Influence of donor and acceptor doping on conductivity in potassium niobate

Lead-free ferroelectrics based on the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mrow> <a:mo>(</a:mo> <a:mrow> <a:mi mathvariant="normal">K</a:mi> <a:mo>,</a:mo> <a:mi>Na</a:mi> </a:mrow> <a:mo>)</a:mo> </a:mrow> <a:msub> <a:mi>NbO</a:mi> <a:mn>3</a:mn> </a:msub> </a:mrow> </a:math> solid solution have emerged as strong candidates to replace lead-containing ceramics due to their excellent piezoelectric and thermal properties. However, their practical application is hindered by high leakage currents and low ceramic density. Doping provides a strategy for enhancing material performance, highlighting the need for a deeper understanding of defect chemistry. For this purpose, we investigated the equilibrium of intrinsic defects and the doping behavior of Ca and Fe in the boundary phase <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:msub> <c:mi>KNbO</c:mi> <c:mn>3</c:mn> </c:msub> </c:math> . We have found all elements to be prone to vacancy formation. Moreover, Ca acts as a donor and leads to a large concentration of charge carriers and therefore larger electronic conductivity. On the other hand, Fe behaves as an acceptor and shows oxidation states ranging from <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:msup> <d:mn>2</d:mn> <d:mo>+</d:mo> </d:msup> </d:math> to <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:msup> <e:mn>5</e:mn> <e:mo>+</e:mo> </e:msup> </e:math> , whose concentrations depend on the total Fe concentration. As a consequence, the number of charge carriers and the electronic conductivity are reduced.