We describe the preparation of wurtzite Al1-xScxN bulk single crystals by physical vapor transport by introducing scandium to the aluminum nitride (AlN) source material. The Al1-xScxN single crystals obtained exhibit a maximum scandium atomic concentration of about x = 0.55% in axial c-direction while being uniform within the c-plane. The high structural quality as determined by high resolution x-ray diffraction is comparable to that of AlN bulk crystals. However, the incorporated scandium concentration is about 15 times lower than expected from thermodynamic considerations. This behavior is ascribed to a pronounced scandium segregation or agglomeration in the gas phase and parasitic deposition/loss of ScN during growth.