DFT theoretical and experimental studies unraveling the structural and electronic properties of niobium doped calcium apatite ceramics

Calcium apatite ceramics consisting of a biphasic structure of hydroxyapatite (HAP) and tricalcium phosphate (β-TCP) have been widely explored by biomedical communities for hard-tissue engineering applications, especially bone replacement as it has the potential to mimic the properties of the natural bones. Doping of cations in the hydroxyapatite (NbHAP) is expected to promote the alkaline phosphatase activity and thereby generation of new bone, which, however, depends on its structural and electronic properties. In the present investigation, the variation in the structural and electronic properties of NbHAP prepared by the chemical precipitation method is determined theoretically by employing density functional theory (DFT) using the B3LYP functional method in the Gaussian 09 package program and experimentally using various characterization tools. The experimental investigation by X-ray diffraction (XRD) studies revealed notable variation in the β-TCP content in NbHAP when compared to the pristine HAP, while the Inductively Coupled Plasma spectroscopic results revealed the presence of Nb ions in doped HAP and along with the percentage increase in β-TCP content in NbHAP by Ca/P molar ratio variation, when compared with the HAP system. In addition, experimentally determined Raman spectroscopic values revealed the formation of substitution-type solid solutions in the NbHAP system. The doping of niobium ions was shown to cause noticeable variations in the reactivity indices, as indicated by a number of electronic characteristics that were calculated using DFT. Furthermore, variations in the theoretical parameters such as dipole moment, polarizability, and electric susceptibility revealed that NbHAP is better bioceramic material for hard-tissue engineering applications in comparison to HAP.