Optical Materials xxx (xxxx) xxx Please cite this article as: A.G. Doroshenko, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109505 0925-3467/© 2019 Elsevier B.V. All rights reserved. Effect of the sintering temperature on the microstructure and optical properties of YAG:Cr,Mg ceramics A.G. Doroshenko a, * , R.P. Yavetskiy a , S.V. Parkhomenko a , I.O. Vorona a , O.S. Kryzhanovska a , P. V. Mateychenko a , A.V. Tolmachev a , E.A. Vovk a , V.A. Bovda a, b , G. Croitoru c , L. Gheorghe c a Institute for Single Crystals, National Academy of Sciences of Ukraine, 60 Nauky Ave., Kharkiv, 61072, Ukraine b National Science Center Kharkiv Institute of Physics and Technology, 1 Akademicheskaya Str., Kharkiv, 61108, Ukraine c National Institute for Laser, Plasma and Radiation Physics, Laboratory of Solid-State Quantum Electronics, 409 Atomistilor Str., Magurele, 077125, Ilfov, Romania A R T I C L E INFO Keywords: YAG:Cr ceramics Vacuum sintering Optical losses Valence transformation Q-switching operation ABSTRACT YAG:Cr, Mg optical ceramics were prepared by the solid-state reactive sintering method within the temperature range of 1750–1800 � C. The infuence of the sintering temperature on the microstructure, phase composition, and optical properties of YAG:Cr, Mg ceramics was studied. It has been shown that optimal obtaining temper- ature of YAG:Cr, Mg ceramics doped with 0.5 аt.% of chromium ions and 0.5 аt.% of magnesium ions is 1765 � C. YAG:Cr, Mg ceramics sintered at T ¼ 1765 � C possess the lowest porosity and optical losses in the visible wavelength range. They are characterized by the average grain size of about 3 μm and in-line transmittance above 80%. Thermal annealing of ceramics results in transition of a part of Cr 3þ ions to Cr 4þ charge state. A decrease in concentration of Cr 4þ ions in tetrahedral coordination with increase the sintering temperature has been observed. This effect was attributed to different charge compensation mechanisms of Mg 2þ ions taking place in YAG:Cr, Mg ceramics obtained at various temperatures. Finally, laser performance of YAG:Cr, Mg ce- ramics was estimated in the passive Q-switch mode. 1. Introduction YAG:Cr crystals and optical ceramics are widely used as a photo- tropic material for lasers emitting in one micron range [1]. Advantages of YAG:Cr optical ceramics for passive Q-switchers include high optical and thermal properties of YAG matrix [1], improved processability, wider range of compositions available, as we as possibility to create composite YAG:RE/YAG:Cr architectures [2,3]. One of the most important parameters of passive Q-switcher is the absorption at the laser generation wavelength (1064 nm for YAG:Nd 3þ and 1030 nm for YAG: Yb 3þ ), which depends on the concentration of Cr 4þ ions in the tetra- hedral environment. The pulse power and duration of a microlaser operated in Q-switched mode also strongly depends on the initial transmission of YAG:Cr ceramics [4]. Chromium ions enter the garnet structure in the Cr 3þ charge state [5]. When annealed in an oxidizing atmosphere, these ions change the valence state to Cr 4þ , which could be stabilized if a bivalent impurity is added as a charge compensator (for example, Ca 2þ or Mg 2þ ions). Valence transformation of chromium ions occurs when the charge-compensating ions are located nearby Cr 3þ ions. It is obvious that the probability of creation of such a pair of replacement defects is directly proportional to the concentration of chromium and Me 2þ ions. The introduction of high concentrations of Me 2þ ions is limited by the low solubility of Ca 2þ and Mg 2þ ions in the garnet structure [6,7]. The solubility of Ca 2þ ions in YAG is low; doping even by 300–400 ppm of Ca 2þ leads to precipitation of impurity phases [7]. Unlike MgO and CaO, chromium ions dissolve in the garnet crystal structure up to 25 at.% [8]. “Pure” transparent YAG ceramics with CaO dopant have been syn- thesized in Refs. [9–11]. The authors obtained garnet ceramics and found that the optimal concentration of CaO is 0.045 wt% [9]. However, the solubility of MgO in YAG structure is higher than that of CaO [6]; therefore, MgO could be considered as promising additive to obtain YAG:Cr ceramics. Moreover, MgO acts not only as a charge compen- sator, but also as a sintering aid. According to Ref. [6], MgO accelerates densifcation and could be used to control grain size of the transparent ceramics. It is well known that only a small portion of chromium ions is transferred to Cr 4þ state in YAG crystals or ceramics during thermal * Corresponding author. Institute for Single Crystals, NAS of Ukraine, 60 Nauky Ave., Kharkiv, 61072, Ukraine. E-mail address: dorochenko@isc.kharkov.ua (A.G. Doroshenko). Contents lists available at ScienceDirect Optical Materials journal homepage: http://www.elsevier.com/locate/optmat https://doi.org/10.1016/j.optmat.2019.109505 Received 9 July 2019; Received in revised form 17 October 2019; Accepted 3 November 2019