Active materials for rechargeable batteries have a general formula Aa(MO)bM′cXO4 where A represents an alkali metal or mixture of alkali metals, a is greater than about 0.1 and less than or equal to about 2; MO is an ion containing a transition metal M not in its highest oxidation state, M′ represents a metal, or mixture of metals, and b is greater than 0 and less than or equal to about 1, c is less than 1 wherein a, b and c are selected so as to maintain the electroneutrality of the compound, and X is phosphorus, arsenic, or sulfur, or mixtures thereof. Preferred active materials are alkali metal vanadyl metal phosphates of general formula Aa(VO)bM′cPO4 where a and b are both greater than 0 and c may be zero or greater. New synthetic routes are provided to alkali metal mixed metal phosphates where at least one of the starting materials is a metal-oxo group (MO)3+, where M represents a metal in a +5 oxidation state. Reductive methods are provided for reacting a metal-oxo phosphate with an alkali metal compound in the presence of reducing carbon. The carbothermal reduction step can for example result in the reduction of a +5 metal to a +4 state or lower.