Spin Decoherence Dynamics of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msup> <mml:mrow> <mml:mi>Er</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> </mml:mr

Developing telecom-compatible spin-photon interfaces is essential towards scalable quantum networks. Erbium ions (Er^{3+}) exhibit a unique combination of a telecom (1.5 μm) optical transition and an effective spin-1/2 ground state, but identifying a host that enables heterogeneous device integration while preserving long optical and spin coherence remains an open challenge. We explore the potential of Er^{3+}:CeO_{2} films on silicon and study the Er^{3+} spin coherence, offering low nuclear spin density and the potential for on-chip integration. We demonstrate a 38.8 μs spin coherence, which can be extended to 176.4 μs with dynamical decoupling. Pairing experiments with cluster correlation expansion calculations, we identify spectral diffusion induced by bath Er^{3+} spin flip-flops as the dominant decoherence mechanism and provide pathways to millisecond-scale coherence.