Fig. 2: 31P NMR of Fe–Si–CaHPO4 (1:2:0.1 by mole) mixture heated to 1000 °C for 8 h.
From: Routes to reduction of phosphate by high-energy events
The NMR spectrum, in which individual anions are referenced to a frequency of 161.907 MHz for H3PO4 (0 ppm), identifies P species based on parts per million deviations (δ) from this frequency. The peaks correspond to (left to right) hypophosphate (P4+), phosphite (P3+), phosphate (P5+), and pyrophosphate (P5+). Note that phosphite is a broad doublet with a J-coupling constant of 570 Hz when coupled to 1H. About 7% of the phosphorus in this NMR spectrum has been changed from HPO42–, which is consistent with reduction of phosphate to phosphide. Phosphide, specifically Fe3P, reacts with water to form a 1:5:5:1 mixture of hypophosphate: phosphate: phosphite: pyrophosphate. In contrast, a change to CaHPO3 would yield only phosphite. This suggests that reduction of phosphate to phosphite occurs much faster than the timescale of 8 h reaction, proceeding to Fe3P within 8 h.
