Structure and Microwave Dielectric Properties of Solid Solution in SrLaAlO₄‐Sr₂TiO₄ System

Abstract

Ceramics in (1−x)SrLaAlO4‐xSr2TiO4 system were synthesized in the entire range of 0 ≤ x ≤ 1 by a standard solid‐state reaction method. X‐ray diffraction patterns revealed a single solid solution phase Sr1+xLa1−xAl1−xTixO4 with K2NiF4‐type structure. The cell refinement showed the linear variation of lattice parameters a, c, and cell volume. With increasing x, εr increased from 17.6 to 31.0, whereas τf was tuned from −32 to 122 ppm/oC. The Qf value first increased and then decreased with increasing x due to the result of competition balance between the effect of layer mismatch and interlayer polarization. The degradation of Qf value in the vicinity of x = 0.2 could be attributed to the compositional fluctuation and inhomogeneity originated from the positive mixing enthalpy ΔHmix, which could be confirmed by TEM observation. The best combination of microwave dielectric characteristics was achieved at x = 0.4: εr = 18.5, Qf = 95 000 GHz, τf = −8.9 ppm/oC.

Structure and microwave dielectric properties of SrSmAlO4-Sr2TiO4 solid solutions

“…The Q × f value decreases with x until x = 0.15, from 71,680 GHz at x=0 to 52,960 GHz at x=0.15, which corresponds to the result of Mao et al [19,20]. While, with further increasing x, a gradual rise of Q×f value occur until x=0.60, followed by a soaring increase to the maximum (87,750 GHz) at x=0.90, which is about 1.67 times of that for x=0.15, and then decreases rapidly to 35,400 GHz at x=1.0.…”

“…The similar results were obtained in Sr 1+x Nd 1-x Al 1-x Ti x O 4 solid solutions [18]. However, the previous work on Sr 1+x Sm 1-x Al 1-x Ti x O 4 (x=0, 0.05, 0.10, 0.15) ceramics indicated an unexpected decrease of Q×f value [19,20]. Therefore, it comes to be an important issue to understand what happens in (1-x)SrSmAlO 4 -xSr 2 TiO 4 system and whether the Q×f value can be improved by further increasing x.…”

“…Considering the presence of Sr 2 TiO 4 end-member with a same K 2 NiF 4 structure [13,14], the Sr 1+x R 1-x Al 1-x Ti x O 4 solid-solution could be expected in the entire composition range, and more effective modification of microwave dielectric properties should be expected. Actually, this idea was realized in (1-x)SrLaAlO 4 -xSr 2 TiO 4 solid solution system where the significant improvement of microwave dielectric properties was achieved [15], though the microwave dielectric properties of Sr 2 TiO 4 ceramics were barely satisfactory [16,17]. The similar results were obtained in Sr 1+x Nd 1-x Al 1-x Ti x O 4 solid solutions [18].…”

“…Moreover, the large difference in ion radius of Sr 2+ and R 3+ may lead to the microstructure inhomogeneity and distortion which has been confirmed by EDS and TEM analysis in SrRAlO 4 -Sr 2 TiO 4 (R=La, Nd) solid solution systems [15,18]. The depressed Q×f values between x=0.15 and x=0.60 in Sr 1+x Sm 1-x Al 1-x Ti x O 4 ceramics may attribute to this reason and a further study on the microstructure is necessary.…”

“…According to the previous study [20,33], the IR active modes of Sr 1+x Sm 1-x Al 1-x Ti x O 4 with K 2 NiF 4 structure and I4/ mmm space group have been confirmed to be 3A 2u +4E u by factor group analysis. Therefore, seven modes are used to fit the IR reflectivity spectra and the calculated frequencies of these modes and the dielectric loss of each mode are listed in Table 1.…”