Comparison of BST film microwave tunable devices based on (100) and (111) MgO substrates.

We have increased the figure-of-merit (FOM) of a (Ba,Sr)TiO3 (BST) film microwave tunable device by approximately three times for MgO(111) compared with a MgO(100) substrate at a frequency range of 20 GHz. Differences in permittivity and tunability in a BST film may be closely related to the difference in the film strain. The ratio of calculated permittivities of BST(100) and BST(111) films nearly corresponds to that of the FOM in the microwave range, which was rather unexpected because a higher permittivity leads to both larger tunability and dielectric loss in ferroelectrics. From a series of results, it is suggested that there are additional influences of orientation (other than the direct influence of strain itself) on the tunable properties in BST films especially in the high-frequency region.

Low temperature preparation of bismuth-related ferroelectrics powder and thin films by hydrothermal synthesis.

Bi(4)Ti(3)O(12) (BIT) thin films were prepared by low temperature hydrothermal synthesis on Pt/TiO(x)/SiO(2)/Si. Bi(4)Ti(3)O(12) or TiO(2) gel solution was formed and annealed at 350 degrees C. The BIT thin films were crystallized as a Bi-layer structural ferroelectric. During the hydrothermal treatment, the TiO(2) anatase (101) peak appears and seems to play the role as an intermediate layer. Randomly oriented BIT thin films were obtained. As a result, the BIT thin films have ferroelectric property. The as-deposited BIT thin films include spherical grains with the grain size of 120 nm.

A mechanism for the 150 µC cm(-2) polarization of BiFeO(3) films based on first-principles calculations and new structural data.

Following our experimental report of a giant ferroelectric polarization in the region of 150 µC cm(-2) in BiFeO(3) (BFO) films, we have performed first-principles calculations based on the local density approximation to density functional theory, aiming to clarify its mechanism. Upon optimization of lattice constants we have shown that the natural tetragonal structure of BFO has a giant tetragonality ratio of 1.26 and large ionic off-centring. Experimentally this structure has been detected in BFO films deposited on La-doped SrTiO(3) substrates. The spontaneous polarization calculated ab initio for this structure is 143.5 µC cm(-2), in agreement with the remanent polarization of hysteresis loops measured at 90 K. These results suggest that the giant polarization of our BFO films may occur upon stabilization of the optimal tetragonal phase with giant tetragonality. Future experimental effort aiming to routinely obtain such values of spontaneous polarization should concentrate on how to isolate this phase without compromising the insulating and switching properties of BFO.