Universality of the $1/9$ Magnetization Plateau and Quantum-Disordered States in the Kagome Family $\mathrm{Cs_8AB_3Ti_{12}F_{48}}$ ($A=\mathrm{Rb},\mathrm{Li}$; $B=\mathrm{K},\mathrm{Na}$)

Abstract

The microscopic origin of the low-field $1/9$ magnetization plateau in spin-$1/2$ kagome antiferromagnets remains unresolved. Here, we show that chemical pressure reshapes the hierarchy of fractional magnetization plateaus in the titanium-based kagome family $\mathrm{Cs_8AB_3Ti_{12}F_{48}}$ ($A=\mathrm{Rb},\mathrm{Li}$; $B=\mathrm{K},\mathrm{Na}$). High-field magnetization measurements up to 60 T reveal a robust $1/9$ plateau-like phase in the expanded $\mathrm{Cs_8RbK_3Ti_{12}F_{48}}$ and $\mathrm{Cs_8LiK_3Ti_{12}F_{48}}$ compounds, despite the absence of the conventionally more robust $1/3$ plateau. In contrast, compressed $\mathrm{Cs_8LiNa_3Ti_{12}F_{48}}$ exhibits neither the $1/9$ plateau-like phase nor a quantum-disordered ground state. Specific-heat measurements and first-principles calculations show that lattice expansion preserves a frustrated, fully connected kagome exchange network and gapless quantum-disordered ground states, whereas compression reorganizes the exchange network into weakly coupled quasi-one-dimensional subsystems and induces successive magnetic transitions. These results demonstrate that the $1/9$ and $1/3$ plateaus need not share a common microscopic origin and suggest that the $1/9$ plateau may represent a more universal feature of frustrated spin-$1/2$ kagome magnetism.