Pressure-induced ferroelectric to paraelectric transition in LiTaO₃ and (Li,Mg)TaO₃

Journal Article · 16 February 2016 · Journal of Applied Physics

DOI: https://doi.org/10.1063/1.4941383 · OSTI ID:1338347

Yamanaka, Takamitsu ^[1]; Nakamoto, Yuki ^[2]; Takei, Fumihiko ^[2]; Ahart, Muhtar ^[1]; Mao, Ho-kwang ^[3]; Hemley, Russell J. ^[1]

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1. Carnegie Institution of Washington, Washington, DC (United States)
2. Osaka Univ., Osaka (Japan)
3. Carnegie Institution of Washington, Washington, DC (United States); Center for High Pressure Science and Technology, Shanghai (China)

X-ray powder diffraction and Raman scattering of LiTaO₃ (LT) and (Li,Mg)TaO₃ (LMT) have been measured under pressure up to 46 GPa. Above 30 GPa, the ferroelectric rhombohedral phase (R3c, Z – 6) of LiTaO₃ transforms to a paraelectric orthorhombic phase (Pnma with Z – 4) with a large hysteresis. Rietveld profile fitting analysis shows that the Li-O bond is compressed and approaches that of Ta-O with pressure. The cation distribution analysis of the orthorhombic perovskite structure shows that Li and Ta are located in the octahedral 8-fold coordination sites. Difference Fourier |F_obs(hkl)| - |F_cal(hkl)| maps of LiTaO₃ and (Li,Mg)TaO₃ indicate polarization in the c axis direction and a more distinct electron density distribution around the Ta position for (Li,Mg)TaO₃ compared to LiTaO₃. The observed effective charges indicate that for (Li,Mg)TaO₃ without vacancies Ta⁵⁺ becomes less ionized as a function of Mg substitution. Considering both site occupancy and effective charge analysis, Ta⁵⁺ is reduced to Ta^4.13+. Mg²⁺ and O^2- change to Mg^1.643+ and O^{1.732 -}, respectively. The space- and time-averaged structures of the dynamical vibration of atoms can be elucidated from the electron density analysis by difference Fourier and temperature factors T(hkl) in the structure refinement. The refinement of the temperature factor is consistent with the cation distribution assuming full stoichiometry. The residual electron density induced from the excess electron in (Li,Mg)TaO₃ indicates more electrons around the Ta site, as confirmed by the effective charge analysis. Raman spectra of LiTaO₃ and (Li,Mg)TaO₃ show notable changes over the measured pressure range. Raman peaks centered at 250 cm^–1 and 350 cm^–1 at ambient pressure merge above 8 GPa, which we associate with the diminishing of difference in distances between Li-O and Ta-O bonds with pressure in both materials. Finally, Raman spectra show significant changes at 28 GPa and 33 GPa for LT and LMT, respectively, due to the structural transition from R3c to Pnma consistent with the x-ray diffraction results.

• Research Organization: Carnegie Inst. of Washington, Washington, DC (United States); Energy Frontier Research Centers (EFRC) (United States). Energy Frontier Research in Extreme Environments (EFree)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: NA0002006; AC02-06CH11357; FG02-99ER45775; NA-0002006; NA0001974; SC-0001057
• OSTI ID: 1338347
• Alternate ID(s): OSTI ID: 1238078
• Journal Information: Journal of Applied Physics, Vol. 119, Issue 7; ISSN 0021-8979
• Publisher: American Institute of Physics (AIP)Copyright Statement
• Country of Publication: United States
• Language: English