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

Abstract

The crystal structure of pentatitanium tetraoxide tetrakis(phosphate), Ti 5 O 4 (PO 4 ) 4 , has been determined and refined from X-ray diffraction single-crystal data [ P 2 1 2 1 2 1 (No. 19), Z = 4, a = 12.8417 (12), b = 14.4195 (13), c = 7.4622 (9) Å (from Guinier photographs); conventional residual R 1 = 0.042 for 2556 F o > 4σ( F o ), R 1 = 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-Fe 2 O(PO 4 )-type phosphates and synthetic lipscombite, Fe 3 (PO 4 ) 4 (OH). While these consist of infinite chains of face-sharing M O 6 octahedra, in pentatitanium tetraoxide tetrakis(phosphate) only five-eighths of the octahedral voids are occupied according to □ 3 Ti 5 O 4 (PO 4 ) 4 . Four of the five independent Ti 4+ O 6 show high radial distortion [1.72 ≤ d (Ti−O) ≤ 2.39 Å] and a typical 1 + 4 + 1 distance distribution. The fifth Ti 4+ O 6 is an almost regular octahedron [1.91 ≤ d (Ti−O) ≤ 1.98 Å]. Partial disorder of Ti 4+ over the available octahedral voids is revealed by the X-ray structure refinement. High-resolution transmission electron microscopy (HRTEM) investigations confirm this result.