Transparent fluoride glass-ceramics with phase-selective crystallization for middle IR photonics

The density functional theory calculations in AIMD are performed using the Vienna ab initio simulation package (VASP).32 The projected augmented-wave (PAW) method is used to describe the electron–ion interaction, and the generalized gradient approximation (GGA) in the Perdew–Burke– Ernzerhof (PBE) form is employed for the exchange-correlation energy functional. The G point was used to sample the Brillouin zone of the simulation cell containing 330 atoms.

The constant number of atoms, volume and temperature (NVT) ensemble is applied with Nose–Hoover thermostats. The Verlet algorithm is used to integrate Newton’s equation of motion, using a time step of 3 fs. The initial configuration is randomly generated and equilibrated for over 6000 time steps at 1750 K, which is much higher than the melting points.

Then the sample is cooled down to 1150 K. The atomic structures at 1750 K, 1550 K, 1350 K and 1150 K are collected separately to perform further isothermal annealing for more than 12 000 time steps. The first 2000 time steps are not used in the analysis.