## 1 Introduction
Here, one assumes that in the case of homogeneous nucleation, the precipitating phase results from crystallization of the parent matrix, thus forming a new crystallite with constituents now depleted (in full or part) from the parent glass. The presence of this secondary crystalline phase in the amorphous glass matrix can increase the load required for crack initiation and subsequent crack propagation. Therefore, by controlling the size and size distribution of the secondary phase to ~1/10 th the wavelength of light within the optical application domain, GCs can be used as an optical component with enhanced mechanical and in select cases, thermo-mechanical properties [16; 17; 18]. For optical GCs, the attributes of the secondary phase(s) must meet more stringent criteria. Specifically, for optically transparent composites, the refractive index (RI) mismatch between the glass and crystal should be small, to minimize scatter loss. In an ideal case, a homogenous optical GC with subwavelength crystallites would promote transparency as well as enhanced mechanical strength. In special cases, selective crystallization of the high refractive index crystals in a lower index glass can result in a matrix with an effective refractive index greater than its precursor, and if this distribution can be spatially varied, a gradient in the index can be realized. Here, the formation of the secondary crystalline phase can be controlled and spatially varied via exposure to high intensity laser writing [19; 20; 21]. Hence, in order to produce an optical GC that still transmits in the desirable spectral region, the induced crystals must be uniform in chemistry, narrow in distribution and sub-wavelength in size [22]. The main limitation in realizing a greater number of candidate glass families suitable for controlled crystallization is poor glass-forming ability and the change in optical properties when a nucleating agent is introduced [4]. The addition of nucleating agents has been shown to result in undesirable effects such as phase separation, spontaneous crystallization of the entire glass upon cooling, as well as reduced crystal growth control [23]. Some groups have investigated the formation of GCs in ChGs to increase mechanical hardness and toughness [17; 24; 25; 26; 27; 28; 29; 30; 31; 32], but a systematic correlation between starting glass morphology (homogeneous glass versus phase separated glass) and glass-crystal volume fractions in a GC, and the respective corresponding impact on the mechanical properties of ChG ceramics, has not to our knowledge been carried out. The change in the mechanical properties due to controlled crystallization is most frequently measured in terms of the media's fracture toughness, Young's modulus and Vickers hardness. In the case of GCs, the propagation energy required for a crack (and its extension) to pass through or around the crystalline particle is higher than passing through the typically weaker parent glass matrix phase. The crystalline particles divert the propagating crack front, resulting in a slowing of the crack's propagation, in turn leading to an increase in mechanical properties. While a great deal of work has been carried out in the field of measurement of mechanical properties in ChGs [15; 17; 33; 34; 35; 36], few studies have carried out measurements related to parent glass morphology free of any nucleating agent such as examined in the effort here. Previous efforts to examine mechanical property changes in ChGs doped with alkali halides has been extensive, highlighting numerous promising candidates for optical glass and GC applications [26; 27; 28; 29; 30]. A well-studied example is for Ge-Ga-Se glasses doped with KX or CsX where (X is Cl or Br); here, the addition of metal halides facilitates homogenous crystallization, which results in improved hardness as well as reduced expansion coefficient [31]. The addition of RbI in the Ge-Ga-Se glass composition also results in synthesis of a moldable GC without any loss of transmission window [32].