Molecular-dynamics simulations of some BaXF4compounds

Abstract

We have carried out molecular-dynamics simulations on BaXF 4 compounds, where X is Mg, Mn, or Zn. Ab initio potentials, with no adjustable parameters, were used to obtain short-range interactions between ion pairs. We found a polar ground-state structure which is in agreement with the A2 1 am space group reported experimentally. We were able to reverse polarization in BaMgF 4 at high temperatures, using large fields, but were unable to reverse polarization in the other compounds. The second-order phase transition in the Mn compound at 250 K was reproduced. We believe this to be the first extension of molecular dynamics to materials consisting of chains of F octahedra. We have carried out molecular-dynamics simulations on BaXF, compounds, where X is Mg, Mn, or Zn. Ab initio potentials, with no adjustable parameters, were used to obtain short-range interactions between ion pairs. We found a polar ground-state structure which is in agreement with the A2,am space group reported experimentally. We were able to reverse polarization in BaMgF, at high temperatures, using large fields, but were unable to reverse polarization in the other compounds. The second-order phase transition in the Mn compound at 250 K was reproduced. We believe this to be the first extension of molecular dynamics to materials consisting of chains of F octahedra. ©1994 The American Physical Society

Ferroelectric and optical properties of single crystal BaZnF4

“…6 The nonlinear coefficients have been estimated relative to d 11 quartz, 7 d 32 Ϸ 0.13d 11 and d 33 Ϸ 0.05d 11 , and relative to d 36 potassium dihydrogen phosphate, 8 d 32 Ϸ 0.085d 36 and d 33 Ϸ 0.023d 36 . 10 The polarization hysteresis, however, has not been observed yet. The transmittance of BaZnF 4 has been reported to be similar to that of BaMgF 4 ͑0.17-8 m͒, while the nonlinear coefficients are larger, d 32 Ϸ 0.25d 11 and d 33 Ϸ 0.11d 11 .…”

A variable temperature solid-state nuclear magnetic resonance, electron paramagnetic resonance and Raman scattering study of molecular dynamics in ferroelectric fluorides

“…However, the paraelectric -ferroelectric transition and its mechanism have been strongly evidenced by both experimental and theoretical studies [16,20,21,24]. There is also strong experimental evidence of the existence of the "soft" Raman bands which disappear at the Curie point [24,34].…”

“…For further analysis the differences in chemical The origin of this discontinuity lays in the discontinuous change of the fluorine coordination sphere, which is directly related to the change of the octahedra geometry. The largest discontinuities observed for iso 21 and iso 23 ( figure 3(b)), indicate that geometrical rearrangement takes place in the plane formed by F(1), F(2) and F (3) No significant changes in spectral parameters have been observed between 292 and 150 K ( figure 2(c)), despite the potential of 25 Mg to be a very sensitive probe of the local environment through the changes in quadrupolar interaction [51]. However, such "insensitivity" cannot provide definitive evidence of the high structural stability because the observed broad line spectrum could mask very subtle changes in 25 Mg coordination.…”

“…For three members of the BaMF 4 family, namely BaMnF 4 BaCoF 4 and BaNiF 4 , multiferroic properties were reported and are now widely accepted [13][14][15]. The molecular dynamics of geometric fluorites have also attracted attention, but most studies are limited to understanding the origin of spontaneous polarisation and the paraelectric -ferroelectric phase transition, which occurs around 100 K above the melting point (in the range of 990 to 1240 K, respectively for BaFeF 4 to BaNiF 4 ) [20,21]. The low temperature studies have been predominantly focused on magnetic properties of BaMnF 4 and BaNiF 4 [12,15,22,23].…”

Structural phase transitions in ionic molecular solids

“…2 we show the &projection of the room temperature phase of BaMnF4 (Flocken et al, 1994) calculated by the Gordon-Kim approach. What one observes are parallel chains of MnFi-units which have rotated about the c' axis through an angle sufficient to permit contact with the Ba2+ to stabilize these otherwise intrinsically unstable linear arrays.…”