Rare-earth Size Research Articles

Infrared-active phonons in RTiO3 perovskites (R=La,Ce,Pr,Nd,Sm,Gd).

The room-temperature reflectance (100--1000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) of the rare-earth titanites (R${\mathrm{TiO}}_{3}$ R=La,Ce,Pr,Nd,Sm,Gd) has been measured. The spectrum for ${\mathrm{LaTiO}}_{3}$, ${\mathrm{CeTiO}}_{3}$, and ${\mathrm{PrTiO}}_{3}$ consists of the three main perovskite vibrational bands. The central band, near 340 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, consists of several barely resolved modes which begin to separate in ${\mathrm{NdTiO}}_{3}$ and ${\mathrm{SmTiO}}_{3}$ and which split into two clusters of modes in ${\mathrm{GdTiO}}_{3}$. These features are correlated with an increasing tetragonal distortion of the ${\mathrm{TiO}}_{6}$ octahedron as the rare-earth ionic size decreases. Using the LO-TO splitting, the effective ionic charges are estimated and the R${\mathrm{TiO}}_{3}$ perovskites are shown to be less ionic than ${\mathrm{SrTiO}}_{3}$ or ${\mathrm{BaTiO}}_{3}$.

The crystal structures of Dy 2Ba 4Cu 7O 14+δ and Nd 2Ba 4Cu 7O 14+δ: A high resolution powder neutron diffraction study

The structures of Dy 2Ba 4Cu 7O 14+δ and Nd 2Ba 4Cu 7O 14+δ (247), which can be considered to be derived from alternate blocks of the LnBa 2Cu 3O 7 (123) and LnBa 2Cu 4O 8 (124) structures, have been refined using high resolution powder neutron diffraction data. Accurate structural parameters, determined by this oxygen-sensitive technique, are presented for these superconducting oxides. In both materials oxygen was found to be randomly distributed over the formally single chain and inter-chain sites of the type seen in 123 compounds; no crosslinking of the 124 double chains was observed. Notable differences in oxygen contents were found between the two samples; the neodymium sample exhibited a high average copper oxidation state as a result of considerable occupancy of both of the chain and inter-chain sites. The orthorhombic distortions of these materials are markedly reduced compared to those recorded for the corresponding 123 and 124 materials. A flattening of the copper-oxygen planes sandwiching the lanthanide site with increasing lanthanide size, as previously found in Ln123 and Ln124 compounds, is observed.

The crystal structures of LnBa 2Cu 4O 8 and LnBa 2Cu 3O 7− σ (Ln=Er, Ho, Dy) : A high resolution powder neutron diffraction study

The structure of LnBa 2Cu 4O 8 (124) and LnBa 2Cu 30O 7− σ (123), (Ln=Er, Ho, Dy), have been refined from very high resolution powder neutron diffraction data in order to accurately delineate changes in the superconductor structure which occur as a function of the lanthanide ion size. Accurate positional parameters and anisotropic temperature factors, determined by this oxygen-sensitive technique, are presented for the structures of non-yttrium 124 compounds. Refined parameters demonstrate that the dimensions of the CuO 2 planes alter in a similar fashion in the two series of materials with a reduction of the orthorhombic distortion with increasing lanthanide size and a slight flattening of the planes. Similar changes are seen in the copper-oxygen sublattice for both structure types: the apical bond length of the CuO 2 square pyramids is found to decrease slightly despite the increase in the c-parameter as Er→Ho→Dy. No occupation of the inter-chain site is observed in the Ln124 materials.

New quadruple perovskites Ln2Ba2TixSn2–xCu2O11, (0⩽x⩽2): controlling copper–oxygen distances in potential superconducting parent phases

The compounds Nd2Ba2Cu2SnTiO11, Sm2Ba2Cu2Sn0.5Ti1.5O11 and Gd2Ba2Cu2Ti2O11 have been synthesized by reaction of the component oxides at 1025 °C and their structures refined from powder X-ray diffraction data. The materials crystallise with a quadrupled perovskite unit cell, similar to that of La2Ba2Cu2Sn2O11 containing alternating double layers of copper and titanium/tin. Decreasing lanthanide size and increasing titanium content is associated with a marked unit cell contraction and a reduction in the mismatch between the different double-layer dimensions. A reduction in the copper–oxygen distances in the CuO2 planes from 2.007 A in La2Ba2Cu2Sn2O11 to 1.951 A in Gd2Ba2Cu2Ti2O11 produces a copper–oxygen interaction similar to that found in high-temperature superconductors.

Structural characterization of R2BaCuO 5 ( R = Y, Lu, Yb, Tm, Er, Ho, Dy, Gd, Eu and Sm) oxides by X-ray and neutron diffraction

R 2BaCuO 5 compounds with R = rare earth atom from Sm to Lu have been studied by neutron and X-ray powder diffraction. All of them are isostructural, and belong to the space group Pnma ( Z = 4). A comparative study of the influence of the rare earth size on the structural parameters is presented. The stability of the structure is estimated from the agreement between valence bond sums and formal valence states. A systematic variation of the structural stability is found depending on the size of the rare earth atom.

In search of chemical clues: Single-crystal structural studies of PrBa 2Cu 3O 6 and PrBa 2Cu 3O 7

Complete refinements of single-crystal X-ray diffraction intensity data for two tetragonal PrBa 2Cu 3O 6 crystals and for two detwinned orthorhombic PrBa 2Cu 3O 7 crystals, along with a more precise refinement for YBa 2Cu 3O 6, have confirmed that the praseodymium compounds are strictly isostructural with the yttrium congeners. Crystallographic data available for the superconducting orthorhombic RBa 2Cu 3O 7 analogs (R=Y, rare earth) were used to establish overall structural trends as a function of lanthanide size. In comparison with these trends, the present results obtained for orthorhombic PrBa 2Cu 3O 7 indicate no markedly unusual structural features; the crystallographic behavior of Pr corresponds to that of the trivalent ion. However, subtle structural anomalies in the PrBa 2Cu 3O x system were revealed by detailed comparisons of structural trends in RBa 2Cu 3O x systems as a function of oxygen content. The axial CuO bond distance is unusually short in both PrBa 2Cu 3O 6 and PrBa 2Cu 3O 7, indicating stronger chemical linkage of the chains and planes as compared to the superconducting systems. Trends in the bond-valence sums and polyhedral volumes about R 3+ suggest that in PrBa 2Cu 3O 7 the increased spatial extension of the 4f-orbitals (as compared to neighboring Nd 3+) mediates strong interactions with the CuO manifold that are capable of inhibiting superconductivity. The retention of superconductivity in Pr 1.85Ce 0.15CuO 4 may arise from significant crystal-chemical differences between the T′ — and the YBa 2Cu 3O 7-type structures: the T′ structure features (1) oxygen layers that are bonded only to Pr, not Cu, and (2) a larger R 3+ polyhedral volume that results in greater isolation of the 4f-orbitals.

Rare-earth ionic size effect on the Pr-induced T c depression in the series R 1- xPr xBa 2Cu 3O 7- δ (R=Sm, Gd, Y, Er, Tm)

The rare-earth ionic size effect on the depression of superconducting transition temperature T c by Pr substitution has been studied in the R 1- x Pr x Ba 2Cu 3O 7- δ (R=Sm, Gd, Y, Er, Tm) series. For each system, T c decreases with increasing Pr content. The larger the ionic size difference of R and Pr ions, the less T c decreases and hence the smaller the critical Pr concentration for destruction of superconductivity. In contrast to the hydrostatic pressure effect on T c, this anomalous behaviour can be explained by considering that the large size difference of R and Pr ions will lead to the high concentration of the neighbouring relation R-R, and thus the relatively high T c for the sample.

Effect of the lanthanide ion on the structure and low-temperature phase transitions in RbL(SO4)2.4H2O (L identical to La-Er) crystals

Crystallographic unit cell parameters of RbL(SO4)2.4H2O (L identical to La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Er) are 296 K and specific heat capacity (Cp) data on RbL(SO4)2.4H2O (L identical to Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Er) for 110<T<305 K are presented. The RbL(SO4)2.4H2O lattices form an isostructural monoclinic series and have the layered structure of RbSm(SO4)2.4H2O. The unit cell parameters a, b, c and beta reflect an unusual trend with lanthanide size, unlike other isostructural lanthanide series where cell parameters are monotonically related to the lanthanide contraction. The variation in the cell parameters was schematically used to deduce the trend in the buckling of lanthanide-sulphate layers in RbL(SO4)2.4H2O, and it is argued that the observed nonmonotonic behaviour in cell parameters is a consequence of this buckling. The Cp measurements indicate that RbL(SO4)2.4H2O (L identical to Pr, Nd, Eu, Gd, Tb and Dy) lattices undergo a lambda transition. Additional structural transitions are observed in RbPr(SO4)2.4H2O, RbNd(SO4)2.4H2O, and RbDy(SO4)2.4H2O, while no transitions occur in RbEr(SO4)2.4H2O for 110<T<305 K. The lattice stability model, inclusive of the buckling effects, successfully maps the observed trend in Tlambda as well as in the order-disorder transition T2 in isostructural NH4L(SO4)2.4H2O (L identical to La-Tb) lattices.

Eight rare-earth-substituted Pb-based 1:2:1:2 superconducting compounds: (Pb,Cu)Sr2(Ca,R)Cu2O7 (R=Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm).

Rare-earth substitution has given a series of Pb-based 1:2:1:2 superconducting compounds (Pb,Cu)${\mathrm{Sr}}_{2}$(Ca,R)${\mathrm{Cu}}_{2}$${\mathrm{O}}_{7}$ (R=Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm). ${\mathit{T}}_{\mathit{c}}$'s vary inversely with rare-earth ionic size from 72 K (Tm) to 60 K (Gd) and then remain constant for larger ions up to Nd. This behavior (${\mathit{T}}_{\mathit{c}}$ decreasing for larger ions separating the two ${\mathrm{CuO}}_{2}$ superconducting sheets) is similar to the behavior of ${\mathrm{RBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ (1:2:4) and ${\mathit{R}}_{2}$${\mathrm{Ba}}_{4}$${\mathrm{Cu}}_{7}$${\mathrm{O}}_{\mathit{y}}$ (2:4:7), but not R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (1:2:3). As prepared in air the samples were not superconductors; superconductivity was induced by annealing in oxygen under 100 bar of oxygen pressure at 940 \ifmmode^\circ\else\textdegree\fi{}C followed by fast cooling from 940 to 100 \ifmmode^\circ\else\textdegree\fi{}C (50 \ifmmode^\circ\else\textdegree\fi{}C/min). Lattice constants of the compounds correlate with the ionic radii of the rare-earth elements, as in the 1:2:3, 1:2:4, and 2:4:7 compounds.

Control of microstructure and component shape in rapidly solidified/powder metallurgy titanium alloys

Extensive use of titanium rapidly solidified/powder metallurgy (RS/PM) components requires not only careful control of the microstructure for optimum mechanical properties but also cost-effective processing. A new direct reduction process for production of titanium alloy powder will be presented. Control of the microstructure in conventional alloys such as Ti-6Al-4V and in non-conventional dispersion strengthened terminal and intermetallic alloys will be discussed. It will be shown that RS/PM processing allows production of a fine grain size and useful dispersions of rare earth and metalloid phases; phases which normally form as gross undesirable particles. The use of hydrogen as a temporary alloying element, thermochemical processing, will be discussed and it will be demonstrated how this treatment can lead to refined microstructures with enhanced mechanical behaviour. Cost-effective processing using near-net shape techniques such as the ceramic mold process, rapid omnidirectional compaction (ROC), and the use of RS/PM preforms for subsequent isothermal forging will be presented. Microstructural control and shape-making used in unison should lead to increased use of titanium components in advanced aerospace systems.

Magnetic properties of Gd2CuO4 crystals.

Magnetic-susceptibility and magnetization measurements on single crystals of the ${\mathrm{CuO}}_{2}$-planar compound ${\mathrm{Gd}}_{2}$${\mathrm{CuO}}_{4}$ indicate copper moment ordering near 260 K, producing an internal field at the gadolinium site that induces substantial anisotropy in the magnetic response. Dilute substitutions for gadolinium and copper leave the essential physics unaffected. We suggest that copper ordering also occurs in ${\mathrm{Eu}}_{2}$${\mathrm{CuO}}_{4}$ on the basis of substitutional studies but is not detected in other rare-earth-based compounds in this series. Rare-earth size appears to play an important role.

Superconducting and magnetic properties of Ba2LnCu3O7?x compounds (Ln=lanthanides)

The superconducting and magnetic properties of a series of compounds Ba2LnCu3O7−x (Ln=Pr, Nd, Sm ... Gd, Dy ... Yb) are reported and compared with corresponding measurements on the isostructural model substance Ba2YCu2O7−x . The unknownx in the oxygen concentration is obviously responsible for a nonmonotonic variation of bothT c and the lattice constants with the rare-earth size. No remarkable increase ofT c was achieved on replacing Y by a magnetic Ln. A partial substitution of Ba by Sr reducedT c, but increasedT N of Ba2GdCu3O7−x from 2.3 K to 2.65 K in BaSrGdCu3O7−x . Intrinsic pinning effects in external magnetic fields appear to be surprisingly weak at higher temperatures where technical applications of this sort of superconductors might be of interest.

Clustering in rare-earth-doped alkaline earth fluorides (dielectric relaxation)

Audio-frequency capacitance and conductance measurements have been carried out for calcium fluoride doped with 1.0 mol.% of thirteen rare earths, and 10 and 20 mol.% of erbium and lanthanum; strontium fluoride doped with 0.1 and 1.0 mol.% of twelve rare earths, yttrium and lanthanum; and barium fluoride doped with 0.1 mol.% of twelve rare earths, yttrium and lanthanum. For the moderately doped calcium fluoride samples, it is found that the low-temperature (RIII) relaxation dominates the spectrum for small rare earths. For large rare earths, the dimer-associated RIV ('gettered' 2:2:2) relaxation dominates the spectrum. The primary result for strontium fluoride is the observation of a new relaxation in the heavily doped samples with an activation energy which depends strongly upon the size of the rare earth and thus is attributed to RIV dimers. Other results may be summarised as follows. The RIV dimer becomes less stable as the host size increases but also as the rare earth size decreases. For smaller rare earth dopants, RIII is more stable in the smaller hosts and appears at lower concentrations in smaller hosts. Correspondingly, the complexity of the dielectric spectrum increases for smaller hosts and smaller rare earth dopants.

The structures of the Na 3Ln( XO 4) 2 phases ( Ln = rare earth, X = P, V, As)

Due to interest in new luminophors, a complete crystal chemical study of phases with the formulation Na 3 Ln( XO 4) 2 ( Ln = rare earth, X = P, V, As) has been undertaken. An overall description of these phases is given and relationships among various structural types are presented. The structural evolution is analyzed and discussed as a function of the rare-earth ion size and of the temperature.

Les orthoarseniates de sodium et de terre rare

Sodium rare-earth arsenates Na 3 Ln(AsO 4) 2 may occur in five allotropic varieties. Their structural evolution has been studied as a function of rare-earth size, temperature, and pressure. The structures are isotypic with those of homologous phosphates and vanadates previously described.

Magnetic properties of amorphous alloys of Fe with La, Lu, Y, and Zr

In order to study the systematics of the Fe-Fe exchange in amorphous rare-earth---Fe alloys, without the complications associated with the magnetic characteristics of the rare-earth elements, amorphous films of Fe alloyed with La, Lu, Y, and Zr have been prepared with a wide range of Fe concentrations. Magnetization and M\ossbauer-effect measurements were made. The magnetic properties of the alloys depended critically on the choice of rare earth (or rare-earth-like element). YFe and LuFe alloys were found to have spin-glass characteristics while LaFe and ZrFe alloys were found to be ferromagnetic, but with evidence that exchange fluctuations were nearly as large as the average exchange. Thus the nature of the Fe-Fe exchange interaction depends critically upon the species of the rare earth. The most important parameter in determining the magnetic behavior of these alloys appears to be the size of the rare-earth atom, with large rare-earth atoms resulting in a smaller ratio of exchange fluctuations to exchange. The same dependence of the magnetic properties upon rare-earth size appears to be important in the case of magnetic-rare-earth atoms; however, the effect of rare-earth---Fe exchange also becomes important and these effects are discussed.