Electronically driven structural transitions in A10(PO4)6F2 apatites (A = Ca, Sr, Pb, Cd and Hg)

Area of Science:

• Solid-state chemistry
• Materials science
• Crystallography

Background:

• Apatites are a class of calcium phosphate minerals with a hexagonal structure.
• Fluorapatites with divalent A-site cations can exhibit unique structural properties.
• Understanding lattice dynamics is crucial for predicting material behavior.

Purpose of the Study:

• To investigate the dynamic lattice instability in A10(PO4)6F2 fluorapatites.
• To determine the driving mechanism behind structural distortion.
• To elucidate the role of A-site cation electronic configurations.

Main Methods:

• Theoretical calculations were employed to study the aristotype P63/m structure.
• Analysis focused on fluorapatites containing divalent cadmium (Cd) or mercury (Hg) cations.
• Investigated cations with (n-1)d10ns0 electronic configurations.

Main Results:

• A dynamic lattice instability was identified in the P63/m structure.
• The instability leads to a distortion towards a low-symmetry P\bar{1} triclinic structure.
• The distortion is primarily driven by an electronic mechanism.

Conclusions:

• Electronic configurations of A-site cations significantly influence fluorapatite lattice stability.
• The observed structural distortion is an electronic effect, not solely due to ionic size.
• This work offers key insights into the structure-property relationships in fluorapatites.