Quantum-chemical modeling of the electronic structure and magnetic properties of Sn1 − x − y M x Sb y O2, M = Cr, Mn, Co, or Ni (x = 0.25; y = 0, 0.25)

The electronic and magnetic structures of the Sn_0.75 M _0.25O₂ and Sn_0.5 M _0.25Sb_0.25O₂ (M = Cr, Mn, Co, Ni) compounds with a structure that is derivative of the rutile structure are modeled using the ab initio spin-polarized tight-binding linear muffin-tin orbital (TB-LMTO) method. The magnetic moments of the transition metal atoms are calculated. The data obtained are used to analyze the influence of the composition of Sn_{1 ? x ? y} M _x Sb _y O₂ phases on their electronic spectra and the magnetic and transport characteristics.     

Superionic conductivity of the heterovalent solid solutions R 1−x M x F 3−x (R=REE, M=Ca, Ba) with tysonite-type structure

The fluorine-ion conductivity of anion-deficient solid solutions R _1−x Ca_xF_3−x and R _1−x Ba_xF_3−x having the tysonite (LaF₃) structure was investigated by the impedance spectroscopy method. R _1−x Ca_xF_3−x (R=La, Pr, Nd, Sm, Gd, Tb, Dy, Ho) and R _1−x Ba_xF_3−x (R=La, Pr, Nd) single crystals were grown from the melt by the Bridgman-Stockbarger method. The electrophysical measurements were performed in the frequency range 5−5×10⁵ and temperature range 300–700 K. The temperature dependences of the electrical conductivity for the crystals studied is determined by the migration of fluorine anions along various structural positions. It is shown that, from the standpoint of increasing the conductivity of tysonite matrices RF₃ (R=La, Pr, Nd), doping by CaF₂ and BaF₂ is less promising than SrF₂. Fiz. Tverd. Tela (St. Petersburg) 41, 638–640 (April 1999)     

Mössbauer Study of La1−x Ca x Mn1−y 57Fe y O3 with x=0,0.25; y=0.01

Temperature dependent Mössbauer measurements are done on the samples of La_1–x Ca _x Mn_1–y ⁵⁷Fe _y O₃ with x=0 and 0.25, and y=0.01. With decreasing temperature, the specimen with x=0.25 shows a paramagnetic to ferromagnetic transition around 175 K. In the specimen x=0.0, the temperature dependence of both the center shift () and the recoilless fraction (f) can be fitted very well with the Debye theory with a _D=320±50 K. But for the specimens with x=0.25, f and show distinct deviations from the Debye behavior in the temperature range in which the resistivity shows a sharp decrease. Dips observed in both the f and around the transition temperature suggest that the Jahn–Teller distortion observed in these systems is dynamic in nature.     

Mössbauer study of hydrided amorhous Fe−M−Zr (M=Co,Cr) alloys

Mössbauer spectra of hydrided amorphous Fe₉₀?_xCo_xZr₁₀ (A_x) and Fe₉₀?_yCr_yZr₁₀ (B_y) (x=4, 10, 20; y=0, 4, 7, 13, 16, 20) are studied. For low hydriding time values (t), the average hyperfine field \(\left( {\bar B_{hf} } \right)\) and isomer shift δ_is show a drastic increase which can mainly be associated with the volume effect. For higher (t) values, an electron transfer effect can be responsible for the hyperfine parameter variation. The influence of hydrogenation on magnetic anisotropy is also discussed.     

Electronic structure and magnetic properties of RCo5−xMx (R=Y,Pr and M=Al,Si) system

Detailed theoretical and experimental investigations of the electronic and magnetic properties of the RCo_5?xM_x compounds (R=Y, Pr and M=Si, Al) have been performed. All theoretical investigations of the electronic and magnetic properties of the system have been done using the Korringa–Kohn–Rostoker (KKR) band structure method. The Si for Co substitution in RCo₅ does not change the magnetic ordering: the RCo_5?xSi_x with R=Y, Nd and Pr is ferromagnetic, whilst the heavy rare-earth containing compounds are ferrimagnetic. The important modifications induced by this substitution concerns the magnetic properties of the system: the Curie temperature and the magnetic moments of Co decrease with Si content, indicating the weakening of the Co–Co exchange interaction. The band structure calculations evidence the hybridization between the 3d electronic states of Co and the 3p states of Si as possible reason for the diminishing of Co–Co exchange interaction. Also, the volume effect on the magnetic properties of the YCo₄Si was investigated using theoretical methods. The results are compared with the experimental measurements in order to distinguish the origin of magnetization reduction in YCo₄Si compared with YCo₄Al.     

Luminescence properties of Sr3−x−3y/2MxCeyAlO4F (M=Ca,Ba, 0≤x≤0.9, 0.001≤y≤0.05) phosphors

Luminescent materials composed of Sr_3?x?3y/2M_xCe_yAlO₄F (M=Ca, Ba, 0≤x≤0.9, 0.001≤y≤0.05) were prepared by the solid-state reaction method. X-ray diffraction (XRD) patterns of the obtained oxyfluorides are exhibited for indexing peak positions. Dynamic excitation and emission spectra of the Ce³⁺-activated oxyfluoride phosphors are clearly monitored. The critical emission quenching as a function of Ce³⁺ contents in Sr_2.5?3y/2M_0.5Ce_yAlO₄F phosphors is revealed at quite low concentrations of the activator. CIE coordinates of blue and green Sr_2.5?3y/2M_0.5Ce_yAlO₄F phosphors are clearly measured. The relative quantum efficiency of Sr_2.4985Ca_0.5Ce_0.005AlO₄F based on the integrated emission is determined. The Sr_3?x?3y/2M_xCe_yAlO₄F phosphors excited near 410 nm light could be prominent phosphors in applications of NUV-LED.     

Influence of structurization of amorphous metallic alloys Al87Y5 − x Gd x Ni8 − y (x = 0, 1, 5; y = 0, 4) on their mechanical properties

Amorphous metallic alloys (AMAs) Al₈₇Y₅Ni₈, Al₈₇Gd₅Ni₈, Al₈₇Y₄Gd₁Ni₈, Al₈₇Y₄Gd1Ni₄Fe₄, and Al₈₇Gd₅Ni₄Fe₄ produced by melt spinning on a cooled substrate have been studied. Based on the data of differential scanning calorimetry and X-ray diffraction, the activation energies of individual stages of crystal-lization, the sizes and volume fractions of nanocrystals in an amorphous matrix have been calculated. The alloying of AMAs with Y or Gd is shown to lead to the formation of particles with sizes of 9–15 nm at the first stage of nanocrystallization, which provides their high microhardness.     

Optical coefficients of Hg1 − x − y Cd x Eu y Se crystals

Optical properties of Hg_{1 ? x ? y} Cd _x Eu _y Se crystals grown by the Bridgman method have been investigated based on the independent reflectance and transmittance measurements, which were performed on a Nicolet 6700 spectrometer at T = 300 K in the wavelength range 0.9 ≤ λ ≤ 26.6 μm. The values of refractive index n, absorption index k, and absorption coefficient α have been determined for the crystals studied. Based on the dependences α = f(hν), the presence of direct allowed interband optical transitions in the crystals is established and the band-gap values are determined. The influence of temperature on the transmittance and band gap are investigated in the range T = 114–300 K.     

Effects of Co substitution on structural and magnetic properties of R3(Fe1−xCox)29−yVy (R=Tb,Dy)

Synthesis of two novel series of intermetallic compounds Tb₃(Fe_1−xCo_x)_27.4V_1.6 (x=0,0.1, 0.2, 0.3, 0.4) and Dy₃(Fe_1−xCo_x)_27.8V_1.2 (x=0, 0.1, 0.2, 0.3) with the monoclinic Nd₃(Fe,Ti)₂₉-type structure (3:29) is presented. In the Dy series for x=0.4 a disordered variant of the hexagonal Th₂Ni₁₇-type structure is formed. The cell parameters decrease and the Curie temperature increases with increasing of the Co content. In the case of the Tb₃(Fe_1−xCo_x)_27.4V_1.6 series in the M(T) curve a magnetic transition is observed which is attributed to spin reorientation phenomena. This critical temperature decreases with increasing Co from 473 K for x=0.1 to 393 K for x=0.3, and was not observed in the case of 0.4. XRD patterns of magnetically aligned powder samples reveal the presence of a tilted magnetic structure.     

Substituent effects on gas-phase acidities of formic acid and its silicon and sulphur derivatives R − M(= X)XH(M = C., Si; X = O,S; R = H,F, CI,OH, NH2 and CH3)

Ab initio molecular orbital methods at the CBS-Q level of theory have been used to study the effect of substituent (F, Cl, NH₂, OH and CH₃) on the gas-phase acidities of formic acid, HCOOH, its silicon and sulphur derivatives R-M(= X)XH(M = C., Si; X = 0, S; R = F, Cl, OH, NH₂ and CH₃). For formic acid and its thio and dithio derivatives the acidity changes upon substitution are irregular and depend on both the type of substituent, position and degree of replacement of oxygen atoms by sulphur atoms. For sila carboxylic acids and their thio and dithio derivatives the calculated acidities regularly increase in the order: R-SiOOH < R-Si(=S)OH ? R-Si(=O)SH < R-SiSSH(R = H, F, Cl, OH, NH₂ and CH₃). The chloro derivatives are the strongest among the acids studied. The highest gas phase acidity (1277.6 kJmol^?1) has been calculated for ClC(=S)OH.     

Evaluation of the electrical capacitance of M,O2/O2− interfaces (M=Pt, Au, Pd, In2O3; O2− = zirconia based electrolyte) exhibiting constant phase element behavior

The behavior of the electrode systems M,O₂/O² (M = porous Pd, Pt, A and dense In₂O₃; O²⁻ = ZrO₂-based single-crystal solid electrolyte) was studied by means of impedance measurements. The examination of the Pt,O₂/O²⁻ electrode system showed that the constant phase element (CPE) can be attributed to a nonuniform distribution of current at the electrode surface. It was observed that the CPE parameters n and B in the expression Y_CPE = B (jω)ⁿ may be related by B=(C_dl)ⁿ (R_Ω)_n-1, where C_dl is the double layer capacitance and R_Ω the resistance of the electrolyte in the cell. Then, C_dl of the electrode - electrolyte interface could be determined. The specific C_dl of the oxidized noble metals and india electrodes is nearly one order of magnitude lower than C_dl of the electrodes in the metallic state. The C_dl value of all the electrodes studied depends little or is independent of temperature and oxygen pressure. It is concluded that the Helmholtz model of double layer structure does not contradict the C_dl behavior.     

Structure and magnetic properties of RFe13−xSix (R = Pr,Nd or Gd,x = 2.5–5)

RFe_13−xSi_x (R = Pr, Nd and Gd) alloys with x = 2.5–5 have been synthesized and characterized in a magnetic field up to 17 kOe and in a temperature range 10–1173 K to ascertain whether they might be useful as high temperature, high-energy permanent magnet materials. It was found that a body-centered tetragonal (BCT) Ce₂Ni₁₇Si₉-type structure forms in PrFe_13−xSi_x alloys when x ⩾ 4. The Curie temperatures T_c of this BCT phase are in a range 50–90 K, higher than that of the corresponding PrCo_13−xSi_x BCT phase (∼ 20 K). The PrFe_13−xSi_x alloys with x⩾ 4 show spin reorientation at cryogenic temperatures (15–47 K) and exhibit significant coercivity in loose powder samples below their spin-reorientation temperature. Using the information about the magnetization of LaFe_13−xSi_x BCT alloys, one can estimate the moment of Pr ion in PrFe_13∼-xSi_x alloys. It is found to be very close to that in PrCo_13−xSi_x BCT alloys, around 2 μ_2/atom. For RNd or Gd, the RFe_13−xSi_x alloys occur only as mixtures of RFe₂Si₂, R(Fe, Si)₁₁ and FeSi. The Ce₂Ni₁₇Si₉-type structure cannot be formed in these alloys even when x = 5. The low T_c for the PrFe_13−xSix alloys precludes their use as a permanent magnet material, except perhaps at low temperature.     

Structure and properties of the anions MF4−, MCl4− and MBr4− (M = C,Si, Ge)

Density functional theory (DFT), Møller–Plesset (MP2) and coupled cluster with single and double substitutions including non-iterative triple excitations (CCSD(T)) calculations on the anions MX₄^?, with M = C, Si, Ge and X = F, Cl, Br, show that GeF₄^?, SiCl₄^?, GeCl₄^? and SiBr₄^? prefer a C_2v conformation, but CCl₄^? is an elongated C_3v structure. CBr₄^? has T_d symmetry in MP2, but is slightly more stable in elongated C_3v form with DFT and CCSD(T). GeBr₄^? has T_d symmetry. CF₄^? and SiF₄^? are unstable with respect to loss of an electron. Vertical electron affinities (EAs) are negative also for CCl₄ and SiCl₄, and close to zero for GeF₄ and SiBr₄. Adiabatic EAs range from 0.47 eV for SiCl₄ to 1.78 eV for GeBr₄. The lowest excited states at T_d symmetry are ²T₂ resonances with energies of 2.1–3.5 eV, resulting from excitation of the a₁ singly occupied molecular orbital to vacant t₂ orbitals. Vertical excitation energies (VEEs) and vibrational frequencies are given for the most stable anionic geometries. Comparison with experimental VEEs for CCl₄^? is made. From dissociation energies of MX₄, MX₄^?, MX₃ and MX₃^?, appearance energies of X^?, MX₃^?, X₂^? and MX₂^? were calculated. Most were found to be in reasonable agreement with experimental values. Theoretical spin densities and g-factors have been compared with experimental results available for CCl₄^?, SiCl₄^? and GeCl₄^?.     

Al,F-doped new perovskite lithium fast ion conductor Li3x La2/3−x□1/3−2x Ti1−y Al y O3−y F y (x=0.11)

Al, F-doped new perovskite lithium ion conductors (x=0.11) have been prepared by solid state reaction. It is found that a pure perovskite-structured phase with space group of P4mm(99) exits in the composition range of 0<y≤0.10. The sample with y=0.02 possesses the highest ionic conductivity of 1.06×10⁻³ S/cm at room temperature, and its decomposing voltage is 2.3 V. The factors affecting the conductivity of this system are discussed.     

Hyperfine interactions in R x Gd1− x Fe3 intermetallics R = Tb,Y

The structural and magnetic properties of rare earth iron intermetallic compounds Tb _x Gd_1?x Fe₃ and Y _x Gd_1?x Fe₃ (x = 0. 0, 0. 1, 0. 2, 0. 4, 0. 5, 0. 6, 0. 8, 1. 0) was studied by X-ray diffraction, the ⁵⁷Fe Mössbauer effect and SQUID measurements. All investigated compounds crystallize in the rhombohedral PuNi3-type of crystal structure. The investigation of magnetic properties of R _x Gd_1?x Fe₃ proved their ferrimagnetic behavior. The Curie temperature of the investigated compounds decreases with the increase of R concentration from 721K (GdFe₃) to 655K (TbFe₃) and 533K (YFe₃). The saturation magnetic moment MS in the R _x Gd _1?x Fe₃ system increase with x parameter. The Mössbauer spectra are analyzed using four sextets, corresponding to three crystallographically (b, c, h) and four magnetically (b, c, h₁, h₂) inequivalent sites for iron. The mean hyperfine magnetic field increases with increase of the Gd concentration     

Crystal structures, phase relationships, and magnetic phase transitions of RsM4 compounds （R = rare earths, M = Si, Ge）

Our recent studies of the crystal structures, phase transitions, and magnetic properties of intermetallic compounds RsM4 （R = rare earths; M = Si, Ge） are reviewed briefly. First, crystal structures, phase relationships, and magnetic prop- erties of several 5：4 compounds, including Nd5 Si4-xGex, Pr5 Si4_xGex, Gds-xLaxGe4, La5 Si4, and Gd5 Sn4, are presented. In particular, the canted spin structures as well as the magnetic phase transitions in PrsSi2Ge2 and PrsGe4 investigated by neutron powder diffractions and small-angle neutron scattering are reviewed. Second, the crystal structures and magnetic properties of the most studied compounds Gds（Si,Ge）4 are summarized. The focus is on the parent compound GdsGe4, which is an amazing material exhibiting magnetic anisotropy, angular dependent spin-flop transition, metastable magnetic response, Griffiths-like phase, thermal effect under pulsed fields, antiferromagnetic and ferromagnetic resonances, pro- nounced effects of impurities, and high-field induced magnetic transitions.     

Tellurium-125 Mössbauer spectroscopy study of molybdenum metal tellurides of composition Mo6−x M x Te8 (M=Ru,Rh)

Tellurium-125 Mössbauer spectra have been recorded from some recently reported superconducting compounds Mo_6?x Ru _x Te₈ (x=0, 0.5, 1.0, 1.5) and Mo_5.5Rh_0.5Te₈. The spectra are characterised by single lines which probably contain contributions from the crystallographically dissimilar tellurium sites in these compounds. The trends in the linewidth and chemical isomer shift data are consistent with the enhanced polarisation of tellurium 5p electrons with increasing ruthenium content in the compounds. The results can be related to the nature of chemical bonding in the cluster of metal atoms.     

Tunneling and point-contact spectroscopy of high-T c superconductors M−Ba2Cu3O7 (M=Y,La, Eu)

Tunneling and point-contact measurements have been performed on different high-T _c superconductors M–Ba₂Cu₃O₇ with M=Y, La, Eu. The average energy gaps deduced from the tunneling spectra are about 2=33 meV for the Y- and La-type samples and about 2=38 meV for the Eu-type samples, which give ratios 2/k _B T _c in the range from 4 to 6. The point-contact characteristics show a distinct minimum in the differential resistance about zero bias and additional sharp spikes up to 40 mV. From these pointcontacts we can give an estimate of the critical current, yielding values in the range from 0.2 to 3 mA.     

Diffusion of Fe,Co, Nd,and Dy in R2(Fe1−xCox)14B where R=Nd or Dy

Transition metal and rare earth diffusion coefficients at 1323 K in Dy_2−yNd_y(Fe_1−xCo_x)₁₄B were determined by field emission energy dispersive spectroscopy compositional analysis of diffusion couple specimens. Various arrangements of component materials and temperatures were examined in order to understand the mechanisms affecting diffusion of the components and to predict the stability of functionally graded microstructures consisting of a dysprosium-rich (Dy_2−yNd_y(Fe_1−xCo_x)₁₄B) outer layer and a neodymium-rich (Nd₂(Fe_1−xCo_x)₁₄B) interior. Estimates of the mutual interdiffusion coefficients of Dy, Nd, Fe, and Co in this system were obtained from the preparation of arc melted and annealed polycrystalline specimens, assuming that the diffusion coefficients were independent of concentration (Grube solution). Fifteen diffusion couples were prepared and heat treated at 1323 K for various times in order to provide data for calculation of the diffusion coefficients. The results indicate that the diffusion coefficients of Fe and Co (D_Fe=3.28×10⁻¹⁰ cm²/s and D_Co=7.63×10⁻¹⁰ cm²/s) were significantly higher at 1323 K in this system than those for Dy and Nd (D_Nd=2.3×10⁻¹² cm²/s and D_Dy=2.9×10⁻¹² cm²/s).