Effect of the (Ba + Sr)/Ti ratio on the microwave‐tunable properties of Ba _0.6 Sr _0.4 TiO ₃ ceramics

<jats:title>Abstract</jats:title> <jats:p> The impact of the (Ba + Sr)/Ti (A/B) ratio on the microwave‐tunable characteristics of diffuse phase transition ( <jats:styled-content style="fixed-case">DPT</jats:styled-content> ) ferroelectric Ba <jats:sub>0.6</jats:sub> Sr <jats:sub>0.4</jats:sub> TiO <jats:sub>3</jats:sub> (0.6‐ <jats:styled-content style="fixed-case">BST</jats:styled-content> ) ceramics was investigated. The reduction in the lattice constant with increasing nonstoichiometry was attributed to introduced partial Schottky defects, i.e., <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jace14700-math-0001.png" xlink:title="urn:x-wiley:00027820:media:jace14700:jace14700-math-0001"/> and <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jace14700-math-0002.png" xlink:title="urn:x-wiley:00027820:media:jace14700:jace14700-math-0002"/> . 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, <jats:italic>T</jats:italic> <jats:sub>m</jats:sub> , because <jats:italic>T</jats:italic> <jats:sub>m</jats:sub> was close to room temperature for the 0.6‐ <jats:styled-content style="fixed-case">BST</jats:styled-content> . 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 <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jace14700-math-0003.png" xlink:title="urn:x-wiley:00027820:media:jace14700:jace14700-math-0003"/> and <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jace14700-math-0004.png" xlink:title="urn:x-wiley:00027820:media:jace14700:jace14700-math-0004"/> were mainly compensated by oxygen vacancies and likely partly compensated by holes, h <jats:sup>•</jats:sup> , which contributed to the electrical conduction. The tunability, <jats:italic>T</jats:italic> , at 100 MHz was almost constant at 20%–25% for A/B≥1.00 despite the reduction of the ε′, whereas <jats:italic>T</jats:italic> decreased for A/B<1.00 to <jats:italic>ca</jats:italic> . 10% for A/B=0.95 having the greatest A‐site vacancy loading. The results implied that the <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jace14700-math-0005.png" xlink:title="urn:x-wiley:00027820:media:jace14700:jace14700-math-0005"/> for larger A/B values was more efficient in generating nucleation sites in the polar nanoregions ( <jats:styled-content style="fixed-case">PNR</jats:styled-content> s) than the <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jace14700-math-0006.png" xlink:title="urn:x-wiley:00027820:media:jace14700:jace14700-math-0006"/> for smaller A/B values, thereby providing greater dipole polarization. Consequently, the figure of merit, <jats:styled-content style="fixed-case">FOM</jats:styled-content> , reached its maximum of 250 at A/B=0.9875, which was <jats:italic>ca</jats:italic> . 155% higher than that of the stoichiometric <jats:styled-content style="fixed-case">BST</jats:styled-content> . </jats:p>