Intermediate Tetragonal Phase Research Articles

Role of A and B site doping on antiferrodistortive phase transition in sodium bismuth titanate

We report the results of temperature-dependent powder X-ray diffraction studies on lead-free piezoelectric sodium bismuth titanate (NBT) modified by partially substituting Ba at the A site or Nb at the B site. We observed the disappearance of certain reflections and a change in the main perovskite peaks of the rhombohedral phase suggesting a structural phase transition. Detailed Rietveld refinements revealed that both the ferroelectric and the antiferrodistortive phase transitions occur together at high temperature leading to a change in crystal structure from the low-temperature ferroelectric rhombohedral phase (space group: R3c) to the high-temperature paraelectric cubic phase (space group:Pm3¯m). This suggests the suppression of the intermediate tetragonal phase of pure NBT. We found that substituting Ba onto the A site enhances the lattice strain and lowers the antiferrodistortive phase transition temperature.

Unveiling Nanoscale Compositional and Structural Heterogeneities of Highly Textured Mg0.7Ti0.3Hy Thin Films.

Thin films often exhibit fascinating properties, but the understanding of the underlying mechanism behind such properties is not simple. This is partially because of the limited structural information available. The hurdle in obtaining such information is especially high for textured thin films such as Mg-rich MgxTi1-x, a promising switchable smart coating material. Although these metastable thin films are seen as solid solution alloys by conventional crystallographic methods, their hydrogen-induced optical transition is hardly understood by a solid solution model. In this study, we collect atomic pair distribution function (PDF) data for a Mg0.7Ti0.3Hy thin film in situ on hydrogenation and successfully resolve TiH2 clusters of an average size of 30 Å embedded in the Mg matrix. This supports the chemically segregated model previously proposed for this system. We also observe the emergence of a previously unknown intermediate face-centered tetragonal phase during hydrogenation of the Mg matrix. This phase appears between Mg and MgH2 to reduce lattice mismatch, thereby preventing pulverization and facilitating rapid hydrogen uptake. This work may shed new light on the hydrogen-induced properties of Mg-rich MgxTi1-x thin films.

(Nd/Pr)2NiO4+δ: Reaction Intermediates and Redox Behavior Explored by in Situ Neutron Powder Diffraction during Electrochemical Oxygen Intercalation.

Oxygen intercalation/deintercalation in Pr2NiO4+δ and Nd2NiO4+δ was followed by in situ neutron powder diffraction during electrochemical oxidation/reduction, in a dedicated reaction cell at room temperature. For both systems three phases, all showing the same line width, were identified. The starting phases Pr2NiO4.23 and Nd2NiO4.24, considered with an average orthorhombic Fmmm symmetry, although both show a slight monoclinic distortion, get reduced in a two-phase reaction step to tetragonal intermediate phases with 0.07 ≤ δ ≤ 0.10 and P42/ ncm space group, which on further reduction transform, again in a two-phase reaction step, toward the respective stoichiometric (Pr/Nd)2NiO4.0 phases, with Bmab space group. Electrochemical oxidation does, however, not proceed fully reversibly for both cases: while the reoxidation of Nd2NiO4+δ is limited to the tetragonal intermediate phase with δ = 0.10, the homologous Pr2NiO4+δ can be reoxidized up to δ = 0.17, showing orthorhombic symmetry. For the intermediate tetragonal phase, we were able to establish for Pr2NiO4.09 a complex anharmonic displacement behavior of the apical oxygen atoms, as analyzed by single-crystal neutron diffraction and maximum entropy analysis, in agreement with a low- T diffusion pathway for oxygen ions, activated by lattice dynamics.

An insight into formation mechanism of rapid chemical Co-precipitation for synthesizing yttrium iron garnet nano powders

The rapid chemical co-precipitation method is supposed to be a stable way to synthesize high quality cubic yttrium iron garnet (YIG) nanopowders. In this paper, focusing on clarifying its formation mechanism, the thermal behaviors and the elemental distributions of the chemical co-precipitated precursors have been analyzed by simultaneous differential scanning calorimetry plus thermal gravimetric analysis (DSC-TGA) and energy disperse spectroscopy (EDS) mapping technique on a transmission electronic microscope. The results indicated that the homogeneity of Y and Fe elemental distributions in the precursor powders determines the process through which the cubic YIG phase would form: one experiences the intermediate tetragonal YIG phase; the other takes YFeO3 as the intermediate phase. The homogenous distributions of Y and Fe elements in the precursor powders at the scale below 50 nm forms the intermediate tetragonal YIG phase rather than YFeO3, which leads to reduction of the formation temperature for synthesizing the single phase YIG. The obtained YIG nanopowders with the average particle size below 100 nm show excellent magnetic properties with saturation magnetization of 26 emu/g and coercive field of 45 Oe. This clear formation mechanism of YIG nanopowders is suitable for optimizing the processing conditions for the chemical precipitation synthesis of YIG nanopowders at large scale.

Dielectric Response: Answer to Many Questions in the Methylammonium Lead Halide Solar Cell Absorbers

AbstractDue to the unprecedented rapid increase of their power conversion efficiency, hybrid organic–inorganic perovskites CH3NH3PbX3 (X = I, Br, Cl) can potentially revolutionize the world of solar cells. However, despite tremendous research activity, the origin of the exceptionally large diffusion length of their photogenerated charge carriers, that is, their low recombination rate, remains elusive. Using frequency and temperature‐dependent dielectric measurements across the entire frequency spectrum, it is shown that the dielectric constant conserves very high values (>27) for frequencies below 1 THz in all three halides. This efficiently prevents photocarrier trapping and their recombination owing to the strong screening of charged entities. By combining ultrasonic and Raman spectroscopy with dielectric analysis, similarly large contributions to the dielectric constant are attributed to the dipolar disorder of the CH3NH3 + cations as well as lattice dynamics in the gigahertz range yielding dielectric constants of εstat = 62 for the iodide, 58 for the bromide, and about 45 for the chloride below 1 GHz at room temperature. Disorder continuously reduces for decreasing temperature. Dipole dynamics prevail in the intermediate tetragonal phase. The low‐temperature orthorhombic state is antipolar. No indications of ferroelectricity are found.

Is CH3NH3PbI3 Polar?

In view of the continued controversy concerning the polar/nonpolar nature of the hybrid perovskite system, CH3NH3PbI3, we report the first investigation of a time-resolved pump-probe measurement of the second harmonic generation efficiency as well as using its more traditional form as a sensitive probe of the absence/presence of the center of inversion in the system both in its excited and ground states, respectively. Our results clearly show that SHG efficiency, if nonzero, is below the limit of detection, strongly indicative of a nonpolar or centrosymmetric structure. Our results on the same samples, based on temperature dependent single crystal X-ray diffraction and P-E loop measurements, are entirely consistent with the above conclusion of a centrosymmetric structure for this compound in all three phases, namely the high temperature cubic phase, the intermediate temperature tetragonal phase and the low temperature orthorhombic phase. It is important to note that all our experimental probes are volume averaging and performed on bulk materials, suggesting that basic material properties of CH3NH3PbI3 are consistent with a centrosymmetric, nonpolar structure.

Absence of nematic order in the pressure-induced intermediate phase of the iron-based superconductorBa0.85K0.15Fe2As2

The hole doped Fe-based superconductors Ba$_{1-x}$A$_x$Fe$_2$As$_2$ (where A=Na or K) show a particular rich phase diagram. It was observed that an intermediate re-entrant tetragonal phase forms within the orthorhombic antiferromagnetically-ordered stripe-type spin density wave state above the superconducting transition [S. Avci et al., Nature Comm. 5, 3845 (2014), A. E. B\"ohmer et al., arXiv:1412.7038v2]. A similar intermediate phase was reported to appear if pressure is applied to underdoped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ [E. Hassinger et al., Phys. Rev. B 86, 140502(R) (2012)]. Here we report data of the electric resistivity, Hall effect, specific heat, and the thermoelectrical Nernst and Seebeck coefficients measured on a Ba$_{0.85}$K$_{0.15}$Fe$_2$As$_2$ single crystal under pressure up to 5.5 GPa. The data reveals a coexistence of the intermediate phase with filamentary superconductivity. The Nernst coefficient shows a large signature of nematic order that coincides with the stripe-type spin density wave state up to optimal pressure. In the pressure-induced intermediate phase the nematic order is removed, thus confirming that its nature is a re-entrant tetragonal phase.

Microstructures and Magnetic Properties of Fe–Ga and Fe–Ga–V Ferromagnetic Shape Memory Alloys

The microstructures and magnetic properties of bulk Fe73Ga27 and Fe73Ga18V9 (at.%) ferromagnetic shape memory (FSM) alloys were investigated by transmission electron microscopy (TEM), a magnetostrictive-meter setup, and a superconducting quantum interference device magnetometry. Results showed that in the Fe73Ga27 FSM alloy solution treated (ST) at 1100 °C for 4 h and quenched in ice brine, the antiphase boundary segments (APBs) of the $D0_{3}$ domain were in the A 2 (disordered) matrix and the Fe73Ga27 FSM alloy had an optimal magnetostriction and a higher magnetostrictive susceptibility ( $\Delta \lambda^{s}_{\|} \Delta {H}$ ). In Fe73Ga27 FSM alloy ST, aged at 700 °C for 24 h and furnace cooled (7 HTFC), $D0_{3}$ nanoclusters underwent phase transformation to an intermediate tetragonal phase (i.e., $L1_{0}$ -like tetragonal martensite) via Bain distortion, and finally $L1_{2}$ (Fe3Ga) structures precipitated, as observed by TEM. The $L1_{0}$ -like tetragonal martensite and $L1_{2}$ phases in the aged Fe73Ga27 FSM alloy drastically decreased the magnetostriction from positive to negative. However, by adding V into the Fe73Ga27 alloy system to produce Fe73Ga18V9 FSM alloy ST and ageing at 7 HTFC, the phase transformation of $D0_{3}$ to an intermediate tetragonal martensite phase and precipitation of L 12 structures were not found. The results indicate that the aged Fe73Ga18V9 FSM alloy maintained stable magnetostriction. However, the addition of V into the Fe73Ga27 alloy system to produce Fe73Ga18V9 does not result in a higher saturation magnetization ( $M_{s}$ ) or magnetocrystalline anisotropy energy constant ( $K_{u}$ ) in the Fe73Ga18V9 FSM alloy.

High P–T Raman study of transitions in relaxor multiferroic Pb(Fe0.5Nb0.5)O3

The vibrational and structural properties of Pb(Fe0.5Nb0.5)O3 have been investigated using Raman spectroscopy up to 40 GPa at 300 K and from 300 to 415 K at selected pressures. The measurements reveal three phase transitions, at 5.5, 8.7, and 24 GPa at room temperature. The temperature dependences of the spectra indicate transitions at 1.5 GPa, at 335 and 365 K. The results are consistent with the appearance of an intermediate tetragonal P4mm phase between the ferroelectric R3m and paraelectric Pm‐3m phases. A P–T phase diagram is proposed that allows further insight into the magnetoelectric coupling present in this material. Copyright © 2015 John Wiley & Sons, Ltd.

Effects of phase transformation on the microstructures and magnetostriction of Fe-Ga and Fe-Ga-Zn ferromagnetic shape memory alloys

The phase transformation and magnetostriction of bulk Fe73Ga27 and Fe73Ga18Zn9 (at. %) ferromagnetic shape memory alloys (FSMs) were investigated by transmission electron microscopy (TEM), x-ray diffraction (XRD), and a magnetostrictive-meter setup. For the Fe73Ga27 FSM alloy solution treated at 1100 °C for 4 h and quenched in ice brine, the antiphase boundary segments of the D03 domain were observed in the A2 (disordered) matrix, and the Fe73Ga27 FSM alloy had an optimal magnetostriction (λ‖s = 71 × 10−6 and λ⊥s = −31 × 10−6). In Fe73Ga27 FSM alloy as-quenched, aged at 700 °C for 24 h, and furnace cooled, D03 nanoclusters underwent phase transformation to an intermediate tetragonal phase (i.e., L10-like martensite) via Bain distortion, and finally L12 (Fe3Ga) structures precipitated, as observed by TEM and XRD. The L10-like martensite and L12 phases in the aged Fe73Ga27 FSM alloy drastically decreased the magnetostriction from positive to negative (λ‖s = −20 × 10−6 and λ⊥s = −8 × 10−6). However, in Fe73Ga18Zn9 FSM alloy as-quenched and aged, the phase transformation of D03 to an intermediate tetragonal martensite phase and precipitation of L12 structures were not found. The results indicate that the aged Fe73Ga18Zn9 FSM alloy maintained stable magnetostriction (λ‖s = 36 × 10−6 and λ⊥s = −31 × 10−6). Adding Zn can improve the ferromagnetic shape memory effect of aged Fe73Ga18Zn9 alloy, which may be useful in application of the alloy in high temperature environments.

Shape memory deformation mechanisms of Ru–Nb and Ru–Ta shape memory alloys with transformation temperatures

Ru-based high temperature shape memory alloys show different structures at different temperatures and composition. Equiatomic Ru–Ta and Ru–Nb show two subsequent phase transformations, but their individual influence is still unclear because of the difficulty of comparing the high temperature cubic phase, the intermediate tetragonal phase and the low temperature monoclinic phase. It is important to find a way to compare these structures in order to better understand the microstructure of these alloys and their shape memory behaviour. In order to get the necessary information the alloys have been investigated with neutron diffraction technique during in-situ heating up to about 1100 °C.This study shows the evolution of the lattice parameters and the shape of the unit cells at different temperatures for the three phases these materials exhibit. It proposes a way to compare the different structures and it gives a mathematical expression of the phase transformations. The ultimate goal of this work is to enable a better understanding of the deformation mechanisms in the unit cell and a possible anticipation of the existence of a shape memory effect in these systems.

The improvement of dispersibility of YIG precursor prepared via chemical coprecipitation

To prevent YIG (Y3Fe5O12) precursor from agglomeration in the chemical coprecipitation process, the relationship between YIG precursor’s surface electric charge and its dispersibility was studied through the combination of electrophoresis and SEM test. The thermochemical characteristics of YIG precursor with improved dispersibility were also investigated. The results showed: when the coprecipitation pH is smaller than 10.5, the YIG precursor is positively charged which is in favor of YIG precursor’s good dispersibility. The YIG precursor coprecipitated at pH of 9.5 has the least agglomeration due to its most positive surface electric charge. The effect of three typical surfactants viz., SDS (anionic surfactant), CATB (cationic surfactant) and p123 (non-ionic surfactant) on the surface electric charge of YIG precursor was studied. The CATB surfactant is found to be superior to the other two surfactants in the aspect of improving YIG precursor’s dispersibility. The YIG precursor with improved dispersibility has superior homogeneity. The crystallization of cubic YIG phase can be initiated directly from amorphous YIG precursor just below 758°C and completed above 758°C, without the formation of other intermediate phase and tetragonal YIG phase, which are often involved in the preparation of YIG or doped-YIG by applying chemical coprecipitation method.

Temperature–field phase diagrams in Pb(Zn1/3Nb2/3)O3–4.5%PbTiO3 II

Temperature and field dependences of the dielectric constants under the DC biasing fields along the [011]- and [111]-directions in the cubic coordinate in Pb(Zn1/3Nb2/3)O3–4.5%PbTiO3 were investigated. The temperature–field phase diagrams were constructed in the field range below 10kV/cm. It was confirmed that in Pb(Zn1/3Nb2/3)O3–4.5%PbTiO3 the intermediate tetragonal phase as a ground state of the system exists even without the DC field, and the tetragonal phase disappears in the external field above 4 and 3kV/cm along the [011]- and [111]-directions, respectively. The field-induced orthorhombic-phase in the field along the [011]-direction was also found.

Uniaxial versus hydrostatic pressure-induced phase transitions in CaFe2As2and BaFe2As2

We present uniaxial pressure structural relaxations for CaFe2As2 and BaFe2Ase2 within density functional theory and compare them with calculations under hydrostatic pressure conditions as well as available experimental results. We find that CaFe2As2 shows a unique phase transition from a magnetic orthorhombic phase to a nonmagnetic collapsed tetragonal phase for both pressure conditions and no indication of a tetragonal phase is observed at intermediate uniaxial pressures. In contrast, BaFe2As2 shows for both pressure conditions two phase transitions from a magnetic orthorhombic to a collapsed tetragonal phase through an intermediate nonmagnetic tetragonal phase. We find that the critical transition pressures under uniaxial conditions are much lower than those under hydrostatic conditions manifesting the high sensitivity of the systems to uniaxial stress. We discuss the origin of this sensitivity and its relation to superconductivity.

Temperature-Field Phase Diagrams in Pb(Zn1/3Nb2/3)O3-4.5%PbTiO3

Temperature dependence of the dielectric constants under the DC biasing fields along the [001]- and [011]-directions in the cubic coordinate was measured in Pb(Zn1/3Nb2/3)O3- 4.5%PbTiO3. The temperature-field phase diagrams were clarified in the field range below 15.0 kV/cm. The existence of the critical end point at 155°C and 1.2 kV/cm was confirmed under the electric field along the [001]-direction. The stable orthorhombic phase was found in the field along the [011]-direction. It was confirmed that in Pb(Zn1/3Nb2/3)O3- 4.5%PbTiO3 the intermediate tetragonal phase can exist in the absence of the DC field.

Unconventional mechanism of stabilization of a tetragonal phase in the perovskite ferroelectric(PbTiO3)1−x(BiFeO3)x

Temperature-dependent x-ray powder-diffraction study of the tetragonal compositions of ${\text{PbTiO}}_{3}{\text{-BiFeO}}_{3}$ series has revealed that, unlike for all the known ferroelectric perovskites, the compositions exhibiting giant tetragonality is stabilized from the cubic phase via a complex transition pathway which involve (i) formation of minor monoclinic phase with a large pseudotetragonality along with an intermediate tetragonal phase (major) with a small tetragonality, (ii) gradual vanishing of the intermediate tetragonal phase and concomitant increase in the monoclinic regions, and finally (iii) gradual transformation of the monoclinic phase to the tetragonal phase with giant tetragonality. The system seems to adopt such a complex transition pathway to create a microstructure with very large number of domains and interfaces for stress relief, which would not have been possible in case of a direct cubic-tetragonal transition.

A primitive tetragonal intermediate in the orthorhombic–cubic phase transition of perovskite-type strontium niobate Sr 0.92NbO 3

The Sr deficient perovskite Sr 0.92NbO 3 was synthesized from Sr 5Nb 4O 15 and Nb and its crystal structure was determined using powder neutron diffraction. At room temperature the structure is orthorhombic in space group Pnma with both in-phase and out-of-phase tilting of the NbO 6 octahedra. High temperature measurements have shown that the oxide undergoes a sequence of phase transitions with increasing temperature: Pnma→ P4/ mbm→ Pm3¯ m. The intermediate tetragonal phase has only in-phase tilts of the NbO 6 octahedra, rather than the out-of-phase tilts present in the more commonly observed I4/ mcm structure, due to initial softening at the M point rather than R point. The tetragonal phase exists only over a very narrow temperature range. The importance of M– M and M–O bonding in controlling the transition temperatures in Sr MO 3 perovskites is discussed.

Stability of phases in (Ba, Gd)MnO 3 solid solution system

The existing phases in Ba xGd 1-xMnO 3 solid solution system (0 ≦ x ≦ 1) were studied by analyzing the detailed crystal structure of each composition from the results of the Rietveld method using powder X-ray diffraction data. For a small substitution of Ba for Gd (0 ≦ x < 0.1), the orthorhombic phase with a perovskite type structure ( Pnma space group) was stably formed and this fact was supported by the electron diffraction data. There existed an intermediate phase of Ba 0.33Gd 0.67MnO 3, which was characterized as the tetragonal phase with perovskite structure. The composition range of this phase was narrow and almost line compound. Between the regions of these phases, there existed two-phase region. There was also a two-phase region between the intermediate tetragonal phase and BaMnO 3. Measurement of electrical conductivities of these orthorhombic solid solutions and tetragonal phases showed semiconducting behaviors for both phases and the existence of the phase transition at high temperature for the orthorhombic phase. The transition temperature decreased as the Ba content increased.

Pressure-Driven Phase Transitions in NaBH4: Theory and Experiments

We have investigated pressure-induced structural transitions in NaBH4 through density-functional theory calculations combined with X-ray and neutron diffraction experiments. Our calculations confirm that the cubic phase is stable up to 5.4 GPa and an orthorhombic phase occurs above 8.9 GPa, as observed in X-ray diffraction experiments. Both the calculations and X-ray diffraction measurements identify an intermediate tetragonal phase that appears between 6 and 8 GPa; that is, between the cubic and orthorhombic phases. This result is also confirmed by high-pressure neutron diffraction experiments performed on NaBD4. Our calculations and X-ray diffraction measurements show that the space group of the orthorhombic phase above 8.9 GPa is Pnma and the orthorhombic phase remains stable up to 30 GPa. The calculated equations of state are in excellent agreement with experiments.

Crystal structure and phase transitions of [formula omitted

The crystal structure of Sr 2 CdWO 6 , prepared by solid state reaction, was determined by high-resolution X-ray diffraction at different temperatures. At room temperature, this compound has a monoclinic structure (space group P 2 1 / n ) with a = 5.7463 ( 1 ) , b = 5.8189 ( 1 ) , c = 8.1465 ( 1 ) , β = 90.071 ( 1 ) . At 1105 K the structure is converted to tetragonal (space group I 4 / m ). Diffraction data also suggest that a cubic phase exists above 1220 K. Comparing the phase transition temperatures of Sr 2 CdWO 6 with those of other compounds of the Sr 2 MWO 6 family reported previously, it was observed that the transition temperatures are higher in compounds with low-tolerance factors. At the same time, the temperature range in which the intermediate tetragonal phase exists is reduced.