  Within a mean-field approximation, the ground state and finite temperature
phase diagrams of the two-dimensional Kondo lattice model have been carefully
studied as functions of the Kondo coupling $J$ and the conduction electron
concentration $n_{c}$. In addition to the conventional hybridization between
local moments and itinerant electrons, a staggered hybridization is proposed to
characterize the interplay between the antiferromagnetism and the Kondo
screening effect. As a result, a heavy fermion antiferromagnetic phase is
obtained and separated from the pure antiferromagnetic ordered phase by a
first-order Lifshitz phase transition, while a continuous phase transition
exists between the heavy fermion antiferromagnetic phase and the Kondo
paramagnetic phase. We have developed a efficient theory to calculate these
phase boundaries. As $n_{c}$ decreases from the half-filling, the region of the
heavy fermion antiferromagnetic phase shrinks and finally disappears at a
critical point $n_{c}^{*}=0.8228$, leaving a first-order critical line between
the pure antiferromagnetic phase and the Kondo paramagnetic phase for
$n_{c}<n_{c}^{* }$. At half-filling limit, a finite temperature phase diagram
is also determined on the Kondo coupling and temperature ($J$-$T$) plane.
Notably, as the temperature is increased, the region of the heavy fermion
antiferromagnetic phase is reduced continuously, and finally converges to a
single point, together with the pure antiferromagnetic phase and the Kondo
paramagnetic phase. The phase diagrams with such triple point may account for
the observed phase transitions in related heavy fermion materials.
