Structural and Magnetic Transitions Caused by Dimer Formation in the CrCl₃–MoCl₃ Solid Solution

In this work, we apply the concept of solid solutions to the two 2D transition metal trihalides CrCl₃ and MoCl₃. While CrCl₃ belongs to the magnetically active CrX₃ family, the magnetism in MoCl₃ is intrinsically suppressed by the formation of aligned Mo–Mo dimers, which also distort the regular honeycomb lattice that is typical for the 2D transition metal trihalides. We report suitable synthesis conditions for the gapless solid solution and crystal growth by chemical vapor transport. The CrCl₃–MoCl₃ solid solution was initially synthesized at 650 °C for 100 h under addition of MoCl₅ as mineralizer, and bulk crystals were subsequently grown by vapor transport in a temperature gradient from 600 °C → 550 °C for 60 h. The obtained solid solution exhibits multiple composition-dependent phase transitions at room temperature, as confirmed by powder X-ray diffraction measurements. The possible presence of Mo–Mo dimers in the solid solution was further investigated by infrared, Raman, electron energy loss, and nuclear quadrupole resonance spectroscopies. The combined results indicate that the Mo–Mo dimers are present over a wide range of compositions. Their orientation changes from parallel alignment for compositions from 70% ≤ c(Mo³⁺) ≤ 100% to random orientation for samples with c(Mo³⁺) < 70% content, which significantly affects the course of lattice parameters. Finally, the magnetic properties of the powder samples show a correlation between the Mo³⁺ content and the transition temperature into the low-temperature phase.