  In this paper, the phase diagrams of a polar spin-1 Bose gas in a
three-dimensional optical lattice with linear and quadratic Zeeman effects both
at zero and finite temperatures are obtained within mean-field theory. The
phase diagrams can be regrouped to two different parameter regimes depending on
the magnitude of the quadratic Zeeman effect $Q$. For large $Q$, only a
first-order phase transition from the nematic (NM) phase to the fully magnetic
(FM) phase is found, while in the case of small $Q$, a first-order phase
transition from the nematic phase to the partially magnetic (PM) phase, plus a
second-order phase transition from the PM phase to the FM phase is obtained. If
a net magnetization in the system exists, the first-order phase transition
causes a coexistence of two phases and phase separation: for large $Q$, NM and
FM phases and for small $Q$, NM and PM phases. The phase diagrams in terms of
net magnetization are also obtained.
