# Simulating phase transition (structure change) with temperature

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

From a quick look the problem could be related to using the “iso” option with the NPT ensemble.

Carlos

yes, if you use “iso”, then the box aspect ratio
is locked in. If you want it to be able to
change you need different fix npt options.

See the doc page for details.

Steve