Tetragonal Region Research Articles

Ta 2O 5–Y 2O 3–ZrO 2 system: Experimental study and preliminary thermodynamic description

Some aspects of stable and equilibrium phases in the Ta 2O 5–Y 2O 3–ZrO 2 system have been studied in this work. Phase stability of Ta 0.16Y 0.16Zr 0.68O 2 and Ta 0.10Y 0.10Zr 0.80O 2 has been studied by X-ray powder diffraction. A secondary phase segregation of YTaO 4 has been observed in both compositions below 1500 °C. However, once treated at 1500 °C, YTaO 4 phase disappears. The preliminary phase diagram of the Ta 2O 5–Y 2O 3–ZrO 2 system at 1200 °C and 1500 °C have been constructed using THERMOCALC package. Thermodynamic description is obtained by critically evaluating experimental data on the binary systems Y 2O 3–Ta 2O 5 and Ta 2O 5–ZrO 2. Binary system data have been combined and parameters for ternary tetragonal and cubic structures have been assessed to construct ternary phase diagram. Calculated isothermal section at 1200 °C is similar to 1500 °C. There is no change in the tetragonal solid solution region observed with temperature, which indicates the same equilibrium phases at 1500 °C and at 1200 °C.

Structural properties of (Bi 0.5Na 0.5) 1− xBa xTiO 3 lead-free piezoelectric ceramics

A systematic XRD investigation of poled and unpoled ceramics of the system (1 − x) Bi 0.5Na 0.5TiO 3– x BaTiO 3 (0 ≤ x ≤ 0.2) (BNBT) was performed. The variation of the lattice parameters confirms the existence of a morphotropic phase boundary at 0.06 ≤ x ≤ 0.08; however, significant differences in unit cell parameters between poled and unpoled states appear. Lattice distortions of the rhombohedral and tetragonal phases are significantly increased in poled samples. Dramatic changes in peak intensities of the pseudo-cubic (2 0 0) reflections between poled and unpoled samples reveal a strong enhancement of the tetragonal volume fraction in the poled state. Temperature-dependent XRD studies confirm a transition into a cubic high-temperature phase. This transition is rather smooth in the unpoled state. In poled samples, the tetragonal distortion and the tetragonal volume fraction display a different temperature variation and tetragonal regions seem to persist into the cubic phase field.

Transformation stress induced metastable tetragonal phase in (93–92)%Pb(Zn1/3Nb2/3)O3–(7–8)%PbTiO3 single crystals

High-resolution synchrotron radiation diffraction results indicate that the room temperature tetragonal (T) phase in (93–92)%Pb(Zn1/3Nb2/3)O3–(7–8)%PbTiO3 [PZN–(7–8)%PT] single crystals is a metastable phase stabilized by the transformation-cum-cooling stress in the crystal. This is manifested by the “missing” (001)T diffractions when the diffractions were taken from (001) fractured surfaces. The stress relaxation in this case has caused the metastable (001)T domains in the surface layer to transform to the equilibrium rhombohedral phase. Temperature and particle size effects on the metastable domains have been examined and discussed. Based on the present finding, a new PZN–PT phase diagram showing extended rhombohedral+tetragonal region is proposed.

Ultra‐Low Firing High‐k Scheelite Structures Based on [(Li0.5Bi0.5)xBi1−x][MoxV1−x]O4 Microwave Dielectric Ceramics

In this work, dense ceramic samples across the solid solution of the [(Li0.5Bi0.5)xBi1−x][MoxV1−x]O4 system are prepared using the solid‐state reaction process. The monoclinic phase region could be obtained for 0≤x<0.098 and the scheelite tetragonal solid solution region could be obtained for 0.098<x≤1. When x=0.098, the boundary composition [(Li0.5Bi0.5)0.098Bi0.902][Mo0.098V0.902]O4 could be readily sintered at 650°C and also have excellent microwave properties with a high relative permittivity of 81, and a Q×f value of 8000 GHz and a low temperature coefficient of +9.7 ppm/°C. This ceramic is chemically compatible with both Al and Cu electrode materials at its sintering temperature. It can be an excellent candidate for ultra‐low‐temperature co‐fired ceramic technology and is the highest permittivity microwave material found to date with sintering considerations as low as 650°C.

Phase transitions and phase diagram of the ferroelectric perovskite(Na0.5Bi0.5)1−xBaxTiO3by anelastic and dielectric measurements

The complex elastic compliance and dielectric susceptibility of ${({\text{Na}}_{0.5}{\text{Bi}}_{0.5})}_{1\ensuremath{-}x}{\text{Ba}}_{x}{\text{TiO}}_{3}$ (NBT-BT) have been measured in the composition range between pure NBT and the morphotropic phase boundary included, $0\ensuremath{\le}x\ensuremath{\le}0.08$. The compliance of NBT presents sharp peaks at the rhombohedral/tetragonal and tetragonal/cubic transitions, allowing the determination of the tetragonal region of the phase diagram, up to now impossible due to the strong lattice disorder and small distortions and polarizations involved. In spite of ample evidence of disorder and structural heterogeneity, the R-T transition remains sharp up to $x=0.06$, whereas the T-C transition merges into the diffuse and relaxorlike transition associated with broad maxima of the dielectric and elastic susceptibilities. An attempt is made at relating the different features in the anelastic and dielectric curves to different modes of octahedral rotations and polar cation shifts. The possibility is also considered that the cation displacements locally have monoclinic symmetry, as for PZT near the morphotropic phase boundary.

Dimensionality and magnetic interactions in CaFe 2As 2: An ab initio study

We present detailed electronic structure calculations for CaFe 2As 2. We investigate in particular the ‘collapsed’ tetragonal and orthorhombic regions of the temperature–pressure phase diagram and find properties that distinguish CaFe 2As 2 from other Fe-pnictide compounds. In contrast to the tetragonal phase of other Fe-pnictides the electronic structure in the ‘collapsed’ tetragonal phase of CaFe 2As 2 is found to be strongly 3D. By an analysis of the non-interacting susceptibility, χ 0 ( q ) , of CaFe 2As 2 and LaFePO we discuss the role of magnetic interactions in iron-pnictides. From this we propose an intuitive explanation for the outstanding question relating to why the predicted antiferromagnetic moment depends strongly on coordinate relaxation and the choice of correlation functional.

Dielectric and acoustical high frequency characterisation of PZT thin films

Pb(Zr, Ti)O3 (PZT) is an interesting material for bulk acoustic wave resonator applications due to its high electromechanical coupling constant, which would enable fabrication of large bandwidth frequency filters. The major challenge of the PZT solid solution system is to overcome mechanical losses generally observed in PZT ceramics. To increase the understanding of these losses in textured thin films, thin film bulk acoustic resonators (TFBAR's) based on PZT thin films with compositions either in the tetragonal region or at the morphotropic phase boundary and (111) or {100} textures were fabricated and studied up to 2 GHz. The dielectric and elastic materials coefficients were extracted from impedance measurements at the resonance frequency. The dispersion of the dielectric constant was obtained from impedance measurements up to 2 GHz. The films with varying compositions, textures and deposition methods (sol-gel or sputtering) were compared in terms of dielectric and acoustical properties.

Decomposition of 8.5 mol.% Y 2O 3-doped zirconia and its contribution to the degradation of ionic conductivity

Transmission electron microscopy (TEM) in combination with energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy was applied to analyze the microstructure and the local chemical composition of as-sintered and aged Y 2O 3-doped ZrO 2 (YDZ) thick-film substrates with yttria contents of 8.5 mol.% (8YDZ) and 10 mol.% (10YDZ). Selected-area electron diffraction and dark-field TEM imaging show nanoscale precipitates of the tetragonal YDZ phase, which are coherently embedded in grains consisting of the cubic phase. A distinct coarsening of the tetragonal regions is observed after 2500 h annealing of 8YDZ at 950 °C in an oxidizing atmosphere. This coarsening is accompanied by decomposition on the cationic sublattice on the nanoscale, leading to the depletion of yttrium in the tetragonal regions. The decomposition is discussed in terms of spinodal decomposition which takes place even at a high doping concentration of 8.5 mol.% Y 2O 3.

Domain Wall Engineering in Lead-free Piezoelectric Materials

For [111] poled barium titanate (BaTiO3) single crystals, piezoelectric properties increased with decreasing domain sizes. To explain the phenomenon, the multidomain crystals were regarded as composite of (a) a distorted 90° domain wall region with ultrahigh piezoelectric property and (b) a normal tetragonal domain region. This model suggested that the d 31 of the lead-free piezoelectric crystals with domain sizes below 1 μ m can be over 1,000 pC/N. Thus, to induce the finer engineered domain configurations, a patterning electrode was used. As the results, the domain size of 3 μm was successfully induced, and the d31 was obtained at −243.2 pC/N.

Dielectric and ferroelectric properties of (BixBa1−x)(Znx/2Ti1−x/2)O3 ceramics

(BixBa1−x)(Znx/2Ti1−x/2)O3 [xBi(Zn1/2Ti1/2)O3-(1 − x)BaTiO3, x ⩽ 0.30] solid solution ceramics with a pure perovskite structure were synthesized by a conventional solid-state reaction method. A change in symmetry from tetragonal to cubic was observed as the mole fraction of Bi(Zn1/2Ti1/2)O3 increased. All compositions show a dielectric anomaly over the temperature range 30–120 °C. In the tetragonal phase region, the temperature Tm of the maximum dielectric constant decreases with increasing Bi(Zn1/2Ti1/2)O3 content. In the cubic phase area, the compositions show general dielectric relaxation of linear dielectrics, and Tm increases with increasing Bi(Zn1/2Ti1/2)O3 content. The phase transition, dielectric and ferroelectric properties of (BixBa1−x)(Znx/2Ti1−x/2)O3 ceramics are sensitive to nano-regional chemistries.

Crystal structure and orthorhombic–tetragonal phase transition of nanoscale (Li 0.06Na 0.47K 0.47)NbO 3

Lead-free nano (Li 0.06Na 0.47K 0.47)NbO 3 powders, with pure perovskite structure and various grain sizes between about 30 and 60 nm, have been successfully synthesized by a novel sol–gel method, in which Nb 2O 5 was used as the Nb source. The refining XRD and Raman spectra were used in combination with TEM to investigate the evolution of lattice structure and phase transformation behavior as a function of grain size. The results demonstrated that the growth process of grains has been divided into two stages, and the distortion of unit cell apparently decreases with decreasing grain size. At around 35 nm, the phase structure of (Li 0.06Na 0.47K 0.47)NbO 3 changed from orthorhombic to tetragonal. This phenomenon is related to a grain size-induced structural phase transition. For the well accepted wisdom that niobates get super piezoelectric properties in orthorhombic–tetragonal transition region, our results suggested a critical size for the application of (Li 0.06Na 0.47K 0.47)NbO 3 in nano piezoelectric devices.

Structural and magnetic characterization of multiferroic(BiFeO3)1−x(PbTiO3)xsolid solutions

A series of multiferroic $(1\ensuremath{-}x){\text{BiFeO}}_{3}\ensuremath{-}x{\text{PbTiO}}_{3}$ solid solution ceramics were prepared by solid-state reactions. Structural characterization by x-ray diffraction reveals the existence of a morphotropic phase boundary (MPB) region in this system in which a tetragonal, a rhombohedral, and an orthorhombic phase exist simultaneously with a large tetragonality in the tetragonal phase region. The temperature variation of magnetic moment of the samples with MPB compositions, measured under zero-field cooling (ZFC) mode, shows three anomalies arising from the antiferromagnetic orderings of the rhombohedral, tetragonal, and orthorhombic phases, respectively. The significant difference in antiferromagnetic ordering temperatures of the rhombohedral and tetragonal phases is attributed to the different structural effects on the magnetic interactions between the rhombohedral and the tetragonal phases, and the effect of the magnetic dilution on the magnetic ordering strength. The magnetic phase diagram of the $(1\ensuremath{-}x){\text{BiFeO}}_{3}\ensuremath{-}x{\text{PbTiO}}_{3}$ solid solution system was established.

Stability of ferroic phases in the highly piezoelectric Pb(ZrxTi1−x)O3ceramics

The morphotropic phase boundary in the phase diagram of the technologically important Pb(ZrxTi1-x)O3 (PZT) ceramics has been traditionally believed to separate ferroelectric tetragonal and rhombohedral phase regions. This old picture has come under close scrutiny during the last eight years following the discovery of new monoclinic phases in the Cm and Cc space groups. This article presents a brief overview of these discoveries in which the use of multiple diffraction probes (X-ray, electron, neutron diffraction) in conjunction with physical property measurements has played a crucial role. A new phase diagram of PZT showing the stability fields of these structures below room temperature is also presented.

Domain Wall Engineering in Lead-Free Piezoelectric Crystals

For [111] poled barium titanate (BaTiO3) single crystals, piezoelectric properties increased with decreasing domain sizes. To explain the phenomenon, the multidomain crystals were regarded as composite of (a) a distorted 90° domain wall region with ultrahigh piezoelectric property and (b) a normal tetragonal domain region. This model suggested that the d 31 of the lead-free piezoelectric crystals with domain sizes below 1 μ m can be over 1,000 pC/N. Thus, to induce the finer engineered domain configurations, a patterning electrode was used. As the results, the domain size of 3 μm was successfully induced, and the d31 was obtained at −243.2 pC/N.

Optimum Compositions for Piezoelectric Properties of Pb-free (Bi0.5Na0.5)(1-x)BaxTiO3 Ceramics

Optimum compositions for piezoelectric properties of <TEX>$(Bi_{0.5}Na_{0.5})_{(1-x)}Ba_xTiO_3$</TEX> ceramics were investigated in the range of <TEX>$x=0{\sim}0.1$</TEX> covering rhombohedral to tetragonal phase regions. No impurity phases other than a perovskite phase were found and the grain size decreased with increasing x. A two-phase coexisting morphotropic phase area rather than boundary dividing rhombohedral and tetragonal phase regions appeared to exist at <TEX>$x=0.05{\sim}0.08$</TEX>. As for piezoelectric properties within morphotropic phase compositions, the piezoelectric constant (<TEX>$d_{33}$</TEX>) and the electromechanical coupling factor (<TEX>$K_p$</TEX>) showed peak values at x=0.065, 192 pC/N and 34%, respectively, indicating x=0.065 as an optimum composition for piezoelectric <TEX>$(Bi_{0.5}Na_{0.5})_{(1-x)}Ba_xTiO_3$</TEX> ceramics.

Opportunities for TBCs in the ZrO 2–YO 1.5–TaO 2.5 system

An examination of the ZrO 2–YO 1.5–TaO 2.5 system reveals several promising attributes for use in thermal barrier coating applications. The rather unique presence of a stable, non-transformable tetragonal region in this ternary oxide system allows for phase stability to high temperatures (1500 °C). Selected compositions with high levels of yttria and tantala have also shown superior resistance to vanadate corrosion than the commercially utilized 7YSZ. In addition, Y + Ta stabilized zirconia compositions within the non-transformable tetragonal phase field exhibit toughness values comparable or somewhat higher than those of 7YSZ, which bodes well for their durability as TBCs. These promising attributes are discussed in this paper in the context of recent experimental work.

Investigation of dipolar defects in (1−x)Pb(Zn1∕3Nb2∕3)O3–xPbTiO3 single crystals using different poling methods

The temperature dependence of piezoelectric, pyroelectric, and dielectric properties have been measured for [001] and [111] oriented 0.92Pb(Zn1∕3Nb2∕3)O3–0.08PbTiO3 (PZN-8%PT) and 0.955Pb(Zn1∕3Nb2∕3)O3–0.045PbTiO3 (PZN-4.5%PT) single crystals poled by two different poling methods, i.e., room temperature (RT) poling and poling through field cooling (FC). Compared with the RT poling case, some interesting features have been found in the FC poled [001] oriented crystal but not in the [111] oriented crystal. These features include the enhancement of piezoelectric properties, lowering of the transition temperature from rhombohedral to tetragonal phase, increasing the amplitude of the spontaneous polarization in the tetragonal phase region, and the decrease of the dielectric constants in the tetragonal phase. These results revealed that the dipolar defects in the crystals are formed between the B and O sites in the ABO3 perovskite structure.

Hyperfine interaction measurements on ceramics: PZT revisited

The solid solution of PbZr 1− x Ti x O 3, known as lead-zirconate titanate (PZT), was probably one of the most studied ferroelectric materials, especially due to its excellent dielectric, ferroelectric and piezoelectric properties. The highest piezoelectric coefficients of the PZT are found near the morphotropic phase boundary (MPB) (0.46⩽ x⩽0.49), between the tetragonal and rhombohedral regions of the composition-temperature phase diagram. Recently, a new monoclinic phase near the MPB was observed, which can be considered as a “bridge” between PZT's tetragonal and rhombohedral phases. This work is concerned with the study of the structural properties of the ferroelectric PZT (Zr/Ti=52/48, 53/47) by hyperfine interaction (HI) measurements obtained from experiments performed by using the nuclear spectroscopy time differential perturbed angular correlation (TDPAC) in a wide temperature range.

Phase transitions in PZN–4.5%PT in the range 4.2–450 K

Phase transitions in the relaxor ferroelectric PZN–4.5%PT in the temperature range 4.2–450 K were studied using very high resolution neutron powder diffraction. Three phases, rhombohedral ( R3 m), tetragonal ( P4 mm) and cubic ( Pm3 m) occur in this range with increasing temperature. The transitions are approximately 15 K lower than shown in the conventional phase diagram and are first order. A single phase tetragonal region was not observed and hence is very narrow (<5 K) or non-existent.

Analysis of Composite Structures on Barium Titanate Fine Particles Using Synchrotron Radiation

The crystal structures of barium titanate (BaTiO3) fine particles with a size around 140 nm were investigated using a synchrotron radiation X-ray diffraction method. The observed diffraction pattern was analyzed by Rietveld method assuming several models. As a result, it is found that the BaTiO3 fine particles have composite structures include (a) a tetragonal region with a constant c/a ratio, (b) a tetragonal region with gradient c/a ratios and (c) a cubic region. In order to estimate these structures, the partial profile relaxation technique was applied in the Rietveld refinement. It is considered that the composite structure is important for the size effect on BaTiO3 fine particles.