Ideal and Real Structure of Ti₅O₄(PO₄)₄: X-ray and HRTEM Investigations

The crystal structure of pentatitanium tetraoxide tetrakis(phosphate), Ti₅O₄(PO₄)₄, has been determined and refined from X-ray diffraction single-crystal data [P2₁2₁2₁ (No. 19), Z = 4, a = 12.8417 (12), b = 14.4195 (13), c = 7.4622 (9) Å (from Guinier photographs); conventional residual R₁ = 0.042 for 2556 F_o > 4σ(F_o), R₁ = 0.057 for all 3276 independent reflections; 282 parameters; 29 atoms in the asymmetric unit of the ideal structure]. The structure is closely related to those of β-Fe₂O(PO₄)-type phosphates and synthetic lipscombite, Fe₃(PO₄)₄(OH). While these consist of infinite chains of face-sharing MO₆ octahedra, in pentatitanium tetraoxide tetrakis(phosphate) only five-eighths of the octahedral voids are occupied according to □₃Ti₅O₄(PO₄)₄. Four of the five independent Ti⁴⁺O₆ show high radial distortion [1.72 ≤ d(Ti−O) ≤ 2.39 Å] and a typical 1 + 4 + 1 distance distribution. The fifth Ti⁴⁺O₆ is an almost regular octahedron [1.91 ≤ d(Ti−O) ≤ 1.98 Å]. Partial disorder of Ti⁴⁺ over the available octahedral voids is revealed by the X-ray structure refinement. High-resolution transmission electron microscopy (HRTEM) investigations confirm this result.

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