sentences:  Keggin structure
  
  Keggin structure is the best known structural form for heteropoly acids.It is the structural form of α-Keggin anions, which have a general formula of [XM12O40]"n"−, where X is the heteroatom (most commonly are P5+, Si4+, or B3+), M is the addenda atom (most common are molybdenum and tungsten), and O represents oxygen.