Chiral superconductivity exhibits the formation of novel electron pairs that breaks the time-reversal symmetry and has been actively studied in various quantum materials in recent years. However, despite its potential to provide definitive information, effects of disorder in the crystal structure on the chiral superconductivity has not yet been clarified, and therefore the investigation using a solid-solution system is desirable. We report muon-spin-relaxation $\left(\mu \mathrm{SR}\right)$ results of layered pnictide ${\mathrm{BaPtAs}}_{1-x}{\mathrm{Sb}}_x$ with a honeycomb network composed of Pt and (As, Sb). We observed an increase of the zero-field muon-spin relaxation rate in the superconducting (SC) state at the Sb end of $x=1.0$, suggesting the occurrence of a spontaneous magnetic field due to the time-reversal symmetry breaking in the SC state. On the other hand, a spontaneous magnetic field was almost and completely suppressed for the As-Sb mixed samples of $x=0.9$ and 0.2, respectively, suggesting that the time-reversal symmetry-breaking SC state in $x=1.0$ is sensitive to disorder. The magnetic penetration depth estimated from transverse-field $\mu \mathrm{SR}$ measurements at $x=1.0$ and 0.2 behaved like weak-coupling $s$-wave superconductivity. These seemingly incompatible zero-field and transverse-field $\mu \mathrm{SR}$ results of ${\mathrm{BaPtAs}}_{1-x}{\mathrm{Sb}}_x$ with $x=1.0$ could be understood in terms of chiral $d$-wave superconductivity with point nodes on the three-dimensional Fermi surface.

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• Pairing mechanisms
• Superconducting order parameter
• Superconductivity
• Unconventional superconductors
• DC susceptibility measurements
• First-principles calculations
• Muon spin relaxation & rotation