Noncubic Sites Research Articles

Probing Site Symmetry Around Eu3+ in Nanocrystalline ThO2 Using Time Resolved Emission Spectroscopy

ThO2:Eu3+ nanoparticles were synthesized at 300°C by combustion route using urea as a fuel and characterized by thermogravimetric/differential thermal analysis, X‐ray diffraction, transmission electron microscopy, and photoluminescence techniques. To investigate the effect of annealing temperature, as synthesized powder were heated further at 500°C, 700°C, and 900°C. It was observed that extent of asymmetry around Eu3+ at 700°C/900°C is very high as compared to as‐prepared or 500°C annealed sample. Based on the time resolved emission spectroscopic investigations, it was inferred that two different types of Eu3+ ions were present in the ThO2 nanoparticles. In ThO2 structure, Eu3+ ions occupy two sites; cubic (Oh) and noncubic (<C2v) as can be confirmed from our emission studies. Short‐lived species T1 (~1.3–3.4 ms) predominates at higher annealing temperature arises because of Eu3+ ions occupying noncubic (<C2v) sites without inversion symmetry, whereas long‐lived species T2 (~4.6–6.6 ms) can be ascribed to Eu3+ ions occupying cubic sites (Oh) with inversion symmetry.

Invariant Form of Hyperfine Interaction with Multipolar Moments – Observation of Octupolar Moments in NpO2 and Ce1-xLaxB6 by NMR –

The invariant form of the hyperfine interaction between multipolar moments and the nuclear spin is derived, and applied to discuss possibilities to identify the antiferro-octupolar (AFO) moments by NMR experiments. The ordered phase of NpO$_{2}$ and the phase IV of Ce$_{1-x}$La$_{x}$B$_{6}$ are studied in detail. Recent $^{17}$O NMR for polycrystalline samples of NpO$_{2}$ are discussed theoretically from our formulation. The observed feature of the splitting of $^{17}$O NMR spectrum into a sharp line and a broad line, their intensity ratio, and the magnetic field dependence of the shift and of the width can be consistently explained on the basis of the triple $\bq$ AFO ordering model proposed by Paix\~{a}o {\it et. al.} Thus, the present theory shows that the $^{17}$O NMR spectrum gives a strong support to the model. The 4 O sites in the fcc NpO$_2$ become inequivalent due to the secondary triple $\bq$ ordering of AF-quadrupoles: one cubic and three non-cubic sites. It turns out that the hyperfine field due to the antiferro-dipole and AFO moments induced by the magnetic field, and the quadrupolar field at non-cubic sites are key ingredients to understand the observed spectrum. The controversial problem of the nature of phase IV in Ce$_{1-x}$La$_{x}$B$_{6}$ is also studied. It is pointed out that there is a unique feature in the NMR spectra, if the $\Gamma_{5}$($T^{\beta}_{x}=T^{\beta}_{y}=T^{\beta}_{z}$) AFO ordering is realized in Ce$_{1-x}$La$_{x}$B$_{6}$. Namely, the hyperfine splitting of a B atom pair on the $({1/2},{1/2},\pm u)$ sites crosses zero on the $(1\bar{1}0)$ plane when the magnetic field is rotated around the $[001]$ axis.

Investigation of 4f 7 levels of Eu 2+ ions in KMgF 3 crystal by two-photon excitation spectroscopy

Intraconfigurational 4f 7–4f 7 transitions of Eu 2+ in KMgF 3 are investigated by two-photon excitation (TPE) spectroscopy. The emissions due to the 6 P 7/2→ 8 S 7/2 and 6 I 7/2→ 8 S 7/2 transitions are obtained by the two-photon excitation into the 6 D 9/2 at 10 K. It is found that the TPE spectrum of the 6 I 7/2→ 8 S 7/2 emission is different from that of the 6 P 7/2→ 8 S 7/2 emission. The result is discussed in terms of the relative position between the lowest crystal-field component of the 6 I 7/2 level and the lowest 4f 65d level of Eu 2+ at the cubic and non-cubic sites in KMgF 3.

Ligand field analysis of the 3dN ions at orthorhombic or higher symmetry sites

A computer package has been developed to calculate the energy levels and the ligand field states for a full Hamiltonian including the electrostatic terms, Trees correction, the spin-orbit interaction and the ligand field interactions within the 3dN configuration (N = 2 to 8). The program can deal with various cubic and non-cubic site symmetries of the 3d ion, namely the tetragonal, trigonal and orthorhombic ones. The package may be very useful in analyzing the EPR spectra of 3dN ions and in correlating the ligand field parameters with spin Hamiltonian parameters derived from the orbital singlet states for some F-term (S = 1, 32) and S-term (S = 52) ions as well as the 3d4 and 3d6 ions with high-spin S = 2. Numerous examples of these ions in various crystals exist in the literature. Other spin states arising from strong ligand fields can also be dealt with.

PAC and XPS studies of II–VI compounds

The perturbed angular correlation (PAC) method has considerable potential for studying near-surface defects of materials on a microscopic scale, especially in combination with standard surface techniques. We have characterized the near-surface and surface regions of II–VI compounds using PAC and X-ray photoelectron spectroscopy (XPS). PAC was used to identify point defect complexes containing the radioactive 111In probe atoms, which were diffused into Cd 0.96Zn 0.04Te, Cd 0.8Mn 0.2Te and Hg 0.79Cd 0.21Te (MCT) samples under vacuum from 200 to 550 °C. After the Cd(Zn)Te samples were annealed at 450 °C for 30 min, 60% of the In atoms occupied unique non-cubic sites, characterized by an interaction frequency ν Q = 60 MHz and an asymmetry parameter η=0.2. These sites were attributed to In-(cadmium vacancy) complexes, since this signal was eliminated by subsequent annealing in a cadmium atmosphere. Similar results were obtained for the Cd(Mn)Te samples, but the vacancy complex occured at a lower annealing temperature. The XPS data for Cd(Mn)Te showed that its oxide characteristics differed considerably from those of CdTe. Although oxide formation of CdTe occurs very slowly after sputter cleaning, a significant TeO layer was formed on the Cd(Mn)Te surfaces after only 10 min exposure to air. In addition, deionized water removed the native oxide completely for CdTe but not for Cd(Mn)Te. For MCT, annealing at 350 °C caused the formation of two defect complexes, characterized by frequencies of 83 and 92 MHz, and η ≈ 0.1. These PAC signals vanished after annealing under a mercury atmosphere, indicating that they were also due to In-vacancy complexes.

Asymmetry of the 57Fe Mössbauer quadrupole doublet in oriented powders and in single crystals of high- Tc superconductors

This paper is devoted to the asymmetry ratio of the quadrupole doublet observed on the 14.4 keV Mössbauer transition (I e= 3 2 →I f= 1 2 ) of 57Fe, in a non-cubic site of a diamagnetic or paramagnetic orthorhombic compound. The intensity ratio of the two lines is computed for perfectly aligned powders, imperfectly aligned powders, untwinned single crystals and twinned single crystals. The resulting formulas are discussed in connection with recent experiments performed by various authors on oriented powders and on single crystals of high- T c superconductors.

PAC Studies of Defects in AgCI and II-VI Compounds

ABSTRACTThe Perturbed Angular Correlation (PAC) method has been used to study In -vacancy complexes in AgCl and II - VI compounds. Although these materials are quite different in structure and properties, they are related by photoconductive properties which make them interesting intrinsically and for application to photography (silver halides) and to the design of photodetectors and photovoltaic cells (II - VI compounds). We have obtained the first PAC results for silver halides and ternary II-VI compounds.For AgCl, measurement at 77 K clearly reveals two different 111In - vacancy complexes characterized by quadrupole interaction frequencies Vq1 = 21.5 MHz and Vq2=43 MHz, both having axially symmetric electric field gradient (EFG) tensors with principle axes along <100> directions. Temperature dependence of the PAC signal allows a determination of the activation energy and attempt frequency for vacancy motion by measurement of the damping arising from dynamic effects.For 4% Zn - doped bulk CdTe samples, vacuum annealed in the range 400°C to 550°C, PAC shows that a large fraction of the 111 In probe atoms occupy non-cubic lattice sites having an EFG characterized by vQ = 60 MHz, η = 0.2. The concentration of this complex was decreased by annealing between 400°C and 600°C in a Cd overpressure, suggesting tha the observed EFG is caused by a 111In - cadmium vacancy complex. Similar measurements of Cd0.8Mn0.2Te showed an identical EFG which, however, appeared at a lower annealing temperature.

Local distortion and zero-field splittings of 3d5ions in oxide crystals

A lattice relaxation model is employed to calculate the distorted ligand positions of Mn2+ and Fe3+ doped in cubic sites of some alkaline-earth metal oxides and the non-cubic sites of Fe3+:MgO as well. A superposition model analysis is then carried out for the spin parameters of the 6S5/2 ground-state zero-field splittings of both 3d5 ions. A consistent set of intrinsic parameters is obtained which gives good predictions for the spin parameters of Fe3+ in non-cubic sites of MgO and Mn2+ in the C3i site of some garnets.

Photoluminescence of neodymium-implanted gallium phosphide and gallium arsenide

Characteristic intra 4f-shell luminescence spectra of Nd3+(4f3) have been observed from neodymium-implanted gallium phosphide and gallium arsenide. The luminescent centers were found to occupy different types of noncubic lattice sites, presumably formed by association of the neodymium implants with radiation damage centers. The relative intensities of the neodymium luminescent centers were found to depend critically on the conditions of postimplantation thermal annealing.

Variation of the local properties in TmxSe (0.8 < x ≤ 1.0) : 169Tm Mössbauer and X-ray measurements

169Tm Mossbauer absorption measurements have been made on the system TmxSe in order to investigate the variation of the local properties with stoichiometry. Five samples spanning the whole range of composition where the overall symmetry remains cubic were examined from 1.4 to 295 K. For the samples with the highest lattice constants, a = 5.712 and 5.711 A, there is no evidence of any significant departure from local cubic symmetry in the paramagnetic region, and in the magnetically saturated region we observe a distribution of hyperfine fields. For each of the samples with intermediate lattice constants (a = 5.685 and 5.672 A) a distribution of quadrupole splitting is observed in the paramagnetic region probably related to a distribution of noncubic environments. Surprisingly, for the most nonstoichiometric sample (a = 5.626 A) this distribution vanishes and there is only one dominant noncubic site present. A room temperature X-ray examination of this sample shows the presence of sharp superstructure reflections indicating a cationic vacancy segregation in every second {111} plane within a multi-domain structure. For the three smallest lattice parameters samples, in the paramagnetic region at low temperatures, the Mossbauer line shapes are influenced by magnetic relaxation. Adequate fits are obtained using a fluctuating hyperfine field model.

Luminescence of Eu 3+ -activated thorium oxide

The luminescence of ThO 2 : Eu 3+ has been studied in order to compare the luminescence of Eu 3+ produced by photonic excitation and by chemical excitation during a catalytic reaction. The photoluminescence varies with the Eu 3+ concentration, the preparation process and the excitation modes (in the charge transfer band or in the 4f 6 levels). This is explained by the existence of cubic and non-cubic sites for the Eu 3+ ions, the populations of which vary with the concentration and the preparation process. The chemical excitation involves only non-cubic sites.

Variation of hyperfine splitting constants for 3d 5-ions with interatomic distance

The hyperfine splitting constants A in the EPR spectra of the d 5-ions of 55Mn and 57Fe as impurities in crystals increase with the size of the host ion and seem to approach an upper limiting value. Also, for divalent manganese in noncubic sites larger splittings along the directions of bond elongation are observed. Both effects can for 55Mn be represented by a linear increase of about 0.1–0.2·10 −4 cm −1 ppm. For 57Fe the increase relative to the free ion value is about four times as large. An explanation for this variation with bond distance based on accompanying changes of covalency is proposed. The size of A can be used to decide which one of several possible sites is actually occupied by the transition metal ion. Other potential applications for the solution of structural details in amorphous or crystalline systems are indicated.

Optical absorption of MgO: Fe

The optical absorption spectra of as-grown, oxidized and reduced MgO: Fe crystals at low temperatures are reported. It is found that reduction at 1500°C converts more than 99% of the iron to Fe 2+ revealing optical spectra due to Fe 2+ in both cubic and non-cubic sites. Oxidation, however, converts only 60–70% of the iron to Fe 3+ and no clear Fe 3+ crystal-field spectrum is observed. A cubic Fe 2+ band at 4.9 eV exhibits a progression of 13 vibronic sidebands typical of a Jahn-Teller distortion.

Divalent manganese in SrO†

The first observation of Mn/sup 2+/ in a non-cubic site of SrO single crystals was reported. Also, since the reported values for the cubic crystal field parameter a in SrO vary from 1.6 to 4.3 x 10/sup -14/ cm/sup -1/, an independent measurement of the SrO cubic spectrum was made.

Impurity induced Raman spectra in MgO+Fe3+and in MgO+Cr3+crystals

Raman spectra of Fe3+ and Cr3+ doped MgO single crystals have been obtained, and are interpreted with the help of the projected densities of states of pure MgO. The spectra of the two crystals are different, presumably because the Cr3+ ions are at non-cubic sites.

The interpretation of the crystal field groundstate of Dy 3+ in Pd

A recent interpretation of the crystal field ground state of Dy 3+ in Pd is discussed in terms of a model where isotropic conduction electron/ion exchange is included. The new results suggest the ratio of fourth and sixth order crystal field parameters to be 292±4. The magnitude of the crystal field parameters cannot be determined. A resonance originating possibly from Dy ions in non-cubic sites is identified.

Correlated Electron Paramagnetic Resonance and Optical Study of CdF2:Er3+. I.C2vLocal-Site Symmetry

A correlated E PR and optical study has been performed on Cd${\mathrm{F}}_{2}$: (${\mathrm{Er}}^{3+}$, $U$) ($U\ensuremath{\equiv}\mathrm{unintentionally}\mathrm{compensated}$) and Cd${\mathrm{F}}_{2}$: (${\mathrm{Er}}^{3+}$, ${M}^{+}$) (${M}^{+}={\mathrm{Li}}^{+}, {\mathrm{Na}}^{+}, {\mathrm{Ag}}^{+}, or {\mathrm{K}}^{+}$) crystals with the following objectives: (i) generating a high concentration of a specific (i.e., ${C}_{2v}$) ${\mathrm{Er}}^{3+}$ site through the addition of monovalent cations and characterizing this site by EPR; (ii) unambiguously determining selected optical properties of ${\mathrm{Er}}^{3+}$ in ${C}_{2v}$ symmetry; and (iii) determining the crystal field splitting of the $^{4}I_{\frac{15}{2}}$ ground site of ${\mathrm{Er}}^{3+}$ for ${C}_{2v}$ symmetry. The orthorhombic (${C}_{2v}$) symmetry, produced at the erbium site when ${M}^{+}$ ions are introduced for charge compensation, has been identified through the angular dependence of the ${\mathrm{Er}}^{3+}$ EPR spectrum (at 4.2 K). Moreover, the EPR results reveal that the ${C}_{2v}$ (${\mathrm{Er}}^{3+}$, ${M}^{+}$) site accounts for nearly all (> 98%) of the noncubic sites recorded for (${\mathrm{Er}}^{3+}$, ${M}^{+}$) specimens. This result has permitted an unambiguous determination of the emission, excitation, absorption, lifetime, and efficiency properties of ${\mathrm{Er}}^{3+}$ in ${C}_{2v}$ symmetry. These characteristics have been found to be similar for each of the ${M}^{+}$ ions listed, but different from those obtained from (${\mathrm{Er}}^{3+}$, $U$) crystals. In particular for (${\mathrm{Er}}^{3+}$, ${\mathrm{Na}}^{+}$) the green ($^{4}S_{\frac{3}{2}}\ensuremath{\rightarrow}^{4}I_{\frac{15}{2}}$) quantum yield is observed to increase from 2.3 to 19.7%, whereas the red ($^{4}F_{\frac{9}{2}}\ensuremath{\rightarrow}^{4}I_{\frac{15}{2}}$) quantum yield decreases from 26 to 2.9%. The large variation in these radiative-quantum yields is analyzed in terms of multiphonon decay processes ($^{4}S_{\frac{3}{2}}\ensuremath{\rightarrow}^{4}F_{\frac{9}{2}}$), which are seen to be sensitive to ${\mathrm{Er}}^{3+}$-site symmetry, but relatively insensitive to the exact nature of the compensating species. The crystal field splitting of the $^{4}I_{\frac{15}{2}}$ ground state of ${\mathrm{Er}}^{3+}$ in ${C}_{2v}$ symmetry is in good agreement with that expected from the cubic-field approximation of Lea, Leask, and Wolf, with crystal field parameters, ${A}_{4}〈{\mathcal{r}}^{4}〉=\ensuremath{-}245$ ${\mathrm{cm}}^{\ensuremath{-}1}$ and ${A}_{6}〈{\mathcal{r}}^{6}〉=40$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The noncubic portion of the total orthorhombic field may be accounted for in terms of an axial distortion along the ${\mathrm{Er}}^{3+}$-${M}^{+}$ direction, as verified by the excellent agreement obtained between the EPR $g$ values and the optical splitting of the ${\ensuremath{\Gamma}}_{8}^{(1)}$ state of the $^{4}I_{\frac{15}{2}}$ multiplet.

Mössbauer Study of Some 2–17 Lanthanide-Iron Compounds

We have studied the iron-rich, rare-earth-iron intermetallic compounds R2Fe17 (R = Pr, Gd, Tm, Lu), using the Mössbauer effect in 57Fe. The measured average hyperfine field H(T/Tc) acting on the 57Fe nuclei is a unique, monotonically decreasing function of T/Tc, with no observed irregularity at temperatures corresponding to a suspected ferromagnetic-antiferromagnetic phase transition in these compounds. The onset of paramagnetism occurs at temperatures close to the upper transition points measured by Strnat et al., namely at Tc = 282°, 472°, 271°, and 263°K for Pr2Fe17, Gd2Fe17, Tm2Fe17, and Lu2Fe17, respectively. Low-temperature (≃90°K) spectra indicate the presence of at least four magnetically nonequivalent Fe sublattices, while for T>Tc broadened 2-line absorption spectra typical of 57Fe at noncubic sites in paramagnetic material are observed.

Stimulated Emission from Nd3+ and Yb3+ in Noncubic Sites of Neodymium- and Ytterbium-Doped CaF2

Three new laser lines are reported: Two lines, at 1.0448 and 1.0661 μ, occur for a CaF2 crystal which is heavily doped with NdF3; a line at 1.0336 μ occurs for a CaF2 crystal containing NdF3 and YbF3. Details of the absorption and fluorescence spectra indicate that the laser oscillations arise from Nd3+ ions at noncubic sites in the first crystal and from Yb3+ ions in the second crystal. The growth techniques are summarized; the special growth procedures minimize the oxygen content, thereby reducing the number of impurities in sites for Nd3+ and Yb3+.

Reactions between Vacancies and Impurities in Magnesium Oxide. II. Mn4+-Ion and OH−-Ion Impurities

Reactions between cation vacancies and Mn4+- and OH−-ion impurities in MgO are studied as a function of temperature using optical techniques to determine the concentrations of reaction components. The emission spectrum of the Mn4+ ion in both cubic and noncubic sites in single crystals of MgO is reported, and several new features of the OH−-ion absorption spectrum are discussed. The experiments have enabled the identification of the spectral lines due to Mn4+ and OH− in isolated and associated sites. Calculation of the OH−-ion eigenfrequencies for ions in various sites, using a point-charge model, are found to be in good agreement with the experimental results. Finally, observations of the x-ray-induced V′ center (the Seitz V1 center) absorption are found to be consistent with the results of impurity-ion—vacancy interaction.