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Structural and superconducting properties of single-phase La-substituted compounds of the formula Dy(Ba2−yLay)Cu3Oz (0.0⩽y⩽0.5) have been investigated by neutron diffraction, electrical resistivity, and magnetization measurements. The compound DyBa2Cu3O7−δ (orthorhombic, space group Pmmm) exhibits superconductivity with transition temperature, Tc, of about 90 K. Rietveld analysis of the neutron diffraction data on La containing compounds reveals that the orthorhombicity decreases with increasing La concentration and the compounds become tetragonal for higher La content. The overall oxygen content increases as divalent Ba is replaced by trivalent La and approximately compensates for the difference in the valence state of these two ions. The superconducting transition temperature, obtained from low field magnetization and/or four-probe resistivity measurements, decreases from ∼90 K for y=0.0 to ∼34 K for y=0.5. The depression in Tc in Dy(Ba2−yLay)Cu3Oz system is attributed to hole filling/localization cause...

A series of Sm2−xTbxFe17−ySiy solid solutions with x=0, 1, and 1.5 and y=1, 2, and 3 were prepared by induction melting stoichiometric amounts of high purity elements. The x-ray diffraction data confirm that the postannealed samples are 2:17 intermetallics of the R3m space group. The lattice parameters and the unit cell volumes were calculated using a modified Rietveld program. The fitted intensities showed behavior related to a disordered rhombohedral structure as inferred from neutron data. It was observed that for a particular y, the unit cell volume decreased almost linearly with increasing Tb content (x=0, 1, and 1.5). The unit cell volume decreases with increasing Si content (y) for a particular x as is also observed for the single rare-earth 2:17 compounds. The Curie temperature measurements show that with increasing Tb content x a decrease in the Curie temperatures of about 3% for y=1, 10% for y=2, and 15% for y=3 was observed. For a particular x, a nearly linear type increase in the Curie temper...

Magnetization vs temperature behavior of RuSr 2 GdCu 2 O 8−δ (Ru-1212) measured in an field of 5 Oe, shows a clear branching of zero-field-cooled (ZFC) and field-cooled (FC) curves around 140 K, a cusp at 135 K, and a diamagnetictransition around 20 K (in the ZFC branch). The cusp at 135 K is due to the antiferromagnetic ordering of the Ru moments. The magnetization-field isotherms, below 50 K, show a nonlinear contribution from a ferromagnetic component. The resistance vs temperature behavior of the compound, in applied fields of 0, 3, and 7 T, confirms that the sample is superconducting at around 20 K. The superconducting transition exhibits field broadening of a type different than that known for conventional high T c superconductors. The magnetoresistance(MR) is negative above the Ru magnetic orderingtemperature of 135 K, while below this temperature,MR displays a positive peak in low fields and becomes negative in higher fields. A maximum of 2% is observed for the negative MR value at the Ru magnetic orderingtemperature. An electron diffraction pattern obtained for this Ru-1212 sample shows two types of superstructure; one with a weak spot at the center of the a–b rectangle and the other only along the b direction. It is possible that either Ru/Cu or Ru 4+ / Ru 5+ ordering of 2b periodicity takes place along the b direction.

Magnetic interactions in perovskite compounds of the type La1−xSrxMO3−δ (M=3d transition such as Mn and Fe) are presumed to arise through a super exchange between 3d electrons of the magnetic ions via oxygen orbitals. The magnetic structure of La0.6Sr0.4FeO3−δ has been studied with neutron diffraction. Oxygen vacancies were created by annealing samples under various gases including N2, air and mixtures of CO/CO2. All La0.6Sr0.4FeO3−δ compounds maintain the rhombohedral structure (space group R3c). The air- or oxygen-annealed samples have almost no oxygen vacancies while those made in the reducing atmosphere show 7%–11% oxygen vacancies. The rhombohedral distortion decreases in the reduced samples. All the samples exhibit antiferromagnetic ordering at room temperature, although a small ferromagnetic moment may also be present. The samples with little or no oxygen vacancies show a room temperature magnetic moment of ∼1.4 μB at the Fe site while those having >7% oxygen vacancies show a moment of ∼4.0 μB. Ma...

Magnetic and neutron diffraction studies have been carried out on PrMnSb2. Magnetization data in the temperature range of 5–400 K show two magnetic transitions: one at ∼175 K attributed to the antiferromagnetic ordering of the Mn moments, and the other at ∼35 K possibly due to the antiferromagnetic ordering of the Pr moments. The magnetization-field isotherms at various temperatures are consistent with the above. Neutron diffraction data obtained at various temperatures can be fitted with a nearly antiferromagnetically coupled Mn moment of ∼3μB at 70 K. At 10 K, moments both on Mn (∼3.5μB) and Pr (∼0.94μB) are ordered antiferromagnetically with small canting angles.

The magnetic and crystallographic structures of R(Fe/Mn) 6A6 ~R5Nd or Sm and A5Ge or Sn! intermetallics have been investigated using x-ray and neutron diffraction techniques and superconducting quantum interference device magnetic measurements. For both stannides ~A5Sn! and germanides ~A5Ge!, the lattice contracts with increasing iron content. In the case of the stannides, substitution of manganese by iron enhances the saturation magnetization and Curie temperature at low iron concentrations ( x

Samples of La0.6Sr0.4Fe3−δ with varying oxygen vacancy contents were prepared by heating them in different gas flows. Magnetization measurement showed that samples with low oxygen vacancies have a magnetic ordering temperature in the range of 300–325 K while those with 9%–12% oxygen vacancies have a magnetic ordering temperature of 800 K and higher. Mossbauer spectra at 300 K exhibit paramagnetic or weak magnetic characteristics for the N2, O2, and air-quenched samples, whereas an average hyperfine field of 52 T is found for the CO/CO2 reduced samples. The heat treatment in the reducing atmosphere creates oxygen vacancies and increases unit cell volume. However, the Fe–O bond length remains nearly constant, resulting in distortion/rotation of the oxygen octahedra which increases the Fe–O–Fe bond angle as much as 12 degrees. This dramatically affects the Fe–O–Fe superexchange coupling and plays a key role in the increase of the Neel temperatures.

Three samples of Nd2Fe16TiCx, with x equal to 0.0, 0.3, and 2.8, with the Th2Zn17‐type rhombohedral structure, have been studied by powder x‐ray and neutron diffraction, magnetic measurements, and Mossbauer spectroscopy. Nd2Fe16Ti and Nd2Fe16TiC0.3 were synthesized by induction melting stoichiometric amounts of the constituent elements, whereas Nd2Fe16TiC2.8 was synthesized by methane‐derived gas phase insertion of carbon into finely ground Nd2Fe16Ti at 600 K. The neutron diffraction determined titanium site occupancies are similar in both Nd2Fe16Ti and Nd2Fe16TiC2.8 in which titanium preferentially occupies the 6c transition metal site. In contrast, the titanium occupancies in Nd2Fe16TiC0.3 are markedly different in that titanium avoids the 6c transition metal site and randomly occupies the other three transition metal sites. This difference in occupancies most likely occurs because the titanium diffusion rate during the quenching of Nd2Fe16TiC0.3 is affected by the presence of carbon in the melt. Even t...

Several NdnFem−x−yVxAly [(n,m)=(1,12), (2,17), (3,29)] samples were prepared and analyzed using neutron powder diffraction. Rietveld analysis of the neutron diffraction data indicates that the V and Al substituents take those sites with similar environments in all three phases, as observed in our previous study of Ti‐substituted compounds. It was confirmed that the diffraction data of the 3:29 compound can be better refined using the A2/m space group than using the P21/c space group. The SQUID measurements show that all samples have Curie temperatures well above room temperature. The neutron diffraction results show that the easy direction is along the a axis for the 3:29 compounds, along the c axis for the 1:12 compounds and in the a‐b plane for 2:17 compounds, respectively. The average site magnetic moments, the metal–metal bond lengths and the unit cell parameters of these compounds are compared with those of the Ti‐substituted compounds.

Six samples of Ce2Fe17−xGax with nominal Ga content x equal to 0, 0.3, 0.5, 0.7, 1.0, 2.0 have been studied by powder neutron diffraction at room temperature. Both crystalline and magnetic refinements have been carried out. All six samples adopt the Th2Zn17‐type rhombohedral structure. The only additional phase found is α‐iron. Gallium atoms are found to have high affinity for the iron 18h site, and are absent from the 9d and 18f sites. The Ga substitution for Fe leads to an expansion of both the a and c axes. The Curie temperature increases from 238 K for Ce2Fe17 to 406 K for Ce2Fe15Ga2. Magnetic refinements on the samples with x=0.3, 0.5, 0.7, 1.0, and 2.0 reveal that the magnetic moments of the four Fe sites are in the basal plane and that their values increase with increasing Ga content.

In order to understand the magnetic properties of the substituted rare‐earth‐iron alloys, it is especially important to know the location of the substitutional atoms within the iron lattice. The site distributions of some nontransition‐metal substituents in the substituted Nd2Fe17−x T x alloys have previously been reported. Here we report the site distributions of some transition‐metal substituents (Cu,Zr,Nb,Ti,V) in the Nd2Fe17−x T x alloys and compare them with those of the nontransition‐metal substituted compounds.Rietveldanalysis of neutronpowderdiffraction data indicates that the nontransition‐metal substituents show very similar site affinity at low substituent content. For example Al, Ga, and Si all prefer the 18h sites. The transition‐metal substituents show a more complex site affinity. Ti and V atoms strongly prefer the 6c sites, Cu atoms prefer the 9d and 18f sites, Nb atoms prefer the 6c and 18h sites, and Zr atoms prefer 6c and 18f sites. It was also noted that the site affinity can change if carbon is included in the melting procedure of the sample preparation. The superconducting quantum interference device measurements show that all the substituted compounds have a Curie temperature higher than the unsubstituted parent compound. The relationship between the site distribution of substituents and the magnetic properties of the substituted Nd2Fe17−x T x alloys is discussed.

The compounds CeRhSb and CeNiSn show hybridization gap in the electronic density of states. It is of interest to look for a similar gap in a related compound, CeNiSb. We present here the results of the structural, magnetic, and transport studies on CeNiSb. Rietveld analysis of the room‐temperature neutron diffraction data reveals that this compound crystallizes in the hexagonal structure (space group P63/mmc). Magnetization measurements show that this compound is ferromagnetically ordered with a transition temperature of ∼4 K. The magnetic susceptibility of CeNiSb follows Curie–Weiss behavior between 10 and 300 K with effective paramagnetic moment close to that of Ce3+ ion. The heat capacity of CeNiSb shows a rise at about 4 K due to the ferromagnetic ordering and a moderately large value of γ is obtained. Its electrical resistivity shows a weak shallow minima at 15 K which is a characteristic feature of Kondo lattice systems. However, no rise in the resistivity at low temperatures, as seen in CeRhSb and ...

The neutron diffraction powder patterns of Pr2Fe17 and Pr2Fe17N2.6 have been measured at 295 K and refined within the Th2Zn17 structure type to give the structural changes which occur upon nitrogenation; changes which include a 6.4% expansion of the unit cell volume. The Mossbauer spectra of Pr2Fe17 and Pr2Fe17N2.6 have been measured between 85 and 295 K and analyzed with a model which is based on the Wigner–Seitz cell environment of each iron site, the basal orientation of the magnetization, and a comparison with the Mossbauer spectra of Nd2Fe17 and Nd2Fe17N2.6. Upon nitrogenation of Pr2Fe17, the 85‐K‐weighted average isomer shift increases from 0.049 to 0.156 mm/s, and the isomer shifts of the four crystallographically distinct iron sites increase in agreement with the increase found in their Wigner–Seitz cell volumes and the presence of a nitrogen near‐neighbor for the 18f and 18h sites. The temperature dependence of the isomer shifts indicates an increase in covalency upon the formation of the nitride...

Recently discovered R2 Fe17Nx (R=rare earth, x∼3) compounds look promising as permanent‐magnet materials. The addition of N to R2Fe17 compounds almost doubles their Curie temperatures.1 It also give rise to uniaxial anisotropy in the Sm compound. We have performed spin‐polarized self‐consistent electronic structure calculations in order to understand the magnetic properties. Neutron diffraction and photoemission measurements are used to obtain information on nitrogen site occupancies and electronic structure. For Y2Fe17N3, the calculated site‐dependent Fe moments vary significantly from those for Y2Fe17 (up to ∼1.7 μB), and the total moment per cell agrees very well with experiment. The expansion of the lattice due to nitrogenation lowers the overlap among the Fe atoms in the ion‐only planes thereby increasing their magnetic moments. This leads to an increase in their interatomic exchange interactions and hence the Curie temperature. The Fe atoms in the vicinity of the N atoms overlap strongly with the la...

Rietveld analyses of room-temperature neutron diffraction data for seven Nd2(Co/x/Fe/1-x/)14B alloys (x = 0,0.1, 0.3, 0.5, 0.7, 0.9, 1) are reported. Throughout the entire composition range the Nd2Fe14B-type tetragonal crystal structure is maintained, with the lattice constants decreasing significantly as the Co concentration x increases. It is found that the J2-type transition-metal sites are preferentially occupied by Fe ions in the pseudoternary systems, a result which is analogous to the preferential Fe occupation of c sites previously observed in hexagonal Nd2(Co/x/Fe/1-x/)17 alloys.

— The ternary system Y6(Fe1_xMn ;e)23 exhibits unusual magnetic behavior. Although both Y6Mn2 3 and YgFe2 3 are magnetically ordered, these ternaries show a striking reduction in both Curie temperature and magnetization. In the compositional range of JC = 0.5 to 0.75, there exists no magnetic ordering at liquid helium temperatures. The Mn and Fe atoms are found to exhibit strong site preference in the Y6(Fe0 2 5Mn0 7 5)23 unannealed, Y6(Fe0 2 5Mn0 7 5 ) 2 3 annealed and Y6(Fe0 5Mn0 5 ) 2 3 unannealed samples. The magnetization of these compounds is field-dependent and suggests possible spin glass behavior.

The Debye-Waller factor of sodium has been measured as a function of temperature from 80 to 295 K using Moessbauer ..gamma..-ray scattering. The high energy resolution provided by this technique allowed experimental separation of the elastic scattering from the inelastic thermal diffuse scattering. The results were compared with the harmonic model using integrations over dispersion curves from the neutron-scattering measurements of Woods et al. and the lattice-dynamics calculations of Glyde and Taylor. The Debye-Waller exponent was shown to exceed the harmonic prediction by 23% at room temperature. This difference is attributed to anharmonic terms in the interatomic potential.

A microfoil internal conversion electron (MICE) detector is described which permits direct Doppler shifting of resonance radiation up to velocities of ± 20 cm/s, and gives large improvements in signal-to-background ratios for many Mossbauer isotopes, when compared with transmission geometry. The detector described has efficiency of nearly unity, and it allows for cooling the reciprocating microfoil module to 100 K, which improves signal-to-background substantially over room temperature operation. We give an analysis of the signal-to-background that can be expected for this MICE detector, and for a corresponding transmission experiment. In a table of neutron produced isotopes we find more than 10 cases which are favorable to the MICE approach as compared with the more conventional transmission geometry. The signal-to-background enhancement predicted for several Mossbauer isotopes is substantial for the MICE geometry compared to transmission geometry. Direct measurements of the Mossbauer conversion electron spectrum for the 46.5 and 99.1 keV transitions of 183 W and the 100.1 keV transition in 182 W are reported and compared with our analysis. In the case of 183 W (46.5 keV) we observe over 500% signal-to-background, and this experimental result agrees well with our analysis of the expected size of the effect. Satisfactory agreement is also found for the 99.1 keV 183 W and 100.1 keV 182 W spectra. Based on the analysis given it is possible to determine nuclear resonance cross sections and thereby infer internal conversion coefficients for the resonance transitions. Thus, we are able to determine the internal conversion coefficient for the 46.5 keV transition in 183 W to be α = 13 ± 3.

The flipping ratios for polarized neutron scattering from the first six reflections in Ni have been measured on a series of samples of varying thickness and perfection and at a number of wavelengths. The results were corrected for secondary extinction on a point-by-point basis, using the measured absolute reflectivities, and multiple scattering was detected using conservation of neutrons. No adjustable parameters were used, and all data, except those from a rather perfect specimen which showed both primary and secondary extinction, agree to within the experimental accuracy after extinction correction. A diffuse scattering process with spin dependence was also detected, and the flipping ratios corrected for this. The corrected values differ somewhat from the values given by Mook [Phys. Rev. (1966), 14B, 495-501].

A new device has been designed to facilitate the cleaving of thin sections from bulk crystals of mercuric iodide. Crystallographic perfection of the machine-cleaved sections was established by gamma-ray diffraction rocking curves and was found to be higher than the perfection of hand-cleaved sections or of as-grown thin platelets, approaching the perfection of string-sawn sections of HgI/sub 2/. A correlation was found between the perfection and thickness of the machine-cleaved sections, i.e., the thicker the section the more perfect it is. Reproducibility of the fabrication was significantly improved by using machine cleaving in the fabrication process.

The Mossbauer diffraction instrument at the Missouri University Research Reactor has been used to measure the elastic and inelastic contributions to the 444 Bragg reflection in dynamic silicon at room temperature. These measurements used the 46.5-keV gamma rays from high intensity183Ta sources cooled to liquid nitrogen temperature. The main feature of this study compared to similar measurements on silicon is the significantly improved momentum space resolution. ΔQ values of 0.011 A−1 and 0.11 A−1 were measured for the transverse and longitudinal directions, respectively.

Neutron diffraction studies of the nomagnetic compositional range of the Y 6 (Fe 1− x Mn x ) 23 system reveal the presence of preferential ordering of Fe and Mn atoms on the four transition metal crystallographic sites. Throughout the entire compositional range of the ternary system, Mn atoms prefer to occupy the f 2 site and Fe atoms the f 1 site. Refinements of the data were carried out using the Rietveld profile method.

A Mossbauer gamma-ray diffraction instrument has been developed which utilizes high intensity sources produced by neutron irradiation. Most of the early work has used 70 Ci, 5.1 d 183Ta sources produced by double neutron capture which yield 12 × 1010 photons/s for the 46.5 keV Mossbauer transition in 183W. A dewar is enclosed in the shielding cask which allows sources to be cooled to 77 K. Samples are located approximately midway between source and detector, which are separated by 155 cm. Collimators in front of the sample and detector commonly limit the beam to 3 mm wide by 25 mm high. A 27% minimum is seen in the velocity spectrum for the 46.5 keV photons scattered from the 200 Bragg reflection in LiF taken in transmission. Enriched, room temperature absorbers mounted on a rotor can be Doppler-shifted up to 17 meV. Q-space resolution of 0.01 A−1 is significantly better than that reported by other groups using Mossbauer scattering. When a microfoil internal conversion electron (MICE) detector is used, the instrument has the potential for directly measuring the energy transferred by the inelastically scattered photons, especially for low energy excitations in solids which require high energy resolution.

We describe detailed analyses of neutron powder diffraction data on Nd2Fe14B at several temperatures and discuss relationships between the crystal structure and the magnetic properties via comparison with other rare earth‐transition metal systems, Nd2Fe17 in particular. Diffraction studies have also been performed on optimum energy product melt‐spun Nd‐Fe‐B ribbons. Those results demonstrate that the ribbons are comprised of Nd2Fe14B particles with diameters of a few hundred angstroms.

For the application of Rietveld profile analysis to neutron powder diffraction data a precise knowledge of the peak profile, in both shape and position, is required. The method now in use employs a Gaussian-shaped profile with a semi-empirical asymmetry correction for low-angle peaks. The integrated intensity is taken to be proportional to the classical Lorentz factor calculated for the X-ray case. In this paper an exact expression is given for the peak profile based upon the geometrical dimensions of the diffractometer. It is shown that the asymmetry of observed peaks is well reproduced by this expression. The angular displacement of the experimental profile with respect to the nominal Bragg angle value is larger than expected. Values for the correction to the classical Lorentz factor for the integrated intensity are given. The exact peak profile expression has been incorporated into a Rietveld profile analysis refinement program.

Rietveld analyses of room‐temperature neutron powder diffraction data for seven Nd2(CoxFe1−x)17 alloys (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) are reported. In the ternary systems we find that the c‐type transition metal sites are preferentially occupied by Fe ions; concomitantly, the d and h sites have Co occupations larger than those predicted by stoichiometry, while the f sites deviate only slightly from random occupation. The magnetic moments obtained from the data refinements indicate a transition from a low to a high spin state on the d, f, and h sites as Fe is incorporated into the structure; a reverse trend is exhibited by the moments on the c site.

We have studied the lattice dynamics, the magnetic ordering and excitations, and the crystallographic distortions associated with the metal-insulator transitions in pure and Cr-doped vanadium sesquioxide using neutron-scattering techniques. In the paramagnetic metallic phase of pure ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$ and in the higher-temperature paramagnetic metallic phase of 1.5-at. %-Cr-doped ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$ we see an anomalous splitting of the transversely polarized acoustic phonon mode, propagating along the hexagonal [hh0] direction near the center of the Brillouin zone. Upon lowering the temperature, a substantial softening of a large portion of the transverse acoustic branch in the [00l] direction is observed in Cr-doped samples in the paramagnetic insulating phase as the transition to the antiferromagnetic insulating phase is approached. The loss of antiferromagnetic order with increasing temperature in 3-at. %-Cr-doped and 6-at. %-Cr-doped samples appears to be nearly continuous. The temperature dependence of the spin-wave energy gap in the 3-at. %-Cr-doped sample appears to approach zero continuously up to the N\'eel temperature. The monoclinic distortion of the trigonal phase at this transition is much reduced in the 6-at. %-Cr-doped sample compared to pure ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$. These and other results are described in detail, and discussed in connection with the recent theoretical work on metal-insulator transitions.

We have carried out inelastic neutron scattering measurements of the spin waves propagating along the [$00l$] trigonal axis and along the [$\mathrm{hh}0$] axis in the antiferromagnetic phase of ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$. We find an energy gap in the spin-wave spectrum at the zone center of 4.75 meV. We obtain exchange parameters and the anisotropy energy from these data using a nearest-neighbor and next-nearest-neighbor Heisenberg Hamiltonian model.

We report the results of neutron diffraction and magnetization measurements on powder samples of Er2(Fe,Mn)14B. We find that the unit cell expands with increased concentration of manganese as larger manganese atoms replace the smaller iron atoms within the transition metal sublattice. The preferential occupancy of transition metal sites is also affected since manganese prefers positions with larger site volumes, such as the j2, while iron prefers smaller site volumes, such as the k2. At low temperatures (4.2 K) the simple collinear moment model of the magnetic structure, valid at higher temperatures, is not sufficient to explain the existence of reflections forbidden in the space group P42/mnm. A five‐sublattice model is used in which each of the two rare‐earth sites splits into two distinct sublattices whose moments are oppositely canted from the direction antiparallel to the transition metal moment. The linear field dependence of the magnetization provides support for this moment configuration. Neutron ...

Use of gamma-ray diffraction has now been successfully demonstrated for accurate extinction-free structure factor measurements, but no complete crystallographic studies have been performed using gamma-ray techniques. Recently, we have taken data using 0.12 A gamma-rays (153Sm) on Mg2SiO4 and BeO which demonstrate for the first time the ability to perform “normal” crystallographic studies with gamma-rays. To make this technique accessible to a more general community we have designed a new instrument using 0.2 A gamma radiation (169Yb) and a special detector which could be set up in any X-ray laboratory. We have also demonstrated the feasibility of powder diffraction studies with gamma-rays which hold significant advantages over X-ray techniques and could also be set up in an X-ray laboratory. A gamma-ray Compton scattering spectrometer using 320 keV radiation (51Cr) is being designed and will be used to determine momentum density distributions. Finally, a quasi-elastic scattering instrument using an intense (1000 Ci) Mossbauer source and a conversion electron detector is partially completed and should open some new areas of research when operational.

Using 100 Ci183Ta and 5 Ci182Ta sources, with LiF and NaCl crystal monochromating filters, we have measured the lineshape parameters for the 46.5 keV and 99.1 keV Mossbauer effect (ME) transitions of183W and the 100.1 keV transition of182W. Using an analytic representation of the convolution integral and utilizing asymptotic analyses of the lineshape, we find, for both transmission and microfoil internal conversion (MICE) experiments, accurate values of all ME parameters including width, position, cross section, and interference. This new approach allows deconvolution of source and absorber spectra and gives a simple analytic expression for both as well as their Fourier transforms. The line widths for the 46.5, 99.1, and 100.1 keV transitions are 3.10(10), 0.369(18), and 0.195(12) cm/s, respectively. The interference parameters are −0.00257(9), −0.0093(12), and −0.0107(12) in the same respective order. The agreement between transmission and MICE /1,2/ measurements for the above lineshape parameters is within the experimental errors. We believe these measurements are the first having sufficient precision to allow a quantitative comparison with dispersion theory /3,4/ and they indicate interference parameters 10 to 20% smaller than predicted. Our measured line widths are less than earlier reported values. This is because our analysis of the true lineshape and the study of line asymptotics permits a quantitative determination of the isomer lifetimes rather than the usual lower bound found in earlier ME experiments.

We report a simple analytic form for the convolution integral in transmission Moessbauer spectroscopy allowing accurate representation of the line shape even for very thick absorbers (t = 10), and permitting easy fitting to the true line-shape function. This representation permits the accurate determination of all Moessbauer-effect (ME) parameters, including position, width, cross section, and interference. This analytic method can be applied to deconvolute accurately information contained in either source or absorber, and an explicit analytic form for the emission and absorption Fourier transforms is given. We show that from the asymptotics of the line shape, it is possible to determine all line-shape parameters, and that line-shape asymptotics can circumvent short-ranged hyperfine or instrumental broadening contributions to the observed spectrum. A formula for the correction to the line shape caused by source self-absorption is given, and it is shown that when there is significant source resonance self-absorption a ''good'' fit to data, judged by a chi-squared analysis, can yield completely wrong ME line-shape parameters. We find an equation for the dependence of the area under the absorption curve and the resonance peak height, and give its explicit dependence on the interference parameter and source broadening parameters. Although these effects havemore » been neglected in earlier work, their contribution may be of order 10% in many cases of interest.« less

The use of reactor-produced radioisotopes is reviewed, with emphasis on those applications which require high neutron fluxes. Considerable opportunities are noted for new or improved utilization if neutron fluxes in the 10 20 /m 2 s range can be achieved. New uses are seen in medical applications, condensed matter research and in a variety of industrial applications.

A linear position-sensitive detector has been constructed with an array of three proportional counters and installed on the neutron powder diffractometer at the University of Missouri Research Reactor. The event position is determined by the charge division method using digital arithmetic. Consideration has been given to the corrections necessary to account for the differences in detection efficiency, solid angle, dead time and resolution for different points along the detector, caused by the finite thickness and tangential orientation of the detectors. An important feature of the instrument is an oscillating radial collimator with high transmission placed between the sample and detector which enables the system to have low background and a high signal-to-noise ratio. By considering the angular rate of accumulating data, the peak intensity per channel and the resolution of a Bragg diffraction peak, the enchancement factor for the position-sensitive detector is estimated to be about 50 compared to a single detector with a step scan. This allows experiments to be performed with better statistics, better resolution and smaller samples, and enables marginal experiments to be worthwhile. A number of structures have been refined with this detector using Rietveld analysis including a new rare-earth-transition-metal phase whose structure has been solved from the powder diffraction data.

The crystalline perfection of wire sawn pieces of vapor grown single crystals of mercuric iodide was compared with the perfection of (00l) cleaved sections of the same crystal from which nuclear radiation detectors have been fabricated. The crystalline perfection was studied using neutron and gamma‐ray diffraction rocking curves. Most of the gamma‐ray data were obtained using a high intensity source of 153Sm gamma rays with a wavelength of λ = 0.12 A. Some of the data were obtained using highly penetrating 198Au gamma rays with a shorter wavelength of λ = 0.03 A. The neutrons had a wavelength of λ = 1.07 A. It was found that, in terms of the mosaic spread of the crystals, the cleaved detector plates have a much lower crystalline perfection than the thicker uncleaved detector plates. At the same time, the results show that for detectors cut from the same crystal, the one with the lower spectral resolution for radiation detection will also have a lower perfection and larger width of the gamma‐ray rocking cu...

A new method for the treatment of secondary extinction in polarized neutron diffraction data has been developed. As in previous models, the Zachariasen solutions to the Darwin intensity transfer equations are used, but in this case the extinction corrections are made on a point-by-point basis across the rocking curve and the corrections are determined by the absolute reflectivity at each point. There are no adjustable parameters (other than background). Measurement of the reflectivity also provides a simple test for multiple scattering, since the sum of diffracted and transmitted intensities should equal the direct-beam intensity, corrected for absorption, if no multiple scattering is present. The present method should give more reliable results than parametrized models where the correlation between the extinction and other parameters, such as the scale factor and temperature factors, are important.

The magnetic ordering of Er/sub 2/Fe/sub 14-//sub x/Mn/sub x/B alloys has been studied by powder neutron diffraction for xless than or equal to8. At high x, ferromagnetic ordering of the Fe atoms was observed only on the k/sub 1/ and c sites, consistent with extrapolations from lower concentrations. The j/sub 2/ site, however, which contains a high concentration of Mn atoms, orders with an antiferromagnetic arrangement. This ordering corresponds to strong coupling along the major disclination line in the system and is an entirely new feature in these compounds.

We report a rapidly convergent analytic representation for the Mossbauer effect (ME) lineshape and its Fourier transform, which gives an exact description of transmission and conversion electron cases. This representation permits the accurate determination of all Mossbauer effect (ME) parameters, including position, width, cross section, and interference and can be used to deconvolute hyperfine information contained in either source or absorber. The limitations of lorentzian and exponential-lorentzian fits to ME experiments are clarified.

Neutron powder diffraction has been used to characterize the magnetic ordering and crystallographic behavior of Er2(Co0.55Fe0.45)17 which crystallizes in the hexagonal form (Th2Ni17). The f site, which is the dumbbell site, analogous to the c site in the rhombohedral 2‐17 phase (Th2Zn17) and to the j2 site in the 2‐14‐1 phase, is characterized by long TM–TM bonds (>2.55 A) and a large number of TM near neighbors, and like its analogues, is strongly preferred by the Fe. The other three sites are slightly cobalt rich, compensating for the Co deficiency on f. The magnetic structure at 293 and 580 K is axial with erbium moments antiparallel to the transition‐metal moments as expected. At 8.7 K, basal plane components develop in addition to the axial moments on the erbium sites. Further studies will investigate the spin reorientation as a function of temperature and composition across the entire composition diagram.

We have carried out inelastic neutron scattering measurements of the spin waves propagating along the (00l) trigonal axis and along the (hh0) axis in the antiferromagnetic phase of pure V2O3. We find an energy gap in the spin wave spectrum at the zone center at 4.75 meV. We obtain exchange parameters and the anisotropy energy from this data using a nearest neighbor and next nearest neighbor Heisenberg Hamiltonian Model. Elastic neutron scattering on V2O3.0198 has shown two distinct transitions. Magnetic ordering occurs at ∼100 K and the magnetic moment reaches ∼0.5μB at 50 K where the metal‐insulator (M–I) transition occurs and the moment saturates at 0.9μB. This contrasts sharply with pure V2O3 in which the magnetic and M‐I transitions coincide and with higher oxygen concentrations for which the M‐I transition is completely suppressed. In addition, magnetic intensity observed at the 3/2,3/2,3 position in both pure and metal‐deficient specimens suggests a magnetic contribution from the oxygen sites.

The (CrxV1_x)203system undergoes two metal-insulator transitions as a function of temperature. In this report, we present elastic and inelastic neutron scattering data on these two transitions for samples with 0%, 1.5%, 3% and 6% Cr concentration and discuss the apparent suppression of magnetism by the intermediate metallic phase in this system.

Two simple counters included in the charge-digitization circuitry of a position-sensitive proportional counter using the charge division method for position encoding have enabled us to determine the dead time losses for the system. An interesting positional dependence of the dead time τ is observed, which agrees with a simple model. The system enables us to correct the experimental data for dead time and to be indifferent to the relatively slow analog-to-digital converters used in the system.

Determination of the crystal structure of ${\mathrm{Nd}}_{2}$${\mathrm{Fe}}_{14}$B, a new ternary phase, is reported. It has recently been demonstrated that permanent magnets having large coercivities and energy products can be formed from this phase, underscoring its potential technological importance. We relate the crystal structure and intrinsic magnetic properties by considering analogies with previously known rare-earth---transition-metal materials.