(1-x)Pb(Mg_(1/3)Nb_(2/3))O_(3)-xLa(Mg_(1/2)Ti_(1/2))O_(3) dielectrics were prepared by a solid-state reaction process and the structures were evaluated together with the dielectric characterization. The pyrochlore-free dielectrics were obtained with increasing La(Mg_(1/2)Ti_(1/2))O_(3) content due to the increased electronegativity difference. The 1/2(111) superlattice reflection showed 1:1 ordering on the B site in all compositions. Antiphase tilting of oxygen octahedra and B-site cation displacing appeared at x=0.9. Low loss (tan δ～10~(-4)) dielectrics with high dielectric constant (36-900) were obtained in the present system, and the good microwave dielectric properties were achieved in the compositions with x≥0.7. The best combination of microwave dielectric properties was obtained for x=0.9: ∈=34, Qf=21 280 GHz, and τ_(f)=-60 ppm/℃.
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