  The phase field method is an effective tool for modeling microstructure
evolution in materials. Many efficient implicit numerical solvers have been
proposed for phase field simulations under uniform and time-invariant model
parameters. We use Eyre's theorem to develop an unconditionally stable implicit
solver for spatially non-uniform and time-varying model parameters. The
accuracy, unconditional stability, and efficiency of the solver is validated
against benchmarking examples. In its current form, the solver requires a
uniform mesh and may only be applied to problems with periodic, Neumann, or
mixed periodic and Neumann boundary conditions.
