions to the Gibbs energy. Present DFT-GGA calculations indicate that this thermodynamically predicted suppression of the Al doping disorder in the metallic sublattice of MgB2 occurs in parallel with the collapse of the superconductive properties of the material.

Influence of Fe substitution on the structure and magnetic properties in Gd(Ni 1− xFe x) 3 intermetallic compounds

In this paper, we present crystal structure, electronic structure and magnetic properties of pollicrystalline Gd(Ni 1− x Fe x ) 3 intermetallic compounds. The rhombohedral PuNi 3 type of crystal structure for the whole series was confirmed with the use of X-ray powder diffraction. The partial replacement of Ni by Fe atoms up to x ≤ 0.4 causes the increase in Curie temperature T C and next for 0.4 < x ≤ 1 a slow decrease in T C value. As it was also shown saturation magnetization of the examined samples monotonically decreases with increasing Ni/Fe substitution. The magnetocaloric effect (MCE) for several samples with T C values around the room temperature was estimated from magnetic isotherms. The decrease in magnetic entropy connected with the increase in magnetic disorder in 3d magnetic sublattice has been observed. Moreover some correlations between magnetic properties and electronic structure studied by X-ray photoemission spectroscopy (XPS) have been found.

Amorphization and dynamic annealing of hexagonal SiC upon heavy-ion irradiation: Effects on swelling and mechanical properties

Structural, mechanical, and dimensional evolutions of silicon carbide (SiC) induced by heavy-ion irradiations are studied by means of Rutherford backscattering spectrometry and channeling (RBS/C), nanoindentation, and surface profilometry measurements. 4H- and 6H-SiC single crystals were irradiated with 4 MeV Au2+ and 4 MeV Xe+ ions at room temperature (RT) or 400 °C. Using a Monte Carlo program to simulate the RBS/C spectra (MCCHASY code), we find that Au ion irradiation at RT induces a total silicon sublattice disorder related to full amorphization at a dose of about 0.4 displacement per atom (dpa). A two-step damage process is found on the basis of the disordered fractions deduced from RBS/C data. Complete amorphization cannot be reached upon both Au and Xe ion irradiations at 400 °C up to about 26 dpa because of the dynamic annealing of defects. When complete amorphization is reached at RT, the Young’s modulus and Berkovich hardness of irradiated 6H-SiC samples are lower by, respectively, 40% and 45% than those of the virgin crystals. The out-of-plane expansion measured by surface profilometry increases versus irradiation dose and the saturation value measured in the completely amorphous layer (normalized to the ion projected range) is close to 25%. We show that the modifications of the macroscopic properties are mainly due to the amorphization of the material. The macroscopic elasticity constants and dimensional properties are predicted for a composite material made of crystalline matrix containing dispersed amorphous inclusions using simple analytical homogenization models. Voigt’s model seems to give the best approximation for disordered fractions larger than 20% in the second step of the damage process.

Multi-range high-frequency EPR spectroscopy of LiYF4 and LiLuF4 crystals doped by rare-earth ions

EPR spectra of isostructural LiYF4 and LiLuF4 crystals doped by Dy3+, Er3+, and Ho3+ ions are measured at 4.2 K in the frequency range 40–800 GHz. The effects caused by isotopic disorder in the lithium sublattice, the random crystal field, and the interaction between paramagnetic impurity ions are detected and studied. The results of the measurements are used to determine the spectral characteristics of the compounds and the crystal field parameters. It is demonstrated that the formation of the isotope structure of the EPR signal is dominated by local deformations of the crystal lattice induced by mass defects.

Metal-Insulator Transition by Isovalent Anion Substitution inGa1−xMnxAs: Implications to Ferromagnetism

We have investigated the effect of partial isovalent anion substitution in Ga1-xMnxAs on electrical transport and ferromagnetism. Substitution of only 2.4% of As by P induces a metal-insulator transition at a constant Mn doping of x=0.046 while the replacement of 0.4% As with N results in the crossover from metal to insulator for x=0.037. This remarkable behavior is consistent with a scenario in which holes located within an impurity band are scattered by alloy disorder in the anion sublattice. The shorter mean free path of holes, which mediate ferromagnetism, reduces the Curie temperature T_{C} from 113 to 60 K (100 to 65 K) upon the introduction of 3.1% P (1% N) into the As sublattice.

Spectral signatures of hyperfine and isotopic effects and of Tm 3+–Tm 3+ pairs in LiYF 4:Tm

We report on the high-resolution optical Fourier-transform spectroscopy of the LiYF 4:Tm 3+ crystals. Splitting of several lines in the optical low-temperature polarized spectra was observed. We show that these splittings are caused by (i) the hyperfine interaction, (ii) the isotopic disorder in the lithium sublattice, and (iii) the interionic interaction between neighboring Tm ions. It is the first observation of the hyperfine splitting in the spectra of the Tm 3+ ions in crystals. From the experimentally measured hyperfine splitting we evaluate the magnetic field at the thulium nucleus and calculate the magnetic g-factors of the excited crystal-field levels.

Bulk nanocrystalline superconducting YBa2Cu3O7-x

A method to fabricate high density nanocrystalline superconducting YBa2Cu3O7-x(YBCO) is presented in which commercially available YBCO powder is ball milled and then hot isostatically pressed (HIP'ed) at 0.2 GPa and 400 C. Ball milling decreased the average particle size from 2 μm to ~ 20 nm within 2 hours and to ~ 4 nm after milling for 30 hours with no noticeable strain. Ball milling increases the oxygen sublattice disorder and the Y/Ba anti-site disorder and drives the orthorhombic YBCO phase towards a disordered metastable cubic phase Y1/3Ba2/3CuO3-x consistent with the work by Simoneau et al. [14, 19]. Powders milled for 30 hours were HIPed at temperatures from 400 C to 550 C. At 450 C and above, the powder decomposed into the parent oxides. At 400 C, the metastable cubic phase reordered to form bulk YBCO with an average grain size of 5 - 18 nm and a relative mass density of ~ 96%. The bulk YBCO material has an onset melting temperature and associated enthalpy of 970 °C and 157 J.g-1 respectively which can be compared to 965 °C and 156 J.g-1 for the starting commercial powder. These results are consistent with XRD data and they suggest a predominantly single-phase nanocrystalline YBCO has been fabricated. The superconducting transition temperature (TC) of the nanocrystalline YBCO determined from ac. susceptibility measurements is 88.8 K, with a slope of upper critical field near TC of ~ 0.12 T.K-1.

Oxide Ion Disorder in Nd2Hf2O7

The compound Nd2Hf2O7 was synthesized via a solid‐state route and the crystal structure determined by electron diffraction and Rietveld refinement of both X‐ray and neutron diffraction data. A small amount of disorder was found on the anion sublattice, while no evidence of disorder in the cation sublattice could be verified. The compound is cubic with space group Fd3m, structurally lying between the ideal pyrochlore and fluorite forms, with 0.26% of the oxide ions residing on the 8a sites usually vacant in pyrochlores. The lattice constant is 10.6469(2) Å, and the overall fit parameter was Rwp=0.0398.

Temperature Behaviour of Nanoclusters in Mixed Crystals Hg2(Br,I)2

The structural and polar nanoclusters have been found and studied out in model ferroelastics—mixed Hg 2 (Br,I) 2 crystals that were comprehensive investigated. The structural clusters are the nuclei of the low-temperature orthorhombic phase in the high-temperature tetragonal matrix, observed in a diffuse X-ray scattering at X-points of a Brillouin zone. They are related to phase transition and induced by fluctuations of an order parameter. The polar clusters, found out as quasi-local vibrations in a low temperature Raman-spectra, are due to disorder in an anionic sublattice and are induced by the polar mixed Br-Hg-Hg-I molecules and their proximate environment, which form ferroelectric and antiferroelectric nanoclusters.

The structure and conductivity of new fluorite-type Bi 2O 3–Er 2O 3–PbO materials

Fluorite-type fcc phases have been synthesised in the system Bi 2O 3–Er 2O 3–PbO by solid state reaction, and a partial air-quenchable domain of the fluorite-type phase has been established. Some of these materials display high oxide ion conductivities, notably (BiO 1.5) 0.80(ErO 1.5) 0.11(PbO) 0.09 and (BiO 1.5) 0.85(ErO 1.5) 0.12(PbO) 0.03, which have conductivities of 0.49 and 0.72 S cm − 1 at 750 °C, respectively, placing them among the most conductive Bi 2O 3-based materials. Conductivity was found to increase with increasing Pb 2+/Er 3+ ratio and decreasing (Er 3+ + Pb 2+)/Bi 3+ ratio. Positional disorder in the oxide ion sublattice was characterised by neutron powder diffraction. At room temperature, the oxide ion sublattice appeared to be completely disordered, with oxide ions only in 32 f and 48 i sites, and changes in occupancy with increasing Pb 2+/Er 3+ and (Er 3+ + Pb 2+)/Bi 3+ ratios were not significant. At 700 °C, there appeared to be oxide ions in 8 c sites for the material (BiO 1.5) 0.80(ErO 1.5) 0.11(PbO) 0.09, with a correspondingly smaller occupancy of the 32 f sites, whilst the occupancy of the 48 i sites had not changed significantly.

Nonstoichiometric fluorides—Solid electrolytes for electrochemical devices: A review

The solid electrolytes with fluorine-ion conductivity that were revealed during the analysis of the phase diagrams of the MF m -RF n systems within the program of search for new multicomponent fluoride crystalline materials carried out at the Shubnikov Institute of Crystallography, Russian Academy of Sciences, are described. The most widespread and promising materials are the nonstoichiometric phases with fluorite (CaF2) and tysonite (LaF3) structures, which are formed in the MF2-RF3 systems (M = Ca, Sr, Ba, Cd, or Pb; R = Sc, Y, or La-Lu). These phases have superionic fluorine conductivity due to the anion sublattice disorder. The ionic conductivity of crystals of both structure types has been studied and the limits of its change with composition and temperature are determined. Nonstoichiometric fluorides are used as solid electrolytes in chemical sensors, fluorine sources, and batteries. The prospects of the use of fluorine-ion conductors in solid-state electrochemical devices, principles of their operation, and the problems of optimization of their composition are discussed.

Investigation into the influence of the degree of cation ordering on the magnetic properties of 1:1 double perovskites in the framework of the Heisenberg model

The influence of disorder in the cation sublattice on the magnetic properties of 1: 1 double perovskites is investigated in the framework of the Heisenberg model. It is revealed that the dependence of the sublattice magnetization on the degree of atomic ordering for strontium ferromolybdate exhibits a linear behavior. The results of simulation allow the assumption that disordered lead ferroniobate has a spin-glass structure.