The Influence of Defects on Phase Transitions.

The phase transitions of SrTiO_3 and a d = 3 random field Ising model (Fe _{0.46}Zn_{0.54 }F_2) were studied. The phase transition of the d = 3 random field Ising model was studied by measuring the specific heat using a heat pulse type experiment. The magnetic component of the specific heat was determined by subtracting the nonmagnetic background. The magnetic component of the specific heat was also measured previously using the technique of linear optical birefringence. The results of the two independent measurements of the magnetic specific heat, one using the technique of birefringence and the other using a heat pulse technique, were shown to be in good agreement. The birefringence of SrTiO_3 was measured as a function of temperature and pressure. Runs with pressure along the <100 > and the < 111> axes were performed. With no pressure applied, the critical exponent s was measured, and determined to equal 0.84 +/- 0.02. This agrees well with theory, which predicts s = 0.865 +/- 0.015. The runs with pressure applied were used to determine phase boundaries. The results are consistent with the predictions of renormalization group theory.