Improvement of microwave dielectric characteristics in SrNdAlO₄ ceramics by La-substitution

Single-phase Sr(Nd_1−xLa_x)AlO₄ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1) solid solutions with tetragonal K₂NiF₄-type structure were obtained by a standard solid-state reaction approach in the entire composition range. Their microwave dielectric characteristics were investigated together with microstructures, and were significantly modified by La-substitution. The X-ray diffraction results presented that the samples were SrNdAlO₄ and SrLaAlO₄. And the dielectric constant (ε _r ) changed slightly, the temperature coefficient of resonant frequency (τ _f ) varied from −9 to −34 ppm/°C. The significantly improved Q × f value was achieved in the present ceramics with x = 0.9 and reached the maximum of 65,900 GHz. The ionic radius difference of A-site affects the distortion in the rock-salt layer and the tolerance t affects the interlayer mismatch, both of them will predominantly affect the Q × f value. The best combination of microwave characteristics was achieved at x = 0.9: ε _r  = 18.49, Q × f = 65,900 GHz, τ _f  = −27 ppm/°C.