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.

arXiv comment24 pages, 4 figures, supporting information

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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