Molecular relaxation versus orientational ordering ofH2ina-Si:H

Recently reported experimental results on a-Si:H have led to suggestions that ${\mathrm{H}}_2$ in the microvoids of a-Si:H undergoes a phase transition to an orientational ordered state at temperatures ranging from 3 to 20 K. The interpretation is based upon the onset of a Pake doublet in NMR experiments that is suggestive of an order parameter and also by peaks in the heat capacity of ${\mathrm{H}}_2$ in a-Si:H. We show that orientational ordering of microvoid ${\mathrm{H}}_2$ is extremely unlikely at any temperature and certainly not above the critical temperature of bulk single-crystal ${\mathrm{H}}_2$. Instead we shall show that the observed Pake doublet is a manifestation of the slow molecular relaxation regime and that the heat capacity can be explained entirely by a distribution of electric field gradients. Neither phenomenon requires a phase transition driven by the electric quadrupole-quadrupole molecular interactions. However, the sharp onset of the Pake doublet reflects the size and surface conditions of the microvoids.