Studio ab initio di cristalli di Lu2SiO5 utilizzati come scintillatori nell'ambito della diagnostica per immagini: sviluppi recenti e modellizzazione con il codice CRYSTAL

LSO is one of the best scintillators aviable for medical imaging due to its unique combination of properties including high emission intensity, fast decay time, high density, and high atomic number. In this work, an initial characterization of the LSO crystal has been made, using DFT method as implemented in the CRYSTAL code. The basis set for lutetium, silicon and oxygen has been chosen, taking into account the for this selection the values of the ground state atomic charges and the reproducibility of the value of the energy gap with respect to the experimental one. After the basis sets have been fixed, the shrinking parameters (used for the sampling in the reciprocal space) and the tolinteg parameters (which fix the tolerances for the Coulomb and exchange integral calculations) are selected. A study of the convergence of the calculation with respect to the ground state properties has been made and it has been noted that with a shrink value of 6 6 and a tolinteg 8 8 8 8 16 convergence is reached. Therefore, these two parameters have been used for the calculations performed in this work. At this point, the structural parameters of LSO bulk have been computed with a fully relaxation of the structure, by means of a geometrical optimization. Since the exchange-correlation functional in DFT methods have an influence on the ground state properties, different functionals have been used for the geometry optimization, namely, PBE, PBE0, B3LYP and HSE06. Finally, the electronic properties of the optimized LSO crystal are studied, computing the band structures with PBE functional. We also discussed a possible way of modeling defects within the LSO, by adopting a periodic method based on local atomic basis to expand the crystalline orbitals and a supercell approach. Using this modeling tools, we can simulate the presence of Ce atoms in the LSO matrix, and we can compute the structural and electronical properties of LSO:Ce. As regards the comparison with experimental energy gap, our result with the PBE functional is best in agreement with the experimental one (with a difference of 1.31 eV) with respect to the other theoretical PBE values found in literature. On the other hand, the values of the energy gap obtained with hybrid functionals does not represent the experimental value as well as that computed with DFT-HSE06 by L. Ning et al.. As we can see the lattice parameters and volume values are reasonably comparable to the one already present in literature, highlighting the reliability of the Crystal code in describing LSO structure. The value of the energy gap that we obtain using the PBE functional is 0.5 eV higher than the one computed with the same functional present in literature. With hybrid functionals our resulting values are around 7,6 eV. Using specifically the HSE06 functional the value we obtain differs of about 0.83 eV from the one calculated by L. Ning et al. with the same functional. The origin of this divergence could be attributed to the fact that the other theoretical values are computed with plane waves codes (ABINIT and VASP), while in the Crystal code the crystalline orbitals are defined in terms of local functions in the direct space. Although, other calculations can be done in order to assess the role of the basis set in the Crystal code and to obtain more reliable values of the energy gap with hybrid functionals.