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We report the results of theoretical calculations of copper K-edge x-ray-absorption near-edge spectra (XANES) in ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ and related oxides ${\mathrm{Cu}}_{2}$O, CuO, and ${\mathrm{KCuO}}_{2}$. The final bound states were obtained from the self-consistent-field discrete-variational X\ensuremath{\alpha} method (SCF DV X\ensuremath{\alpha}), and continuum states were found by the multiple-scattering method using the muffin-tin truncation of the SCF DV X\ensuremath{\alpha} potentials. Composition of the final-state wave functions was analyzed. The 1s ionization potentials of the three reference compounds obtained from the SCF DV X\ensuremath{\alpha} transition-state calculations were used to set up the relative energy scale for the calculated cross section. The principal features of measured Cu K-edge XANES for ${\mathrm{Cu}}_{2}$O, CuO, and ${\mathrm{KCuO}}_{2}$ were reproduced satisfactorily by our calculations along with their relative energy positions. Our calculated polarized XANES of ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ were compared with measured spectra, and two shakeup features were identified with radiation polarized along the crystal c axis. By quantitatively comparing the measured spectra with a model based on our calculated cross sections, their intensities were found to be \ensuremath{\sim}24% and 10% of the main transition with shakeup energies of 5.7 and 9.5 eV, respectively. We suggest that these multielectron excitation features involve Cu 3d\ensuremath{\rightarrow}4p transitions.

The M\"ossbauser effect of the 80.6-keV resonance in $^{166}\mathrm{Er}$ has been used to study the magnetic and electronic properties of Er${\mathrm{H}}_{2}$. The onset of magnetic hyperfine splitting shows a magnetic transition occurring at 2.4 \ifmmode\pm\else\textpm\fi{} 0.1 K. The extrapolated saturation hyperfine field is 2480 \ifmmode\pm\else\textpm\fi{} 20 k${\mathrm{O}}_{\mathrm{e}}$, corresponding to a magnetic moment of $(2.75\ifmmode\pm\else\textpm\fi{}0.08){\ensuremath{\mu}}_{B}$ on the Er ion in the ordered state. Measurements of the hyperfine splitting in an external magnetic field show that the crystal-field ground state is a ${\ensuremath{\Gamma}}_{6}$ Kramers doublet. These data, as well as an analysis of previously published magnetic-susceptibility data, indicate that the first excited crystal-field level is \ensuremath{\gtrsim} 150 K above the ground state. Measurements of the hyperfine spectra of Er as a solute in Y${\mathrm{H}}_{2}$ give a ${\ensuremath{\Gamma}}_{7}$ ground state, which can arise from a small change in the ratio of fourth-order to sixth-order crystal-field parameters on going from this case to that of Er${\mathrm{H}}_{2}$. All of the above results can be understood on the basis of a "hydridic" (i.e., negatively charged) model for hydrogen in this compound.

The measurement of Cu K-edge x-ray absorption near-edge structure of monovalent, divalent, and trivalent copper oxides has been used to identify the presence of different valence of copper in the La/sub 2-//sub x/(Sr,Ba)/sub x/CuO/sub 4/ (x = 0.0--0.3) system. The results indicate the coexistence of Cu II and Cu III states in these compounds. Theoretical calculations based on a cluster of CuO/sub 4/O/sub 2/La/sub 8/LaSr predict the presence of nearly degenerate electronic ground states representing nominally Cu II and Cu III configurations.

We present here the results of our detailed M\"ossbauer measurements on the mixed-valence system ${\mathrm{EuNi}}_{2}$${\mathrm{P}}_{2}$. The linewidth of the resonance exhibits a broad peak at \ensuremath{\sim}40 K. We argue that this feature is anomalous in that it cannot be understood in terms of the known mechanisms that can give rise to line broadening. The system exhibits strong mixed valence---the average valence being \ensuremath{\sim}2.5---even at T=1.4 K. This is the first Eu-based material having truly mixed valence behavior as T\ensuremath{\rightarrow}0; all others tend to approach the trivalent configuration as T\ensuremath{\rightarrow}0.

The M-edge x-ray-absorption near-edge spectra was calculated for bulk ${\mathrm{UO}}_{2}$ and gas-phase ${\mathrm{UCl}}_{4}$ from a multiple-scattering approach within the muffin-tin--potential approximation. The muffin-tin potential was constructed from the self-consistent relativistic discrete-variational X\ensuremath{\alpha} potential. The spectra were interpreted by analyzing the photoelectron-trapping time in different channels inside each atomic sphere. The energy levels of bound states down to the valence levels of the ligands were calculated within the same framework as the continuum in order to set the absolute energy scale. The calculated spectra were compared and found to agree quantitatively with our experimental results.

The magnetic moment on the Er atom in the reentrant superconductor Er${\mathrm{Rh}}_{4}$${\mathrm{B}}_{4}$ at 100 mK is found from M\"ossbauer-effect measurements to be $(8.3\ifmmode\pm\else\textpm\fi{}0.2){\ensuremath{\mu}}_{B}$. This value is 30% larger than that found by neutron diffraction, indicating that only a fraction of the moment shows long-range magnetic order. The combination of M\"ossbauer and neutron diffraction results suggest that it is the presence of superconducting fluctuations in the magnetically ordered state which affects the nature of the ordered state.

The compound Ce${\mathrm{Rh}}_{3}$${\mathrm{B}}_{2}$ orders magnetically with a high Curie temperature (${T}_{\mathrm{Cj}}$) of 115 K but with a low moment of only ${0.4\ensuremath{\mu}}_{B}$. ${L}_{\mathrm{III}}$ absorption-edge measurements show a dominant absorption peak at the energy corresponding to trivalent cerium. Magnetic studies on the compounds $\mathrm{Ce}{({\mathrm{Rh}}_{1\ensuremath{-}x}{T}_{x})}_{3}{\mathrm{B}}_{2}$ with $T=\mathrm{Ru}\mathrm{and}\mathrm{Os}$ and Os reveal that the magnetic state is very rapidly broken up with the replacement of Rh by Ru and Os. These results, along with the observation of a high ${T}_{C}$ in Ce${\mathrm{Rh}}_{3}$${\mathrm{B}}_{2}$, suggest that magnetism in this compound arises from a strong hybridization of nearly localized or slightly delocalized Ce $4f$ electrons with conduction electrons. The small moment may be due to a Kondo-type interaction coupled with crystal-field effects.

We have used rare gas matrix isolation techniques in combination with extended x-ray absorption fine structure (EXAFS) to study the variation in interatomic distances for small Fe molecules in solid neon. A considerable contraction in the interatomic distances was observed for the metal molecules. An Fe-Fe distance of 2.02 \ifmmode\pm\else\textpm\fi{} 0.02 \AA{} for the lowest concentration of metal was observed. This is in good agreement with early EXAFS measurements in ${\mathrm{Fe}}_{2}$-Ar. We also carried out a careful study of the x-ray-absorption near-edge structure (XANES), and observed the appearance of considerable structure for a 1.5-at.% Fe sample. The XANES spectra were analyzed in terms of $1s$-to-($3d,4s$) and $1s$-to-$4p$ transitions.

The /sup 155/Gd Moessbauer-effect measurements in GdMo/sub 6/S/sub 8/ and GdMo/sub 6/Se/sub 8/ are reported. A quadrupole interaction of 545 +- 40 and 470 +- 60 MHz is measured at /sup 155/Gd in GdMo/sub 6/S/sub 8/ and GdMo/sub 6/Se/sub 8/, respectively. This indicates the presence of a large B/sub 2//sup 0/O /sub 2//sup 0/ term in the crystalline-electric-field (CEF) Hamiltonian in all rare-earth Chevrel-phase sulfides and selenides. The results imply that the previous analyses of CEF effects in these materials based on a cubic Hamiltonian are not valid. The importance of the CEF in understanding various measured magnetic and superconducting properties of the RMo/sub 6/S/sub 8/ compounds, where R denotes rare earth, is discussed.

Moessbauer-effect studies and susceptibility measurements have been carried out to characterize the magnetic and structural properties of the Chevrel-phase compound FeMo/sub 6/S/sub 8/. The Moessbauer spectra indicate that a structural phase transition occurs at T/sub 0/approx. =100 K from an ordered form at low temperature to a disordered form of the material at high temperature, accompanied by a hardening of Fe vibrational modes. Susceptibility measurements give average magnetic moments of 1.61..mu../sub B//Fe atom and 1.42..mu../sub B//Fe atom in the regions below and above T/sub 0/, respectively. An antiferromagnetic transition is observed at a temperature T/sub N/approx. =41 K. Moessbauer spectra obtained below T/sub N/ show a distribution of magnetic hyperfine fields which are attributed to Fe atoms being distributed among the crystallographically equivalent sites in the low-temperature structure.

The electronic and magnetic structures of Gd in superconducting GdBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/ have been probed using the 86.5-keV Moessbauer resonance transition in /sup 155/Gd. The values of the hyperfine interaction parameters provide direct evidence for an absence of conduction electrons at the Gd site in this lattice and hence no exchange coupling of the Gd ion with the conduction electrons. The results explain why the superconducting transition temperature of YBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/ is not suppressed by the substitution of Y atoms by magnetic Gd atoms.

The compound ${\mathrm{CeRh}}_{3}$${\mathrm{B}}_{2}$ is magnetically ordered with a low saturation moment of \ensuremath{\sim}0.4 ${\ensuremath{\mu}}_{B}$ per formula unit and an unusually large Curie temperature of 115 K while isostructural ${\mathrm{LaRh}}_{3}$${\mathrm{B}}_{2}$ is superconducting with a transition temperature of \ensuremath{\sim}2.3 K. Magnetic and superconducting studies on the series of compounds ${\mathrm{Ce}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{La}}_{\mathrm{x}}$${\mathrm{Rh}}_{3}$${\mathrm{B}}_{2}$ reveal that the magnetic state persists for 0\ensuremath{\le}x\ensuremath{\le}0.8. The magnetic moment per Ce ion goes through a maximum and the Curie temperature decreases as La is substituted for Ce. On the La-rich side, superconductivity is destroyed even with 1% replacement of La by Ce, indicating a strong coupling of Ce 4f moments with conduction-electron spins. The temperature dependence of the upper critical field of ${\mathrm{LaRh}}_{3}$${\mathrm{B}}_{2}$ has been measured and is discussed in terms of Werthammer-Helfand-Hohenberg theory.

The Moessbauer resonance of the 42.9-keV ..cap alpha.. transition in /sup 240/Pu has been observed. As source activity, the ..cap alpha.. parent /sup 244/Cm, with half-life of 18.1 yr, was used. For a CmO/sub 2/ source and a PuO/sub 2/ absorbed at 4.2 and 77 K, a single resonance line with a width of about 20% larger than the natural width of 40 mm/sec was observed. In the hexavalent plutonyl compound PuO/sub 2/C/sub 2/O/sub 4/, a quadrupole interaction of vertical-bar e/sup 2/qQ vertical-bar = 95 mm/sec was found. No isomer shifts could be detected.

Extended x-ray-absorption fine structure was used to determine the nearest-neighbor distance for silver particles isolated in solid argon. The silver particles were produced in the gas phase with the use of the gas-aggregation technique. In this technique the metal atoms are evaporated in an argon atmosphere in the pressure range 0.1-10 Torr; a cooling of the metal atoms takes place through collision with cold gas atoms. The metal aggregates are then transported by the gas stream through an aperture into a liquid-helium cryopump, where most of the gas is condensed. The metal flow rate is monitored by a quartz oscillator. The size of the metal clusters can be selected by changing the argon pressure, density of the metal atom vapors, and geometric configuration of the cell. The size distribution was determined by introducing a sample holder into the molecular beam, sampling an atomic quantity of approximately ${10}^{15}$ atoms/${\mathrm{cm}}^{2}$, and evaluating the corresponding electron micrographs. Particles with average sizes between 25 and 130 \AA{} were studied. A careful analysis of the data was performed with a thin silver foil at 78 K as a standard. No remarkable effects were observed in the near-edge region of the $K$ edge, but there was a noticeable contraction of the nearest-neighbor distance. This contraction can be explained if we assume the presence of surface stresses. There is good agreement between the observed contraction and the calculated values.

Moessbauer spectroscopy has been used to investigate the crystal-field levels of Yb/sup 3 +/ in Yb/sub 2/Ti/sub 2/O/sub 7/. The hyperfine field observed when an external field is applied provides a local magnetization measurement which has been interpreted in terms of an appropriate crystal-field Hamiltonian. A procedure is described for performing a powder average to include the effects of magnetic anisotropy in the calculations. Combining the current results and previous measurements on isomorphic compounds gives crystal-field values of B/sup 0//sub 2/ = 15--21 K and B/sup 0//sub 4//B/sup 0//sub 2/ = 0.01.

There are many restrictions on using the white-noise approximation to describe the line shape of radiation emitted by a Mossbauer atom experiencing paramagnetic relaxation. Using a resolvent operator method we have obtained an expression for the line shape without making such an approximation. As a result the relaxation matrix depends on the hyperfine frequencies as well as on correlation functions of the bath variables. The situation has been illustrated for the case of spin-spin relaxation of a "rigid lattice" using an exponential form for the correlation function. The calculations have been utilized to analyze the Mossbauer relaxation spectra of Cs 2 NaYbCl 6 measured using the 84.6-keV transition in 170 Yb. The analysis permits us to deduce the dipolar field and the correlation time for the spin bath in this material.

Extended x-ray-absorption fine-structure (EXAFS) and x-ray-diffraction studies have been conducted on the Chevrel-phase superconductor SnFe/sub 0.05/Mo/sub 6/S/sub 8/. The site symmetry of the iron is found to be similar to that of copper in the high-temperature phase of Cu/sub 1.8/Mo/sub 6/S/sub 8/. EXAFS analysis yielded interatomic distances, e.g., Fe-Sn and Fe-S, in this compound. The Fe-S distances are found to be characteristic of iron sulfides (e.g., FeS and Fe/sub 7/S/sub 8/). Through the observation of focused multiple scattering, a distance corresponding to a linear array of Fe-Sn-Fe atoms was measured. Iron atoms were found to be situated in two different configurations, one where iron has only a tin neighbor and the other where a linear-chain-like configuration of Fe-Sn-Fe atoms occurs.

Electronic structure calculations were performed for clusters representing the Chevrel-phase ${\mathrm{SnMo}}_{6}$${\mathrm{S}}_{8}$, with and without oxygen doping. In order to obtain the local structure around the Sn atom, extended x-ray-absorption fine-structure (EXAFS) measurements were made with synchro- tron radiation. The interatomic distances obtained experimentally were used in the calculations. The effect of oxygen doping on the M\"ossbauer isomer shift and quadrupole splitting values of $^{119}\mathrm{Sn}$ was investigated theoretically and compared with reported experimental values. The effect of oxygen substitution on the density of states at the Fermi energy of the [${\mathrm{Mo}}_{6}$${\mathrm{S}}_{8}$${]}^{2\mathrm{\ensuremath{-}}}$ cluster was also studied. The results suggest that oxygen doping does not alter significantly the electronic structure of ${\mathrm{SnMo}}_{6}$${\mathrm{S}}_{8}$.

Magnetic susceptibility and /sup 119/Sn Moessbauer studies have been carried out on a new series of rare-earth-containing Heusler alloys having the formula Pd/sub 2/RSn (R = Tb--Yb). The Tm and Yb compounds in this series along with Lu, Sc, and Y compounds are superconducting. The magnetic susceptibility of Pd/sub 2/TmSn and Pd/sub 2/YbSn deviates from the Curie-Weiss behavior at low temperatures indicating that crystalline electric field effects are appreciable. Analysis of the magnetic susceptibility data in terms of crystalline electric fields shows that the ground state of the Tm/sup 3 +/ ion is nonmagnetic which, therefore, does not affect the superconducting state. The susceptibility of Yb/sup 3 +/ ions at low temperatures is also reduced from the free-ion value due to the crystalline electric field interaction. The Tb- and Dy-containing compounds order antiferromagnetically with Neel temperatures of 9 and 15 K, respectively, while Er- and Ho-containing compounds are paramagnetic down to 1.4 K. The hyperfine field at the Sn site in Pd/sub 2/TbSn is about 50 kOe but is only about 10 kOe in Pd/sub 2/DySn at 4.2 K.

Theoretical x-ray-absorption near-edge structure (XANES) and extended x-ray-absorption fine structure (EXAFS) of molecular and crystalline ${\mathrm{KrF}}_{2}$ and gas-phase ${\mathrm{FeCl}}_{2}$ have been obtained by using a modified X\ensuremath{\alpha} scattered-wave procedure. EXAFS calculations are made in the single-scattering approximation. Effects of multiple scattering near the absorption edge have been investigated by comparing predicted XANES and EXAFS spectra in the near-edge region. Two exchange-correlation potentials, X\ensuremath{\alpha} and energy-dependent Dirac (ED), were incorporated in the EXAFS calculations. Comparison between experimental and theoretical results confirm that the ED potential is more valid for EXAFS spectra. Molecular field splitting as well as resonance coupling were demonstrated by comparing the theoretical XANES of the isolated ${\mathrm{KrF}}_{2}$ molecule to that of ${\mathrm{FeCl}}_{2}$. The single-particle XANES calculation of molecular ${\mathrm{FeCl}}_{2}$ is compared with the experimental crystalline ${\mathrm{FeCl}}_{2}$ results of Stern.

Detailed line shapes have been calculated of M\"ossbauer spectra for comparison to Oosterhuis and Lang's data of ${\mathrm{K}}_{3}$Fe${(\mathrm{CN})}_{6}$ diluted in diamagnetic ${\mathrm{K}}_{3}$Co${(\mathrm{CN})}_{6}$. Spin-relaxation effects have been included by utilizing the full hyperfine interaction (including all off-diagonal terms) within the super-operator formalism of relaxation theory. In order to describe the data, it is necessary to assume a distribution of relaxation rates corresponding to differing Fe near-neighbor and next-near-neighbor environments. Using a simplified model containing three relaxation rates, the concentration and temperature dependence of the spectra have been explained. The relaxation rates obtained are compared with those from ESR studies, and the differences in the results of the two methods discussed.

We have used crystal-field theory to discuss susceptibility data for the compounds ${\mathrm{Cs}}_{2}\mathrm{Na}R{\mathrm{Cl}}_{6}$ where $R=\mathrm{C}\mathrm{e},\phantom{\rule{0ex}{0ex}}\mathrm{N}\mathrm{d},\phantom{\rule{0ex}{0ex}}\mathrm{T}\mathrm{b},\phantom{\rule{0ex}{0ex}}\mathrm{D}\mathrm{y},\phantom{\rule{0ex}{0ex}}\mathrm{H}\mathrm{o},\phantom{\rule{0ex}{0ex}}\mathrm{T}\mathrm{m},\phantom{\rule{0ex}{0ex}}\mathrm{a}\mathrm{n}\mathrm{d}\phantom{\rule{0ex}{0ex}}\mathrm{Y}\mathrm{b}$. These materials have an octahedral coordination for the lanthanide ion which remains undistorted even at low temperatures. A combined analysis of all the susceptibility data provides the crystal-field interaction parameters ${A}_{4}〈{r}^{4}〉=254〈{r}^{4}〉$ and ${A}_{6}〈{r}^{6}〉=4〈{r}^{6}〉$ K with $〈{r}^{4}〉$ and $〈{r}^{6}〉$ in atomic units. The numerical values ${A}_{4}$ and ${A}_{6}$ are independent of the lanthanide ion within experimental error. Attempts to explain these results within a simple point-charge model were unsuccessful.

Results of magnetization, /sup 57/Fe, /sup 155/Gd, and /sup 166/Er Moessbauer-effect measurements on Gd/sub 2/Fe/sub 3/Si/sub 5/ and Er/sub 2/Fe/sub 3/Si/sub 5/ are presented. There is no magnetic moment on Fe. The Gd Moessbauer results are used to construct an approximate model of the crystalline--electric-field effects in the heavy rare-earth members of the R/sub 2/Fe/sub 3/Si/sub 5/ family. This model predicts large magnetic anisotropy in these materials, explains the well-developed static hyperfine interaction seen in the Er resonance spectra up to 4.2 K, and overestimates the deviations from de Gennes--law scaling in the magnetic transition temperature.

The compound ${\mathrm{Pd}}_{2}$YSn, having the cubic Heusler L${2}_{1}$ structure, is superconducting with a transition temperature (${T}_{c}$) of 4.55 K. The effect on ${T}_{c}$ of replacing Y by other magnetic rare-earth metal has been investigated in the series ${\mathrm{Pd}}_{2}$${\mathrm{Y}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{R}}_{\mathrm{x}}$Sn (R=Gd, Dy, Ho, Er, Tm, and Yb). The depression in ${T}_{c}$ is the largest with Gd substitution. The initial rate of depression of ${T}_{c}$ shows deviation from the de Gennes function. The results of ${T}_{c}$ measurements are analyzed in terms of Abrikosov-Gor'kov theory, taking into account the effects of crystalline electric fields acting on rare-earth ions. Values of the density of states times the exchange constant, [N(0)${J}_{\mathrm{sf}{}^{2}}$], are derived from these measurements.

The electric-quadrupole hyperfine interaction at dilute tantalum impurities in lutetium metal has been measured between 10 and 655 K using time-differential perturbed angular correlations of the 133--482-keV cascade in /sup 181/Ta. The temperature dependence of the quadrupole frequency is described by the linear temperature relation n/sub Q/(T) = )338.7(11)(1-4.05(14) x 10/sup -4/T)) MHz. This result contradicts the assumptions of the phenomenological model for interaction between open 4f-orbital hosts and dilute closed-shell impurities previously proposed by the authors, whereby the system LuTa should exhibit the usual T/sup 1.5/ temperature dependence. The effect on closed-shell probes of the crystal-field-induced quadrupolar distortion of open-orbital host ions, if present at all, is much more complex than previously assumed.