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Pairing and clustering of o-H2 impurities in solid p-H2 are discussed. The anomalous temperature dependencies observed experimentally earlier are explained not only on the basis of transitions between different mechanisms of quantum diffusion, but also taking into account the large differences between diffusion trajectories associated with different mechanisms. The most interesting and important is a description of diffusion trajectories related to a two-phonon mechanism of quantum diffusion when because of strong ‘‘kinetic repulsion’’ the characteristic pairing trajectories are very long. The considerable decrease in pairing times at low temperatures is caused by a transition from this regime to another one with much shorter trajectories. Two major simplifications permitted to cut considerably the number of unknown (fitting) constants involved staying still within the reasonable agreement with experimental data. The concentration dependence of the pairing and clustering times is also discussed. At lower o-H2 concentrations intermediate stages of clustering should be studied, taking into account a coherent motion of pairs or triads of o-H2 particles.

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© 1990 The American Physical Society