Showing papers on "Tetragonal crystal system published in 1996"
TL;DR: In this article, the results of an investigation into the grain-size dependence of lattice structure for barium titanate (BaTiO) ceramics prepared by a sol-gel method were reported.
Abstract: We report the results of an investigation into the grain-size dependence of lattice structure for barium titanate (${\mathrm{BaTiO}}_{3}$) ceramics prepared by a sol-gel method. Raman and infrared spectroscopy, x-ray diffraction, and differential scanning calorimetry were used in combination with electron microscopy to study the evolution of lattice structure and phase transformation behavior with heat treatment and grain growth from the nano scale to the micron scale for ${\mathrm{BaTiO}}_{3}$ polycrystals. Raman spectroscopy and optical second-harmonic-generation measurements indicated the onset of local room-temperature acentric crystal symmetry with heat treatment and crystallite growth, well before the observation of any tetragonal structure by x-ray diffraction. Analysis of the room-temperature Raman spectra for ultrafine grain (grain size 0.1 \ensuremath{\mu}m) polycrystals suggested that a locally orthorhombic structure preceded the globally tetragonal form with grain growth. In support of this observation, differential scanning calorimetry suggested the orthorhombic-tetragonal phase transformation shifts up through room temperature with decreasing grain size. Hot-stage transmission electron microscopy studies revealed that fine grain (grain size \ensuremath{\approxeq}0.1 \ensuremath{\mu}m) ceramics, which showed a thermal anomaly associated with the cubic-tetragonal phase transformation, were untwinned at room temperature, as well as on cycling through the normal Curie temperature, suggesting a single-domain state for individual grains. The findings are discussed in light of a number of possible causes, including the presence of processing-related hydroxyl defects and the effect of elastic constraints on phase transformation behavior for ${\mathrm{BaTiO}}_{3}$ grains in a polycrystalline microstructure. \textcopyright{} 1996 The American Physical Society.
814 citations
TL;DR: In this article, a tetragonal phase with an axial ratio of unity is interpreted to appear in the compositional region around 16 mol.% YO1.5.
295 citations
TL;DR: Crystals of tetragonal hen egg-white lysozyme were grown using Advanced Protein Crystallization Facility (APCF) apparatus under a microgravity environment (SpaceHab-01 mission) and ground control conditions and some residues of the two refined structures show multiple alternative conformations.
Abstract: Crystals of tetragonal hen egg-white lysozyme were grown using Advanced Protein Crystallization Facility (APCF) apparatus under a microgravity environment (SpaceHab-01 mission) and ground control conditions. Crystals were grown from NaCl as a crystallizing agent at pH 4.3. The X-ray diffraction patterns of the best diffracting ground- and space-grown crystals were recorded using synchrotron radiation and an image plate on the W32 beamline at LURE. Both ground- and space-grown crystals showed nearly equivalent maximum resolution of 1.3–1.4 A. Refinements were carried out with the program X-PLOR with final R values of 18.45 and 18.27% for structures from ground- and space- grown crystals, respectively. The two structures are nearly identical with the root-mean-square difference on all protein atoms being 0.13 A. Some residues of the two refined structures show multiple alternative conformations. Two ions were localized into the electron-density maps of the two structures: one chloride ion at the interface between two symmetry-related molecules and one sodium ion stabilizing the loop Ser60–Leu75. The sodium ion is surrounded by six ligands which form a bipyramid around it at distances of 2.2–2.6 A.
263 citations
TL;DR: Barium titanate (BaTiO3) powders of varying particle sizes were grown under hydrothermal conditions at a temperature of 240 °C as mentioned in this paper, where the hydroxide concentration of the r...
Abstract: Barium titanate (BaTiO3) powders of varying particle sizes were grown under hydrothermal conditions at a temperature of 240 °C. This was accomplished by varying the hydroxide concentration of the r...
212 citations
TL;DR: In this paper, the effect of heat treatment on the grain size, phase assemblage, and mechanical properties of a 3 mol% Y-TZP ceramic was investigated.
Abstract: The effect of heat treatment on the grain size, phase assemblage, and mechanical properties of a 3 mol% Y-TZP ceramic was investigated. Specimens were initially sintered for 2 h at 1450 C to near theoretical density; some specimens were then heat-treated at 1550, 1650, 1750, or 1850 C to coarsen the microstructure. The average grain size increased with heat treatment from 1750 C. The maximum fraction of tetragonal phase that transformed during fracture corresponded with the largest tetragonal grain size of {approximately}5--6 {micro}m. Strength was on the order of 1 GPa, and was surprisingly insensitive to heat-treatment temperature and grain size, contrary to previous studies. The fracture toughness increased from 4 to 10 MPa{center_dot}m{sup 1/2} with increasing grain size, owing to an increasing transformation zone size. Grain sizes larger than 5--6 {micro}m spontaneously transformed to monoclinic phase during cooling. Such critical grain sizes are much larger than those found in past investigations, and may be due to the greater fraction of cubic phase present which decreases the strain energy arising from more » crystallographic thermal expansion anisotropy of the tetragonal phase. « less
197 citations
TL;DR: In this paper, the axial ratio c a decreases with an increase of RO15 content and becomes unity at 14 or 16mol% RO15 for arc-melted ZrO2-X mol%RO15 specimens.
190 citations
TL;DR: The effect of pressure on chemical interactions in molecular mixtures is important for problems spanning fundamental chemistry, planetary science, and materials science as mentioned in this paper, and diamond-anvil cell studies reveal pressure-induced chemistry in the CH4-H2 system.
Abstract: The effect of pressure on chemical interactions in molecular mixtures is important for problems spanning fundamental chemistry, planetary science, and materials science. Diamond-anvil cell studies reveal pressure-induced chemistry in the CH4-H2 system. The system, which has no known compounds at ambient conditions, formed four molecular compounds, CH4(H2)2, (CH4)2H2, CH4(H2)4, and CH4H2, at pressures up to 10 gigapascals. These have been characterized by synchrotron single-crystal x-ray diffraction, polycrystalline x-ray diffraction, Raman spectroscopy, and visual observation. Although CH4(H2)2 crystallizes in the MgZn2-type, hexagonal Laves phase structure, (CH4)H2 has a body-centered tetragonal structure that is similar to that of Al2Cu. The 1:1 and 1:2 compounds are stable to at least 30 gigapascals.
189 citations
TL;DR: In this article, high-temperature powder x-ray diffraction data is presented for perovskite between 293 and 1523 K. The temperature-dependence of superlattice intensities and cell parameters suggests a sequence of phase transitions from the room temperature orthorhombic (Pbnm) structure to a tetragonal (I4/mcm) polymorph at temperatures in the range 1373 - 1423 K, followed by transformation to the cubic aristotype at.
Abstract: High-temperature powder x-ray diffraction data are presented for perovskite between 293 and 1523 K. The temperature-dependence of superlattice intensities and cell parameters suggests a sequence of phase transitions from the room temperature orthorhombic (Pbnm) structure to a tetragonal (I4/mcm) polymorph at temperatures in the range 1373 - 1423 K, followed by transformation to the cubic aristotype at . The intensity of the diffuse background increases on transformation to the cubic structure and is associated with disorder (and anionic mobility) of the oxygen sub-lattice. The I4/mcm - Pbnm transition induces a large spontaneous strain, but the tetragonal spontaneous strain in the I4/mcm phase due to the - I4/mcm transition is small, below the resolution of this experiment. These results add weight to suggestions from recent computer simulations that orthorhombic may transform to a tetragonal (rather than a cubic) polymorph under the conditions of the Earth's mantle, in which case the effects on electrical conductivity would not be expected to be as great as for a transition to a cubic polymorph, although the consequences for elastic properties may be more significant.
160 citations
TL;DR: In this article, a high-resolution scanning superconducting quantum interference device microscopy study of tetragonal single-layer Tl 2 Ba 2 CuO β+δ films, deposited on tricrystal SrTiO 3 substrates, demonstrates the effect of spontaneously generated half flux quanta.
Abstract: A high-resolution scanning superconducting quantum interference device microscopy study of tetragonal single-layer Tl 2 Ba 2 CuO β+δ films, deposited on tricrystal SrTiO 3 substrates, demonstrates the effect of spontaneously generated half flux quanta. This observation shows that in addition to YBa 2 Cu 3 O 7 , the order parameter symmetry in Tl 2 Ba 2 CuO β+δ is consistent with that of a d x 2 - y 2 pair state. This result also rules out any bilayer or twinning effects and any pairing that is incompatible with the fourfold rotational symmetry as in the Tl 2 Ba 2 CuO β+δ superconducting system.
142 citations
TL;DR: A high temperature flux solution method was used to grow (1-x)PZN-(x)PT, where x= 0.0, 0.1 and 0.15 single crystals as mentioned in this paper.
Abstract: Relaxor ferroelectric single crystals of Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN-PT) are of interest as high performance transducers due to their very large piezoelectric coupling and dielectric properties. A high temperature flux solution method was used to grow (1- x)PZN-( x)PT, where x= 0.0, 0.1 and 0.15 single crystals. Processing conditions were optimized to increase the size and yield of the perovskite crystals, including variation of the flux to composition ratio, cooling rate, soak time and soak temperature. The crystals varied in size from 0.01 cm to 1.5 cm on an edge, and in color from opaque to brown due to the changes in processing conditions. The crystals were characterized by XRD, dielectric constant and dielectric loss measurements. As the content of PT increased the transition from the paraelectric to the ferroelectric phase approached first order behavior and the crystal structure transformed from rhombohedral to tetragonal. This structure transition caused the lattice constant along the c-axis to elongate as the c/a ratio increased. At room temperature, the dielectric constants for the PZN-PT compositions along the [111] or [001] axes were as great as 5000 and the dielectric losses were as low as 0.01.
140 citations
TL;DR: The ferroic phase transition behavior of two (Na 1/2 Bi-1/2 )TiO 3 (NBT) crystals grown by flux and by the Czochralski method has been investigated in this article.
Abstract: The ferroic phase-transition behavior of two (Na 1/2 Bi 1/2 )TiO 3 (NBT) crystals grown by flux and by the Czochralski method has been investigated in the present study. Although both the tetragonal and the rhombohedral phases of NBT are expected to be ferroelastic, these crystals exhibit different ferroelastic behavior. The two NBT crystals also show differences in the amount of temperature hysteresis and the thermal expansion coefficients. These differences can be attributed to nonstoichiometry and structural variations dependent on the growing method. The present investigation has revealed a second maximum at ∼450°C in dielectric constant (e(T)) curves, which could indicate that the intermediate tetragonal phase is either polar or antipolar. This maximum, however, originates from space-charge polarization and interaction between the charge carrier and the electrode, such that the tetragonal phase, in fact, is paraelectric. The diffuse phase transition (DPT) of the NBT crystal, therefore, is from a paraelectric and ferroelastic tetragonal phase to a ferroelectric and ferroelastic rhombohedral phase. The crystallographic supergroup-subgroup relationships in the ferroic phase transitions of NBT crystals are discussed.
TL;DR: In this paper, the structural and electronic properties of bulk cubic and tetragonal tungsten trioxide are investigated using a periodic boundary condition ab initio Hartree−Fock method; a posteriori density function is defined.
Abstract: The structural and electronic properties of bulk cubic and tetragonal tungsten trioxide are investigated using a periodic boundary condition ab initio Hartree−Fock method; a posteriori density func...
TL;DR: In this paper, a thin La1−xCaxMnOδ with tetragonal symmetry was fabricated and the measured magnetoresistance is anisotropic: initially positive for applied magnetic field perpendicular to the film plane and negative for field applied parallel to the plane.
Abstract: We have fabricated thin films of La1−xCaxMnOδ with tetragonal symmetry. For low temperatures and magnetic fields the measured magnetoresistance is anisotropic: initially positive for applied magnetic field perpendicular to the film plane and negative for field applied parallel to the film plane. At high temperatures the magnetoresistance is negative for all fields and field orientations. We also observe an in‐plane magnetoresistance anisotropy with an angular dependence corresponding to that observed in transition metal ferromagnets. We suggest an interpretation requiring a substantial spin‐orbit interaction in the material.
TL;DR: In this paper, fine particles of barium titanate single crystal with an average particle size of about 66 nm were prepared by a hydrothermal method, and the crystal structure was assigned to cubic with an expanded lattice using a Rietveld method.
Abstract: Fine particles of barium titanate single crystal with an average particle size of about 66 nm were prepared by a hydrothermal method. In the as‐prepared sample, there was a large amount of the hydroxyl group and barium vacancy, and its crystal structure was assigned to cubic with an expanded lattice using a Rietveld method. The hydroxyl group desorbed with increasing calcination temperature, especially desorbing remarkably in the range from 200 to 300 °C. Above 700 °C, there was no hydroxyl group in the particle, and the crystal structure of the sample treated at 800 °C was assigned to tetragonal with tetragonality of 1.001 using a Rietveld method. The state of the lattice vibration was measured by an infrared reflection method, and analyzed using a nonlinear least‐squares method with a four‐parameter semiquantum model. As the result, the crystal structure of the as‐prepared sample estimated using Fourier transform infrared and Raman was assigned to tetragonal. On the O6 octahedra deformation mode, the re...
TL;DR: In this paper, thermal barrier coatings (TBCs) were used to deposit TBCs on rotating cylindrical electron beam physical vapour deposition (EB-PVD) NiCoCrAlY-coated IN 100 substrates by reactive high rate EB PVD.
Abstract: ZrO2-based ingot sources with stabilizing oxides of 6.5 and 20 wt.% Y2O3 and 25 2.5 wt.% CeO 2 Y 2 O 3 respectively were used to deposit thermal barrier coatings (TBCs) on rotating cylindrical electron beam physical vapour deposition (EB-PVD) NiCoCrAlY-coated IN 100 substrates by reactive high rate EB-PVD. The TBCs were investigated by scanning electron microscopy, X-ray fluorescence and X-ray diffraction. The phases within the TBCs were t′ tetragonal for 6.5 wt.% Y2O3ZrO2 and cubic for 20 wt.% Y2O3ZrO2 TBCs. The phases found in ceria-stabilized TBCs were t′, cubic and monoclinic depending on the compositional fluctuations within the layers. Compositional variations were due to wide differences in the respective vapour pressures of the various oxides in ceria-stabilized ingot sources which become effective in single-source EB-PVD processing. The apparent crystal habits in the TBCs were correlated with process parameters, phases and chemistry and with regard to structural growth models.
TL;DR: In this paper, the surface phase of sulfated zirconia is reconstructed from tetragonal to monoclinic phase during the deactivation process while the bulk remains in the Tetragonal phase after deactivation.
Abstract: Ultraviolet Raman spectroscopy (UVRS) has been demonstrated to be a powerful new tool for catalysis and surface science studies. UVRS can successfully avoid the surface fluorescence which frequently occurs in normal Raman spectra of many catalysts. Fresh, deactivated and regenerated sulfated-zirconia catalysts have been characterized using this new method. The UV Raman spectrum of the fresh sample is dominated by the tetragonal phase, however, the spectrum of deactivated sulfated zirconia is nearly identical to that of the monoclinic phase of pure zirconia. A regeneration of the deactivated catalyst restores the spectrum back to that of the fresh sample. In addition, a new band at 750 cm−1 associated with the fresh sample attenuates with the deactivation process. This band is tentatively assigned to the surface [ZrO4]4− unit which is assumed to bond to the surface sulfate group. The results indicate that the surface phase of sulfated zirconia is reconstructed from tetragonal to monoclinic phase during the deactivation process while the bulk remains in the tetragonal phase after the deactivation. It is proposed that the surface tetragonal phase is stabilized by sulfate groups, and is associated with the catalytic activity.
TL;DR: In this paper, Sol-gel prepared plasma spray powders of Sc2O3 (SYSZ) were used to give better tetragonal phase stability at 1400°C than present-day Y 2O3-stabilized ZrO2 thermal barrier coating (TBC) material.
Abstract: Sol-gel prepared plasma spray powders of Sc2O3, Y2O3-stabilized ZrO2 (SYSZ), where the total amount of stabilizer is ∼ 7 mol. %, and the stabilizer contains 2.5–20% Y2O3, have been found to give significantly better tetragonal (t′) phase stability at 1400°C than present-day Y2O3-stabilized ZrO2 thermal barrier coating (TBC) material. The results are discussed in the context of the ongoing effort to develop ZrO2-based (or other) ceramics suitable for ultra-high temperature TBC usage.
TL;DR: This paper proposed a new model based on the stability of lattice vibrations, in which the amount of the lattice defects can determine the correlational size of barium titanate dipoles, and the tetragonality (c/a) is dependent on the correlation size.
Abstract: Barium titanate fine particles were prepared by a hydrothermal method using titanium tetrahydroxide gel and barium hydroxide. In particles with an average size of 89.62 nm, the crystal structure changed from cubic to tetragonal accompanied with a decrease in the amount of lattice defects such as lattice hydroxyl groups, without a change in particle size. This result revealed that a physical quantity with size much smaller than the particle size could determine the crystal structure of barium titanate. To explain this phenomenon, we proposed a new model based on the stability of lattice vibrations, in which the amount of the lattice defects can determine the correlational size of barium titanate dipoles, and the tetragonality (c/a) is dependent on the correlational size. Moreover, the model can be used to explain the size effect on relative permittivity and Curie temperature in barium titanate.
Patent•
27 Nov 1996
TL;DR: In this article, the structure of and a method for fabricating a ferroelectric film on a nonconductive substrate is described, and a seed layer, e.g., a compound containing lead, lanthanum, titanium, and oxygen, with a controlled crystal lattice orientation, is deposited on the conductive layer.
Abstract: There is disclosed a structure of and a method for fabricating a ferroelectric film on a non-conductive substrate. An adhesion layer, e.g., a layer of silicon dioxide and a layer of zirconium oxide, is deposited over a substrate. A conductive layer, e.g., a noble metal, a non-noble metal, or a conductive oxide, is deposited over the adhesion layer. A seed layer, e.g., a compound containing lead, lanthanum, titanium, and oxygen, with a controlled crystal lattice orientation, is deposited on the conductive layer. This seed layer has ferroelectric properties. Over the seed layer, another ferroelectric material, e.g., lead zirconium titanate, can be deposited with a tetragonal or rhombohedral crystalline lattice structure with predetermined and controlled crystal orientation.
TL;DR: In this article, four crystal structures of the title complexes have been analyzed by X-ray crystallography, and the interplanar distances between the two phthalocyanato dianions have a linear relationship to the ionic radii of the central Ln(III) cations.
Abstract: Four crystal structures of the title complexes have been analyzed by X-ray crystallography 1: C80H68N17Nd, a = 170545(6) A, c = 22807(3) A, tetragonal, P4/ncc, Z = 4 2: C80H68N17Gd, a = 17399(8) A, c = 11377(6) A, tetragonal, P4212, Z = 2 3: C80H70N17OHo, a = 18096(5) A, c = 11079(6) A, tetragonal, P4212, Z = 4 4: C80H70N17OLu, a = 18171(3) A, c = 10987(4) A, tetragonal, P4/nmm, Z = 2 The [LnIIIPc2]- and [NBun4]+ ions in each crystal stack alternately in a column along the 4-fold axis The Ln(III) cation of each [LnIIIPc2]- is eight-coordinated by two phthalocyanato dianions (Pc) The interplanar distances between the two Pc rings have a linear relationship to the ionic radii of the central Ln(III) cations The skew angles between the two Pc rings are 62, 344, 432, and 450° for the Nd, Gd, Ho, and Lu complexes, respectively The skew angle seems to depend on the strength of the π−π interaction between the two Pc rings
TL;DR: In this paper, the authors show that the maximum electromechanical response at a given composition (x) is a function of temperature (T) and occurs in the tetragonal phase field just outside the two-phase MPB region in the (x,T) plane.
Abstract: The temperature variation of planar electromechanical coupling coefficient (kp) in Pb(ZrxTi1−x)O3(PZT) ceramics near the morphotropic phase boundary (MPB) compositions has been correlated with structural changes to show that the maximum electromechanical response at a given composition (x) is a function of temperature (T) and occurs in the tetragonal phase field just outside the two‐phase MPB region in the (x,T) plane. This clearly disproves the hypothesis that the coexistence of rhombohedral and tetragonal phases at MPB is responsible for the enhancement of the electromechanical response. In fact, coexistence of the rhombohedral phase with the tetragonal phase is shown to lead to the lowering of the electromechanical response.
TL;DR: In this article, the authors investigated metallic photonic crystals built around a layer-by-layer geometry and obtained rejection rates of 7-8 dB per layer from both metallic crystals. But their measurements were in good agreement with theoretical simulations.
Abstract: We have investigated metallic photonic crystals built around a layer‐by‐layer geometry. Two different crystal structures (face‐centered‐tetragonal and tetragonal) were built and their properties were compared. We obtained rejection rates of 7–8 dB per layer from both metallic crystals. Defect modes created by removing rods resulted in high peak transmission (80%), and high quality factors (1740). Our measurements were in good agreement with theoretical simulations.
TL;DR: In this article, a phase transition from the tetragonal structure of zirconia to the monoclinic phase takes place above 800 degrees c. The grain size dependence upon the annealing temperature has been evaluated using the Scherrer equation.
Abstract: Low-temperature (400-800{degrees}C) stabilization of tetragonal zirconia, prepared by a hydroxide gel route and low-temperature annealing, has been achieved by partial substitution of Zr{sup 4+} with Bi{sup 3+}. XRD and Raman spectroscopy have been used for characterization of the doped zirconia powders as well as for identification of the crystal symmetry. XPS data provide evidence of the presence of Bi(III) and of a small amount of a lower Bi sub-oxide. A phase transition from the tetragonal structure of zirconia to the monoclinic phase takes place above 800{degrees}C. The grain size dependence upon the annealing temperature has been evaluated using the Scherrer equation. Particle morphology and size have been directly imaged with the aid of SEM. 62 refs., 6 figs., 2 tabs.
TL;DR: In this paper, single crystals of PbWO 4 have been obtained by the Czochralski method using X-ray diffraction, and the atomic structure has been determined by using the scheelite type, tetragonal, space group I 4 1 / a, a = 5.456(2), c = 12.018(2) A).
TL;DR: In this article, the phase transition sequence of (Na1/2Bi 1/2)1−xPbxTiO3 was shown to be rhombohedral-tetragonal-cubic.
Abstract: The region of the (Na1/2Bi1/2)1−xPbxTiO3 phase diagram from 0≤x≤0.2 was established through analysis of structure and thermal anomalies associated with changes in temperature and composition. The rhombohedral‐tetragonal morphotropic phase boundary (MPB) was investigated considering the dependence of the rhombohedral lattice distortion of (Na1/2Bi1/2)TiO3 (NBT) on chemical stoichiometry and fabrication processes. It was found that the MPB would lie in the composition range 0.13
TL;DR: The layered perovskite compounds NaLnTiO4 (Ln = La, Pr, Nd, Sm, Eu, Gd, Y and Lu) were synthesized by the solid state reaction as discussed by the authors.
TL;DR: In this article, high-temperature X-ray line profile measurements were performed to maximal temperatures of 1050 °C on samples of the nickel-base superalloy SRR 99.
Abstract: High-temperature X-ray line profile measurements were performed to maximal temperatures of 1050 °C on samples of the nickel-base superalloy SRR 99. The samples with rod axes near the [001] direction were investigated in the initially undeformed state and after creep deformation at different temperatures and stresses. For the measurements of the (002) and (020) line profiles, a special X-ray double crystal diffractometer with negligible line broadening was used which was equipped with a high-temperature vacuum chamber. The line profiles were evaluated for the lattice parameters of the matrix phase γ and the precipitated γ′ phase and for values of the lattice mismatch parallel and perpendicular to the stress axis, respectively, which were found to be different. Elastic, tetragonal distortions of the phases γ and γ′ could be determined between room temperature and about 900 °C. These distortions are thermally induced due to the different thermal expansion coefficients of the two phases and deformation induced due to interfacial dislocation networks which were built up during deformation. At the high temperatures of the X-ray measurements, at least partial recovery of the deformation-induced internal stresses occurred, depending on the temperature of the X-ray measurements. The results are discussed and compared with data obtained by complementary techniques.
TL;DR: In this article, the infrared optical constants of cubic and tetragonal zirconias were determined from the analysis of the reflectance spectra of yttria doped Zirconia single crystals and undoped Tetragonal ZIRconia powders, respectively.
Abstract: The infrared optical constants of cubic and tetragonal zirconias were determined from the analysis of the reflectance spectra of yttria doped zirconia single crystals and undoped tetragonal zirconia powders, respectively. In the latter case, the reflectance spectrum was analyzed by an effective medium Theory, which has been proven to successfully determine the dielectric properties of small powder particles. The symmetry of the resulting modes of tetragonal zirconia was elucidated by performing theoretical calculations of the vibrational properties.
TL;DR: In this article, the effect of the crystal structure of ferroelectric phases was discussed in terms of the relaxation of internal stresses by the formation of 90{degree domains, resulting in high Curie temperatures.
Abstract: The Curie temperature of normally sintered and hot-pressed Pb{sub 1{minus}x}Ba{sub x}Nb{sub 2}O{sub 6} (x = 0.30, 0.37, 0.41, and 0.50) was dependent on the sample preparation conditions. Samples with large amounts of pores and cracks had higher Curie temperatures than dense samples. Dense samples with fine grains had higher Curie temperatures than those with coarse grains. Comparison between microstructure and the Curie temperature revealed that the variation in Curie temperature was caused by internal stresses developed in the paraelectric-to-ferroelectric phase transition. Large internal stresses increased the free energy of the ferroelectric phase, and thus decreased the Curie temperature. The magnitude of internal stresses depended on the microstructure. Pores and cracks relaxed internal stresses, resulting in high Curie temperatures. Grain boundaries also relaxed internal stresses; samples with small grains had high Curie temperatures. Furthermore, the magnitude of internal stresses was dependent on the crystal structure of ferroelectric phases; samples with the tetragonal ferroelectric structure had internal stresses larger than those with the orthorhombic ferroelectric structure. The effect of crystal structure was discussed in terms of the relaxation of internal stresses by the formation of 90{degree} domains.
TL;DR: In this article, the authors calculate the total energy variation along the tetragonal distortion path connecting the body centred cubic (bcc) and the face centered cubic (fcc) structures, providing a first-principles energy surface with two minima as a function of c/a.
Abstract: Employing a high-precision band structure method (FP LAPW - full potential linearized augmented plane wave) we calculate the total energy variation along the tetragonal distortion path connecting the body centred cubic (bcc) and the face centred cubic (fcc) structures. The total energy along this Bain transformation is calculated, varying c/a and volume, providing a first-principles energy surface which has two minima as a function of c/a. These are shallow and occur for the sp metals at the two cubic structures, while Ti (V) has a minimum at fcc (bcc) but a saddle point (i.e. a minimum in volume and a maximum with respect to c/a) at the other cubic structure. These features can be analysed in terms of an interplay between the Madelung contribution and the band energies. Our total energy results allow us to calculate the elastic constants and and to study the influence of pressure on the phase stability. These energy surfaces will be used in part II of this paper to investigate finite-temperature effects by mapping them to a Landau - Ginzburg expansion.