[en] Full text: Two-dimensional (2D) materials such as SnSe, SnS and SnS1-xSex compounds have received increasing attention as new thermoelectric and tunable band gap materials. At ambient condition SnS1-xSex compounds crystallize in a layered orthorhombic structure, for any x value. A nanostructured layered SnS0.5Se0.5 solid solution was synthesized by mechanical alloying under argon atmosphere. The structural behavior under pressure was investigated using in situ angle-dispersive X-ray diffraction in a synchrotron source, LNLS. The pressure range was from 1.5 up to 20 GPa, quasi-hydrostatic conditions were maintained throughout the experiments, monitored by ruby fluorescence. At ambient condition, structure stability was investigated by XRD measurements. The variations of unit cell of the phases were described by a third-order Birch–Murnaghan equation of state, DFT calculation was applied to simulate the behavior of lattice parameters as function of pressure. The thermal stability was assessed through calorimetry measurements. In the as-milled sample only the Pnma phase was observed, the crystallite size calculated through Scherrer equation was 12.6 nm. The DSC measurements shown that the solid solution is destabilized near 200 C. The in-situ X ray measurements shown the phase transition from the Pnma (62) space group to the Cmcm (63), occur in the pressure range of 7.9-12.4 GPa. The experimental lattice parameters as function of pressure shown an almost isotropic behavior, which clashed with the DFT calculations due possibly to defects in the milled sample and the interfaces between the nanometric solid solution[1][2]. [1] H. Tian, J. Tice, R. Fei, V. Tran, X. Yan, L. Yang, H. Wang, Low-symmetry two-dimensional materials for electronic and photonic applications, Nano Today. 11 (2016) 763–777. [2] Z.G. Chen, X. Shi, L.D. Zhao, J. Zou, High-performance SnSe thermoelectric materials: Progress and future challenge, Prog. Mater. Sci. 97 (2018) 283–346. (author)