A process for producing LiMXO 4, wherein M is a transition metal with a degree of oxidation of 2+ chosen from Fe 2+, Mn 2+ and mixtures thereof, unsubstituted or partially substituted on iron or manganese sites by one or more additional metal with a degree of oxidation of 1+ to 5+, and X is P 5+, unsubstituted or partially substituted by an oxyanion forming element, the process comprising the steps of: a) providing a source of lithium, a source of M, and a source of X, b) reacting the source of lithium, the source of M, and the source of X together: i. in a melted state at a reaction temperature between 900 to 1450°C, ii. in the presence of an excess of: a solid-solid reducing couple having an oxygen partial pressure at equilibrium (pO 2) comprised between 10 -8 and 10 -15 atm at said reaction temperature according to an Ellingham-Richardson diagram for oxides, or one component of said solid-solid reducing couple together with a gas-gas reducing couple having an oxygen partial pressure at equilibrium (pO 2) comprised between 10 -8 and 10 -15 atm at said reaction temperature according to an Ellingham-Richardson diagram for oxides, and iii. under thermic equilibrium and thermodynamic equilibrium, thereby producing molten LiMXO 4, c) isolating the LiMXO 4 from the reducing couple, d) solidifying the LiMXO 4, and wherein step c) can be carried out before and/or after step d). a) providing a source of lithium, a source of M, and a source of X, b) reacting the source of lithium, the source of M, and the source of X together: i. in a melted state at a reaction temperature between 900 to 1450°C, ii. in the presence of an excess of: a solid-solid reducing couple having an oxygen partial pressure at equilibrium (pO 2) comprised between 10 -8 and 10 -15 atm at said reaction temperature according to an Ellingham-Richardson diagram for oxides, or one component of said solid-solid reducing couple together with a gas-gas reducing couple having an oxygen partial pressure at equilibrium (pO 2) comprised between 10 -8 and 10 -15 atm at said reaction temperature according to an Ellingham-Richardson diagram for oxides, and iii. under thermic equilibrium and thermodynamic equilibrium, thereby producing molten LiMXO 4, i. in a melted state at a reaction temperature between 900 to 1450°C, ii. in the presence of an excess of: a solid-solid reducing couple having an oxygen partial pressure at equilibrium (pO 2) comprised between 10 -8 and 10 -15 atm at said reaction temperature according to an Ellingham-Richardson diagram for oxides, or one component of said solid-solid reducing couple together with a gas-gas reducing couple having an oxygen partial pressure at equilibrium (pO 2) comprised between 10 -8 and 10 -15 atm at said reaction temperature according to an Ellingham-Richardson diagram for oxides, and a solid-solid reducing couple having an oxygen partial pressure at equilibrium (pO 2) comprised between 10 -8 and 10 -15 atm at said reaction temperature according to an Ellingham-Richardson diagram for oxides, or one component of said solid-solid reducing couple together with a gas-gas reducing couple having an oxygen partial pressure at equilibrium (pO 2) comprised between 10 -8 and 10 -15 atm at said reaction temperature according to an Ellingham-Richardson diagram for oxides, and iii. under thermic equilibrium and thermodynamic equilibrium, c) isolating the LiMXO 4 from the reducing couple, d) solidifying the LiMXO 4, and