La2 O3 and SrO-doped CaO-MgO-Nb2 O5-TiO2 system ceramics were prepared by solid-state ceramic technique. The microstructure and microwave dielectric properties of CaO-MgO-Nb2 O5 -TiO2-La2 O3 ceramics can be adjusted by varying the amount of La3+ or Sr2+ ions respectively. The replacement of Ca2+ by La3+ at A-site of the ceramics increases the quality factor Q value ( at 7.6 GHz) as well as the temperature coefficient of resonant frequency τfand decreases the dielectric constant εr and the substitution of Sr2+ at A-site in this ceramics system exhibits opposite characteristics. The microwave properties of La3+ , Sr2+ -doped CaO-MgO-Nb2 O5 - TiO2 system ceramics depend on the degree of octahedral distortion inside materials .
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