Hasta el 28 de agosto de 2021 puede accederse gratis a la versión publicada del artículo en este enlace: authors.elsevier.com/c/1dNyu_UwYsE~Gd The manuscript addresses the solidification of Y2O3-MgO eutectic composites for the first time, and the study of their microstructure, mechanical properties and infrared transmittance. The composites with eutectic composition (20 % vol MgO) have been solidified using the laser floating zone method, at solidification rates between 25 mm/h and 750 mm/h. Their microstructure has been quantified and the conditions for coupled or cellular regime determined. They are dense, with elastic modulus, as determined from nanoindentation tests of around 200 GPa. The highest 3-point flexural strength (256±25 MPa) values and highest infrared optical transmittance has been observed for the material solidified with the finest, still homogeneous microstructure that could be achieved. This microstructure, which is achieved at a solidification rate of 50mm/h, consists of MgO fibers of 600 nm diameter embedded in the Y2O3 matrix. The thin MgO rods and the associated residual stress field contribute to the material strength as they deviate the propagating crack. The Vickers Hardness ranged from 9.5 to 11.5 GPa, with mild increase with the decrease of the microstructural size which has been rationalized as the MgO rods blocking the movement of dislocations in the matrix, as in other studied composites. Indentation fracture toughness values around 3 MPa.m1/2, independent of the sample microstructure, have been measured. In the longitudinal direction transmittance values higher than 80 % in the wavelength range from 3.5 to 7 μm, for 1 mm thick slices have been measured. Light scattering limits the short wavelength cut-off to around 2 μm for light travelling in the longitudinal direction, while a transmittance gap is observed in the transverse direction, of photonic crystal like nature. This work was supported by the Ministerio de Economía y Competitividad (Government of Spain) and FEDER Funds under Grants No. MAT77769-R and Gobierno de Aragón, Subvención a Grupos de Investigación No. T02_20R. BMM wants to acknowledge the support of the Spanish MINECO by means of a “Juan de la Cierva-Incorporación” fellowship during her sabbatical stay in Zaragoza and is grateful to the Ministerio de Economía y Competitividad for the research funding through the project PID2019-103847RJ-I00 and ‘Junta de Andalucía’ through the project P18-RTJ-1972. We want to thank Rubén Gotor for his help in several tasks of material preparation. Authors would like to acknowledge also the use of the Servicio General de Apoyo a la Investigación SAI, Universidad de Zaragoza, namely, the Service of Electron Microscopy of Materials and the Service of X-ray Diffraction and Fluorescence Analysis. Peer reviewed