NMR studies of a D₂ single crystal in the ordered phase

Abstract

We report the first experimental study of the NMR lineshape anisotropy in the orientationally ordered cubic phase of a D₂ single crystal. The para-D₂ concentration covered the range 0.72<X<0.82. The observed line shapes were obtained by means of Fourier transform techniques from the solid echoes recorded with a pulsed NMR spectrometer operating at 5.9 MHz. Theoretical line shapes and their second moments forp-D₂ (with angular momentumJ=1 and spinI=1) and foro-D₂ (withJ=0, I=2) were calculated as a function of the applied field direction from the theory by A. B. Harris. Good agreement was obtained for all directions, provided that, just as for solid H₂, a Gaussian function for the distribution of molecular axial alignment was assumed with an rms spreading angle of ∼6 deg forX=0.77. No change in the anisotropy of the second moment in the disordered phase was detected between the initial (hcp) crystal and after repeated thermal cyclings through the martensitic transition. This result implies no change in the orientation of the plane for the sliding nets during repeated passages through the transition. Furthermore, we report measurements of the longitudinal relaxation time for bothp-D₂ ando-H₂ versusT over the temperature range 0.5<T<3.5 K. The complex behavior reported previously for both H₂ and D₂ was confirmed. Measurements of the relaxation times as a function of the applied field direction in the ordered phase at 1.2 K showed no anisotropy within experimental error, and this result is discussed in the light of predictions by Hardy and Berlinsky. Finally, the polarization of theJ=0 molecules is studied as a function of the concentrationX of theJ=1 molecules and their state of orientational order over a wide range ofX andT in the ordered and disordered states. The results are compared with predictions of A. B. Harris and with previous data on polycrystalline samples.