Thermodynamic and muon spin relaxation measurements have recently highlighted ${\mathrm{BiYbGeO}}_5$ as a new example of a rare-earth-based quantum dimer magnet with isolated ${\mathrm{Yb}}^{3+}$ spin-1/2 dimers. However, direct spectroscopic evidence of the triplet excitations and measurements of the structural disorder are lacking. In this work, polycrystalline ${\mathrm{BiYbGeO}}_5$ was synthesized using conventional high-temperature solid-state methods and investigated via high-resolution neutron powder diffraction and inelastic neutron scattering. Diffraction measurements down to 58 mK reveal no signatures of magnetic order and indicate that nearly 20% of ${\mathrm{Yb}}^{3+}$ sites are replaced by nonmagnetic ${\mathrm{Bi}}^{3+}$, introducing significant structural disorder. Inelastic neutron scattering shows dispersionless triplon excitations, consistent with localized, noninteracting spin dimers. Fits to the triplet excitation spectrum with an XXZ-type anisotropic exchange give $J_{\text{XX}}=0.100\left(1\right)\text{meV}$ and $J_{\text{Z}}=0.202\left(1\right)\mathrm{meV}$. These findings establish ${\mathrm{BiYbGeO}}_5$ as a structurally disordered but magnetically well-isolated quantum dimer system, providing a model platform for studying the resilience of entangled spin states to site dilution.

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• Exchange interaction
• Magnetic interactions
• Polycrystalline materials
• Dilution refrigerator
• Inelastic neutron scattering
• Neutron diffraction
• Neutron scattering
• Time-of-flight neutron spectroscopy