## 1 Introduction
As mentioned above, the EPR method is a well-established approach for the study of disordered materials such as CPs. However, we should accept the fact that this method has several limitations (low spectral resolution and technical restrictions) that do not allow us to resolve weak hyperfine interactions. The main problems arise from line broadening and signal overlapping. Therefore, important information about the local nuclear environment remains unexplored. More complicated approaches in hardware and better physical understanding have led to the advancement of magnetic resonance techniques. However, methods based on double resonance techniques for the study of disordered materials are still not used as widely as would be expected. In this article, we will demonstrate that there are a number of interesting techniques, which are targeted to the study of weak hyperfine splittings, allowing us to study local atomic (nuclear) surroundings. Such methods as electron spin echo envelope modulation (ESEEM), electron nuclear double resonance (ENDOR), and electron-electron double resonance (ELDOR) detected NMR (EDNMR) provide information that is related to the NMR spectroscopy approach and obviously cannot be obtained by conventional EPR. The corresponding experimental data (Larmor frequencies of nuclei, the values of hyperfine and quadrupole interactions with angular-selected measurements) significantly complement and deepen the existing understanding of the structural behavior of CP materials. The diverse field of pulse EPR spectroscopy allows us to obtain a complete set of spectroscopic and dynamic parameters for the radiation-induced centers in calcium phosphate materials. The parameters of the spin Hamiltonian (anisotropic \(g\)-factor, hyperfine \(A\) and quadrupole \(P\) interaction matrices) reflect the local symmetry of the defect position. The extent of anisotropy and principal values of the tensors depend on the electron density, the distribution of nearby ions, and the local gradients of electric fields. Additional quantum chemical methods can be employed to calculate the listed energy characteristics, which are determined by the structural features of defects as well as the localization of impurity ions in the vicinity. Thus, a comparative analysis of experimental data from pulse EPR spectroscopy with further theoretical calculations is crucial for a more complete understanding of the structural features of the crystal lattice of calcium phosphate materials. The explicit identification of certain paramagnetic centers is important for biphasic materials (HA + TCP), which are now generating great interest in dentistry [52]. For the successful characterization of biphasic compounds, a comprehensive understanding of structural nuances in single-phase materials is required. The experimentally obtained data in this article (anisotropic \(g\)-factors, hyperfine \(A\) and quadrupole \(P\) interaction values) for different impurity centers can be used as a reference point for further theoretical calculations based on quantum chemistry (e.g., the density functional theory--DFT approach), which would allow for the acquisition of additional structural information about the features of the doped crystal lattice.