Quenched Disorder in the Triangular Lattice Antiferromagnet YbZn₂GaO₅

We investigate the crystal electric field (CEF) excitations of Yb³⁺ ions in powder samples of the triangular-lattice rare-earth-based antiferromagnet YbZn₂GaO₅ 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 Yb³⁺ ions due to Ga³⁺/Zn²⁺ 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 Yb³⁺ ions leads to broadened CEF peaks. First-principles calculations demonstrate that the random Ga³⁺/Zn²⁺ distribution can produce the distortions of the YbO₆ 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 YbZn₂GaO₅, including broad low-energy magnetic excitations and the absence of magnetic ordering down to 0.3K.