Martin R. Harrison

Ionic and metallic clusters of the alkali metals in zeolite Y

Abstract Dehydrated samples of zeolite Y containing alkali-metal cations have been reacted with alkali-metal vapor in sealed silica tubes, and the products studied by electron-spin resonance (ESR) spectroscopy. Two distinct species were detected following the reaction of sodium-exchanged zeolite Y with sodium, potassium, or rubidium vapor. Exposure to a low concentration of metal vapor resulted in brightly colored samples with ESR signals characteristic of a stable ionic cluster species Na3+4, in which an unpaired electron is trapped on four equivalent sodium cations in the sodalite cages of the zeolite. Exposure to higher concentrations of metal vapor resulted in dark-colored samples with ESR signals characteristic of small metallic particles located inside the zeolite cavities. A similar ionic cluster species K3+4 was detected following the reaction of potassium-exchanged zeolite Y with sodium or potassium vapor although the potassium cluster was less stable than its sodium counterpart and an ESR signal from small metallic particles was observed at the same time. The corresponding Rb3+4 ionic cluster species was not detected following the reaction of rubidium-exchanged zeolite Y with rubidium vapor; only an ESR signal from small metallic particles was observed. The narrow linewidths of the ESR signals from the small metallic particles suggest an inhibition of the spin-relaxation mechanisms in the bulk metals.

Magnetism and superconductivity in the spinel system Li1−xMxTi2O4 (M = Mn2+, Mg2+)

X-ray powder diffraction measurements show that the M2+ cations substitute almost completely at the tetrahedral sites in the Li1−xMxTi2O4 spinel systems (M = Mn2+, 0 < x < 1; M = Mg2+, 0 < x < 0.2). The magnetic susceptibility of the Li1−xMxTi2O4 system is dominated by high-spin Mn2+ moments for x < 0.5 and shows ferrimagnetic ordering at low temperatures (15–55 K) for samples with higher x compositions. The occurrence of spontaneous magnetism in the Li0.4Mn0.6Ti2O4 sample is interpreted as indicating a compositionally induced metal-nonmetal transition near that composition. The titanium-titanium cation distance at the transition is 3.016(2) A in excellent agreement with Good-enoughs critical Rc value of 3.02 A for Ti3+ ions. The magnetic susceptibility of the Li1−xMgxTi2O4 system is distinguished by large Pauli susceptibilities and the Curie behavior of small concentrations of spins trapped at defect sites. The superconducting transition temperature Tc falls with increasing x and shows a strong correlation with the lattice parameter in both series. A decreasing density of states at the Fermi level resulting from increasing electron correlation and electron-lattice interactions is proposed to account for these observations. Importantly, no magnetic impurity effect on Tc is observed.

Magnetism, superconductivity, and the metal-nonmetal transition in the spinel LiMxTi2-xO4; M=Al3+, Cr3+

Abstract In the Li M x Ti 2− x O 4 ( M = Cr 3+ , Al 3+ ) spinel solid solution systems, the M 3+ cations substitute at the octahedral sites and thus directly interact with the titanium 3 d conduction electrons. The effect is dramatically illustrated in the chromium series where superconductivity is supressed completely at 2.5% Cr 3+ substitution by a magnetic impurity effect. Infrared transmission measurements indicate a metal-nonmetal transition (MNMT) in LiCr x Ti 2− x O 4 at 0.33 x T c persists at a high value (> 6 K) to x = 0.33, where a MNMT is proposed. Percolation, disproportionation, and conduction electron concentration are briefly discussed as possible contributing mechanisms for the MNMT in the cation-substituted LiM x Ti 2− x O 4 ( M = Li + , Al 3+ , Cr 3+ ) system.

A combined ESR/X-ray diffraction study of the Y2O3–BaO–CuO phase diagram

Measurements at both X- and Q-band frequencies of various phases in the Y–Ba–Cu–O system are reported. Signals close to the free-spin region are attributed to localised Cu2+ species with g-value anisotropies noticeably dependent upon the local crystal structure. These materials include BaCuO2, Y2BaCuO5, Y2Cu2O5; characteristic g values are discussed in terms of crystal-field models for electronic structure.Importantly, we find that high purity YBa2Cu3O7–x(x≲ 0.1) superconducting samples (Tc= 92 K), prepared at lower temperature by solution or sol–gel techniques, have no intrinsic ESR signals in the free-spin region. We propose that this ‘ESR silence’ is, in fact, not due to the absence of intrinsic paramagnetism in these superconducting oxides, but rather due to the extremely efficient spin–lattice relaxation of the d-band conduction electrons. Here, electron spin relaxation in these metallic systems for T > Tc proceeds via an Elliott-type mechanism involving the modulation of spin–orbit interactions via itinerant conduction electrons. This efficient relaxation mechanism would lead to extremely large ESR linewidths, typically in the range 50–100 kG.

ESR studies of high critical-temperature superconductors :absorption at low magnetic fields

One of the most intriguing aspects of ESR studies of oxide superconductors is the presence of a broad, and intense, low-field microwave absorption which grows in intensity upon cooling the sample through the superconducting transition temperature (Tc). In the present investigations we exploit the low-field response as a screening technique for superconductivity in a range of oxide materials. The materials investigated, derivatives of the 90 K superconductor YBa2Cu3O7, were specifically chosen so that Tc could be drastically modified either by oxygen loss or cation substitution. In all of these materials a clear correlation exists between the onset temperature of the low-field absorption and the superconducting transition temperature, as gauged by both a.c. inductive and electrical resistivity measurements. The temperature range over which the superconductivity onset was detected by these complementary methods extended from ca. 90 K down to ca. 30 K in the case of a Zn-substituted YBa2Cu3O7. Measurements on the corresponding Bi–Sr–Ca–Cu–O system, which becomes superconducting for temperatures below 110 K, also illustrate that this absorption phenomenon is not unique to YBa2Cu3O7 and hence will have widespread applicability in detecting superconductivity.

Metal particles supported by porous glass

We have investigated the use of porous ‘Vycor’ glass as a supporting matrix for silver and alkali-metal particles. By different methods of preparation it was possible to produce silver particles either within the pores of the glass or on the external surface. The optical spectra of silver particles are reported and discussed. Alkali-metal particles gave strong electron spin resonance spectra, but in the case of silver weak electron spin resonance signals were detected from the surface particles only.

The Superconductor-Semiconductor Transition in Cation-Substituted Lithium Titanate, Li[Mx Ti2–x]O4:M = Li+, Al3+ and Cr3+

Professor Fritzsche’s research, encompassing more than three decades of beautiful experimental work combined with elegant and incisive writing, has centred primarily around two major themes; one old, one (perhaps) new, but both interwoven. The first is his interest and involvement in the study of the Metal-Semiconductor Transition; the second concerns the structural and electronic properties of disordered materials. In this dedication, we outline a further subtlety (or complexity) into these major problems in condensed matter chemistry and physics; namely the possible nature of the composition induced transition from Superconductor to Semiconductor in a system with obvious crystalline order — in association with equally obvious electronic disorder. As far as we are aware Professor Fritzsche has not, so far, ventured into the area of Superconductor-Semiconductor Transitions. We offer the present meagre contribution in the hope of 1 pressing his considerable talents into the study of yet another class 2 of remarkable transmutations!

Magnetic susceptibility and electron spin resonance in quenched metal solutions

We describe magnetic susceptibility and electron spin resonance measurements of solutions of lithium in the non-aqueous solvent hexamethylphosphoric triamide over the temperature range 4–270 K. The concentration of lithium was varied from 1 to 6 mol% lithium. Our results reveal the coexistence of at least two distinct, magnetically isolated species; a localised electron species being predominant at low metal concentrations and low temperatures, and a metallic, itinerant electron state becoming dominant as the lithium concentration is increased. These results point to a considerable degree of aggregation of centres in the freezing process.

Ionic and metallic clusters in zeolites

Na43+ ionic clusters, probably located inside sodalite cages, are formed when zeolite Na+-Y is exposed to Na or K vapour and K43+ cluster when K+-Y is exposed to the vapour of Na or K, but Rb43+ cannot be observed (by e.s.r. spectroscopy) using Rb+-Y under comparable conditions; at higher concentrations of metal vapour intracrystalline metallic particles are formed.