We investigate the crystal electric field (CEF) excitations of ${\mathrm{Yb}}^{3+}$ ions in powder samples of the triangular-lattice rare-earth-based antiferromagnet ${\mathrm{YbZn}}_2{\mathrm{GaO}}_5$ using inelastic neutron scattering (INS). Three CEF excitations from the ground-state Kramers doublet were observed, each exhibiting significant broadening beyond instrumental resolution. Combining temperature-dependent INS and neutron powder diffraction, we identify a significant static contribution to this broadening and attribute it to heterogeneous coordination of ${\mathrm{Yb}}^{3+}$ ions due to ${\mathrm{Ga}}^{3+}/{\mathrm{Zn}}^{2+}$ site mixing. Rietveld refinement of neutron powder diffraction indicates that 35% of Ga occupies the Zn site and 60% of Zn occupies the Ga site. We show with a point charge model for the CEF Hamiltonian that heterogeneous coordination of ${\mathrm{Yb}}^{3+}$ ions leads to broadened CEF peaks. First-principles calculations demonstrate that the random ${\mathrm{Ga}}^{3+}/{\mathrm{Zn}}^{2+}$ distribution can produce the distortions of the ${\mathrm{YbO}}_6$ octahedra observed from neutron diffraction. Because the documented heterogeneity will extend to exchange interactions, our results suggest that disorder is a significant factor in the unusual magnetism previously reported in ${\mathrm{YbZn}}_2{\mathrm{GaO}}_5$, including broad low-energy magnetic excitations and the absence of magnetic ordering down to $0.3\mathrm{K}$.

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• Crystal-field excitations
• Frustrated magnetism
• Quantum spin liquid
• Strongly correlated systems
• Neutron scattering