In this work the physical properties of La and Li-modified tetragonal tungsten bronze (TTB) ferroelectric $Sr_xBa_{1-x}Nb_2O_6$(SBN) solid solutions are studied. Especially much attention is focused on their diffused phase transition (DPT) behavior, normal-incommensurate phase transition and microstructure.
$[(Sr_xBa_{1-x})_{1-0.5y}Li_y]_{1-1.5z}La_zNb_2O_6$ solid solutions in the composition range 0.25 $\leqq$ x $\leqq$ 0.75,0 $\leqq$ y $\leqq$ 0.2,0 $\leqq$ z $\leqq$ 0.1, abbreviated SBLLN-100x/10y/100z were prepared by the conventional oxide techniques.
In the present work, phase transition and incommensurate superstructure were made on such SBLLN ceramics. Phase analysis of sintered specimens has been conducted by dielectric susceptibility, photoacoustic techniques, X-ray diffraction and high resolution electron microscopy(HREM).
The dielectric constants of SBLLN ceramics were investigated as a function of temperature from - 150$^\circ$C to 300$^\circ$C, dc electric field bias and composition. The temperature of dielectric constant maxima shifted to a higher temperature with the increasing Ba and Li concentrations but the maxima shifted to a lower temperature with the increasing La content. While $Sr_{0.25}Ba_{0.75}Nb_2O_6$ ceramic displayed a typical first order transition which showed a sudden steep increase in the dielectric constant curve, the La substitution induced the diffuse dielectric maxima. This diffuse character becomes more pronounced with the increasing La concentration. A much broader dielectric constant peak in SBLLN ceramics implies that the ferroelectric-paraelectric phase transition does not occur at a discrete temperature, but over a Curie temperature range. The breaking of local symmetry of SBLLN ceramics seems to cause the diffuse phase transition and broadening of Raman line-width. The temperature dependence of dielectric constants showed marked two anomalies. Firstly, they had a shoulder around 0$^\circ$C ~ 20$^\circ$C on the lower temperature tail side of usual ferroelectric-paraelectric transition. The peak value of the dielectric constant shifts to a higher temperature and decreases in magnitude with the increasing frequency. Secondly, the dielectric constants of all the SBLLN ceramics showed a strong dependence on the applied dc electric field. The dielectric constant increases at the lower temperature region anomaly, but at the higher temperature anomaly, they decrease with the increasing dc bias field.
We have performed photoacoustic (PA) effect investigations to find the temperature dependence of the PA signal of the SBLLN ceramics. $(Sr_{0.5}Ba_{0.5})_{1-1.5z}La_2Nb_2O_6$ ceramics around 30$^\circ$C displayed a sudden jump in the photoacoustic signal intensity and then the signal decayed exponentially in the higher temperature region. The decaying signal persisted up to 200$^\circ$C. Moreover, well beyond the dielectric constant transition temperature $T_c$ up to a temperature $T_d$, the SBLLN ceramics still possess polarization implying the existence of distributed a randomly microregions of irreversible polarization. These findings are in agreement with the behavior of the index of refraction reported by Burns.
Differential scanning calorimetry measurements were carried out on a Perkin-Elmer DSC-4 diferential scanning calorimeter. The maximum of endothermal anomaly occurred near 60$^\circ$C in all the SBLLN ceramics but the anomaly in the dielectric constant ascribed to the full ferroelectric ordering was not detected even in the background signal. Along with the absence of photoacoustic anomaly, the SBLLN DSC behavior seems to imply a ferroelastic transition around 60$^\circ$C which apparently does not accompany spontaneous polarization excitation.
The permittivity dependence on the magnitude of bias at a constant temperature Ti which is located at the stable incommensurate phase range before and after the specimens are subjected to a static bias field is investigated. In case where no a bias was applied previously to the sample, a permittivity minimum occurs at E=0. On the other hand, in case where a bias is applied previously a minimum of the permittivity occurs at an external bias field Ei. Therefore, the sample displays a phenomenon called field memory effect, i.e., it remembers the sign and magnitude of the exposure field. In parallel with the thermal memory effect, this field memory effect also ensures the presence of an incommensurate phase in SBLLN ceramics.
Lattice parameters of SBLLN ceramics having the TTB structure were investigated as a function of compositions and temperature. They decreased with the increasing Sr and Li and La contents. Pyrochlore type phase was found among tungsten bronze phase $Sr_{0.75}Ba_{0.25}Nb_2O_6$. Such immiscibility may be traced to the instability of the TB structure in $SrNb_2O_6$ itself. The Tb structure became stable with the introduction of Li as an A site occupant. In the X-ray diffractions of SBLLN, a set of weak incommensurate superlattrice peaks were observed at room temperature. A possible ferroelastic transition temperature ~60$^\circ$C, which coincide with the DSC anomaly was manifested by the disappearance of superstructural reflection lines on heating.
An incommensurate(INC) modulation in SBLLN ceramics in conjunction with the aforementioned ferroelastic transition has beenstudied by the high resolution electron microscopy(HREM) and TEM diffraction. INC satellite diffraction spots in teh tetragonal phase along with their images were obtained in [001] and [110] zones. INC superlattice spots were found on both bisector direction of the tetragonal cell at positions (h+[1+$\delta$]/4 k+[1+$\delta$]/4 1/2) along [110].
The diffraction patterns in [001] zone showed diffuse streaking along the [110] and [110] directions. Streak intensity increase with the increasing La and Li contents. In conformance with the diffractionm pattern change with the solute concentration variation, the HREM microstructure showed the presence of zones with habit planes oriented along <110> or {110} platelets of an atomic spacing thickness. (001) reciprocal lattice plane sections of $Sr_{0.5}Ba_{0.5}Nb_2O_6$ showed no superlattice spots, while {h+1/2 k+1/2 0} superlattice spots appeared in the modified ceramic specimens and increase in intensity with the increasing La and Li contents. At the same time the cross-shaped diffuse diffraction streaks appearing along the equivalent <110> directions became stronger. In the heavily substituted samples, e.g. in $[(Sr_{0.5}Ba_{0.5})_{0.88}La_{0.08}Nb_2O_6$ and $[(Sr_[0.75}Ba_{0.25})_{0.9}Li_{0.2}]_{0.91}La_{0.06}Nb_2O_6$, separated two spots around {h+1/2 k+1/2 0} appeared incomensurate with the subcell spots together with the cross-shaped diffuse streaking along <110>.
