  Using first-principles density functional based calculations, we study the
effect of doping and pressure in manganese based pyrochlore
compound,Tl$_{2}$Mn$_{2}$O$_{7}$ that exhibits colossal magneto-resistive
behavior. The theoretical study is motivated by the counter-intuitive
experimental observation of suppression of ferromagnetic transition temperature
upon application of pressure and its enhancement upon substitution of Mn by
moderate amount of nonmagnetic Sb ion. We also attempt to resolve the issue
related to crystal structure changes that may occur upon application of
pressure.
