abstract |
The invention provides a composition having the formula (I): xXO 2 .yY 2 O 5, (wherein: 0.5 < x < 0.7; 0.3 < y < 0.5; X comprises one or more of silicon, titanium, germanium and zirconium; and Y comprises one or more of phosphorus, vanadium arsenic and antimony), or a hydrate thereof, in which the composition comprises more than 50 wt% or more of crystalline material. |