Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)




Professor C.V. Stager


A low temperature single crystal X-ray diffractometer has been designed and constructed. This has been used to determine the crystal structure of dysprosium arsenate and terbium arsenate at 6ºK. These structures are compared with those found at room temperature. This comparison is used to gain information about the the ionic coupling mechanism which drives a cooperative Jahn-Teller transition in these compounds at temperatures below 30ºK. Further information is obtained from a study of the temperature dependence of the lattice strain, which is proportional to the order parameter for the transition. For TbAsO4 it is found that the coupling is long range, and that the dominant mechanism is via the lattice strain. The ionic interaction is DyAsO4 is found to be short range; coupling to both lattice strain and phonons are important in this case.

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