Solubility of vanadium in barium titanate ceramics

Abstract The solubility of vanadium in perovskite-structured $$\text {BaTiO}_{3}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mtext>BaTiO</mml:mtext> <mml:mn>3</mml:mn> </mml:msub> </mml:math> is analyzed via X-ray powder diffraction as a function of two key variables for multi-layer ceramic capacitor applications: A/B cation ratio and oxygen partial pressure. A low solubility limit of around 0.2 mol% V is measured for B-site rich ( $$A/B=0.997$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>A</mml:mi> <mml:mo>/</mml:mo> <mml:mi>B</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.997</mml:mn> </mml:mrow> </mml:math> ) compositions and &lt; 0.2 mol% V for A-site rich ( $$A/B=1.001$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>A</mml:mi> <mml:mo>/</mml:mo> <mml:mi>B</mml:mi> <mml:mo>=</mml:mo> <mml:mn>1.001</mml:mn> </mml:mrow> </mml:math> ) compositions when processed in air at $$1100\,^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>1100</mml:mn> <mml:mmultiscripts> <mml:mspace/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:mrow> </mml:math> C. The limited solubility arises from the propensity to form the competing $$\text {Ba}_{3}\text {V}_{2}\text {O}_{8}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mtext>Ba</mml:mtext> <mml:mn>3</mml:mn> </mml:msub> <mml:msub> <mml:mtext>V</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mtext>O</mml:mtext> <mml:mn>8</mml:mn> </mml:msub> </mml:mrow> </mml:math> phase, which is enhanced in excess Ba compositions. The solubility of V is, however, significantly increased in reducing atmospheres, to at least 0.4 mol% V in the B-site rich compositions. Only a small increase in the V solubility is observed for A-site rich compositions. The higher solubility in reducing atmospheres is attributed to the stabilization of the lower oxidation states of $$\text {V}^{4+/3+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mtext>V</mml:mtext> <mml:mrow> <mml:mn>4</mml:mn> <mml:mo>+</mml:mo> <mml:mo>/</mml:mo> <mml:mn>3</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> </mml:math> in the B-site octahedral coordination.