Raman scattering studies on nanocrystalline BaTiO₃ Part I—isolated particles and aggregates

Abstract

BaTiO 3 powders with various crystallite sizes, which were prepared through microemulsion-mediated synthesis, were thoroughly studied by Raman spectroscopy. Clear evidence for the presence of the tetragonal phase was found for ultrafine powders with an average crystallite size above 30 nm. The lifetime of phonons that are specifically assigned to the tetragonal phase decreased with decreasing crystallite size below a critical size of 100 nm. In particles as fine as 100 nm, the short mean free path of phonons, mainly due to internal pressure, causes decoupling of the coupled A 1 (TO) phonons and a diffuse phase transition behaviour (T C = 115°C). Coupled A 1 (TO) phonons, which give a spectral dip at around 180 cm −1 and a lesser extent of diffuseness, were revealed for powders consisting of particles as large as 0.17 µm (T C = 123°C). Further coarsening upon annealing induced the formation of aggregates, resulting in the shift of phase transition points to higher temperatures for the rhombohedral to orthorhombic and the orthorhombic to tetragonal transitions and to lower temperatures for the tetragonal to cubic transition, respectively. Phase stability in powders is discussed by considering possible factors such as internal pressure in isolated particles and internal stress in aggregates.

Plastic Deformation of BaTiO3Ceramics by High-pressure Torsion and Changes in Phase Transformations, Optical and Dielectric Properties

“…[16][17][18] Details concerning the BTO Raman spectra were reported elsewhere. [21][22][23][24] These results clearly show that, although substantial pressure is required to trigger the cubic-phase formation, the phase transition is facilitated with straining. This finding is consistent with earlier reports concerning the effect of strain on phase transitions in Ti, [38][39][40] Zr, [41] ZrO 2 [30] and some other metallic alloys.…”

“…[9] Moreover, BTO has been widely investigated for its optical properties, [10] photoluminescence (PL), [11,12] photocatalysis, [13] proton conductivity [14] and phase transformations. [15][16][17][18][19][20][21][22][23][24] BTO transforms from a rhombohedral structure to an orthorhombic phase at − 90°C, to a ferroelectric tetragonal phase at 5°C, and to a paraelectric cubic phase at 120°C under ambient pressure. [15][16][17][18] At ambient temperature, it exhibits a tetragonal-to-cubic phase transformation at 2 GPa [15][16][17][18] and it disorders at 5 GPa.…”

“…[15][16][17][18] At ambient temperature, it exhibits a tetragonal-to-cubic phase transformation at 2 GPa [15][16][17][18] and it disorders at 5 GPa. [22] It was reported that the cubic phase can be stable or that several phases can coexist at ambient conditions due to the nanosize effect: 3 nm for nanowires, [19] 30-50 nm for nanograins, [23,24] and 5-26 nm for nanoparticles. [20,21] *Corresponding author.…”

“…[42][43][44] The cubic phase present after HPT processing remains partially stable even after pressure release because of the formation of nanograins. [19][20][21][22][23][24] Figure 4(b) shows Raman spectra for the samples processed for N = 0, 1 and 5 and subsequently sintered at 1,200°C for 2 h. Figure 4(b) suggests that the crystal structure of BTO samples is mainly composed of tetragonal phase after sintering at 1,200°C.…”

Crystallization, phase evolution and ferroelectric properties of sol–gel-synthesized Ba(Ti0.8Zr0.2)O3–x(Ba0.7Ca0.3)TiO3thin films

“…For solution B, to suppress the hydrolysis of Ti(OC 4 H 9 ) 4 , an appropriate amount of glacial acetic acid was rst added to the accurately weighed Ti(OC 4 H 9 ) 4 , stirring for 20 min, followed by adding ZrO(NO 3 ) 2 $2H 2 O and absolute alcohol, and stirring at room temperature for 1 h to form Ti-Zr solution. Finally, solution A was added into solution B with stirring for 2.5 h at room temperature, and then maintained at [40][41][42][43][44][45][46][47][48][49][50] C for about 1 h to obtain a clear and homogeneous gel. Thin lms were prepared by spin-coating the above solution at 800 rpm for 9 s and at 3000 rpm for 30 s on clean platinumcoated silicon substrates.…”