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 (P2/a) with a = 8.7437(1) Å, b = 4.8584(1) Å, c = 10.8600(2) Å, β = 90.124(1)° (Z = 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.