Theory of the quantum diffusion of parahydrogen impurities in solid orthohydrogen

The hopping of parahydrogen impurities in solid orthohydrogen due to resonant conversion processes of para-ortho into ortho-para pairs is investigated theoretically both in the ordered and in the disordered phase. At temperatures below the transition temperature $T_0$ the jump frequency is predicted to be about four times larger than for ortho impurities in solid parahydrogen at the same impurity concentration. The effect on the hopping of the distortion of the ordered state by the impurities is taken into account, and the cohesive and interaction energies of the impurities are calculated. In the disordered phase the jump frequency is about two orders of magnitude smaller than below $T_0$, and the quantum diffusion of the para impurities is expected to be negligible above $T_0$.