Existence of Posner's Cluster in Vacuum

Abstract

Using ab initio methods, a single isomer of D 3h symmetry was found to be preeminent on the Ca 3 (PO 4 ) 2 cluster potential energy surface. This isomer has the distinct feature that each calcium atom is surrounded by four oxygen atoms. We studied the aggregation of this isomer in the [Ca 3 (PO 4 ) 2 ] n system with n being 2 or 3. This gives us an energy reference for analyzing the stability of the so-called Posner's cluster Ca 9 (PO 4 ) 6 which is the core of the actual structural model of amorphous calcium phosphate. Posner's cluster is shown to be significantly stabilized in comparison to the energy reference.

Evolution of the magnesium incorporated amorphous calcium phosphate to nano-crystallized hydroxyapatite in alkaline solution

“…1c and d), consistent with the results of other researchers [21,22]. It is widely accepted that these nano-units, the building blocks of the ACP, may be constructed by assembling of Posner's cluster, Ca 9 (PO 4 ) 6 [23,24].…”

Crystalline nano-coatings of fluorine-substituted hydroxyapatite produced by magnetron sputtering with high plasma confinement

“…The temperature in the plasma sheath (near Z = 29 mm) was 9.0 eV. This temperature was set higher than the average temperature to i) nucleate monomers of Ca 3 (PO 4 ) 2 (3.4 eV) [47], ii) form amorphous HA clusters, i.e., Ca 9 (PO 4 ) 6 precursors (5.2 eV) [48], iii) replace OH − with F − ions in the HA structure to form FHA (0.1 eV) [49] and iv) crystallise FHA at approximately 0.3 eV. During the sputtering process, the energy required for film formation by ions and electrons from the plasma sheath depends on ε s , n e and ion velocity (Bohm velocity) [38].…”

Amorphous Phase Mediated Crystallization: Fundamentals of Biomineralization

“…In ACP, there exists a short-range order, consistent with Ca 9 (PO 4 ) 6 units and an average diameter of 0.95 nm, which are often named "Posner's clusters". Moreover, theoretical investigations show that Posner's clusters are the most stable arrangement compared to different calcium and phosphate clusters [170,171]. In comparison with other calcium phosphate phases (for example, dicalcium phosphate dihydrate (DCPD), HAP, tricalcium phosphate (TCP)), ACP is meta-stable and inclined to form a crystalline phase.…”