###### Contents  
* 1 Introduction
* Glasses are characterized by the absence of the long-range order (>20, \"LRO\") that occurs in crystalline materials. On one hand, this lack may present several advantages, like the ability for glass formation in a continuous way within the glass-forming range, in contrast to the corresponding formation of crystals that require a specific molar ratio between their constituents. On the other, the quantification of the glass structure remains a continuous challenge and currently is in a primitive state compared to the highly developed solid-state theory of crystals [1]. In practice, the crystal structure can be completely identified by XRD diffraction studies by exploiting the lattice periodicity, whereas there is no experimental method for the complete quantification of the glass structure. Borate glasses constitute an attractive family of materials from both a scientific and a technological viewpoint. Actually, borate glasses are encountered in a plethora of technological applications, including linear and non-linear optical glasses [2], luminescence [3], and scintillator [4] applications, laser and radiation detection [5], radiation shielding [6], supercapacitors [7], energy storage and conversion devices [8], as well as bioactive glasses [9]. From a scientific viewpoint, despite extensive research efforts over the years, the detailed structure of borate glasses continues to be an open and intriguing challenge. This challenge is due to a couple of peculiar factors intrinsic to borate structures. The first one is attributable to the change of the B coordination number from 3 to 4, resulting in the conversion of B\(\varnothing_{3}\) triangular to B\(\varnothing_{4}\)\({}^{-}\) tetrahedral entities upon initial addition of a network modifier to pure B\({}_{2}\)O\({}_{3}\) glass (\(\varnothing\) denotes a bridging oxygen (BO) atom, linking two structural units). This is in contrast to other conventional glass systems like silicate glasses, where the initial addition of a network modifier to pure SiO\({}_{2}\) glass depolymerizes the glassy network by the formation of non-bridging oxygen atoms (NBO). In borate glasses, the formation of NBO-containing units occurs only at higher modification levels. The second factor relies on the formation of superstructural units similar to those encountered in crystalline borates, at least up to the diborate composition. The aforementioned special features of borate glasses were firstly investigated and evidenced in the pioneering works of [PERSON] [10; 11; 12] and [PERSON] and coworkers [13; 14; 15]. Nowadays, it is widely accepted that the SRO structure of borate glasses consists of a variety of triangular units, with the number of BO atoms ranging from 3 to 0, namely neutral, B\(\varnothing_{3}\); meta, B\(\varnothing_{2}\)O\({}^{-}\); pyro, B\(\varnothing_{2}\)\({}^{-}\); and ortho, BO\({}_{3}\)\({}^{3-}\). In addition, there are two types of borate tetrahedra, i.e., meta, B\(\varnothing_{4}\)\({}^{-}\), and ortho, B\(\varnothing_{2}\)O\({}_{2}\)\({}^{3-}\). Meta-type triangles and tetrahedra are isomeric species, and the same holds for ortho-type triangular and tetrahedral species. Thus, in principle, these units can participate in high-temperature isomerization equilibria:  
\[\text{B}\varnothing_{2}\text{O}^{-}\rightleftarrows\text{B}\varnothing_{4} \text{${}^{-}$} \tag{1}\]  
\[\text{BO}_{3}\text{${}^{3-}$}\rightleftarrows\text{B}\varnothing_{2}\text{O} _{2}\text{${}^{3-}$} \tag{2}\]  
which, upon cooling the melt, govern the particular speciation of SRO structural units in borate glasses [16; 17; 18]. The particular borate speciation of the glass network depends strongly on the modifier content but also, in the case of glasses having the same nominal composition, on the nature of the modifying cation. Nevertheless, despite the enormous number of research studies over the years, to date, there is no experimental technique capable of completely quantifying the SRO structure of borate glasses.