9501550 Leisure Ultrasonic methods will be used to study hydrogen isotope motion in the rare-earth metals, and possibly other metals as well. The objectives of these studies will be to elucidate the unusual dynamics of hydrogen in these materials. Experiments will be performed on scandium, lutetium, and yttrium single crystals containing hydrogen and/or deuterium. The experimental results will be used to test modern theories of quantum diffusion in solids. The movement of hydrogen ions between the lowest vibrational levels of nearby interstitial sites in a metal may be regarded as a two-level system (TLS). The usual theory assumes weak coupling of the TLS to the environment, so that the quantum coherence of the TLS is not destroyed. It has been realized recently that the usual theory is severely restricted because, in many cases the coupling of the TLS to electrons in metals, or to phonons at higher temperatures in any material, destroys the quantum coherence. Modern theories of dissipative quantum tunneling are not restricted to weak coupling to the electrons or phonons. Novel ultrasonic techniques will be applied to several other problems. Resonant ultrasound spectroscopy will be used to measure the complete set of elastic constants of materials undergoing phase transitions. With the construction of a 10 kbar pressure cell, the complete set of elastic constants of small samples will be measured under pressure. %%% Sound waves in the frequency range 100 kHz to 100 Mhz (ultrasound) will be used to study the motion of hydrogen ions in several rare- earth metals. Hydrogen is highly mobile in these metals and moves by hopping among the interstitial sites provided by the host metal lattice. The hydrogen motion is affected by the interactions of the hydrogen ions with the host metal atoms via tbe thermal vibrations and conduction electrons of th e host metal. Hydrogen motion affects the speed and energy loss of the ultrasonic waves which are the measured quantities. Low-temperature hydrogen motion cannot be understood using classical physics, but requires a quantum-mechanical description. The hydrogen moves between nearby interstitial sites by quantum-mechanical tunneling, and the tunneling particle is strongly affected by interactions with both the lattice thermal vibrations and conduction electrons. The resulting motion is called dissipative quantum tunneling. It is an open question as to whether the theory of dissipative quantum tunneling can quantitatively describe the low temperature motion of hydrogen isotopes in metals, and it is this question which the proposed study intends to answer. A second part of the work will involve the use of novel ultrasonic techniques to measure the elastic properties of materials undergoing a phase change and of materials under pressure. The ability to measure simultaneously the complete set of elastic constants at high pressure will be unique. ***
