B. D. Dunlap

Structural behavior and chemical order of Fe in YBa2(Cu1 − xFex)3O7 + δ

Abstract Neutron and Mossbauer effect measurements have been made as a function of Fe concentration in orthorhombic ( x = 0.01, 0.02) and tetragonal ( x ⩾ 0.05), YBa 2 (Cu 1 − x Fe x ) 3 O 7 + δ . A systematic Rietveld analysis of the neutron data for all concentrations shows that Fe principally occupies a site slightly displaced ( y , y , 0) from the Cul (0, 0, 0) site in order to approach tetrahedral coordination. Neutron data for the x = 0.05 compound at 10 K show no significant difference from those obtained at 297 K. Local ordering of oxygen about an Fe substituent is conducive to placement of a second Fe atom in an adjacent site leading to local aggregation into chains of various lengths. This clustering is reflected in the character of low temperature magnetic Mossbauer spectra and in specific heat measurements. The Mossbauer spectra yield the relative changes in oxygen configurations about Fe atoms as the Fe concentration changes.

Structural relationships in rare earth-transition metal hydrides

Several structural types of form AB/sub 2/, AB/sub 3/, A/sub 2/B/sub 7/, and AB/sub 5/ (A = rare earth, B = transition metal) are known to be closely related, with local environments which are very similar among the different compounds. Based on these relationships, we suggest that hydride phases A/sub l/B/sub m/H/sub n/ should be related according to the relationships n(AB/sub 3/) = 1/3n(AB/sub 5/) + 2/3n(AB/sub 2/) and n(A/sub 2/B/sub 7/) = n(AB/sub 5/) + n(AB/sub 2/) where the n(AB/sub m/) are observed hydrogen phase concentrations. It is shown that the phases observed in pressure-composition isotherms for AB/sub 3/ and A/sub 2/B/sub 7/ systems can be understood from this viewpoint. A discussion is given of the maximum hydrogen concentration possible in the compounds having these structure types.

Magnetic properties of DyFe2H2 from 57Fe, 161Dy Mössbauer effect and magnetization measurements

A single phase stable hydride with nominal composition DyFe2H2 is shown to have the cubic C15 Laves structure with a 5% larger lattice constant. 57Fe Mossbauer spectra show two sites with nearly equal population and with different isomer shifts and hyperfine fields. Based on the average hyperfine field at 4.2 K, an Fe moment of approximately 1.6 μB is estimated. From the 161Dy data at 4.2 K, which show a single site, a nearly free‐ion moment of 9.8 μB is determined. Relaxation effects in the 161Dy spectra above 77 K, combined with a sharp decrease in the moment with increasing temperature, imply a weakening of the Dy‐Fe magnetic coupling induced by the presence of the hydrogen.

Surface segregation of iron by hydrogenation of FeTi: An 57Fe mössbauer conversion electron study

Abstract Mossbauer spectroscopy of 57 Fe conversion electrons was used to study FeTi subjected to 51 hydrogen absorption-desorption cycles. This technique probes both the surface and part of the bulk, and it can also distinguish between magnetic and non-magnetic iron in the lattice. Our measurements suggest the presence of a magnetic overlayer on FeTi made up of iron clusters and the absence of magnetic moments on the iron contained in the bulk. This results shows that the magnetic moment observed in bulk magnetization studies in fact originates from this segregated iron layer on the surface of FeTi.

Pressure-composition phase diagram for hydrides of rare earth-Fe2 laves compounds

Abstract The pressure-composition-temperature phase diagram for the absorption of hydrogen in the cubic Laves phase compound ErFe2 has been investigated. At least three stable hydrides ErFe2Hn are found with n ≈ 2, 3.4 and 4. At 20 °C, the equilibrium pressures for these phases are 0.04, 10 and 4000 Torr, respectively. The implications of these results on the preparation of samples for the measurement of physical properties is discussed.

Mössbauer effect investigations of the electronic and magnetic properties of rare earth metal and intermetallic hydrides

Mossbauer spectroscopy has been used in a number of investigations of elemental rare earth (RE) metal and RE–transition metal intermetallic hydrides. Analysis of hyperfine parameters has established the crystalline electric field (CEF) ground state for Dy3+ in DyH2 and for Er3+ in ErH2 and YH2. In all cases the results are compatible with the presence of negatively charged hydrogen atoms. Long range magnetic ordering is observed in the RE‐H2 compounds. In DyH2, short‐range magnetic ordering is seen at temperatures up to twice the long‐range transition temperature. This arises from the changes in CEF symmetry due to the presence of a few H atoms which are not in regular tetrahedral sites. These effects are much more pronounced in slightly non‐stoichiometric compounds and can result in dramatic changes in the magnetic ordering temperature. As an example of an intermetallic hydride, we have studied the cubic Laves phase RE‐iron compounds. These form several stable hydride phases RE‐Fe2Hx, with the maximum x ...

Crystal field effects in RERh4B4 compounds (RE = rare earth)

Abstract By analysis of a large body of available data, crystal field parameters have been obtained for compounds in the series RERh 4 B 4 (RE = rare-earth). These parameters have then been used to calculate properties related to the electronic and magnetic properties of such compounds, and it is shown that a substantial unification of current information is obtained for properties including bulk magnetization (polycrystalline and single crystal), magnetic susceptibility, magnetic anisotropy, Schottky anomalies in the specific heat and suppression of the superconducting transition temperature due to magnetic impurities. In agreement with previous discussions crystal field effects are found to clarify the systematics of magnetic transition temperature, but not to give a full explanation. Other phenomena, such as discrepancies in the magnetic moment of ErRh 4 B 4 as measured by neutron diffraction and by Mo¨ssbauer spectroscopy cannot be explained as crystal field related phenomena.

Magnetic properties of DyFe/sub 2/H/sub 2/ from /sup 57/Fe, /sup 161/Dy Moessbauer effect and magnetization measurements

A single phase stable hydride with nominal composition DyFe2H2 is shown to have the cubic C15 Laves structure with a 5% larger lattice constant. 57Fe Mossbauer spectra show two sites with nearly equal population and with different isomer shifts and hyperfine fields. Based on the average hyperfine field at 4.2 K, an Fe moment of approximately 1.6 μB is estimated. From the 161Dy data at 4.2 K, which show a single site, a nearly free‐ion moment of 9.8 μB is determined. Relaxation effects in the 161Dy spectra above 77 K, combined with a sharp decrease in the moment with increasing temperature, imply a weakening of the Dy‐Fe magnetic coupling induced by the presence of the hydrogen.

Photoemission study of 5f localization in UPd3−x(Pt,Rh)x

Photoemission measurements in the two systems UPd3−x(Pt,Rh)x show that the 5f spectra are consistent with localized 5f electrons (peak in spectral weight is below EF for all x within the double hexagonal DO24 phase) while at both phase transitions the 5f peaks lock in at EF consistent with intinerancy. A satellite 5f peak which we attribute to d screening is observed in both localized and itinerant systems.

Structural and magnetic properties of Fe impurities in YBa2Cu3O7−δ

Abstract Samples having the general composition YBa 2 (Cu 1−x Fe x ) 3 O 7−δ have been investigated using x-ray and neutron diffraction, Mossbauer spectroscopy and bulk property measurements. X-ray and neutron data show that samples having x ≥ 0.04 are tetragonal. Neutron diffraction shows that this structure is very similar to that found for undoped YBa 2 Cu 3 O 7−δ when the oxygen concentration is low, although, in the present case, the total oxygen content is found to be ≥7 for all concentrations. Contrary to the common statement that Fe substitutes onto the Cu sites, it is found from neutron diffraction that the Cu1 site is not occupied, and that Fe appears to have a substantial substitution for Ba. Both Mossbauer spectroscopy and heat capacity measurements show the presence of magnetic transitions at temperatures well below the superconducting transition temperature, with evidence for a large variation in magnetic transition temperatures.