## 1 Introduction  
In recent years, organometal halide perovskites (OHPs) have gained enormous attention as active materials applicable in several optoelectronic applications, such as solar cells, light emitting devices, photodetectors and field-effect transistors [1; 2; 3; 4; 5; 6]. These materials indeed own exceptional optoelectronic properties, such as high absorption coefficients, the direct photogeneration of free carriers and long carrier diffusion length, and can be manufactured using numerous and low-energy consuming methods [1; 7; 8; 9; 10]. Most of the studies reported in the literature concern the investigation and application of thin film OHPs [9; 11; 12]. However, these films are usually characterized by a grain structure and morphology that directly impact on the optoelectronic features, since they are a collection of crystallites, amorphous phases and unreacted species [13; 14; 15; 4]. Additionally, their optical and electronic properties can be dominated by extrinsic defects, since trap states are more easily generated at the grain boundaries [11]. Therefore, in order to understand the intrinsic properties of these appealing materials, polycrystalline powdered samples represent an ideal model to be examined for the expected lower defect density (resulting from the absence of inter-grain regions), and for certain chemical compositions [5; 8; 12; 13; 16; 17; 18; 19]. Peculiarly, the use of polycrystalline samples has nowadays gained significant attention with regards to the exploitation of the effects of different factors, such as chemical manipulation and external perturbations, on the structural, morphological and optoelectronic properties of the hybrid perovskites [20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34]. In some recent examples, polycrystalline samples were synthesized and investigated in order to (i) measure the impact of rubidium ions' incorporation into multiple-cation perovskites Rb(MAFA)PbI\({}_{3}\) (MA = methylammonium; FA = formamidinium) [21], (ii) study the role of different organic cations [27], (iii) study the effect of illumination [34] on MAPbI\({}_{3}\) properties, iv) explore the consequence of halide substitution in a lead-free BZA\({}_{2}\)SnX\({}_{4}\) (BZA = benzylammonium; X = Cl, Br or I) system [28], (v) analyze the degradation processes triggered by the X-ray, N\({}_{2}\), O\({}_{2}\) or H\({}_{2}\)O exposure of MAPbBr\({}_{3}\) perovskite [29], and (vi) assess its enhanced photoluminescence and photoconductivity attributed to erbium-doping [22]. Perovskites crystals were also employed to evaluate the responses of OHPs to external temperature and pressure, which is considered a physical probe for tailoring the properties of these materials [25; 33]. Indeed, the results of different temperatures and/or pressures on the structural and optoelectronic features of MAPbI\({}_{3}\)[22; 23; 24; 25; 32], MAPbBr\({}_{3}\)[30; 31; 32] and FAPbI\({}_{3}\)[33] crystals have been reported in the literature. Herein, polycrystalline powdered samples of three-dimensional (3D) and two-dimensional (2D) OHPs are examined in order to evaluate the effects of prolonged high-energy ball-milling processes on their structural and optical properties. High-energy ball-milling is a mechanical grinding of materials, in which the milling occurs thanks to the mechanical friction between the material and a grinding medium (i.e., balls) included in a container [35; 36; 37]. This technique can have different consequences on the treated materials, such as different extents of downsizing, efficient mixing, solid-state chemical transformations, or a combination of all of these [36]. Very importantly, it can be applied as an environmentally friendly, reproducible, efficient and simple synthesis method for a variety of solid materials, among which are OHPs [24; 35; 38].