Sintering Aid‐Assisted Growth of 0.95(K_0.5Na_0.5)NbO₃–0.05(Bi_0.5Na_0.5)(Zr_0.85Sn_0.15)O₃ Single Crystals by the Solid‐State Crystal Growth Method and Their Characterization

Abstract

In this work, 0.95(K0.5Na0.5)NbO3–0.05(Bi0.5Na0.5)(Zr0.85Sn0.15)O3 (KNN–BNZS) lead‐free piezoelectric single crystals are grown by the solid‐state crystal growth (SSCG) method and their phase transitions and properties characterized. Addition of an appropriate amount of Li2CO3 and Bi2O3 sintering aids considerably promotes single‐crystal growth. Chemical composition of the single crystal is similar to the nominal composition with a slight deficiency in Na and K as evaluated by electron probe microanalysis (EPMA). X‐Ray diffraction (XRD), electrical property measurements, and temperature‐controlled Raman scattering reveal that the phase transition near room temperature is from a rhombohedral to an orthorhombic or tetragonal phase. Wide temperature range Raman scattering and dielectric properties consistently indicate transitions associated with the phase transformations or boundaries of a phase coexistence region and further details. Nontrivial frequency dependence of dielectric properties is successfully parameterized by capacitance relaxations described by Debye and Cole–Cole responses with special constant phase elements with fixed exponents of 0.5 (Warburg elements). High‐frequency dielectric constants and additional dielectric relaxations and dc relaxations are deconvoluted. The modeling parameters indicate the phase transition temperatures (or boundaries of the phase coexistence region) unambiguously. Polarization–electric field and bipolar strain–electric field hysteresis loops show that the single crystal is a classic lossy ferroelectric material.

0.98(K0.5Na0.5)NbO3–0.02(Bi0.5Na0.5)(Zr0.85Sn0.15)O3 Single Crystals Grown by the Seed-Free Solid-State Crystal Growth Method and Their Characterization

“…This, along with the persistence of the ν 1 and ν 5 modes, indicates that even above the Curie temperature, the structure contains local non-cubic distortions [50], as commonly found in perovskite ferroelectric materials [61][62][63][64]. The Raman spectra of the 98KNN-2BNZS single crystal are similar to those of 0.95(K 0.5 Na 0.5 )NbO 3 -0.05(Bi 0.5 Na 0.5 )(Zr 0.85 Sn 0.15 )O 3 (95KNN-5BNZS) [40]. However, the phase transitions are more clearly visible than in the 95KNN-5BNZS single crystal.…”

“…The matrix appears to have separated from the seed and single crystal on the bottom and right-hand faces during sintering. As for the case of (K 0.5 Na 0.5 )NbO 3 , the growth distance is unusually high for the [001] growth direction (compared with a growth distance of several 10s of microns in previous experiments [40,65]), although the crystal contains a lot of porosity. With further experiments, it may be possible to improve the growth rate and decrease porosity in the single crystals.…”

“…Formation of a liquid phase during sintering and angular grains are common characteristics in materials showing AGG [36][37][38]. By adjusting the amount of the liquid-phase sintering aid, single crystal growth can be promoted [39,40]. Bi 2 O 3 and Li 2 CO 3 were used as liquid-phase sintering aids specifically because of their low melting temperatures (817 • C and 723 • C, respectively) and their use in growing single crystals of KNN by seed-free solid-state single crystal growth [25,27,31].…”

Comparison of (K0.5Na0.5)NbO3 Single Crystals Grown by Seed-Free and Seeded Solid-State Single Crystal Growth

“…The single crystals are all alkali deficient ( Table 3 , Table 5 and Table 6 ) due to the evaporation of the volatile alkali elements during sintering. This is sometimes seen in (K 0.5 Na 0.5 )NbO 3 -based single crystals grown by SSCG [ 19 , 32 ], as well as in (K x Na 1-x )NbO 3 single crystals grown by other methods [ 111 , 112 , 113 ]. Charge neutrality is maintained through the formation of oxygen vacancies according to the defect reaction [ 114 ]: …”

“…The SSCG method has several advantages over conventional solution-growth techniques such as lower operating costs (due to the use of standard laboratory furnaces, obviating the need for specialised crystal growth furnaces and expensive Pt crucibles), reduced processing times, lower processing temperatures, and improved composition control [ 25 ]. KNN and KNN-based single crystals several millimetres in size have been grown by this method, the size being limited by the size and crystallographic orientation of the seed crystal [ 12 , 22 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 ]. However, using the SSCG method to grow KNN single crystals also has some disadvantages.…”