Prolate collectivity inTl187

High-spin states of ${}_{}{}^{187}{}_{}{}^{}\mathrm{Tl}$ populated in the ${}_{}{}^{156}{}_{}{}^{}\mathrm{Gd}$ ${(}^{35}$Cl,4n) reaction have been investigated with γ-ray coincidence techniques. Two decoupled bands feeding the oblate 9/$2^{\mathrm{-}}$ isomeric state have been established. These bands are built on π${\mathit{h}}_{9/2}$ and π${\mathit{i}}_{13/2}$ configurations associated with prolate shapes. The first observation of a low-K π${\mathit{h}}_{9/2}$ structure in an odd-A Tl nucleus is used to draw conclusions on the role of this proton orbital in stabilizing the prolate minimum seen in light Hg isotopes.