As an important branch of lead-free piezoelectric materials, ceramics with a tetragonal tungsten bronze (TTB) structure have a high spontaneous polarization strength, high Curie temperature (T_C), and beneficial dielectric properties, making them particularly useful in ferroelectric applications. Depending on which sites are filled, the TTB structure can be divided into three categories, i.e., the "stuffed" structure, the "filled" structure and the "unfilled" structure. As a typical "filled" TTB compound, the Sr₂NaNb₅O₁₅ (SNN) ceramics can be regulated through compositional design to exhibit a wide range of structures and functional characteristics, which have a broad development potential in the fields of dielectric energy storage, multiferroic materials, nonlinear optics, and pyroelectric sensors. However, SNN ceramics require relatively high sintering temperatures, and the secondary phase Na_0.5Sr_0.25NbO₃ (NSN) readily forms during the sintering process, which adversely affects the electrical properties. Although some previous studies used the methods of eliminating the impurity phase, there could be few studies on the relationship between impurities and the properties of SNN ceramics. In this paper, SNN ceramics (i.e., Sr_1-xNa_xNb₅O₁₅, 0.56 ≤ x ≤ 1.06) were prepared with the SrNb₂O₆ (SN)-NaNbO₃ (NN) binary solid solution system, to examine the relationship among the presence of the secondary phases and the ceramic composition, as well as the sintering temperature.

SN and NN powders were synthesized by a solid-state method with reagent-grade SrCO₃ (99.99%), Na₂CO₃ (99.99%), and Nb₂O₅ (99.99%) as raw materials. The starting raw materials were pre-dried at 120 ℃ for 24 h, and then weighed according to the target stoichiometric ratios. The powders were thoroughly mixed in ethanol in a planetary ball mill with ZrO₂ balls at 401 r/min for 20 h. The mixtures were dried at 80 ℃ for 24 h, and then calcined at their respective suitable temperatures. Subsequently, the SN and NN powders were obtained by separately grinding the calcined products in a mortar after natural cooling. Afterwards, SNN ceramics (Sr_1-xNa_xNb₅O₁₅, 0.56 ≤ x ≤ 1.06) were synthesized with polycrystalline SN and NN according to the desired SNN compositions, calcined at 1220 ℃ for 6 h, and uniaxially compacted into pellets with a binder (namely 5% polyvinyl alcohol). The binder was removed at a high temperature. Finally, SNN ceramics were prepared by sintering at 1280-1340 ℃ for 4 h.

The effect of the secondary phases on the structure of SNN ceramics was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and field emission transmission electron microscopy (FETEM). Furthermore, the ceramics were ground and polished down to a thickness of about 0.6 mm, coated with silver electrodes, and fired at 840 ℃ for 30 min. Their dielectric characteristics and piezoelectric coefficients d₃₃ were measured by the LCR meter and quasi-static piezoelectric meter. The ceramics were also ground and polished down to a thickness of about 0.2 mm and coated with gold electrodes to record the room-temperature ferroelectric hysteresis loops by a Radiant Precision Workstation.

The required composition and sintering temperature ranges of the SNN pure phase are determined. For Na⁺contents (x) of 0.56-0.68, SN-NN can be solidly dissolved to obtain a pure phase at high temperatures (i.e., > 1280 ℃). As x = 0.71 and 0.80, SN-NN can be solidly dissolved at 1200-1360 ℃. At the Na⁺ content of 0.88, the SN-NN solid solution can be obtained, but it decomposes at higher temperatures (> 1320 ℃), accompanying the formation of the secondary NN phase. As the Na⁺ content further increases, it cannot be solidly dissolved at 1200-1360 ℃.

The maximum density is obtained for SNN ceramics sintered at 1340 ℃, and the change in Na⁺ content has little effect on the microstructure. The T_C of SNN ceramics increases from 272 ℃ as x = 0.56 to 323 ℃ as x = 0.88 because the crystal structure changes from "unfilled" to "filled" as the Na⁺ content increases, thus affecting the degree of crystal distortion. T_C gradually decreases because a minor amount of NN appears at x of 0.88. The dielectric constant (ε_r) of SNN ceramics initially increases and then decreases, exhibiting a maximum of 1664 as x = 0.80. The dielectric loss (tanδ) remains at a low value at this point. The remnant polarization P_r (11.49 μC/cm²) and coercive field E_c (20.99 kV/cm) are both nearly maximized as x = 0.80 for SNN ceramics due to the "filled" TTB structure, and the maximum d₃₃ value of 34 pC/N is obtained for SNN ceramics as x = 0.80, which is consistent with the optimal composition for ferroelectric properties.

Based on the XRD patterns, the ceramics with a stable, pure TTB phase could be obtained by sintering at 1200-1360 ℃ when x = 0.71-0.80, and the existence of this phase depended on the ceramic composition and the sintering temperature. As the Na⁺ content increased, T_C initially increased because the filling degree of the TTB structure increased, and then gradually decreasesd when x > 0.88 because the increase in Na⁺ content induced the formation of the secondary NN phase. When x=0.71-0.88, the electrical properties could be optimal. The ceramic with x of 0.80 demonstrated a high ferroelectric performance (i.e., P_r = 11.49 μC/cm², and E_c = 20.99 kV/cm). This study could provide certain theoretical basis and technical support for the subsequent studies of SNN ceramics.