The possibility of varying the compositional long-range order parameter s and the influence of s on the ferroelectric properties of pure and Li-doped Pb(Yb_0.5Nb_0.5)O₃ ceramics have been studied. X-ray diffraction and dielectric measurements were used. A compositional order-disorder phase transition was discovered at 1120 degrees C (in doped ceramics). It was shown that in the course of material synthesis the compositionally ordered perovskite structure is formed at comparatively low temperatures (800 degrees C) and the s value may be reduced during sintering at higher temperatures. The pure ceramic remains ordered after sintering because the rate of s change in it is much lower than in the doped material. Low-temperature sintered doped ceramics are characterized by spatial non-homogeneities of s, causing the appearance of two (or more) anomalies in the temperature dependence of the permittivity. The T-s phase diagram has been investigated. Materials with a low s (s<s_c) show a diffuse transition from high-temperature paraelectric phase to ferroelectric phase (at T_C=115 degrees C when s = 0). For materials with a high s (s>s_c) a paraelectric to antiferroelectric phase transition is observed. Thus there is a triple point at s=s_c. The Curie temperature T_C is quadratic in s and a minimum in the T_C(s) curve is observed at the triple point, which is in agreement with the earlier theoretical treatment of Bokov et al.