## 1 Introduction
An extremely deep understanding of the physical, chemical, or biological processes that occur in crystal structures is required to develop or promote any field of materials science. This is especially important for new or poorly studied materials that have recently been obtained with a new chemical composition. However, it often happens that for a certain type of material, one distinct method is not sufficient to get the whole set of structural information. For disordered materials such as CPs, all available experimental tools are required to determine the more interesting features of doped materials. The magnetic resonance approach is a suitable experimental means that significantly complements and expands the knowledge of doped materials. Magnetic resonance-based methods can provide an insight into interatomic interactions, define their origin/mechanisms, and study the local environment of impurity centers, which cannot be directly analyzed by other techniques. Calcium phosphate materials have been thoroughly studied by various experimental techniques. Most of the published CP-related articles can be categorized into studies of the morphology of nanoparticles, the structure of the crystal lattice, and the biological properties of the synthesized materials [1; 2]. To solve problems with the structural analysis of CP powders, combinational approaches are utilized by applying Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, differential thermal and thermogravimetric analysis, Raman spectroscopy, as well as in vivo/in vitro analysis for the study of biological properties [28]. The abovementioned methods allow for the determination of the size and shape of nanoparticles, the presence of different functional groups, and additional crystalline phases as well as the lattice structure, depending on the type/concentration of impurity ions [3; 5]. However, questions concerning the magnetic behavior/origin of defects and their electron-nuclear interactions with the local ionic environment remain unexplored, requiring additional experimental methods. One of the analytical, non-destructive tools for the investigation of CPs of both biogenic and synthetic origins is electron paramagnetic resonance (EPR) [29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43]. The increased sensitivity, spectral and temporal resolution, along with the improved stability of modern commercial EPR spectrometers has opened new possibilities in the study of CPs during the last decade, taking a step towards in situ EPR imaging (EPRI) [44]. Pure, non-substituted CPs are EPR-silent. Consequently, conventional EPR can be used for the purity check of CP materials, for example, the presence of metal impurities [31]. A standard way of studying EPR-silent CPs is through the creation of certain types of defects with ionizing radiation such as X-, \(\beta\)-, \(\gamma\)-rays or ultraviolet light, and the investigation of their spectroscopic properties. Several radiation-induced paramagnetic species (anion radicals) located at hydroxyl or phosphate sites have been identified in synthetic HA, depending mainly on the nature of the biogenic materials and synthesis route/treatment of the artificial compounds as well as on the radiation conditions [45; 46; 47; 48; 49]. Among them, oxygen radicals O\({}^{-}\), trapped atomic hydrogen centers, holes trapped on OH\({}^{-}\) and PO\({}_{4}^{2-}\), carbonate radicals CO\({}_{2}^{-}\), CO\({}_{3}^{-}\), CO\({}_{3}^{3-}\), and color centers have been observed. In burnt bones, coal-type C-radicals have been identified [41; 50]. Besides material characterization, the spectroscopic and relaxation parameters of the observed paramagnetic centers can be used to track the radiation dose in HA-containing dental enamel [41], to follow the processes of calcification [29] and the doping/co-doping of synthetic materials with various ions [35; 38]. The effect of diverse amino acids on the local microstructure of calcium-deficient hydroxyapatite was also tracked/confirmed by EPR spectroscopy [51].