J. G. Daunt

On the question of isotopic phase separation in solid H2 — D2 mixtures*

Abstract : The purpose of the research was to investigate possible isotopic phase separation in solid H2 - D2 mixtures by observing line widths and line shapes of nuclear resonance. A calculation of the rigid lattice second moment for protons in a homogeneous solid H2 - D2 mixture was made. Observations of the N.M.R. line widths and line shapes of solid mixtures of n-H2 and n-D2 at 4.2 K showed the line shapes to be Gaussian and yielded widths which are illustrated. The calculated shape of the derivative of a resonance line obtained from a phase separation of a sample of original concentration X sub H2 = 0.20 into regions with concentration X sub H2 = 0.95 and X sub H2 = 0.05 is presented. The composite nature of the line is evident. (Author)

A Search for Isotopic Phase Separation in Hydrogen-Deuterium Mixtures

This work was originally started in the hope that information obtained from nuclear magnetic resonance experiments would provide additional data on the type of transition known as isotopic phase separation. In particular, it seemed probable that NMR experiments could readily distinguish the difference between a transition from the randomly mixed state (the high temperature equilibrium configuration) to the isotopically ordered state involving rearrangement of the isotopes into interpenetrating sublattices and a transition involving only a clumping together into small domains. The system hydrogen-deuterium was chosen for investigation, since Kogan, Lazarev, and Bulatova1 had reported observing such a phase separation at the relatively high temperature of 16.4°K. At such high temperatures, the diffusion rates are large enough2 so that domains of macroscopic size can form in a relatively short time. Since the line-width is dependent on the concentration of orthohydrogen molecules, one would expect to observe a transition from a simple line into a composite line with both a very broad and a very narrow component in the event a phase separation occurred. At high temperatures T < 12°K, pulsed NMR techniques were employed, since the true line-width was swamped by magnetic field inhomogeneities. No such transitions were observable from the T1 and T2 data. At temperatures below 12°K, the solid absorption line becomes very broad and steady-state (broad line) techniques are applicable.

Magnetic Susceptibility of Mn++ IN CdS and Effects of Antiferromagnetic Exchange

This paper reports low-temperature measurements of the magnetic susceptibility of single crystals of CdS (in the hexagonal modification) with manganese impurity added. The data provide clear evidence for isotropic antiferromagnetic exchange interactions between next-nearest neighbor and in fact even more distant pairs of Mn++ ions. Antiferromagnetic effects in other dilute paramagnetic insulating crystals have been observed previously, as, for example, in susceptibility measurements on Cr+++ in A12O3 by Daunt et al.,1 on Mn++ in ZnS by Brumage et al.,2 in resonance measurements on platinum-diluted ammonium chloriridate by Griffiths and co-workers,3–5 on Mn++ in MgO by Coles, Orton and Owen,6 on Cr+++ in Al2O3 by Rimai et al.,7 and by Gill,8 and in optical observations on Cr+++ in A12O3 by Schawlow, Wood, and Clogston9, ‡ and on Mn++ in ZnS by McClure.11