Short-range order and F ion diffusion inside the Pb1 − xAlxF2 + x solid solution Part I: F-NMR investigation

A ¹⁹F-NMR investigation of the Pb_{1 − x}Al_xF_{2 + x} (0 ≤ x ≤ 0.12) solid solution and of the ordered Pb₉Al₂F₂₄ phase is undertaken as a function of temperature. Several F⁻ ion groups become progressively mobile at the NMR time scale at increasing temperature, the first, (F_i)_pb and (F_n)_pb, are located in the surroundings of Pb²⁺ cations, the second ones, (F_rd)_AI and (F_ra)_Al, in that of Al³⁺ cations. Four types of fluoride ions are distinguished in the ¹⁹F-NMR spectra of rigid lattice at low temperature (T = 175 K) and the distribution of fluoride ions among the four sites is determined by deconvolution of the spectra registered. The sites occupied by the (F_n)_pb anions are identified with the normal anionic sites of the fluorite network, the others with different interstitial sites.     

Nax/2Bi1−x/2MoxV1−xO4 and Bi1−x/3MoxV1−xO4 New Scheelite-related phases

New Scheelite-related solid solutions of the compositions Na_x/2Bi_1−x/2Mo_xV_1−xO₄ (0≤x≤1) and Bi_1−x/3 Mo_xV_1−xO₄(0≤x≤0.2) have been synthesised by the substitution of Na and Mo at the A and B sites respectively of the ABO₄ type ferroelastic BiVO₄. The phases were characterised using chemical analysis, powder X-ray diffraction, scanning electron microscopy, EDAX, and Raman spectroscopy. While almost a continuous solid solution is obtained for the series Na_x/2Bi_1−x/2Mo_xV_1−xO₄, the absence of Na at the A-site results only in a narrow stability region for the other series, Bi_1−x/3 Mo_xV_1−xO₄ where 0≤x≤0.2. Raman spectra of selected samples at room temperature also suggest that vanadium and molybdenum atoms are disordered at the tetrahedral sites.     

Phonon Raman scattering in Nd1+xBa2−xCu3O7−δ and Pr1+xBa2−xCu3O7−δ single crystals

The polarized Raman spectra of Nd_1+xBa_2−xCu₃O_7−δ (−0.023≤x≤0.107) and Pr_1+xBa_2−xCu₃O_7−δ (0.01≤x≤0.15) single crystals have been investigated. It was found that the Cu(2) A_g mode softens by 6 cm⁻¹ in Nd 1:2:3 and 4 cm⁻¹ in Pr 1:2:3 as x increases. These frequency shifts cannot be explained by the change in the relevant bond lengths due to Nd(Pr)-substitution for Ba. The variations with x of the two low frequency modes may be affected by change of their hybridization and/or change of their force constants. The linewidths of Ba mode in Pr 1:2:3 are broader than those in Y 1:2:3. This result suggests that the Pr substitution on Ba sites occurred even in a very small value of x. In x(yy) geometry the relative intensity of the Ba and O(4) modes in Nd 1:2:3 is greater than those in Pr 1:2:3. The difference between Nd 1:2:3 and Pr 1:2:3 in the relative intensity of the Ba and O(4) modes may be produced by the chains.     

NMR study of coexistence of ferro- and antiferromagnetism in (Zr1−xNbx)Fe2

The magnetic phase diagram has been investigated in the C14 type (Zr_1−xNb_x)Fe₂ with x0.7 from ⁹³Nb NMR and magnetization measurements. In the compound with x = 0.825 a first order-like transition has been found to occur around 25 K from a canted state with the ferromagnetic moment in the basal plane to a ferromagnetic state with decreasing temperature.     

Structural and lithium intercalation studies of Mn(0.5−x)CaxTi2(PO4)3 phases (0≤x≤0.50)

Materials from the Mn_(0.5−x)Ca_xTi₂(PO₄)₃ (0≤x≤0.50) solid solution were obtained by solid-state reaction in air at 1000 °C. Selected compositions were investigated by powder X-ray diffraction analysis, ³¹P nuclear magnetic resonance (NMR) spectroscopy and electrochemical lithium intercalation. The structure of all samples determined by Rietveld analysis is of the Nasicon type with the R space group. Mn²⁺/Ca²⁺ ions occupy only the M1 sites in the Ti₂(PO₄)₃ framework. The divalent cations are ordered in one of two M1 sites, except for the Mn_0.50Ti₂(PO₄)₃ phase, where a small departure from the ideal order is observed by XRD and ³¹P MAS NMR. The electrochemical behaviour of Mn_0.50Ti₂(PO₄)₃ and Mn_(0.5−x)Ca_xTi₂(PO₄)₃ phases was characterised in Li cells. Two Li ions can be inserted without altering the Ti₂(PO₄)₃ framework. In the 0≤y≤2 range, the OCV curves of Li//Li_yMn_0.50Ti₂(PO₄)₃ cells show two main potential plateaus at 2.90 and 2.50–2.30 V. Comparison between the OCV curves of Li//Li_(1+y)Ti₂(PO₄)₃ and Li//Li_yMn_0.50Ti₂(PO₄)₃ shows that the intercalation occurs first in the unoccupied M1 site of Mn_0.50Ti₂(PO₄)₃ at 2.90 V and then, for compositions y>0.50, at the M2 site (2.50–2.30 V voltage range). The effect of calcium substitution in Mn_0.50Ti₂(PO₄)₃ on the lithium intercalation is also discussed from a structural and kinetic viewpoint. In all systems, the lithium intercalation is associated with a redistribution of the divalent cation over all M1 sites. In the case of Mn_0.50Ti₂(PO₄)₃, the stability of Mn²⁺ either in an octahedral or tetrahedral environment facilitates cationic migration.     

Oxygen tracer diffusion in La1 − xSrxCoO3

Tracer diffusion of ¹⁸O in dense, polycrystalline La_1−xSr_xCoO₃ for x = 0.1 has been measured in the temperature range 400 to 600 °C and at 500 °C for x = 0.2 at an oxygen partial pressure of 1 × 10⁵ Pa. Depth profiles were obtained by secondary ion mass spectrometry. The diffusion coefficient for La_0.9Sr_0.1CoO₃ is given by D = (17–247) exp[(−232 ± 8 kJ/mole)/RT] cm²/s. This value is several orders of magnitude lower than D extrapolated from the results for x = 0.2 measured in the 700–900 °C temperature range. One possible explanation for the discrepancy is that the two measurements reflect different diffusion paths. As expected, La_0.8Sr_0.2CoO₃ exhibits a higher diffusivity at 500 °C than does La_0.9Sr_0.1CoO₃.     

NMR-spectroscopic study of Pb in pure and barium diluted lead phosphate

³¹P and ²⁰⁷Pb MAS and static ²⁰⁷Pb NMR spectra of Pb₃(PO₄)₂ and (Pb_1−xBa_x)₃(PO₄)₂ (x=0.08, 0.12) are analysed. The resonances stemming from different cation sites are correlated with the corresponding local symmetry and their oxygen neighbours. The coordination sphere of Pb(1) consists of 12 oxygen atoms and shows characteristics of a near-axial arrangement with a comparatively low anisotropy. The tenfold coordinated Pb(2) atoms are located in a more anisotropically-coordinated site. In Pb-diluted lead phosphate crystals the Pb(2) positions appear to be preferentially substituted by barium. There are indications that the cation distributions in the diluted samples are inhomogeneous. Furthermore, ³¹P MAS NMR experiments indicate a single phosphorus position.     

Normal-state transport properties of Ba1−xKxBiO3 crystals

The normal-state transport properties of Ba_1−xK_xBiO₃ crystals with a wide range of potassium compositions (0≤x≤0.62) were studied. Although the host material BaBiO₃ has a monoclinic structure, the system changes from a monoclinic to an orthorhombic structure with a small doping of potassium (0≤x<0.35) and behaves similar to a doped semiconductor, without exhibiting superconductivity. In the composition range, holes are majority carriers in the transport phenomena. When x exceeds a critical value (0.35), the system goes into a cubic superconducting phase with a single metallic band. The vicinity of the critical composition transport phenomena is easy to understand assuming the existence of two conducting channels that are made up of metallic and semiconducting phases. Maximum T_c exceeding 30 K was observed at x0.4, where carrier density was at its maximum. Overdoping with potassium suppresses superconductivity. In the metallic composition of x>0.45, transport seems to correlate with the phonon mode with an energy distribution of 15–43 meV.     

Estimation of conductivity parameters in Sr1 − xThxF2 + 2x solid solution with fluorite-type structure

Concentration of nn-dipoles, nnn-dipoles and charge carriers as well as parameters related to their reorientation and mobility have been estimated from an analysis of a.c. conductivity data of polycrystalline samples of the Sr_{1 − x}Th_xF_{2 + 2x} solid solution. The molar fraction of charge carriers has been evaluated as a function of composition. Results obtained confirm, in agreement with the clustering process model, that the (F_i)_m ions responsible for long range motions in Sr_{1 − x}Th_xF_{2 + 2x} are interstitial fluoride ions of F type.     

Superconductivity in Ba2−xSrxYCu3Oz prepared at GPa

The compounds Ba_1.4Sr_0.6YCu₃O_z (7.0≤z<7.4) were synthesized at 2 GPa to clarify the effect of high temperature and pressure on the superconducting properties. The structure as observed by powder X-ray diffraction was tetragonal for z≥7.2, and orthorhombic or tetragonal for z<7.1. It has been revealed that high-pressure syntheses suppress the superconductivity in the compounds with z<7.1. Recovery of the superconducting properties occurs above z≥7.2. The effect of Sr substitution on the superconducting properties was also studied. The magnitude of diamagnetism at 5–30 K decreases monotonically from −0.003 to −0.0002 emu/g Oe with increasing x in Ba_2−xSr_xYCu₃O_z (x≥1.2).     

Ionic conduction properties of layer-type oxides NaxMx/2Ti1−x/2O2 (M=Ni, Co; 0.60≤x≤1.0)

Layer-type oxide Na_xM_x/2^IITi_1−x/2^IVO₂ (M=Co, Ni; 0.60≤x≤1.0) has been prepared by solid state reactions. In both series, two structural variants of type -NaFeO₂ (O3) and β-RbScO₂ (P2) have been obtained consecutively as x decreases with a borderline composition around x_c0.7. With the decrease of x, the ionic conductivity has been found to increase up to 8.4×10⁻² S cm⁻¹ at 770 K (Na_0.67Co_0.33Ti_0.67O₂). Compositions of P2 have been found to exhibit the conductivity values two to five times greater than those of O3, primarily due to the larger rectangular threshold available for the diffusion of Na⁺ ions. Such a structural effect has also been considered to depend on the polarizability of alkali ion. HT-XRD and ²³Na-NMR data of Na_0.67Co_0.33Ti_0.67O₂ strongly suggest that the diffusion of Na⁺ ion is deeply related with the local distortion of trigonal prismatic sites, leading to the change of activation energy around 430 K.     

Effect of Pr and Ca substitution on superconductivity in Y(1−x−yPrxCay)Ba2Cu3O7−δ

Pr substituted at constant Ca concentration for Y in (Y_1−x−yPr_xCa_y)Ba₂Cu₃O_7−δ superconductors have been prepared under identical conditions and the temperature dependence of the electrical resistivity of these samples are measured. The resistively determined values of T_c decrease linearly with increasing x (0 ≤ x ≤ 0.2) for constant y = 0.10 and 0.15 which provides convincing evidence that the suppression of superconductivity by Pr is mainly due to magnetic pair breaking. The suppression of superconductivity can also be correlated to the observed changes in oxygen content determined by iodometric analysis and to the average Cu-valences. However, it is found that the observed suppression of T_c cannot be compensated by appropriate hole doping with Ca.     

NMR study of hyperfine interactions in R1−xLuxFe2 (R = Gd, Tb)

Spin-echo NMR measurements on ¹⁵⁵Gd, ¹⁵⁹Tb and ¹⁷⁵Lu have been done for the cubic Laves phase compounds Gd_1−xLu_xFe₂ and Tb_1−xLu_xFe_x. The observed hyperfine fields of Gd and Lu in those compounds increase with increasing Lu concentration. The concentration dependence of the Lu hyperfine field is qualitatively discussed.     

Thermodynamic analysis of BaxK1 − xBiO3 using the Eliashberg theory

In order to understand the mechanism that gives rise to superconductivity in the ceramic superconductors of the type Ba_xK_{1 − x}BiO₃, we analyze the thermodynamic properties with the use of the Eliashberg equations. For these calculations we used the electron-phonon spectral density function ²(ω)F(ω), calculated by Shirai et al. [1] from first principles for two different K concentrations, x=0.5 and x=0.7.     

Oxygen nonstoichiometry and phase transitions of the neodymium-rich Nd1+xBa2−xCu3Oz solid solution

On the basis of chemical, thermal analysis and Cu K-edge X-ray absorption measurements, oxygen content in the Nd_1+xBa_2−xCu₃O_z solid solution was determined between 1000°C in air and 400°C in oxygen for x=0.05–0.9 compositions. It has been observed that the oxygen nonstoichiometry Δz of the Nd_1+xBa_2−xCu₃O_7+x/2−Δz solid solution decreases 2–2.5 times for a large substitution (Δz≈0.3–0.33 for x=0.9), despite of the acclaimed higher total oxygen content. The difference in nonstoichiometry is explained by a higher average value of the copper oxidation state (ACV), which is vital for the solid solution with large x even at elevated temperatures (ACV≈2–2.05 for x>0.3 at 1000°C, PO₂=0.21 atm). On the contrary, the ACV after complete oxygenation is almost constant (about 2.25–2.3) for the whole series. The x-dependence of the oxygen content is not monotonous and structural phase transitions can be observed at x=0.3 and x=0.6, as confirmed by the X-ray diffraction and the Raman scattering spectroscopy. The first well-known transition is connected with the oxygen disorder due to the Nd substitution for Ba at random Ba-sites. In the present work, it is proved by the apical oxygen mode broadening in Raman spectra. Ordering of the Nd and Ba atoms with a subsequent orthorhombic distortion of the lattice may occur even at 1000°C in air due to the second transformation at x≈0.6. The invariable orthorhombicity of the Nd-rich solid solution with x>0.6 is not caused by the oxygen absorption as in the x=0.05 case. Existence of high- and low-temperature orthorhombic modifications of this solid solution has been observed for the first time. Finally, a tentative 3D (z–x–T) diagram is suggested for the Nd_1+xBa_2−xCu₃O_z solid solution up to 1000°C in air, including the new x=0.6–0.9 region.     

Superconductivity and crystallographic properties of La2 − xMxCuO4 − δ (M = Na, K)

Superconductivity and crystallographic properties of La_{2 − x}M_xCuO_{4 − δ} (M = Na, K) are studied. In the La_{2 − x}M_xCuO_{4 − δ} system, superconductivity is detected for x 0.2. Oxygen content analysis shows that the system has more oxygen vacancies than the La_{2 − x}Sr_xCuO_{4 − δ} system. These oxygen vacancies may reduce the hole concentration, and high Na-doping is needed to produce superconductivity. In the La_{2 − x}K_xCuO_{4 − δ} system, superconductivity is observed for the first time. Resistivity and magnetic susceptibility measurements show that T_c(onset) is 40 K and the Meissner volume fraction is about 4% for x = 0.7. The system changes from orthorhombic to a tetragonal K₂NiF₄ structure at x ≈ 0.3 and only tetragonal samples show superconductivity.     

Electrical properties of LiCr1-xAlxTiO4 (0≤x≤0.4)

The conductivity measurements on polycrystalline samples have been obtained by ac techniques and the ionic transference number has been calculated by charge/discharge d.c. technique. In the LiCr_1-xAl_xTiO₄ (0≤x≤0.4) spinel systems, at low x, the solid solutions are predominantly electronic conductors but on increasing x, the level of electronic conductivity decreases giving rise to materials that are mixed conductors.     

Synthesis and characterization of the Aurivillius phases Bi2 − xPbxSr1 − xNdxNb2O9

The n = 2 Aurivillius phase Bi_{2 − x}Pb_xSr_{1 − x}Nd₂O₉ was successfully synthesized as a ceramic material over the whole range of simultaneous, charge compensated substitution x = 0–1.0. Structural investigations were performed by Rietveld refinement applying different space groups Fmmm and A2₁am, and additionally by X-ray absorption spectroscopy (EXAFS) on the Nd L_III-edge, confirming the accommodation of Nd on the atomic sites of Sr, which implies the substitution of Bi³⁺ by the isoelectronic Pb²⁺. The ferroelectric transition temperature T_c = 270 °C of the substituted powders with x = 0.4 and 1.0 is distinctly reduced compared to the unsubstituted sample with T_c = 450 °C. In temperature resolved powder X-ray diffraction patterns no structural phase transition could be detected.     

Neutron diffraction and IR spectroscopy on mechanically alloyed La-substituted PbWO4

Defect structures of the lanthanum-substituted PbWO₄ prepared by mechanical alloying (MA) were investigated by means of powder neutron diffraction and FT-IR, to compare with that of the oxide ion conductive sintered samples. Neutron diffraction revealed that Pb_1−xLa_xWO_4+x/2 prepared by MA possessed the lead site deficiency in the lattice. In addition, it was also found that occupation factors of the lead site were still smaller than the nominal value even for the mechanically alloyed Pb_1−xLa_2x/3WO₄. The vacancy formation at the lead site of the mechanically alloyed Pb_1−xLa_xWO_4+x/2 was also corroborated from the splitting of the absorption band around 800 cm⁻¹ ascribed to the W–O stretching vibration of WO₄ tetrahedron. At the end of this paper, the formation process of the lead site vacancy was discussed in the sintered Pb_1−xLa_xWO_4+x/2 during milling.     

Magnetic properties and percolation threshold in diluted B-spinel ZnCr2xAl2−2xS4: a study through high-temperature expansions

By making use of high-temperature series expansions (HTSE) of the correlation functions, we study the thermal and disorder variation of the short-range order (SRO) in the particular B-spinel ZnCr_2xAl_2−2xS₄. We developed the HTSE for the q-dependent static structure factor S(q) to the order 6 in reciprocal temperature including both the nearest- and next-nearest-neighbour interactions J₁ and J₂, respectively. Respecting the experimental fact that the broad diffuse peak of the neutron is situated at the particular wave vector q₀=[0 0 0.79] and is insensitive to the temperature for a given ratio of dilution x, we have estimated the thermal variation of J₁ and J₂ in the case of the pure compound.

The bond percolation threshold x_p of the ZnCr_2xAl_2−2xS₄ is determined by studying the disorder variation of the correlation length ξ. The x_p is considered as the concentration at which ξ vanishes. The obtained values are x_p=0.27 when only J₁ is considered and 0.23 when both J₁ and J₂ are taken into account.