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We have made M\"ossbauer-effect-spectroscopy (MES) measurements of the dynamical broadening of the M\"ossbauer lines above ${\mathit{T}}_{\mathit{N}}$ for a high-quality single crystal of ${\mathrm{Fe}}_{0.9}$${\mathrm{Zn}}_{0.1}$${\mathrm{F}}_{2}$ with an inhomogeneity-induced N\'eel-temperature (${\mathit{T}}_{\mathit{N}}$) broadening of less than 25 mK. In the reduced-temperature range 3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$t (==T/${\mathit{T}}_{\mathit{N}}$-1)${10}^{\mathrm{\ensuremath{-}}2}$, we find that the excess linewidth diverges as ${\mathit{t}}^{\mathrm{\ensuremath{-}}\mathit{w}}$ with w=0.79(7). On the assumption of a single divergent relaxation time, this measurement of w implies a dynamical exponent z=2.18(10) via dynamic-scaling theory. Our value for z involves an asymptotic approach to ${\mathit{T}}_{\mathit{N}}$, which is an order of magnitude closer than previous MES and neutron-spin-echo measurements and agrees with the renormalization-group prediction z\ensuremath{\approxeq}2.11 for the d=3 random-exchange Ising model. The result does not agree with the predictions of conventional van Hove theory, z=1.97, or the results of previous, fundamentally less asymptotic, measurements.

The three inequivalent Ta sites in the basis of 1T-${\mathrm{TaSe}}_{2}$ predicted by the ${\mathit{n}}_{\mathit{A}}^{\mathrm{Ta}}$:${\mathit{n}}_{\mathit{B}}^{\mathrm{Ta}}$:${\mathit{n}}_{\mathit{C}}^{\mathrm{Ta}}$=1:6:6 charge-density-wave (CDW) models have been observed at 4.2 K in an unstrained single crystal with use of high-resolution $^{181}\mathrm{Ta}$ M\"ossbauer effect. The data show that site A has only slightly higher isomer shift (IS) than site B and is different from site C, thereby determining the sign of the CDW. The IS results suggest that a simple sinusoidal model is insufficient to explain the data; harmonics have to be taken in consideration. The CDW amplitudes of the three sites are +0.48(5), +0.44(5), and -0.52(10) electron relative to ${\mathrm{Ta}}^{4+}$. The results are understood on the basis of a 13-Ta-atom Star-of-David-shaped-cluster model including the effect of three-dimensional ordering of the CDW.

Mesure precise de la susceptibilite magnetique de la phase icosaedrique pour une concentration en fer comprise entre 0 et 5. La reponse magnetique n'est pas completement independante de x. Pour de faibles concentrations, le fer augmente le moment magnetique moyen par metal de transition. Explication possible en termes d'un modele de phase α a «liaison brisee»

The now voluminous and often conflicting literature concerning the number and identification of the locations, valencies and spin quantum states adopted by iron when doped into the high- T c superconductor YBa 2 Cu 3 O y is analyzed. Although it now seems clear that the details depend on sample thermal history as well as stoichiometry, a considerable amount of experimental evidence pertaining to the problem appears to be at least qualitatively well established. By looking at the totality of this evidence a number of conclusions have been reached. It is clear that iron in YBa 2 Cu 3− x Fe x O y always replaces copper in some general sense and can do so both in the unique Cu(2) site as well as in several, but not all, of the n -coordinated (2⪕ n ⪕6) ligand geometries which can be envisaged in an oxygen disordered Cu(1)-plane. The allowed ligand coordinations are identified and, by correlating them with their Mossbauer signatures, their population trends as functions of x and y are established. Associated evidence relating to the oxidation state and spin quantum number of iron at each topologically different location is assembled and analyzed. The preponderance of evidence points to the presence of a mixture of valencies and of both high and intermediate spin states.

Giant magnetoresistance in a spinodally decomposed, bulk 60 Cu‐20 Ni‐20 Fe alloy is reported. An annealed, quenched, and heat‐treated sample with a compositional modulation of ≲50 A size exhibits a ΔR/R value as high as 9% at 4.2 K. Optimization of the ferromagnetic phase particle geometry through a combination of spinodal decomposition and uniaxial deformation led to a locally multilayered, superlattice‐like structure and a dramatic increase in room‐temperature magnetoresistance from ∼0.6 to ∼5%. This improvement in magnetoresistance is accompanied by a decrease in coercivity from ∼620 Oe in the fully decomposed material to ∼45 Oe in the optimized structure. Interestingly, this structure no longer exhibits the commonly observed temperature‐dependent behavior of ΔR/R increasing at low temperatures, but rather shows a decrease at 4.2 K.

The Mossbauer effect has been used to measure the hyperfine field of 57Fe nuclei in the layered antiferromagnet Ba2FeF6. The temperature dependence of the sublattice magnetization was measured an order of magnitude closer to the Neel temperature than previous studies. A range of fit analysis to the power law equation h(t)=Btβ, yields β=0.135(3) and TN=47.96(4) K for 4.31×10−3

A study of the Au197 Mossbauer resonance is reported as a function of oxygen stoichiometry for gold‐doped YBa2Cu3O7−δ. Three separate Mossbauer features are identified and correspond respectively to gold metal particles, Au3+ in a square‐planar Cu(1) site, and Au+ in a two‐ligand c‐axis coordinated Cu(1) site. All the gold successfully incorporated into the 123‐structure is trivalent at full oxygenation (O7.02) but is reduced as a function of decreasing oxygen content until only monovalent gold remains at the orthorhombic to tetragonal transition (≊O6.4). A significant fraction of the Au3+ in the oxygenated sample resides near grain boundaries and is precipitated as gold particles upon reduction. The observation of monovalent gold in the reduced sample is the first report of the existence of Au+ with uniquely oxygen coordination for any material.

The Mossbauer effect of the 37.2 keV transition of 121Sb and the 23.88 keV γ transition of 119Sn has been employed to study the electronic configuration of Sb and Sn ions in the conducting materials BaSn1−xSbxO3 for 0≤x≤0.2. A single‐resonance absorption line is observed for each isotope. The isomer shifts change slightly with Sb content and fall in the region corresponding to Sb(V) and Sn(IV) valence states, respectively. The Sb isomer shift (IS) of 7.5 mm/s is significantly depressed (∼1 mm/s) relative to its value in a typical Sb(V)O6 chromophore and indicates a high degree of covalency, corresponding to significant 5sm5pn hybridization. The Sn IS of 0.1 mm/s indicates that the Sn atoms in BaSn1−xSbxO3 are more covalent than in BaSnO3. Charge disproportionation of the Sn and Sb into Sn(II) and Sn(IV) or Sb(III) and Sb(V) has not been observed.

The Mossbauer effect of the 37.2 keV γ transition of 121Sb has been employed to study the electronic configuration of Sb ions in perovskite superconducting materials. The isomer shift (IS) changes with Sb content and falls in the region of the Sb(V) valence state. The IS increases from 7.00 mm/s for BaPb0.75Sb0.25O3 to 7.25 mm/s for BaPb0.75Bi0.25O3 and to 7.70 mm/s for (Ba0.6K0.4)BiO3 compounds, indicating a high degree of covalency, corresponding to changes in the 5sm5pn hybridization.

The Mossbauer effect of the 37.2 keV γ transition of 121Sb has been employed to study the electronic configuration of Sb ions in BaBi1−xSbxO3 with x=0.2, 0.4, and 0.5. A single resonance absorption line is observed. The isomer shift changes slightly with Sb content and falls in the region corresponding to an Sb(V) valence state. The isomer shift of 7.6 mm/s is significantly depressed (∼1 mm/s) relative to its value in a typical Sb(V)O6 chromophore and indicates a high degree of covalency, corresponding to significant 5sm5pn hybridization.

The Mossbauer effect of the 37.2 keV γ transition of 121Sb has been employed to study the electronic configuration of Sb ions in BaPb1−xSbxO3 superconducting materials. The isomer shift is constant as a function of Sb content and falls in the region correspoding to the Sb(V) valence state. The isomer shift of 7 mm/s is the lowest value yet observed for an Sb(V)O6 chromophore and indicates a high degree of covalency, corresponding to significant 5sm 5pn hybridization. The mixed valence state of an equal amount of Sb(III) and Sb(V) has not been observed.

M\"ossbauer-effect measurements on ${\mathrm{Al}}_{86}$${(}^{57}$${\mathrm{Fe}}_{0.02}$${\mathrm{Mn}}_{0.}$ 98)g14 in the icosahedral phase indicate a distribution of quadrupole-splitting magnitudes at the iron sites. This distribution strongly indicates the existence of more than two inequivalent Mn sites. It can be fitted at least as well with a continuous distribution of sites of the form suggested by an amorphous analog as with a discrete set.

The room-temperature paramagnetic and 4.2K Zeeman 57Fe Mossbauer spectra are reported and analysed for both crystalline (c-) and amorphous (a-) NaFeF4. The results point strongly to the presence, in a-NaFeF4, of distorted FeF6 octahedra which are locally coordinated to (predominantly) four other octahedra via a linkage in which the two non-bridged, or terminal, fluorine bonds are in a trans-configuration ( approximately=180 degrees apart). This locally planar magnetic structure is a distorted form of that which occurs in c-KFeF4 and c-RbFeF4, but is different from that in c-NaFeF4 for which the smaller sized alkali-metal ions induce a puckering of the quadratic layer iron sublattice via a cis-terminal ( approximately=90 degrees ) coordination configuration of the FeF4 octahedra. It is concluded that the crystalline constraint which produces this unusual cis-coordination in c-NaFeF4 is sufficiently relaxed in the amorphous state that the more common trans-coordination is restored. Amorphous NaFeF4 should therefore be pictured as a continuous random network of predominantly four-coordinated trans-terminal FeF6 octahedra with sodium ions filling the holes. Since 4.2K is more than one third of the spin-freezing temperature ( approximately=12K) of a-NaFeF4, significant effects relating to thermally induced spin fluctuations are observed in the amorphous spectrum at liquid helium temperature, and are interpreted self-consistently within the framework of the basic model.

We have found the true stoichiometry of the phase first reported as ${\mathrm{Fe}}_{0.25}$${\mathrm{Nb}}_{0.75}$${\mathrm{Se}}_{12}$, and later as Fe${\mathrm{Nb}}_{3}$${\mathrm{Se}}_{10}$, to be ${\mathrm{Fe}}_{1+x}{\mathrm{Nb}}_{3\ensuremath{-}x}{\mathrm{Se}}_{10}$, with $0.25\ensuremath{\le}x\ensuremath{\le}0.40$. Compounds with the Fe${\mathrm{Nb}}_{3}$${\mathrm{Se}}_{10}$ structure type are relatively rare among transition-metal selenides. We report the synthesis and temperature-dependent magnetic susceptibility and resistivity of the new compounds ${\mathrm{Fe}}_{1.33}$${\mathrm{Ta}}_{0.67}$${\mathrm{Nb}}_{2}$${\mathrm{Se}}_{10}$ and ${\mathrm{Fe}}_{1\ensuremath{-}y}{\mathrm{V}}_{1+y}{\mathrm{Nb}}_{2}{\mathrm{Se}}_{10}$, with $0.0\ensuremath{\le}y\ensuremath{\le}0.5$. The magnetic susceptibility and M\"ossbauer-effect measurements on ${\mathrm{Fe}}_{1+x}{\mathrm{Nb}}_{3\ensuremath{-}x}{\mathrm{Se}}_{10}$ are dominated by the Fe-Nb disorder on the zigzag octahedral chains.

We report magnetic and M\"ossbauer studies of the lithium intercalation compounds ${\mathrm{Li}}_{x}{M}_{y}{\mathrm{V}}_{1\ensuremath{-}y}{\mathrm{S}}_{2}$, where we have $M=\mathrm{Fe}\mathrm{or}\mathrm{Cr}$, and we have $xl~1$ and $yl~\frac{1}{2}$ for Fe or $yl~1$ for Cr. In some cases, the valence and magnetic state of the transition metals can be determined unambiguously from these data. When we have $M=\mathrm{Fe}$ and $x=0$ (no Li), the Fe is divalent and low spin ($S=0$), and some compensating pentavalent V must be produced according to ${\mathrm{Fe}}_{y}^{2+}{\mathrm{V}}_{2y}^{5+}{\mathrm{V}}_{1\ensuremath{-}3y}^{4+}{\mathrm{S}}_{2}$. When $x=1$ (fully lithiated) the Fe remains divalent and low spin only at low-Fe concentrations ($y\ensuremath{\lesssim}0.1$) but some of the V atoms develop a magnetic moment. At higher-Fe concentrations ($x=1$, $y\ensuremath{\gtrsim}\frac{1}{4}$) some of the Fe atoms also develop a moment. These latter materials are quite complicated. Similar results are observed when $M=\mathrm{Cr}$; i.e., no moments are observed at $x=0$, but at $x=1$ moments appear probably associated with the Cr atoms. In either case ($M=\mathrm{Fe}\mathrm{or}\mathrm{Cr}$) phase transitions apparent in pure ${\mathrm{Li}}_{x}\mathrm{V}{\mathrm{S}}_{2}$ are suppressed by the cation substitution.

M\"ossbauer quadrupole splitting (QS) measurements are reported for ${\mathrm{Fe}}^{57}$ in vitreous YIG (${\mathrm{Y}}_{3}$${\mathrm{Fe}}_{5}$${\mathrm{O}}_{12}$). Electric-field-gradient (EFG) calculations from a computer model of amorphous YIG suggest a distribution $p(|V|)$ of principal EFG components $V$ which peaks at a nonzero value, is asymmetric, and falls off exponentially in the wings. A nonlinear least-squares fit to the QS data using a distribution $p(|V|)$ of Lorentzian lines establishes not only the asymmetry but also the detailed non-Gaussian form of the EFG distribution.

Elastic and inelastic neutron scattering experiments were performed on single and polycrystalline samples of BaMn${\mathrm{F}}_{4}$. The system orders magnetically below ${T}_{N}\ensuremath{\approx}26$ K and the magnetic structure is shown to be similar to other members of the $\mathrm{Ba}M{\mathrm{F}}_{4}$ family with $M=\mathrm{Fe}\mathrm{and}\mathrm{Ni}$, i.e., an antiferromagnetic ordering along both the $b$ and $c$ directions with a two-dimensional magnetic character perpendicular to $b$. The antiferromagnetic axis is rotated \ensuremath{\sim}9\ifmmode^\circ\else\textdegree\fi{} from the orthorhombic $b$ axis and the ordered ${\mathrm{Mn}}^{2+}$ moment is 4.8${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$. The spin-wave dispersion curves were measured and the exchange constants and anisotropy determined. The structural phase transition occurring at ${T}_{c}=247$ K was studied in detail and shown to be incommensurate, with superlattice peaks below ${T}_{c}$ at ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{1}=(0.392,0.5,0.5)$ and multiples of this: ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{2}=2{\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{1}$ and ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{3}=3{\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{1}$. These wave vectors do not change with temperature. The phase transition appeared to be continuous with $\ensuremath{\beta}=0.31\ifmmode\pm\else\textpm\fi{}0.05$. The dynamical studies of this transition show a soft mode which becomes overdamped at ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{1}$ but becomes underdamped as $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}$ increases away from ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}}_{1}$ and as $T\ensuremath{-}{T}_{c}$ increases. High-resolution studies also show the existence of a diverging central peak. A theoretical description of the phase transition suggests the existence of four modes below ${T}_{c}$, corresponding to fluctuations of the amplitude and phase of the order parameter. We have as yet been unable to detect these different modes.

Almost nothing is yet known about the nature of the ``frozen disorder'' of magnetic moments in concentrated magnetic spin glasses (or ``speromagnets'') such as (amorphous) a-YIG (yttrium iron garnet), a-${\mathrm{FeF}}_{3}$, and a-${\mathrm{NaFeF}}_{4}$, beyond the qualitative description of ``randomlike.'' This paper probes the nature of the nearest-neighbor spin correlations in such systems by analysis of hyperfine-field distributions as measured by 4.2-K M\"ossbauer spectroscopy. Low-field tails are found to exist on the hyperfine distribution spectra of amorphous speromagnets but not amorphous ferromagnets. They indicate the presence of low-energy exchange-field states in the former caused by the cancellation of nearest-neighbor contributions within the frustrated spin network. Their interpretation has been used to determine the detailed exchange distribution function for a-YIG, a-${\mathrm{FeF}}_{3}$, and a-${\mathrm{NaFeF}}_{4}$ and, by comparison of the results to the expectation for a set of randomly oriented spins, to monitor the nature of nearest-neighbor spin frustration in those amorphous speromagnetic systems.

A combination of magnetic susceptibility and Mossbauer measurements on quasicrystalline i­ Ah4Mn2o-xFexSi6 (0.02 ~ x ~ 7.5) establishes that Mn atoms in i-Ab4Mn2oSi6 occupy two distinct classes of sites, and that Fe substitutes for only one of them. The two classes are distinguished by the possession or otherwise of a localized magnetic moment. The data are consistent with a structure of in­ terconnecting Mackay icosahedra (MI) in which localized moments are possessed only by Mn atoms ad­ jacent to "broken" MI connections. The implied connectivity of the resulting MI network is close to that anticipated for a packing of MI on a three-dimensional Penrose-tile lattice.

A measurement of the six-line M\"ossbauer Zeeman spectrum of $^{57}\mathrm{Fe}$ in amorphous yttrium iron garnet at 4.2 K reveals a linewidth asymmetry involving the three line pairs ${L}_{i}$ and ${L}_{7\ensuremath{-}i}$, $i=1, 2, 3$. The major source of this asymmetry is found to be a positive correlation between isomer shift and hyperfine field with a degree of linearity of 18%. A complete linewidth analysis is given which also leads to separate estimates of the widths of the hyperfine field, isomer shift, and electric-field-gradient distributions in the glass.

The magnetic ordering in CsCo${\mathrm{Br}}_{3}$ has been studied by neutron diffraction, on both powder and single-crystal specimens, and by specific-heat and magnetic-susceptibility measurements. This compound has the hexagonal CsNi${\mathrm{Cl}}_{3}$ type of structure, and the susceptibility shows characteristic one-dimensional behavior, while the specific heat indicates that three-dimensional ordering occurs at about 28\ifmmode^\circ\else\textdegree\fi{}K. The neutron single-crystal study shows that between ${T}_{N}$ and 14\ifmmode^\circ\else\textdegree\fi{}K the magnetic structure is consistent with the orthorhombic space group $\mathrm{Cmc}{2}_{1}$, with one-third of the antiferromagnetic cobalt chains being disordered and the other two-thirds antiferromagnetically coupled in the basal plane. Between 4 and 10\ifmmode^\circ\else\textdegree\fi{}K the structure may be described by the space group $C{m}^{\ensuremath{'}}c{{2}_{1}}^{\ensuremath{'}}$, and is similar to the collinear arrangement reported for CsCo${\mathrm{Cl}}_{3}$ and RbCo${\mathrm{Br}}_{3}$ but with a small canting of about 10\ifmmode^\circ\else\textdegree\fi{}. The transformation from one structure to the other in the region of 10-14\ifmmode^\circ\else\textdegree\fi{}K can be explained equally well by either the formation of an intermediate phase of lower symmetry or by a discontinous process in which the two phases coexist. The powder data reveal a small amount of a second low-temperature phase compatible with the space ${C}_{P}{m}^{\ensuremath{'}}c{{2}_{1}}^{\ensuremath{'}}$, consistent with negative rather than positive next-nearest-neighbor interactions in the basal plane.

Previous studies of the intercalated layered compounds ${\mathrm{Li}}_{x}{\mathrm{V}}_{1\ensuremath{-}y}{\mathrm{Fe}}_{y}{\mathrm{S}}_{2}$ for $0.1\ensuremath{\le}y\ensuremath{\le}0.5$ are extended to lower concentrations, $0.1\ensuremath{\ge}y\ensuremath{\ge}0.01$, and to substitutional metals other than Fe (Co and Ni). When no Li is intercalated ($x=0$), no paramagnetic moments are observed independent of the substitutional metal (Fe, Co, or Ni), consistent with the previous report for Fe. At full intercalation ($x=1.0$), the present results show that the substitutional metals develop a paramagnetic moment. This moment is less than that expected for the divalent state of the substitutional metals in an ionic environment, suggesting partial delocalization of the substitutional metal $d$ electrons into the conduction band of the metallic host.

X-ray- and neutron-diffraction measurements on powder and single-crystal samples of BaMn${\mathrm{F}}_{4}$ have failed to show any evidence for the distortion with wave vector, in reciprocal-lattice units, $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}=(0.4,0,0.5)$ recently reported by Scott, Habbal, and Hidaka. The distorted phase which is observed below 247 K is described by a wave vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}=(\ensuremath{\zeta},0.5 ,0.5)$ with $\ensuremath{\zeta}$ incommensurate. There is a small (\ensuremath{\sim}1%) increase in the magnitude of $\ensuremath{\zeta}$ as the temperature is lowered to 100 K, and the detailed temperature dependence of $\ensuremath{\zeta}$ is found to be different on heating and on cooling. The low-temperature distortion is characterized by a single wave vector $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}=(\ensuremath{\zeta},0.5,0.5)$ rather than by a pair of wave vectors ($\ensuremath{\zeta},0.5,0.5$) and ($\ensuremath{\zeta},\ensuremath{-}0.5,0.5$).

Amorphous samples of Fe${\mathrm{F}}_{3}$, KFe${\mathrm{F}}_{4}$, and RbFe${\mathrm{F}}_{4}$ have been prepared by vapor deposition at liquid-nitrogen temperature, and room-temperature M\"ossbauer quadrupole spectra are reported for each. The spectra are broad and indicate the existence of a distribution of electric-field-gradient (EFG) magnitudes at the iron sites. Using a least-squares computer fit to the data determined from a Lorentz broadened EFG distribution function, we are able to make separate determinations of the Lorentz widths of the individual transitions and of the form of the EFG distribution. We find a natural Lorentz width of 0.20 mm/s and an EFG distribution function of asymmetric Gaussian form. The latter has the same shape for all three materials and is both significantly broader and more asymmetric about its peak than predicted by random-packing models using hard spheres and point-charge summations. An analytic shell-model distribution function is shown to be an accurate description of EFG histograms for random-packing models but is unable to provide a quantitative description of the measured quadrupole spectra.

A parametric representation of critical temperature (T c ) vs Tl/Cu for the crossection Tl x Ba 2 Ca 2 Cu 3 O y shows evidence of all known superconducting compounds and two additional phases as well. Peaks in T c vs Tl/Cu fit expected Tl/Cu values within a few percent using weight analysis.

Recent orthorhombic-tetragonal (O-T) phase equilibria predicted by Khachaturyan and Morris have been observed experimentally in YBa/sub 2/Cu/sub 3-//sub x/Fe/sub x/O/sub 7/ (0.01less than or equal toxless than or equal to0.2) by Moessbauer techniques. The relative proportions of O and T phases are found to be functions of Fe concentration. The T phase grows from, and coexists with, the O phase as x increases. A nonzero fraction of T phase persists down to the smallest values of x. A model is presented to interpret the structural aspects of the O-T transition.

We have prepared the layered compounds $\mathrm{Li}{\mathrm{Ti}}_{1\ensuremath{-}y}{M}_{y}{\mathrm{S}}_{2}$ ($M=\mathrm{V},\phantom{\rule{0ex}{0ex}}\mathrm{C}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{F}\mathrm{e}$) by direct synthesis from a mixture of ${\mathrm{Li}}_{2}$S and the appropriate elements. The structure of these compounds is $1T$ for $M=\mathrm{V}\mathrm{and}\mathrm{Cr}$, and $3R$ for $M=\mathrm{Fe}$. In the Fe case two phases are obtained for $yg0.66$. Magnetic and M\"ossbauer data indicate that Fe is divalent and in the high-spin state for $yl0.5$ (effective magnetic moment \ensuremath{\sim}4.6 Bohr magnetons), that Cr is trivalent (effective magnetic moment \ensuremath{\sim}3.4 Bohr magnetons), and that V is nonmagnetic. The lithium can be removed by reaction with ${\mathrm{I}}_{2}$ in an acetonitrile solution at room temperature, producing layered compounds containing oxidized sulfur if $M=\mathrm{Cr}\mathrm{or}\mathrm{Fe}$. The Cr and Fe compounds are metastable, decomposing above 300\ifmmode^\circ\else\textdegree\fi{}C into Ti${\mathrm{S}}_{2}$ and ${\mathrm{Cr}}_{2}$${\mathrm{S}}_{3}$ or Fe${\mathrm{S}}_{2}$. In ${\mathrm{Ti}}_{1\ensuremath{-}y}{\mathrm{V}}_{y}{\mathrm{S}}_{2}$ the V is in the 4+ state and has a local magnetic moment at low concentrations. Cr and Fe are trivalent, and the latter is in low-spin state. Consequently, the valences are assigned as follows: ${{\mathrm{Ti}}^{4+}}_{1\ensuremath{-}y}{{M}^{3+}}_{y}{({\mathrm{S}}^{(\frac{2\ensuremath{-}y}{2})\ensuremath{-}})}_{2}$. In the Cr case the compounds are poor electrical conductors, indicating that the $y$ holes are localized, either by the cation disorder or by forming stable monovalent and divalent sulfur atoms (the monovalent species might form ${\mathrm{S}}_{2}$ pairs in this case, but they dissociate when lithium is reintercalated). Since the Fe compounds remain conducting, the holes are not localized and a band picture with the Fermi level moved well into the valence band to produce $y$ holes applies. Electrochemical measurements in nonaqueous lithium cells showed behavior similar to that of Ti${\mathrm{S}}_{2}$. Those compounds containing large amounts of Fe could be intercalated with more than one lithium per transition metal, but the reaction beyond $x=1$ was irreversible.

Equations for the mean positions and root mean square (RMS) linewidths of the six-line Mossbauer Zeeman spectrum of 57 Fe in an amorphous environment have been set out and used to interpret the measured room temperature spectrum for the amorphous metallic ferromagnet Fe 76 B 24 . The spectrum exhibits a linewidth asymmetry which is caused by correlations between the various fluctuation variables involved. The major source of this linewidth asymmetry is the correlation between hyperfine field, electric field gradient, and their relative orientation, the dominant physical origin being the angular dependence of the field fluctuations. A smaller, but still significant, additional contribution to linewidth asymmetry arises from correlations between hyperfine field and isomer shift. It is the established beyond question that in neither case can these correlations be represented by a simple linear coupling between the fluctuation variables. The linewidth analysis also provides separate measures of the RMS widths of the hyperfine field, isomer shift, and electric field gradient distributions.

The mixed ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{M}}_{\mathrm{x}}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{y}}$ (M=Ni, Zn, Fe, Co, and Al) phases have been characterized for their structural, magnetic, and superconducting properties. The oxygen content in these phases is dependent on the nature and the amount of doping, especially for Co and Fe. The material remains orthorhombic when Cu is replaced by Ni or Zn whereas it evolves to tetragonal symmetry for the Fe-, Co-, and Al-doped compounds when x exceeds 0.05. Evidence for the major substitution of Co in the Cu-O chains only is obtained by means of thermogravimetric analysis and neutron diffraction measurements. The room-temperature M\"ossbauer spectra of the Fe-doped compounds consist of three doublets; their site assignments are proposed. dc resistance and ac susceptibility have shown that both magnetic and diamagnetic ions destroy ${T}_{c}$ in a similar manner. At x=0.2 the Fe and Co compounds are tetragonal, superconduct at 50 K, and show a Curie-type magnetic behavior associated with a magnetic moment of 3.4${\ensuremath{\mu}}_{B}$ per doping atom. The origin of the orthorhombic-tetragonal transition and the importance of the Cu-O chains for superconductivity is discussed. The behavior of these materials with respect to magnetic impurities is apparently different from conventional Bardeen-Cooper-Schrieffer-type superconductors, and we believe that any new mechanism proposed must be mostly sensitive to local structural disorder.

A new electronic configuration involving localized $6s$ electrons is hypothesized for the Sm ions in Sm${\mathrm{B}}_{6}$. The model is successful in explaining earlier results of susceptibility measurements and new M\"ossbauer spectroscopy data reported here.

The results of magnetic-susceptibility and neutron-diffraction measurements on BaCo${\mathrm{F}}_{4}$ show behavior characteristic of a two-dimensional antiferromagnet. The material is highly anisotropic and exhibits two magnetic phases, $A$ and $B$, both of which order in three dimensions at the same temperature, ${T}_{N}=(69.6\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode^\circ\else\textdegree\fi{}$K. The magnetic structure consists of puckered (010) layers of moments within which nearest neighbors are coupled antiparallel. The ordering of the moments 14 \AA{} apart in the $b$ direction is antiparallel in phase $A$ and parallel in phase $B$. The moments are directed along [100]. If the ${\mathrm{Co}}^{2+}$ moment is assumed to be the same for both phases, then the ordered moments have a magnitude of $(3.4\ifmmode\pm\else\textpm\fi{}0.2){\ensuremath{\mu}}_{B}$ at 4.7 \ifmmode^\circ\else\textdegree\fi{}K.

The ambiguity in the cation valencies of Cu2FeSnS4 was resolved by Mossbauer effect studies in 57Fe and in 119Sn and verified by magnetic susceptibility measurements. The cation valencies were found to be Cu2+Fe2+Sn4+S4.

The hyperfine interactions of the iron nuclei in the orthoferrites were studied between 85 and 770\ifmmode^\circ\else\textdegree\fi{}K using the M\"ossbauer effect. The temperature dependence of the sublattice magnetization ${\ensuremath{\sigma}}_{s}(T)$ is compared with various statistical-mechanical theories. It obeys approximately a $\frac{1}{3}$ power law in the temperature range $0.60l\frac{T}{{T}_{N}}l0.99$, and spin-wave theory and Callen decoupling at low temperature in the range $\frac{T}{{T}_{N}}l0.5$. From ${\ensuremath{\sigma}}_{s}(T)$, it is found that the canting angle in these weak ferromagnets is temperature-independent. The exchange integral calculated from the Oguchi spin-wave theory agrees with that calculated from Rushbrook and Wood's high-temperature series expansion and the Green's-function theories, while that calculated from the molecular field and Kubo's spin-wave theories is considerably lower. The quadrupole coupling constant is in good agreement with values calculated by point-charge lattice sums. The specific heat obtained from the temperature shift at high temperature agrees with the Dulong-Petit law. The value of the isomer shift is equal to that expected for a trivalent iron ion.

The above room temperature dielectric behavior of the class of BaMF compounds is reported where M = Mg, Zn, Mn, Fe, Co and Ni. It is found that the strain-free (clamped) dielectric constant along the 2-fold a-axis obeys a Curie-Weiss law near the melting point, and that the Curie-Weiss transition temperature lies above the melting point in all compounds studies.