Trolleite, AI₄(OH)₃[PO₄]₃: A very dense structure with octahedral face-sharing dimers Available to Purchase

Trolleite, 4Al₄(OH)₃[PO₄]₃, a 18.894(5) Å, b 7.161(1) Å, c 7.162 (2)Å, β 99.99(2)°, space group I2/c, is a valid species. For 2010 independent reflections, R(hkl) converged to 0.036. Estimated standard errors in distances are Me-O ± 0.002 Å and O-O’ ± 0.003 Å. Its crystal structure consists of Al-O octahedral face-sharing dimers which link by corner-sharing hydroxyl groups to form an infinite double-chain which runs parallel to the [001] direction. The [PO₄] tetrahedra bridge these chains to produce a very dense three-dimensional structure

The structure is topologically related to that of lazulite, MgAl₂(OH)₂[PO₄]₂. This is particularly evident if the orientations of the [PO] tetrahedra are noted: the lazulite tetrahedra and the oxygen packing can be approximately superimposed upon those of trolleite.

Severe distortions occur on account of the face-sharing and result from the cumulative effects of cation-cation repulsion and deviations of the anions from local electrostatic neutrality by the coordinating cations. The octahedra are distorted into elongate trigonal antiprisms with the Al-O distances toward the shared face longer (1.94-2.09 Å ) than those to the opposing free face (1.79-1.96 Å ). Average polyhedral distances are Al(l)-O 1.915, Al(2)-O 1.905, P(l)-O 1.528, and P(2)-O 1.528 Å. Difference synthesis provided locations of the hydrogen atoms. The proposed hydrogen bonding scheme involves bifurcated hydrogen bonds from the octahedral groups to phosphate oxygens.