Hi All,
I meet with a problem about simulating ZrO2 phase transition (from tetragonal to cubic) and sincerely wish for your help.
I want repeat the results from a published MD paper (Patric K. Schelling, et al., J. Am. Ceram. Soc., 84[7] 1609-19 (2001)) . According to the paper, as temperature increases, the lattice constants of tetragonal phase (a and c) will become the same (change into cubic phase) at about 2000K.
I tried heating up the system in a 50 K interval from 100K to 2500K. (Firstly use NPT to ramp the temperature, time step ~ 0.5 fs, run for 5 ps, then average the lattice constant in NVT for 1 ps.) But the lattice constant seems always change proportionally.That is to say, the c/a ration never changes, even if I heat it up to 2500K.
Could anyone who has done a similar simulation gives some advice? Any hint/idea/comment is really appreciated.
Here is more details about my simulation and part of the input file:
I used lammps-20Aug12. Each unit cell contains 4 Zr + 8 O. The simulation box is 888 unit cell (so 6144 atoms). I calculated the lattice constant by dividing the box length (lx, ly lz) by 8.
#------------------------------ create lattice ------------------------------
lattice custom 1.0 a1 5.082 0.000 0.000 a2 0.000 5.082 0.000 a3 0.000 0.000 5.126 &
basis 0.000 0.000 0.000 basis 0.000 0.500 0.500 basis 0.500 0.000 0.500 basis 0.500 0.500 0.000 &
basis 0.205 0.205 0.280 basis 0.795 0.795 0.280 basis 0.205 0.795 0.720 &
basis 0.795 0.205 0.720 basis 0.301 0.301 0.622 basis 0.699 0.699 0.622 &
basis 0.301 0.699 0.378 basis 0.699 0.301 0.378
region whole block 0 8 0 8 0 8 units lattice
#------------------------------ interatomic potential------------------------------
neigh_modify one 4000
kspace_style ewald 1.0e-6
pair_style buck/coul/long 20
pair_coeff 1 1 0.0 1.0 0.0
pair_coeff 2 2 9547.96 0.224 32.0
pair_coeff 1 2 1502.11 0.345 5.1
#---------------------------------NPT part ----------------------------------------
fix 2 all npt temp 1900 1950 0.1 iso 0 0 1
run 10000
unfix 2
#---------------------------------NVT part ----------------------------------------
reset_timestep
fix 3 all nvt temp 1950 1950 0.1
fix ave_lat all ave/time 1 2000 2000 v_lengthx v_lengthy v_lengthz v_V file 50_K_average_lattice.txt
run 2000
unfix 3
unfix ave_lat
…