## 1 Introduction  
Chalcogenide glasses (ChGs) are known for their transmittance across the infrared (IR), which makes them desirable in the field of opto-electronics and photonics. However, due to weak chemical bonding between constituent metals and the chalcogens, ChGs typically suffer from inherently low mechanical and poor thermal properties [1; 2; 3; 4]. There is a strong motivation to increase the mechanical strength and chemical robustness of ChGs, while still maintaining transparency in the IR. However, only a handful of studies have been made based solely on this motivation. [5; 6; 7]. Glass ceramics (GCs) on the other hand are composed of crystalline phase(s) in an amorphous matrix and have been shown as multi-phase media that exhibit mechanical properties often better than the base glass alone. The crystalline phases can be introduced in the glass by direct doping of a nucleating agent (heterogeneous nucleation) or nucleated and grown through controlled thermal treatment via a homogeneous nucleation process. Oxide-based GCs have seen integration in a wide range of applications ranging from biomedical and dental applications to use in large mirrors for space applications [8; 9]. GCs based on non-oxide media are less common but a review on these [10] highlights many of these recent efforts. The classical formation route for a GC typically starts with an amorphous parent glass matrix and the introduction of a secondary phase into the parent amorphous matrix. This processing route is common across materials for use in optical and non-optical applications and has been shown to not only improve the mechanical properties [11] but also impact the refractive indices, and in some instances, the coefficient of thermal expansion, and electrical, magnetic and non-linear optical properties [11; 12; 13]. In applications where the resulting GC must be transparent in the spectral region of use, the induced crystal phase(s) must have low optical loss (absorption and scatter loss). Such optical GCs can be used for passive or active optical components if the crystallite is cubic and are small with respect to the wavelength of use, possessing a narrow size distribution to minimize scattering. The controlled crystallization of ChGs is an important prerequisite in the development of IR transparent GCs with improved mechanical properties. Since a comprehensive mathematical theory correlating all the factors responsible for controlled crystallization in non-oxide glasses does not yet exist, studies to date have largely been empirical and related to the effectiveness of the precipitated crystalline phase at retarding or eliminating crack initiation and/or subsequent crack propagation. Studied to a much greater extent in oxides, the role of composition and morphology of the parent chalcogenide glass medium has not previously been correlated to our knowledge with a resulting mechanical property change when subjected to a comparable thermal heat-treatment protocol. Several experimental factors e.g., base glass morphology, nucleating agent (for heterogenous nucleation), and heat-treatment temperatures need to be considered when engineering a GC with enhanced mechanical properties, such that the resulting multi-phase medium retains its optical properties. During cooling from the crystallization temperature to room temperature, residual stresses can arise due to the change in thermal expansion between the crystalline and glassy phase [14], degrading the optical properties or if large enough, adversely impacting the goal of strengthening. In the process of controlled crystallization, the individual residual stresses and the type of crystal lattice, as well as its size, can define the extent of strengthening observed in the GC [15]. By maintaining a 'neutral' residual stress during the controlled crystallization, the change in the mechanical properties can be assumed to be attributable predominantly to the composition, microstructure, and volume fraction of the secondary phase [15].