It is the lowest whole number ratio of ions represented in an ionic compound. Examples include ionic NaCl and K 2 O and covalent networks such as SiO 2 and C as diamond or graphite. In the paper you linked the full phrase is A f u is the per formula total surface area of T i S 2 So the full surface area divided by the number of formula units comprising the surface.
A number in front of a chemical formula in an equation that indicates how many molecules or atoms of each reactant and product are involved in a reaction.
A formula unit is the smallest ratio between atoms of different elements in this kind of structure that can be expressed as whole numbers. An ionic compound is made up of a metallic and non-metallic ion. It is the sum of atomic masses of all the elements present in one formula unit of a compound. A formula unit in chemistry is the empirical formula of any ionic or covalent network solid compound used as an independent entity for stoichiometric calculations.