Quantum aspects of low-temperature properties of crystals: A calorimetric study in interaction with spectroscopy and diffraction*

Some of the molecular and ionic crystals containing hydrogen undergo phase transitions only when they are deuterated. This indicates that not only the intermolecular potential energy, but also the kinetic energy of the nuclei are involved in an essential way in the mechanisms of the transitions. The quantum effects implied by these observations are discussed in terms of tunneling of protons and deuterons. Experimental evidence for deuteration-induced phase transitions and nuclear tunneling states accumulated by low-temperature calorimetry, far infrared spectroscopy, and neutron diffraction is discussed.