Effect of the (Ba + Sr)/Ti ratio on the microwave‐tunable properties of Ba_0.6Sr_0.4TiO₃ ceramics

Abstract

The impact of the (Ba + Sr)/Ti (A/B) ratio on the microwave‐tunable characteristics of diffuse phase transition (DPT) ferroelectric Ba0.6Sr0.4TiO3 (0.6‐BST) ceramics was investigated. The reduction in the lattice constant with increasing nonstoichiometry was attributed to introduced partial Schottky defects, i.e., false[V″″Ti−2normalVnormalO∙∙false]×and false[V″normalBa,normalSr−normalVnormalO∙∙false]×. The magnitude of the dielectric constant, ε′, at room temperature in the absence of an applied electric field was governed by the shift in the dielectric maximum temperature, Tm, because Tm was close to room temperature for the 0.6‐BST. The dielectric loss, tanδ, diminished as the ε′ decreased for 0.98≤A/B≤1.05, while the tanδ was much higher for A/B=0.95 having the greatest A‐site vacancy loading. The negatively charged V″normalBa and V″normalSr were mainly compensated by oxygen vacancies and likely partly compensated by holes, h•, which contributed to the electrical conduction. The tunability, T, at 100 MHz was almost constant at 20%–25% for A/B≥1.00 despite the reduction of the ε′, whereas T decreased for A/B<1.00 to ca. 10% for A/B=0.95 having the greatest A‐site vacancy loading. The results implied that the false[V″″Ti−2normalVnormalO∙∙false]× for larger A/B values was more efficient in generating nucleation sites in the polar nanoregions (PNRs) than the false[V″normalBa,normalSr−normalVnormalO∙∙false]× for smaller A/B values, thereby providing greater dipole polarization. Consequently, the figure of merit, FOM, reached its maximum of 250 at A/B=0.9875, which was ca. 155% higher than that of the stoichiometric BST.

Domain engineering enhanced microwave tunability in nonstoichiometric Ba0.8Sr0.2TiO3

“…The effect of nonstoichiometry in DPT ferroelectrics (0.6 BST) on microwave tunability has also been reported in our previous work . The tunability T at 100 MHz increased with the variation from Ba/Sr‐deficient (0.95 ≤ A/B < 1.00) to Ti‐deficient compositions (1.00 < A/B ≤ 1.05).…”

“…The quality of the tunability is typically expressed with the figure of merit (FOM), composed of T /tan δ, where T is the tunability (%) and tan δ is the loss tangent (%) . Among the tunable ferroelectrics, diffuse‐phase transition (DPT) ferroelectrics, such as Ba x Sr 1− x TiO 3 ( x BST, x = 0.5–0.6), exhibit large tunability relative to a small dielectric loss, yielding excellent FOMs at microwave frequencies . The permittivity in BSTs with typical DPT characteristics (e.g., 0.5‐0.6 BSTs) at room temperature is ˜ 4000‐6000 in bulk ceramic forms .…”

“…However, BSTs with DPT features ( x = 0.5‐0.6) display a large temperature coefficient of the dielectric constant near room temperature due to the ε r anomaly at the dielectric maximum temperature ( T m ) . The T m shifts steadily to a higher temperature with increasing Ba/Sr ratio.…”

“…7,10,11 Among the tunable ferroelectrics, diffuse-phase transition (DPT) ferroelectrics, such as Ba x Sr 1Àx TiO 3 (xBST, x = 0.5-0.6), exhibit large tunability relative to a small dielectric loss, yielding excellent FOMs at microwave frequencies. [12][13][14][15] The permittivity in BSTs with typical DPT characteristics (e.g., 0.5-0.6 BSTs) at room temperature is~4000-6000 in bulk ceramic forms. 12,[16][17][18][19][20][21] Such large polarizabilities originate from the ionic contribution (i.e., optical phonons) as well as dipole fluctuations in the polar nanoregions (PNRs).…”

“…[12][13][14][15] The permittivity in BSTs with typical DPT characteristics (e.g., 0.5-0.6 BSTs) at room temperature is~4000-6000 in bulk ceramic forms. 12,[16][17][18][19][20][21] Such large polarizabilities originate from the ionic contribution (i.e., optical phonons) as well as dipole fluctuations in the polar nanoregions (PNRs). 12,16,[20][21][22][23] The response of these combined polarizations with electric field application is thus the source of large BST tunability.…”