A monofilament (100) for producing an Nb3Sn-containing superconductor wire (33) includes a powder core (1) with an Sn-containing powder, a reaction tube (3) composed of an Nb alloy that includes Nb and at least one further alloy component X. The powder core is disposed within the reaction tube. The monofilament also includes at least one source (4) for at least one partner component Pk. A respective source includes one or more source structures at a unitary radial position in the monofilament. The alloy component X and the partner component Pk form precipitates XPk on reaction annealing of the monofilament in which Sn from the powder core and Nb from the reaction tube react to produce Nb3Sn. The powder core is disposed in a moderation tube, which in turn is disposed within the reaction tube. This provides a monofilament for a powder-in-tube based Nb3Sn-containing superconductor wire with improved current carrying capacity.