Magnetization and high-resolution x-ray diffraction measurements of the
Kitaev-Heisenberg material alpha-RuCl3 reveal a pressure-induced
crystallographic and magnetic phase transition at a hydrostatic pressure of
p=0.2 GPa. This structural transition into a triclinic phase is characterized
by a very strong dimerization of the Ru-Ru bonds, accompanied by a collapse of
the magnetic susceptibility. Ab initio quantum-chemistry calculations disclose
a pressure-induced enhancement of the direct 4d-4d bonding on particular Ru-Ru
links, causing a sharp increase of the antiferromagnetic exchange interactions.
These combined experimental and computational data show that the Kitaev spin
liquid phase in alpha-RuCl3 strongly competes with the crystallization of spin
singlets into a valence bond solid.
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