61st Annual Report on Research 2016

In preliminary work prior to the start of this project, it was found that certain solid state reactions carried out within our group at elevated temperatures (≥ 600 °C) produced a liquid product rather than the expected polycrystalline powder product.  The composition of this liquid product was unknown, though it was found to remain liquid over a very wide temperature range, including well below room temperature.  The first objective in this project period was therefore to better understand the composition of this liquid.

The clearest insights into the composition of this liquid came from ¹H and ³¹P solution NMR (Figure 1).  When the thermostable liquid was dissolved in D₂O, one ¹H and two ³¹P peaks were observed.  The first ³¹P peak could be assigned to PO₄^3- ions (present as phosphoric acid), while the second could not be matched to known species, but based on knowledge of the synthesis and chemical bonding principles was assigned to the nitridophosphate anion, PO₃N^4-, a complex oxoanion whose solution properties have never previously been reported. Investigations of this liquid through infrared spectroscopy (Figure 2) are also consistent with a mixture of PO₄^3- and PO₃N^4- species.  The spectrum of the liquid closely resembles that of a pure PO₄^3- solution, but with a few slight shifts in peak intensity and position. The attribution of the thermostable liquid to a mixture of PO₄^3- and PO₃N^4- species is consistent with the properties measured for this liquid, including a freezing point which is near but significantly lower than that for aqueous H₃PO₄ at any molarity, the rheological response of this liquid, and the very low pH of the thermostable liquid. 

Since the PO₃N^4- species were present in low concentrations, synthesis efforts have been carried out to enhance the yield of this novel solution anion.  The initial synthesis route could not be easily reproduced and tuned, so efforts were then refocused on the synthesis of the known oxynitride compound PON and its use as a precursor.  Depending on the synthesis conditions, either crystalline or amorphous forms of PON could be produced (Figure 3).  The crystalline phase was found to be well-indexed by a cristobalite-type structure with the tetragonal space group of I-42d (#122) and lattice parameters of a = b = 4.619 Å and c = 6.981 Å.  The structure, chemistry, and chemical transformations PON are being further investigated.

Figure 1.  ¹H (left) and ³¹P (right) spectra of the thermostable liquid.  The ³¹P peak at -0.09 ppm was assigned to PO₄^3- ions, while the peak at -11.07 ppm was assigned to PO₃N^4-.

Figure 2. Comparison of the infrared spectra of a sample of PO₄^3- (red) and a sample where both PO₄^3- and PO₃N^4- species are present (blue).

Figure 3. Laboratory X-ray (Cu K_a) diffraction patterns of nominal “PON” samples synthesized under different conditions.  Patterns either appeared crystalline (yellow, magenta, cyan scans) or amorphous (blue scan).  The crystalline samples had a dominant peak just above 20° 2q, but long scans allowed a number of weaker peaks to be resolved and quantified.