[EN] Single crystals built up from octahedral Mo-6 cluster units, with appropriate geometry and size, constitute Fabry-Perottype optical microcavities with well-defined resonances. Such resonances appear in the VIS-NIR range when performing optical transmittance (OT) or photoluminescence (PL) measurements. They strongly depend on the crystal thickness and on the optical constants of the material, specifically the real and imaginary parts of the refractive index. In this work, the accurate measurement of the crystal thickness is used for the determination of the optical constants by means of a fitting process, and the preferred orientation of the crystals was determined by X-ray diffraction.

This research was funded by the Severo Ochoa Program (grant number SEV-2016-0683), Ministerio de Ciencia e Innovacion (grant number PGC2018-099744), and Generalitat Valenciana (grant number AICO/2020/149). The authors also thank the Electron Microscopy Service of the Universitat Polite`cnica de Valencia for their support in FESEM image acquisition.