What does x mean in (1-x)A – xB composite materials, molar ratio, weight ratio or volume fraction?

I've been collecting data about ceramic materials and often I get to see composite materials are described as $$(1-x)\ce{A} - x\ce{B}$$. I am confused what this $$x$$ means. And I haven't seen any of the papers mention what $$x$$ is unless %wt is given specifically.

For an example in $$(1-x)\ce{MgO} - x\ce{LiF}$$ ceramic material, does $$x$$ mean molar ratio or weight ratio? For me it doesn't make sense to think of $$x$$ as a volume fraction as most of the times powders of these materials are taken for sintering.

1. If nothing is mentioned, is it all right to assume $$x$$ as the molar ratio?

2. Can we assume volume fraction is equal to molar ratio even in solids or liquids?

Here, $$x$$ is a variable $$(x\in[0;1])$$ denoting mole fraction of a given constituent in a phase of variable composition (non-stoichiometric compound). This is an algebraic way of denoting a given compound from the phase diagram for a solid solution: $$x$$ corresponds to its abscissa axis.
• @avin Again, unless there are any specific notes, $(1-x)\ce{MgO} - x\ce{LiF} ≡ \ce{Li_xMg_{(1-x)}O_{(1-x)}F_x}$, so by default $x$ implicates mole fraction. – andselisk Feb 12 at 18:15