Use of Halogen Bonding in a Molecular Solid Solution to Simultaneously Control Spin and Charge

Halogen-bonding
interactions have attracted increasing attention
in various fields of molecular science. Here we report the first comprehensive
study of halogen-bonding-utilized solid solution for simultaneous
control of multifunctional properties. A series of anion-mixed molecular
conductors (DIETSe)2MBr4xCl4(1–x) [DIETSe = diiodo­(ethylenedithio)­tetraselenafulvalene;
M = Fe, Ga; 0 < x < 1] were synthesized without
changing crystal structure utilizing strong halogen bonds between
DIETSe molecules and anions. Detailed physical property measurements
(T > 0.3 K, H < 35 T) using
the
single crystals demonstrated simultaneous control of both spin and
charge degrees of freedom. The increase in Br content x gradually suppresses a metal–insulator transition attributed
to the nesting instability of the quasi-one-dimensional Fermi surfaces.
It suggests the dimensionality of π electrons is extended by
increasing the anion size, which is opposite of the typical effect
of chemical pressure. We found that the “negative” chemical
pressure is associated with the characteristic halogen-bonding network.
Br substitution also enhances the antiferromagnetic (AF) ordering
of d-electron spins in the Fe salts, as indicated by the Néel
temperature, AF phase boundary field, and saturation field. Furthermore,
we observed hysteresis in both magnetization and resistivity only
in halogen-mixed salts at very low temperatures, indicating simultaneous
spin and charge manipulation by alloying.