The investigation of Li(Ni 1−x Fe x )PO 4 by optical second harmonic generation yields the competition of the antiferromagnetic (AFM) structures and 180 • domain patterns found in the end compounds LiNiPO 4 (point group mm ′ m, spins along z) and LiFePO 4 (point group mmm ′ , spins along y). While AFM order and the distribution of the AFM domains of LiNiPO 4 are unaffected by the ion substitution at x = 0.06, striking changes are observed at x = 0.20. We find that Fe 2+ is dominant in establishing the magnetic order. For x = 0.20 we find that the magnetic order of the solid solution interpolates the magnetic order of its end compounds by exhibiting an orientation of the spins in the (100) plane which include an angle of 40 • ± 3 • with respect to the y axis toward 0 K (point group m). In contrast to the two end members, the associated AFM domains form rods of a few mm length and ∼ 10 µm width occurring in neither of the end compounds. Mechanisms responsible for the magnetic order and domain pattern (180 • domains still being one of the least explored aspects of AFM materials in spite of their omnipresence) are discussed.