Influence of dopants on ferroelectric phase transitions in PbTe_1-xS_x and Pb_1-xGe_xTe

A study was made of the influence of nonisoelectronic impurities, Cd, Ga, In, Tl, Sb, Bi, and Mn on the temperature of a ferroelectric phase transition induced in PbTe_1-xS_x and Pb_1-xGe_xTe crystals by off-center impurities S and Ge. It was found that, like the influence of substitutional disorder, introduction of nonisoelectronic impurities lowered the phase transition temperature T_c and the influence of charged impurities was stronger than that of electrically inactive impurities. Lowering of T_c was attributed to the influence of random electric and deformation fields, created by the nonisoelectronic impurities, on the ordering of the dipole moments of the off-center impurities. The magnitude of the effect varied from one impurity to another and it was governed by its charge state, geometric dimensions, and position in the lattice. An investigation of the influence of deviation from stoichiometry on T_c of the doped samples provided a more detailed information on the microscope state of impurities in a crystal.

Keywords: semiconductor solid solutions, electrical studies, lead germanium telluride, lead telluride sulfide, Curie temperature, impurity effects, cadmium, gallium, indium, thallium, antimony, bismuth, manganese