First-principles pseudopotential calculations by means of the local density approximation (LDA) within density-functional theory (DFT) are carried out to investigate the negative pressure induced ferroelectric phase transition in rutile TiO(2) in the range of -25 to 25 GPa. The softening behavior of the A(2u)(TO) modes at the Γ point following the decreasing pressure leads to a ferroelectric phase transition from P4(2)/mnm (rutile) space group to P42nm (ferro) space group. The calculated pressure dependence of the phonon frequencies, A(1)(TO), E(TO), and B(2) modes of the ferro TiO(2) relative to A(2u)(TO), E(u)(TO), and B(1u) modes of rutile TiO(2), indicates that the phase transition occurs at around -10 GPa, consistent with the total energy results. The dramatic increase of the c-axial dielectric tensor in the vicinity of the phase transition indicates it to be a typical ferroelectric phase transition. The character of the phase transition is generally identified in terms of the calculated order parameters, displaying a second order.

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Strain-induced ferroelectric phase transitions in incipient ferroelectric rutile TiO 2 *Ni Li-Hong( ) a) , Liu Yong( ) a)b) † , Ren Zhao-Hui( ) a) , Song Chen-Lu( ) a) , and Han Gao-Rong( ) a)

The assignment of microscopic Raman spectra on powdered samples of pre-perovskite PbTiO3 (space group I4/m) has been established according to the correspondence between the experimental and theoretical frequency and relative intensity data. We find no giant longitudinal-optical (LO) and transverse-optical (TO) splittings in pre-perovskite PbTiO3. In contrast to conventional perovskite, the largest pre-perovskite LO-TO splitting comes from its stiffest Au mode instead of from its softest mode. The pre-perovskite’s Born effective charges, dielectric tensors, and infrared frequencies are also calculated to determine its lattice dynamics and optical properties, and compared with the conventional perovskite PbTiO3. The results provide invaluable information for use in further research on PbTiO3 phase transition.

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