What is the formula for Lithium phosphate?

$L {i}_{3} P {O}_{4}$
In aqueous solution, this would give $2 \times L {i}^{+}$, and $H P {O}_{4}^{2 -}$, and $H {O}^{-}$. If you have an old bottle it is probably $L {i}_{2} H P {O}_{4}$....