Molecular Dynamics Simulations of the High-Pressure Phase Transitions in BaFCl

Abstract

The alkaline-earth fluorohalides when doped with divalent rare-earth ions, Sm 2þ , are currently used as a more sensitive pressure gauge [1] than the ruby sensor for high-pressure measurements in a diamond-anvil cell. Structural [1][2][3] and electronic [4] studies under high pressure up to 50 GPa have been experimentally carried out on the PbFCl-type crystals. In this rapid note we have examined the nature of the structural phase transitions in BaFCl under hydrostatic pressure up to 30 GPa by molecular dynamics (MD) simulations.The system including 750 atoms in the computational cell was simulated under an isobaric-isothermal (N, T, P) ensemble, where N is the number of atoms, T the temperature and P the pressure. The time step was 1 fs and a cut-off distance was 10 A. The details of the calculations can be found elsewhere [5,6]. The following two cases of simulations were carried out as a function of pressure using the Born-Mayer-Huggins potential for the atomic interaction, in which the parameters for BaFCl are taken from the studies of Baetzold [7]:(sim1) At 300 K, the pressure was increased from 0.1 MPa to 30 GPa with a rate of 0.2 GPa/ps, and decreased from 30 GPa to 0.1 MPa with the same rate.(sim2) At 300 K, the pressure was increased instantaneously between 0.1 MPa and 30 GPa every 5 GPa. For example, in the case of 30 GPa, the rate was about 6 GPa/ps. Figure 1 shows the time dependence of the lattice parameters (a, b, g; a, b, c) and the volume V, when pressure P is varied with a rate of AE0.2 GPa/ps (sim1; see Fig. 1d). In our simulations, the structural changes suddenly occur at two different pressures. One can see the first-order phase transitions with crystallographic changes at about 28.92 GPa in the upstroke process and at about 18.04 GPa in the downstroke process. The simulated transition pressure, 28.92 GPa, is higher than the experimental value [2], 21 GPa, with a conventional diamond-anvil cell. The difference may be attributed to the ideal crystal used in our simulations without having vacancies and impurities. In Fig. 1a, at the first break in the upstroke process when the phase transition takes place, the angle a changes from 90 to 107 and then at the second break in the downstroke process the angle a changes further from 107 to 122 . Figure 1b shows that the magnitude along the unit-cell axes a and b is split into two lines at 28.92 GPa and then joined together at 18.04 GPa, while the unit-cell axis c shortens at 28.92 GPa and then c lengthens at 18.04 GPa abruptly. On the contrary, in the case of sim2, the behavior of the unit-cell axes, a, b and c, was similar as that of sim1, but the angle b changes from 90 to 72 at the first break and returns to the original angle at the second break after occurring a relaxation oscillation. Figures 2a and 2b show the ionic trajectories on the yz-plane in the case of sim1 at 29.00 GPa, just after the phase transition takes place at the first break and at 18.06 GPa, just after that at the second break, respectively. It was found that each atom in BaFCl ...

Unraveling the Hidden Martensitic Phase Transition in BaClF and PbClF under High Pressure Using an Ab Initio Evolutionary Approach

“…Anisotropic compressibilities of matlockite-type compounds (SrClF, SrBrF and PbBrF) are predicted at high pressure and no structural phase transitions were proposed up to 30 GPa for three compounds . Liu et al predicted a structural phase transition from P 4/ nmm to P 2 1 / m in BaClF at 28.92 GPa based on molecular dynamics simulations.…”

“…14 Anisotropic compressibilities of matlockite-type compounds (SrClF, SrBrF and PbBrF) are predicted at high pressure and no structural phase transitions were proposed up to 30 GPa for three compounds. 15 Liu et al 16 transition from P4/nmm to P2 1 /m in BaClF at 28.92 GPa based on molecular dynamics simulations. Interestingly, a high pressure Raman spectroscopic study 17 on BaClF showed an intermediate orthorhombic (Pnma) phase at around 10.8 GPa which coexists with the ambient P4/nmm phase and above 21.1 GPa these two phases coexist with the previously observed P2 1 /m 10 phase up to the maximum studied pressure of 25.6 GPa.…”

Raman and infrared vibrational frequencies and elastic properties of solid BaFCl calculated with various Hamiltonians: anab initiostudy

“…In recent years, the elastic properties and phonon spectra of the alkaline-earth fluorohalides MFX (M = Ba, Ca, Sr; X = Cl, Br, I) have been the subject of many theoretical and experimental investigations [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. These compounds, when doped with rare-earth ions, present fascinating properties, like those related to image plate x-ray storage [21][22][23][24]; they are also used as pressure calibrants [21] in diamond anvil cells.…”

Electronic structure of ionic PbFCl-type compounds under pressure

“…A similar work was reported by Subramanian et al [13] where the high-pressure phase was found to be of monoclinic structure (P2 1 /m) and the transition pressure is around 22 GPa. Molecular dynamics stimulations of the high-pressure phase transition in BaFCl were performed by Liu et al [14]. The thermal expansion coefficient of BaFX (X = Cl, Br) has been studied by Kurobori et al [15].…”