Location and orientation of lone pairs in apatite-type materials: a computational study

Site preference and local structural stability of Bi(III) substitution in hydroxyapatite using first-principles simulations

Bismuth (Bi(III)) substitution in hydroxyapatite (HAp) lattice confers unique properties such as antibacterial, catalytic, radiosensitization, and conductive properties while preserving the innate bioactivity. Understanding the local structural changes upon Bi3+ substitution is essential for controlling the stability and optimizing the properties of HAp. Despite numerous experimental studies, the precise substitution behaviors, such as site preference and structural stability, remain incompletely understood. In this study, the substitution behavior of Bi(III) into the HAp lattice with formula of Ca9Bi(PO4)6(O)(OH) was investigated via first-principles simulation by implementing density functional theory. Energy calculations showed that Bi3+ preferentially occupies the Ca(2) site with an energy difference of ∼0.02 eV per atom. Local structure analysis revealed higher bond population values and an oxygen coordination shift from 7 to 6 for the Ca(2) site, attributed to the greater covalent interactions and its flexible environment accommodating the bulky Bi3+ ion and its stereochemically active lone pair. This work provides the first comprehensive investigation on Bi3+ ion substitution site preference in HAp using first-principles simulations.

Role of lone-pair electron localization in temperature-induced phase transitions in mimetite

The crystal structure of mimetite Pb₅(AsO₄)₃Cl, a phosphate with apatite structure-type has been investigated in situ at 123, 173, 273, 288, 353 and 393 K. A careful inspection of the diffraction pattern and subsequent structure refinements indicated that mimetite transforms from the monoclinic to the hexagonal polymorph with increasing temperature. At 123 K, a monoclinic superstructure, mimetite-2M, with cell parameters a = 20.4487 (9),  b = 7.4362 (2), c = 20.4513 (9) Å, β = 119.953 (6)°, V = 2694.5 (2) ų and space group P2₁ was observed. From 173 to 353 K, the reflections of the supercell were evident only along one direction of the corresponding hexagonal apatite-cell and the structure transforms to the polymorph mimetite-M with space group P2₁/b and unit-cell parameters a = 10.2378 (3), b = 20.4573 (7), c = 7.4457 (2) Å, β = 120.039 (5)°, V = 1349.96 (9) ų. Only at higher temperature, i.e. 393 K, does mimetite adopt the hexagonal space group P6₃/m characteristic of apatite structure-types. The role of the electron lone pairs of Pb atoms in the phase transition was investigated through the analysis of the electron localization function (ELF) calculated based on the DFT-geometry optimized structures of the three polymorphs. The changes in spatial distribution of the 6s² electron density during the phase transitions were explored by means of the Wannier Function Centres (WFCs) derived from ab initio molecular dynamics trajectories. In the high-temperature hexagonal structure the 6s² electrons are spherically symmetric relative to the position of Pb atoms. At low temperature the maximum of 6s² electron density is displaced relative to the position of Pb atom contributing to the polar interaction in the monoclinic polymorphs.

Average and Local Structure of Apatite-Type Germanates and Implications for Oxide Ion Conductivity

Materials with the apatite structure have a range of important applications in which their function is influenced by details of their local structure. Here, we describe an average and local structural study to probe the origins of high-temperature oxide ion mobility in La₁₀(GeO₄)₆O₃ and La₈Bi₂(GeO₄)₆O₃ oxygen-excess materials, using the low-conductivity interstitial oxide-free La₈Sr₂(GeO₄)₆O₂ as a benchmark. For La₁₀ and La₈Bi₂, we locate the interstitial oxygen, O_int, responsible for conductivity by Rietveld refinement and relate the P6₃/m to P1̅ phase transitions on cooling to oxygen ordering. Local structural studies using neutron total scattering reveal that well-ordered GeO₅ square pyramidal groups form in the structure at low temperature, but that O_int becomes significantly more disordered in the high-conductivity, high-temperature structures, with a transition to more trigonal-bipyramid-like average geometry. We relate the higher conductivity of Bi materials to the presence of several O_int sites of similar energy in the structure, which correlates with its less-distorted low-temperature average structure.

Oxide ion conductors have a number of important applications. We have used a combination of average and local structural methods to understand the origin of conductivity in a range of apatite-derived oxide ion conductors. We probe the coordination environment of the key oxide species believed responsible for conductivity, how it distorts the local coordination geometry of the material, and how this evolves as the material enters its high conductivity regime on heating. We relate this insight to the different conductivities of different members of the family.