Cluster rearrangement by chiral charge order in lacunar spinel GaNb${}_4$Se${}_8$

Abstract

Transition-metal atoms with d electrons sometimes form clusters in crystals, which significantly affects the physical properties. Such a cluster formation frequently accompanies a change in the crystal system, leading to the presence of domains with different crystal orientations. In particular, the cubic symmetry is rarely retained after the cluster formation. Here, we identify a cubic-to-cubic phase transition in lacunar spinel GaNb${}_4$Se${}_8$, where the change in the lattice parameter is less than 0.0001%. Each Nb${}^{3.25+}$ tetramer with seven 4d electrons is distorted into a Nb${}^{3+}$ trimer and a Nb${}^{4+}$ monomer induced by charge disproportionation among Nb ions. While the Nb${}^{3+}$ trimer with six 4d electrons forms spin-singlets in the sigma-bonding orbitals for three Nb-Nb bonds, a localized S=1/2 spin remains on the Nb${}^{4+}$ ion. Furthermore, a local electric dipole moment is induced along the three-fold rotation axis of each distorted tetramer by the cluster rearrangement. The electric dipole moments are regularly arranged to maintain the cubic symmetry, giving rise to chiral order.

24 pages, 4 figures, supporting information

References in corpus (9)

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Cited by in corpus (4)

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• Reexamination of the charge-ordered dimer pattern in the spinel compound CuIr2S4 using single-crystal synchrotron x-ray diffraction
• Linear tetramer formation in nonmagnetic pyrochlore niobate