Structural Characterization of Moganite-Type AlPO₄ by NMR and Powder X-ray Diffraction

Structural characterization of a new high-pressure AlPO₄ phase synthesized at 5 GPa and 1500 °C is reported. The phase is monoclinic (<i>P</i>2/<i>a</i>) with <i>a</i> = 8.7437(1) Å, <i>b</i> = 4.8584(1) Å, <i>c</i> = 10.8600(2) Å, β = 90.124(1)° (<i>Z</i> = 6). ³¹P MAS NMR and two-dimensional (2D) ²⁷Al triple-quantum (3Q) MAS NMR revealed that it contains two tetrahedral P sites of 1:2 abundance ratio, and two tetrahedral Al sites with 1:2 ratio. 2D ³¹P dipolar-recoupled double-quantum (DQ) MAS NMR and ²⁷Al → ³¹P dipolar-based (through-space) and J coupling-based (through-bond) 3Q-heteronuclear correlation (HETCOR) experiments provided direct information on the linkages of these sites. The crystal structure was solved and refined from synchrotron powder X-ray diffraction data utilizing the information from NMR. The phase is isostructural to moganite, a rare SiO₂ polymorph, and its structure can be derived from the latter via an ordered replacement of tetrahedral Si sites by Al and P. The NMR parameters of the phase were also calculated by first-principles method, which are consistent with those observed. Contrary to the other moganite phases known to date (i.e., SiO₂ and PON), moganite-AlPO₄ has a higher-pressure stability field than the corresponding quartz phase. This is the first moganite-type phase found in the ABX₄ system