HPAs can be synthesized through various methods, typically involving the reaction of a heteroatom precursor (such as phosphoric acid) with a suitable metal oxide under acidic conditions. The most common synthesis method is the aqueous phase process, where the heteroatom and metal oxides are dissolved in water and then crystallized out to form the HPA. The resulting compounds can then be purified and characterized using techniques such as X-ray diffraction (XRD) and infrared spectroscopy (IR).