Influence of an <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Al</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math> capping

Superconducting radio-frequency cavities, critical components of modern particle accelerators and quantum computing hardware, fundamentally rely on the surface properties of niobium. However, native oxide formation and impurity uptake in the near-surface region can degrade superconducting performance and increase radio-frequency losses during operation. In this study, we present a systematic x-ray reflectivity investigation of (110) niobium single-crystal surfaces with and without an atomic layer deposited <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mtext>Al</a:mtext><a:mn>2</a:mn></a:msub><a:msub><a:mtext>O</a:mtext><a:mn>3</a:mn></a:msub></a:mrow></a:math> capping layer under ultrahigh-vacuum conditions up to <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mn>650</b:mn><b:msup><b:mspace width="0.16em"/><b:mo>∘</b:mo></b:msup><b:mi mathvariant="normal">C</b:mi></b:mrow></b:math>. Our results reveal a temperature-dependent reduction of the native niobium oxide layers in both capped and uncapped samples, with similarities in the overall behavior but clear differences in the reduction pathways. The <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:msub><e:mtext>Al</e:mtext><e:mn>2</e:mn></e:msub><e:msub><e:mtext>O</e:mtext><e:mn>3</e:mn></e:msub></e:mrow></e:math> capping layer modifies the reduction process, prevents oxide regrowth upon air exposure, and protects the niobium surface against impurity uptake during thermal treatment. These findings demonstrate that <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:msub><f:mtext>Al</f:mtext><f:mn>2</f:mn></f:msub><f:msub><f:mtext>O</f:mtext><f:mn>3</f:mn></f:msub></f:mrow></f:math> capping is an effective strategy to suppress native oxide formation during thermal cycling, offering clear benefits for the performance and operational lifetime of superconducting radio-frequency cavities and related quantum computing technologies.