Phase transitions in KIO₃

Abstract

The high-pressure behavior of KIO(3) was studied up to 30 GPa using single crystal and powder x-ray diffraction, Raman spectroscopy, second harmonic generation (SHG) experiments and density functional theory (DFT)-based calculations. Triclinic KIO(3) shows two pressure-induced structural phase transitions at 7 GPa and at 14 GPa. Single crystal x-ray diffraction at 8.7(1) GPa was employed to solve the structure of the first high-pressure phase (space group R3, a = 5.89(1) Å, α = 62.4(1)°). The bulk modulus, B, of this phase was obtained by fitting a second order Birch-Murnaghan equation of state (eos) to synchrotron x-ray powder diffraction data resulting in B(exp,second) = 67(3) GPa. The DFT model gave B(DFT,second) = 70.9 GPa, and, for a third order Birch-Murnaghan eos, B(DFT,third) = 67.9 GPa with a pressure derivative of [Formula: see text]. Both high-pressure transformations were detectable by Raman spectroscopy and the observation of second harmonic signals. The presence of strong SHG signals shows that all high-pressure phases are acentric. By using different pressure media, we showed that the transition pressures are very strongly influenced by shear stresses. Earlier work on low- and high-temperature transitions was complemented by low-temperature heat capacity measurements. We found no evidence for the presence of an orientational glass, in contrast to earlier dielectric studies, but consistent with earlier low-temperature diffraction studies.

Second harmonic generation measurements at high pressures on powder samples

“…From SHG measurements, it became obvious that around 7 GPa KIO 3 transforms to a first acentric high pressure phase and at 14 GPa there is a second pressureinduced structural phase transition [44] (Fig. 8).…”

“…The presence of strong SHG signals shows that all high pressure phases are acentric. The high-pressure behaviour of KIO 3 was then studied up to 30 GPa using X-ray diffraction, Raman spectroscopy, density functional-theory based calculations and SHG experiments [44] and the crystal structure of the first high pressure phase was solved from single crystal X-ray diffraction experiments. While the structure of the second high pressure phase currently is unsolved, it is clear that it is an acentric structure.…”

“…While the structure of the second high pressure phase currently is unsolved, it is clear that it is an acentric structure. The influence of the deviatoric stress on the transition is significant and can be easily determined using SHG [44].…”

Density functional theory based calculations for high pressure research

“…The use of non-hydrostatic pressure media may lead to pressure-induced amorphization, or lead to a significant lowering of the transition pressure (e.g. Bayarjargal et al (2012)). …”

Rashba spin splitting and anomalous spin textures in the bulk ferroelectric oxide perovskite KIO3

“…KIO, in particular, is experimentally synthesized at high temperature in non-centrosymmetric rhombohedral phase with R3m space group symmetry 22 . A thermodynamically favorable rhombohedral R3c phase was also theoretically predicted 21,23,24 . In both phases, distortion of octahedra centered at I-atom induces spontaneous FE polarization.…”