The use of low-temperature thermal expansion for the detection of paramagnetic ions in dielectrics

The object of the thesis is to demonstrate the existence of Schottky type anomalies in the low temperature thermal expansion of dielectric crystals produced by the presence of a small concentration of strongly coupled paramagnetic impurity ions. Cryogenic equipment, utilising a three terminal capacitance dilatometer, for the semi-automatic measurement of thermal expansion at low temperatures using a dynamic measurement technique has been designed, constructed and commissioned. The results of application of this apparatus to determine the contribution to the thermal expansion at low temperatures of aluminium oxide (Al2O3) due to small concentrations of strongly coupled paramagnetic ions (Cr2+(3d4, 5D), Mn3+(3d4, 5D), v3+ (3d2 , 3F)) is presented. Peaked anomalies in the thermal expansion of Al2O3 doped with these ions have been observed which are not present for undoped samples. The effects observed are in contrast with those predicted using a static crystal field model but may be interpreted in terms of a dynamic Jahn-Teller model.