  Mechanical stacking of two dissimilar materials often has surprising
consequences for heterostructure behavior. In particular, a two-dimensional
electron gas (2DEG) is formed in the heterostructure of the topological
crystalline insulator Pb0.24Sn0.76Te and graphene due to contact of a polar
with a nonpolar surface and the resulting changes in electronic structure
needed to avoid polar catastrophe. We study the spintronic properties of this
heterostructure with non-local spin valve devices. We observe spin-momentum
locking at lower temperatures that transitions to regular spin channel
transport only at ~40 K. Hanle spin precession measurements show a spin
relaxation time as high as 2.18 ns. Density functional theory calculations
confirm that the spin-momentum locking is due to a giant Rashba effect in the
material and that the phase transition is a Lifshitz transition. The
theoretically predicted Lifshitz transition is further evident in the phase
transition-like behavior in the Land\'e g-factor and spin relaxation time.
