Hydrothermal Synthesis and Phase‐, Morphology‐Evolution Mechanism of Lead Titanate Nanostructures Assisted with Tetramethylammonium Hydroxide

Lead titanate nanostructures with different phases and morphologies, layered hexagonal PbTiO₂(CO₃)_0.3‐ (NO₃)_0.35(OH) nanosheets, pyrochlore Pb₂Ti₂O₆ nanodendites, pre‐perovskite PbTiO₃ nanofibres and perovskite PbTiO₃ nanoplates, have been synthesized via a conventional hydrothermal route assisted with different concentrations of tetramethylammonium hydroxide (TMAH). X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high‐resolution TEM (HRTEM) were employed to characterize the phase, morphology and growth behavior of the synthesized samples. The results reveal that at low TMAH concentration the obtained samples are mainly of PbTiO₂(CO₃)_0.3(NO₃)_0.35(OH) nanosheets. With the TMAH concentration increasing, the obtained samples change to pyrochlore Pb₂Ti₂O₆ nanodendites, pre‐perovskite PbTiO₃ nanofibres and perovskite PbTiO₃ nanoplates in turn. With the basis of the experimental results, the phase‐ and morphology‐evolution mechanism of the lead titanate nanostructures is discussed by combining the analysis of the lattice structure feature and the properties of TMAH.     

Amorphous and nanocrystalline titanium nitride and carbonitride materials obtained by solution phase ammonolysis of Ti(NMe2)4

Solution phase reactions between tetrakisdimethylamidotitanium (Ti(NMe₂)₄) and ammonia yield precipitates with composition TiC_0.5N_1.1H_2.3. Thermogravimetric analysis (TGA) indicates that decomposition of these precursor materials proceeds in two steps to yield rocksalt-structured TiN or Ti(C,N), depending upon the gas atmosphere. Heating to above 700 °C in NH₃ yields nearly stoichiometric TiN. However, heating in N₂ atmosphere leads to isostructural carbonitrides, approximately TiC_0.2N_0.8 in composition. The particle sizes of these materials range between 4-12 nm. Heating to a temperature that corresponds to the intermediate plateau in the TGA curve (450 °C) results in a black powder that is X-ray amorphous and is electrically conducting. The bulk chemical composition of this material is found to be TiC_0.22N_1.01H_0.07, or Ti₃(C_0.17N_0.78H_0.05)_3.96, close to Ti₃(C,N)₄. Previous workers have suggested that the intermediate compound was an amorphous form of Ti₃N₄. TEM investigation of the material indicates the presence of nanocrystalline regions <5 nm in dimension embedded in an amorphous matrix. Raman and IR reflectance data indicate some structural similarity with the rocksalt-structured TiN and Ti(C,N) phases, but with disorder and substantial vacancies or other defects. XAS indicates that the local structure of the amorphous solid is based on the rocksalt structure, but with a large proportion of vacancies on both the cation (Ti) and anion (C,N) sites. The first shell Ti coordination is approximately 4.5 and the second-shell coordination ∼5.5 compared with expected values of 6 and 12, respectively, for the ideal rocksalt structure. The material is thus approximately 50% less dense than known Ti_x(C,N)_y crystalline phases.     

Heterometallic lead-titanium oxyalkoxide precursors to lead titanate thin films

Two lead titanium oxyalkoxides with composition Pb₂Ti₄O₂(O₂CCH₃)₂(OC₂H₅)₁₄ and Pb₂Ti₂O₂(O₂CCH₃)₂(OCH(CH₃)₂)₈ have been isolated and characterised by elemental analysis, IR, ²⁰⁷Pb NMR and molecular weight measurements. X-ray structural analysis of the 1:2 complex confirmed the presence of two Pb₂Ti₂O units which are linked by a common Pb···Pb edge and are held together by alkoxide and acetate bridges. The acetate groups have migrated from the Pb to the Ti centres during the reaction. Hydrolysis of the 1:1 Pb/Ti complex produced clear gels providing the H₂O/complex ratio was less than 4. Heat treatment of the gel results in loss of residual organics below 400°C. The XRD pattern indicates the presence of the pyrochlore and perovskite phases after heating at 500°C but the sample is poorly crystalline. Complete conversion to the perovskite phase of lead titanate occurs on heating to 600°C for 1 hour. Thin films of lead titanate were deposited by dip-coating a solution of this complex in isopropanol. Analyses of the films, carried out using electron microprobe, Scanning Auger Spectroscopy and Rutherford Backscattering, indicated that they were of excellent quality, crystalline after heating at 600°C and with relatively sharp substrate-coating interface.     

Structural Order–Disorder Transitions in Ln2Ti2O7 (Ln = Lu,Gd)

The phase generation in the Lu(Gd)–Ti–O systems is studied at 20–1000° using a co-precipitation method. During a thermal treatment of co-precipitation products after a sublimation dehydration, for a composition with the Lu : Ti cation ratio of 1 : 1, an Lu₂Ti₂O₇ phase with a fluorite structure forms at 650°. At 730–750°C the phase undergoes a fluorite pyrochlore transition. Above 750°C its structure is that of disordered pyrochlore, in which antistructural defects occur in Lu and Ti positions (up to 18%). Above 900°C the structure of pyrochlore becomes ordered, and the number of defects in Lu and Ti positions decreases, which affects the temperature dependence of permittivity of Lu₂Ti₂O₇. In Gd–Ti–O system, Gd₂Ti₂O₇ is crystallized, which has a pyrochlore structure only at 740–900°. Electroconductivity and permittivity of Lu₂Ti₂O₇ and Gd₂Ti₂O₇ are measured.     

O NMR studies of local structure and phase evolution for materials in the Y2Ti2O7-ZrTiO4 binary system

¹⁷O MAS NMR and XRD studies of precursor-derived Y_1.6Zr_0.4Ti₂O_7.2 and Y_1.2Zr_0.8Ti₂O_7.4 have been performed to investigate the development of local and long-range order in these materials as they evolve from a metastable amorphous state upon heating. Zirconium titanate (ZrTiO₄) was also investigated to help interpret the ¹⁷O NMR spectra of the ternary compositions. Consistent with earlier studies, crystallization was observed at 800 °C to form a fluorite structure and a small amount of rutile; weak broad reflections were also observed which were ascribed to the presence of small pyrochlore-like ordered domains or particles within the fluorite phase. As the temperature was increased further, the sizes of these domains grew along with the concentration of rutile. At the highest temperature studied (1300 °C), the reflections of the thermodynamic phases, pyrochlore and zirconium titanate (ZrTiO₄), dominated the XRD pattern. The ¹⁷O NMR spectra revealed a series of different peaks that were assigned to different 3- and 4-coordinate O local environments. The data were consistent with the formation of a metastable phase Y_2−xZr_xTi_2−yZr_yO_7+x with pyrochlore-like ordering but with Zr substitution on both cation sites of the pyrochlore structure. At low temperatures, doping on the A (Y³⁺) sites predominates (i.e., x>y), consistent with the fact that the pyrochlore develops out of a more disordered fluorite-like, phase. As the temperature is raised, the Zr doping on the A site decreases and the metastable phase at this temperature can now be written as Y_2−x′Zr_x′Ti_2−y′Zr_y′O_7+x′ (i.e., x′<y′); TiO₂ is also observed, consistent with this suggestion. At high temperatures, doping on the B site decreases and the resonances due to the stoichiometric pyrochlore yttrium titanate (Y₂Ti₂O₇) dominate the NMR spectra. Weaker ¹⁷O NMR resonances due zirconium titanate (ZrTiO₄) are also observed.     

Structural manipulation of pyrochlores: Thermal evolution of metastable Gd2(Ti1−yZry)2O7 powders prepared by mechanical milling

The structural and microstructural characteristics of metastable Gd₂(Ti_1−yZr_y)₂O₇ powders prepared by mechanical milling have been studied by a combination of XRD and Raman spectroscopy. Irrespective of their Zr content, as-prepared powder phases present an anion-deficient fluorite-type of structure as opposed to the pyrochlore equilibrium configuration obtained for the same solid solution by other synthetic routes. These fluorites are stable versus thermal activation, at least up to temperatures of 800 °C. For the Ti-rich compositions, thermal treatments at higher temperatures facilitate the rearrangement of the cation and anion substructures and the relaxation of mechanochemically induced defects whereas for compositions with high Zr content, the fluorite crystal structure is retained even at temperatures as high as 1200 °C. Interestingly enough, transient pyrochlores showing a very unusual cation distribution were observed during the thermally induced defect-recovery process.     

Effect of vapor thermolysis on microstructure development of sol–gel-derived lead zirconate–titanate (PZT) and PZT-Sr(K<Subscript>0.25</Subscript>Nb<Subscript>0.75</Subscript>)O<Subscript>3</Subscript> solid solutions

Ceramic granules of PZT (Pb(Zr₅₃,Ti₄₇)O₃) and fibers of SKN-doped PZT (1 Mol-% Sr(K_0.25Nb_0.75)O₃ substitution in PZT) were fabricated from sol–gel precursors. The transformation of the metal–organic gel to the oxide and its crystallization behavior was investigated by thermogravitmetry, and XRD. The formation of pyrochlore or perovskite phase was sensitive to the preceeding heat treatment conditions in moist atmosphere (termed vapor thermolysis). Lower thermolysis temperatures resulted in the preferred formation of metastable pyrochlore phase. Higher thermolysis temperatures favored the formation of perovskite. Investigation of sintered materials revealed a superior sintering activity for samples with pronounced intermediate pyrochlore content whereas low-pyrochlore samples remained highly porous after sintering. For constant chemical precursors the vapour thermolysis conditions (220–280 °C) siginificantly affected crystallization and sintering behavior of the PZT and SKN-PZT material.     

Comparison of the force field in various pyrochlore families. II. Phases presenting structural defects

A previous study of various A₂B₂O₆O′ pyrochlore families by vibrational spectroscopy allowed the analysis of the ir and Raman spectra and the force fields of pyrochlore phases presenting structural defects. The spectra of the following compounds were compared: (i) ideal pyrochlore Cd₂Ta₂O₇, (ii) lacunary Tl₂Ta₂O₆, (iii) nonstoichiometric Pb_1.5Ta₂O_6.5, and (iv) Pb_2.3Ta₂O_7.3 exhibiting regular shear planes. If the tridimensional network of TaO₆ octahedra is not modified and the perturbations concern only the A₄O′ tetrahedra network, a slight modification of the vibrational spectra and a weak decrease in the TaO stretching and OTaO bending force constants are observed. But, if the octahedra network is perturbed, large modifications appear on the spectra, revealing unambiguously the existence of new types of bonds created by the defects in the structure. The Raman intense lines observed in the low frequency range in lead compounds is also discussed.     

Effect of biphase on dielectric properties of Bi-doped lead strontium titanate thin films

Pb_0.4Sr_0.6TiO₃ (PST) thin films doped with various concentration of Bi were prepared by a sol-gel method. The phase status, surface morphology and dielectric properties of these thin films were measured by X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance analyzer, respectively. Results showed that the thin films with the maximum dielectric constant and minimum dielectric loss were obtained for x=0.15. For x<0.15, only pure PST perovskite phase were in the thin films. For 0.2<x<0.4, the PST/Bi₂Ti₂O₇ biphase were obtained. The thin films with pure Bi₂Ti₂O₇ pyrochlore phase were obtained for x=0.67. The biphase thin films had high tunability and high figure of merit (FOM). The FOM of PST/Bi₂Ti₂O₇ biphase thin film was about 6 times higher than that thin films formed with pure perovskite phase or pure pyrochlore phase.     

Study of the fluorite–pyrochlore–fluorite phase transitions in Ln<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript> (Ln=Lu,Yb, Tm)

The ordering processes in Ln₂Ti₂O₇ (Ln=Lu, Yb, Tm) are studied by X-ray diffraction, thermal analysis, infrared absorption (IR) spectroscopy, and electrical conductivity measurements. The coprecipitation method followed by freeze-drying was used for Ln₂Ti₂O₇ synthesis. The region of low-temperature fluorite phase existence is 600 °C<T<740 °C. The low-temperature fluorite–pyrochlore phase transition in Ln₂Ti₂O₇ takes place at ~740–800 °C. Ln₂Ti₂O₇ (Ln=Lu, Yb, Tm) have the structure of disordered pyrochlore with antisite Ln–Ti defects at 800 °C<T<1,100 °C.The high-temperature pyrochlore–fluorite transformation takes place in Tm₂Ti₂O₇, Yb₂Ti₂O₇, and Lu₂Ti₂O₇ in air at T>1,600 °C. The conductivity values are 5·10^–3 S/cm for Tm₂Ti₂O₇, 6·10^–3 S/cm for Yb₂Ti₂O₇, and 10^–2 S/cm for Lu₂Ti₂O₇ at 740 °C. This order–disorder transition leads to a 2 orders of magnitude conductivity growth and a 10–30 times permittivity increase in Ln₂Ti₂O₇ samples obtained at 1,700 °C.Presented at the OSSEP Workshop Ionic and Mixed Conductors: Methods and Processes, Aveiro, Portugal, 10–12 April 2003     

Influence of Preparation Conditions on Crystallization Behavior of the Complex Alkoxide Derived PbTiO3 Powder

PbTiO₃ powders were prepared by hydrolysis of the complex alkoxide derived from Pb(OCOCH₃)₂ and Ti(OCH₂CH₃)₄, and their crystallization behaviors were investigated by DTA. Influence of conditions of synthesis and hydrolysis of the Pb-Ti complex alkoxide and heating atmosphere on crystallization behavior of powder was examined.Powders consisting of spherical particles with submicron diameter could be obtained by using the reaction time over 16 hr for synthesis, and the use of acetone-ethanol solvent with ammonia catalyst for the hydrolysis of the complex alkoxide. The resultant powders crystallized to tetragonal PbTiO₃ perovskite above 250°C under O₂ flow, and then transformed to cubic perovskite at 490°C. The powder heated at 250°C for 1 hr in O₂ consisted of well-crystallized tetragonal perovskite crystals of cubic shape of 0.2 to 1 µm in size.     

Effect of Li2O Addition on the Preparation of (Y2‐yLiy)Ti2O7‐y

A new series of (Y_2‐yLi_y)Ti₂O_7‐y having an ordered pyrochlore phase was prepared by a solid state reaction method with a solid solution range of 0.05 ≥ y ≥ 0.10. Unit cell parameters obtained by the Rietveld refinement method shows that the a‐axis decreases linearly with increasing the amount of Li ion addition, indicating the successful incorporation of the Li ion into unit cell. The average x‐fractional coordinate of the O(1) site depends on the ionic radius ratio of r(A³+)/r(Ti⁴⁺) in the A₂Ti₂O₇ with a pyrochlore phase. The Ti K‐edge XANES spectra of the (Y_2‐yLi_y)Ti₂O_7‐y show that the valence of the Ti ions is slightly less than 4 so that Ti is in the mixed valence state. Average particle size increases with increasing the amount of extra Li ion addition, which acts as a flux to lower the melting point of the materials.     

Synthesis of transparent mixed vanadia/niobia gels and their decomposition to a new metastable VNb9O25 phase

Controlled hydrolysis and condensation of a mixture of vanadyl-tris-n-propoxide, VO(OPr)₃, and niobium pentaethoxide, [Nb(OEt)₅]₂, at 5 °C in propanol yields clear and transparent gels in which the ratio of V:Nb is 1:1, 1:4.5 or 1:9. Oxalic acid and low temperatures are used to slow down the rate of condensation processes. At 800 °C, the thermal decomposition of a gel with the composition 1:9 forms a thermodynamically metastable, new phase of the composition VNb₉O₂₅. At lower temperatures, metastable solid solutions with TT-Nb₂O₅ structure (600 °C) and M-Nb₂O₅ structure (700 °C) are formed from the amorphous xerogel. The new VNb₉O₂₅ phase is structurally related to M-Nb₂O₅. The solid solution with M-Nb₂O₅ structure acts structure directing, leading preferentially to a monoclinic low-temperature form of VNb₉O₂₅. The full transformation of this metastable phase to the well known tetragonal VNb₉O₂₅ requires a annealing temperature of about 1000 °C.     

Pyrochlore “dynamic spin-ice” Pr2Sn2O7 and monoclinic Pr2Ti2O7: A comparative temperature-dependent Raman study

Here we report a temperature-dependent Raman study of the pyrochlore “dynamic spin-ice” compound Pr₂Sn₂O₇ and compare the results with its non-pyrochlore (monoclinic) counterpart Pr₂Ti₂O₇. In addition to phonon modes, we observe two bands associated with electronic Raman scattering involving crystal field transitions in Pr₂Sn₂O₇ at ∼135 and 460 cm⁻¹ which couple strongly to phonons. Anomalous temperature dependence of phonon frequencies that are observed in pyrochlore Pr₂Sn₂O₇ are absent in monoclinic Pr₂Ti₂O₇. This, therefore, confirms that the strong phonon-phonon anharmonic interactions, responsible for the temperature-dependent anomalous behavior of phonons, arise due to the inherent vacant sites in the pyrochlore structure.     

Phase relations in NaxCrxTi8−xO16 at 1350 °C and crystal structure of hollandite-like Na2Cr2Ti6O16

Phase relations of rutile, freudenbergite, and hollandite structures were examined in the pseudobinary system NaCrO₂-TiO₂ (i.e., Na_xCr_xTi_8−xO₁₆) at 1350 °C. The hollandite structure was obtained in the composition range 1.7?x?2.0. The symmetry of the samples at room temperature was tetragonal for x=1.7 and 1.75, and monoclinic for x=1.8 and above. Single crystals of monoclinic hollandite Na₂Cr₂Ti₆O₁₆ were grown and the structure refinement has been carried out using an X-ray diffraction technique. The space group was I2/m and cell parameters were a=10.2385(11), b=2.9559(9), c=9.9097(11)Å, and β=90.545(9)° with Z=1. The Na ion distribution in the tunnel was markedly deformed from that in the tetragonal form. It was suggested that Cr/Ti ratios were different between the two framework metal sites.     

Structural Behavior of the Four-Layer Aurivillius-Phase Ferroelectrics SrBi4Ti4O15 and Bi5Ti3FeO15

Rietveld refinement of powder neutron diffraction data has been used to study the crystal structures of the four-layer Aurivillius-phase ferroelectrics Bi₅Ti₃FeO₁₅ (at 25°C) and SrBi₄Ti₄O₁₅ (at a series of temperatures up to 800°C). At 25°C both materials adopt the polar orthorhombic space group A2₁am, in common with two-layer analogues such as SrBi₂Ta₂O₉. At temperatures well above the ferroelectric Curie temperature (i.e., at temperatures of 650°C and above, with T_c∼550°C) SrBi₄Ti₄O₁₅ transforms to the centrosymmetric tetragonal space group I4/mmm. However, there is good evidence from the raw diffraction data of a very subtle intermediate paraelectric orthorhombic phase, of Amam symmetry, in the region 550>650°C. The distortion in the ferroelectric phase can be traced to displacements of the cations in the A site of the perovskite block, with cooperative tilting of the BO₆ octahedra. The nature of the octahedral tilt system, cation disorder at the perovskite A and B sites, and the phase transition sequence in SrBi₄Ti₄O₁₅, which parallels that found in SrBi₂Ta₂O₉, are discussed.     

Structural changes of (K,Gd)2Ta2O7 pyrochlore at high pressure

The structure of K-bearing tantalate pyrochlore (K_2-xGd_x)Ta₂O_6+x(x∼0.4) was studied at high pressures using in situ X-ray diffraction and Raman scattering methods. Experimental results indicated that (K_2-xGd_x)Ta₂O_6+x(x∼0.4) retains the pyrochlore structure up to 40 GPa, but partial amorphization occurred at pressures above 23 GPa. The amorphous phase was also confirmed in the quenched sample by means of transmission electron microscopy. The tantalate pyrochlore lattice is more stable than pyrochlore compounds in other systems, such as rare earth titanates, zirconates and stannates. The structural stability of pyrochlore tantalate may be mainly related to the size ratio of cations on the 16d and 16c sites in the lattice.     

Long- and short-range order in stuffed titanate pyrochlores

We report a structural study of the stuffed pyrochlore series Ln₂(Ti_2−xLn_x)O_7−x/2 (Ln=Ho, Yb; 0?x?0.67). Electron microscopy and Rietveld refinements of neutron powder diffraction data for the x=0.67 end members, Ho₂TiO₅ and Yb₂TiO₅, reveal that small domains (∼50 Å or less) exist where the Ln and Ti/Ln sublattices are pyrochlore like, while the average structure is fluorite like. Both the Ho and Yb stuffed pyrochlore series for 0.1?x?0.5 are shown to be a composite of long- and short-range-ordered pyrochlore phases. The relative fraction of long-range vs. short-range pyrochlore order decreases with increasing Ln doping. An additional complex structural modulation of the pyrochlore structure is observed in electron diffraction and high-resolution electron microscopy images.     

Structural phase transitions in the relaxor ferroelectric Pb2Bi4Ti5O18

The relaxor ferroelectric Pb₂Bi₄Ti₅O₁₈ has been studied by Rietveld refinement of powder neutron diffraction data collected at temperatures of 100, 250 and 400 °C. Our refinements are compatible with the ‘average’ crystal structure of Pb₂Bi₄Ti₅O₁₈ undergoing the phase transition sequence F2mm→I4mm→I4/mmm as a function of increasing temperature, with the latter phase being observed above the known ferroelectric Curie temperature, T_m, and the intermediate phase consistent with a previously observed dielectric anomaly around 207 °C. The results are, however, in conflict with both observation of a symmetry lowering (to space group B2eb) in the lowest temperature phase, observed by electron diffraction, and also with electrical property measurements, which suggest both a- and c-axis polarisation up to T_m. Nevertheless, these crystallographic results are consistent with the observation of relaxor behaviour in this material, and underline the importance of considering ‘long-range’ versus ‘local’ structural effects in relaxor materials.     

Direct crystallization of perovskite phase in PMN-PT thin films prepared by polyvinylpyrrolidone modified sol-gel processing and their properties

A modified sol-gel processing has been developed by using polyvinylpyrrolidone (PVP) as modifier and lead nitrate as lead source to synthesize (1−x)Pb(Mg_1/3,Nb_2/3)O₃-xPbTiO₃ (PMN-PT) thin films with x=0.23-0.43. With PVP additions, perovskite phase could directly crystallize from amorphous films at the temperature as low as 430 °C via bypassing the metastable phase-pyrochlore and crystallinity was significantly enhanced. The PVP addictives have been optimized with molecular weight <630 K and the ratio of PVP monomer/PMN-PT at 0.25-1.0. XPS analysis indicates that the chemical states of the elements in the well-crystallized PMN-PT films are close to the literature data for the PMN-PT single crystals and the films possess highly desired electrical and optical properties.