We performed variable-temperature synchrotron powder X-ray diffraction measurements and impedance spectroscopy under pressure for silver iodide (AgI) nanoparticles with a diameter of 11 nm. The superionic conducting α-phase of AgI nanoparticles was successfully stabilized down to at least 20 °C by applying a pressure of 0.18 GPa, whereas the transition temperature was 147 °C in bulk AgI at ambient pressure. To our knowledge, this is the first example of the α-phase of AgI existing stably at room temperature.

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.

We report the first observation of spin-flop-induced sharp positive magnetoresistance as large as 100% and nonvolatile magnetoresistive memory in a π–d hybrid molecular conductor, (DIETSe)2FeCl4 [DIETSe = diiode(ethylenedithio)tetraselenafulvalene]. The unprecedented magnetotransport phenomena originate from the coexistence of the spin density wave (SDW) of the quasi-one-dimensional (Q1D) π electrons and the antiferromagnetic order of d-electron spins, indicating the interplay between the electronic instability of Q1D π electrons and local moments of antiferromagnetic d-electron spins. These findings offer new possibilities in molecular electronics/spintronics.