Orthorhombic Phase Transition Research Articles

Evidence for phase transitions and pseudospin phonon coupling in K0.9(NH4)0.1H2AsO4

This work reports a detailed temperature dependent Raman study on the mixed crystals of K0.9(NH4)0.1H2AsO4 (KADA) from 5K to 300K in the spectral range of 60-1200cm−1, covering tetragonal to orthorhombic structural phase transition accompanied by paraelectric to ferroelectric transition at Tc* ∼ 60K. Multiple phase transitions below transition temperature (Tc* ∼ 60K) are marked by the appearance of new modes, splitting of existing ones as well as anomalies in the self-energy parameters (i.e. mode frequencies and damping coefficient) of the phonon modes. Temperature independent behaviour of damping coefficient and abrupt jump in the mode frequency of some of the internal vibrations of AsO4 tetrahedra as well as external vibrations clearly signal long range ferroelectric ordering and proton ordering below Tc*. In addition, we observed that temperature dependence of many prominent phonon modes diverges significantly from their normal anharmonic behaviour below Tc* suggesting potential coupling between pseudospins and phonons.

Structural and luminescence properties of Gd2Si2O7:Ce prepared by solution combustion followed by heat treatment

A method comprising solution combustion followed by a heat treatment has been employed to synthesize cerium doped gadolinium pyrosilicate (Gd2Si2O7:Ce, or GPS:Ce) compounds. The powder obtained after the combustion was annealed at 1200°C for 4h and 1600°C for 3h to synthesize triclinic and orthorhombic phases of the GPS, respectively. Structural and morphological characterizations of the synthesized compounds were carried out using X-ray diffraction and electron microscopy (SEM, TEM) techniques. A change in the enthalpy was observed in the differential thermal analysis data as a consequence of triclinic to orthorhombic phase transition in the GPS. Luminescence spectra and fluorescence decay time were measured at room temperature to characterize emission centers created in GPS compounds doped with trivalent rare earth ion (Ce3+). The triclinic GPS:Ce phase exhibited photoluminescence peaks at 379nm and 410nm while for the orthorhombic phase emissions at 353nm and 380nm were observed. A multi-component exponential decay pattern of the luminescence is observed for both the GPS:Ce phases. In addition, samples of the orthorhombic GPS:Ce were found to exhibit X-ray excited luminescence (XEL).

Chemically Grown MoO3 Nanorods for Antibacterial Activity Study

In the present investigation, sea urchin like morphology of h-MoO3 nanorods are successfully synthesized by chemical bath deposition (CBD) technique. The thermal stability, structural details, morphology and compositional analysis of MoO3 was done using thermogravimetry (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), selected area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS) techniques respectively. The thermal analysis reveals presence of sharp exothermic peak at 409oC indicating irreversible phase transition. X-ray diffraction pattern showed hexagonal to orthorhombic phase transition after annealing at 450oC. As synthesized h-MoO3 shows well oriented hexagonal rods with sea urchin like architecture while that of annealed MoO3 sample revealed 2D layer by layer growth. The SAED pattern confirms single crystalline nature of as synthesized h-MoO3 and polycrystalline nature of annealed α-MoO3. While XPS study of both confirms Mo+6 and O2- oxidation states of elements. Furthermore, characteristic antibacterial properties of h-MoO3 and α- MoO3 against gram positive Bacillus megaterium, Streptococcus aureus and gram negative Escherichia coli is noted.

Phase transition and piezoelectricity of sol–gel-processed Sm-doped BiFeO3 thin films on Pt(111)/Ti/SiO2/Si substrates

We prepared high-quality Bi1−xSmxFeO3 films on Pt(111)/Ti/SiO2/Si substrates by sol–gel processing and found rhombohedral–orthorhombic phase transition with enhanced piezoelectricity.

The Development of Pseudocapacitive Properties in Nanosized-MoO2

Pseudocapacitive charge storage materials offer the opportunity to bridge the gap between high energy density battery materials and high power density electrical double layer capacitor materials through the rational design of transition metal oxide nanoscale architectures. The research reported in this paper describes the origins and development of pseudocapacitance in MoO2. Micron-size particles of MoO2 exhibit a reversible monoclinic to orthorhombic phase transition upon lithium insertion/deinsertion, however, this phase transformation is suppressed when using 15 nm nanocrystals of MoO2. The nanoscale MoO2 exhibits pseudocapacitive behavior and achieves substantially better energy storage kinetics than the corresponding bulk material. Such size-dependent electrochemical behavior is an essential feature of an extrinsic pseudocapacitor material. The high power capability of nanoscale MoO2 is improved further by synthesizing hybrid materials in which MoO2 nanoparticles are grown on reduced graphene oxide (RGO) scaffolds. Electrode architectures containing MoO2-RGO hybrid materials preserve the pseudocapacitance of MoO2 as lithium capacities of nearly 150 mAh g−1 are obtained at a rate of 50 C.

Thickness-dependent phase boundary in Sm-doped BiFeO3 piezoelectric thin films on Pt/Ti/SiO2/Si substrates.

Sm-doped BiFeO3 thin films were fabricated on platinized silicon substrates via a sol-gel method. Sm contents and thicknesses were varied in a wide range to investigate their effects on the phase structure and piezoelectricity. X-ray diffraction and Raman spectroscopy experiments revealed a rhombohedral to orthorhombic phase transition and the co-existence of both phases in a certain compositional vicinity. It is found that the proportion of a rhombohedral phase increased with film thickness at the compositions corresponding to the phase transition boundary, indicating the influence of the film thickness on the phase structure. The phase transition phenomenon and film thickness effect on the boundary were also studied by piezoresponse force microscopy. Based on the structure analysis and piezoelectric characterization results, a phase diagram of thickness versus composition was proposed, in which the morphotropic phase boundary was located at 9% to 11% in thinner Sm-doped films and shifted towards the Sm-rich side with increasing thickness.

Interface control of a morphotropic phase boundary in epitaxial samarium modified bismuth ferrite superlattices

Interfacial control of a polar-(rhombohedral) to-non-polar (orthorhombic) phase transition in (001)-oriented epitaxial $\mathrm{BiFe}{\mathrm{O}}_{3}\text{/}(\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{m}}_{x})\mathrm{Fe}{\mathrm{O}}_{3}$ superlattices is presented. We demonstrate controlling the composition at which a polar phase transformation takes place by tuning the strength of the interlayer interactions while holding the average composition constant. It is shown that the thickness of the superlattice layers has a strong influence on the interlayer polar coupling, which in turn changes the phase transition. For the shortest periods studied (layers 5- and 10-nm thick) the onset of the phase transition is suppressed along with a significant broadening (as a function of $\mathrm{S}{\mathrm{m}}^{3+}$ concentration) of an incommensurately modulated phase determined by two-dimensional x-ray diffraction mapping. Consequently, a ferroelectric character with robust polarization hysteresis and enhanced dielectric constant is observed even for substitution concentration of $\mathrm{S}{\mathrm{m}}^{3+}$ which would otherwise lead to a leaky paraelectric in single-layer $(\mathrm{B}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{m}}_{x})\mathrm{Fe}{\mathrm{O}}_{3}$ films. The experimental results are fully consistent with a mean-field thermodynamic theory which reveals that the strength of the interlayer coupling is strongly affected by the polar-polar interaction across the interface.

Crystal growth of La3+-substituted BaCl2 by the micro-pulling-down technique and their luminescence properties

Due to the phase transition near 920°C and hygroscopicity, the growth of BaCl2 single crystals is difficult. However, owing to the suppression of the cubic–orthorhombic phase transition by La substitution and careful handling to prevent contact with the starting powders and moisture in the air, crack-free BaCl2 single crystals were grown by the micro-pulling-down technique under ambient pressure. In addition, photo- and radio-luminescence in non-activated and Eu2+-activated compounds were reported for scintillator applications.

Robust antiferromagnetism preventing superconductivity in pressurized (Ba 0.61 K 0.39)Mn2Bi2.

BaMn2Bi2 possesses an iso-structure of iron pnictide superconductors and similar antiferromagnetic (AFM) ground state to that of cuprates, therefore, it receives much more attention on its properties and is expected to be the parent compound of a new family of superconductors. When doped with potassium (K), BaMn2Bi2 undergoes a transition from an AFM insulator to an AFM metal. Consequently, it is of great interest to suppress the AFM order in the K-doped BaMn2Bi2 with the aim of exploring the potential superconductivity. Here, we report that external pressure up to 35.6 GPa cannot suppress the AFM order in the K-doped BaMn2Bi2 to develop superconductivity in the temperature range of 300 K–1.5 K, but induces a tetragonal (T) to an orthorhombic (OR) phase transition at ~20 GPa. Theoretical calculations for the T and OR phases, on basis of our high-pressure XRD data, indicate that the AFM order is robust in the pressurized Ba0.61K0.39Mn2Bi2. Both of our experimental and theoretical results suggest that the robust AFM order essentially prevents the emergence of superconductivity.

Dielectric Properties of Barium Zirconate Titanate Ceramics Prepared using Hydrothermal Method

Barium titanate (BT) with different content of Zr (0.001 and 0.008) is prepared using
 hydrothermal method at 150oC for 2h. It is found that with increased of Zr content, the lattice
 constant a increases while the lattice constant c decreases. It is observed also that the curie
 temperature peak shift to lower temperature and be broadened. The lattice constants have the same
 value at Zr content equal to 0.027. For BT samples sintered at 1000oC, the dielectric constant below
 Curie temperature is about 2000 and becomes 6000 at Curie temperature. With the increased of
 sintered temperature (1200oC), the dielectric constant is about 4000 and becomes 9600 at Curie
 temperature (Tc). At Curie temperature the dispersion in BZT1 (x=0.001) is distinct than that of
 pure BT. At near T1 (temperature at phase transition from tetragonal to orthorhombic) the little
 broadening is observed of BZT1 in comparing with BT. The dispersion of BZT2 (x=0.008) is more
 distinct than that of BZT1 (x=0.001) and BT. After adding gradual increasing of Zr, both T1
 (tetragonal to orthorhombic phase transition) and Tc are shifting to each other.

Thermochemistry and Chemical Vapor Transport of In2M3O12 (M = Mo, W)

AbstractThe standard entropy of compounds In2M3O12 (M = Mo, W) were determined to be S°298 = 390 ± 6 (M = Mo] and S°298 = 404 ± 6 J·mol–1·K–1 (M = W). The entropies of formation from the binary oxides are ΔS°R, 298 = 52 J·mol–1·K–1 (M = Mo] and ΔS°R, 298 = 72 J·mol–1·K–1 (M = W). Both compounds possess positive enthalpies of formation from the oxides at 298 K and are entropically stabilized. Transition enthalpies of the monoclinic to orthorhombic phase transition were measured for In2M3O12 by differential scanning calorimetry. Crystals of In2M3O12 (size up to a few mm) were deposited by means of chemical vapor transport along a temperature gradient ΔT = 100 K using transport agent chlorine. Deposition rates are between 0.2 and about 15 mg·h–1, depending on mean transport temperature and amount of chlorine. Experimental results were compared to predictions by means of thermodynamic modelling. A detailed analysis of the chemical vapor transport behavior was carried out regarding gaseous compounds involved in the transport process, dominant transport reactions and influence of moisture content.

Phase-field simulation study on size effect of the microstructure evolution of a single-domain barium titanate 2D lattice square

Microstructure evolution and size effect of a two-dimensional barium titanate lattice sqaure have been studied using phase-field simulation. Based on the Landau phenomenological model, we calculated the microstructure evolution of the tetragonal and orthorhombic ferroelectric BaTiO3 started from a single domain with various lattice sizes. A direction-dependent hysteresis loop can be quantitatively described. The results show that both the phase transition with temperature and polarization switching under an external electric field are accompanied by multidomain and multiphase production in a large single-domain lattice, but synchronous changes in polarization in small lattice; As the grain size decreases, the orthorhombic → tetragonal phase-transition temperature increased, while the tetragonal → cubic, cubic → tetragonal, and tetragonal → orthorhombic phase-transition temperatures decrease. An interesting conclusion is that it shows triple hysteresis loops along the [11]-direction when the lattice size decreases to a certain size. The simulations of microstructure evolutions not only produce a good domain-switching process under various electric fields, but also provide a comprehensive microstructure evolution in the vicinity of the phase-transition points. The physics underlying the continuous evolution of microstructure with electric field and temperature is discussed.

Correlation Between Piezoelectric Properties and Phase Coexistence in (Ba,Ca)(Ti,Zr)O3 Ceramics

High piezoelectric properties are desired for lead‐free piezoelectric materials in consideration as a replacement for lead‐based materials in applications. Due to the high piezoelectric coefficient, (Ba100−xCax) (Ti100−yZry) O3 (BCTZ) piezoelectric ceramics have been considered as a promising lead‐free alternate piezoelectric material. Here, six compositions were selected based on a prediction that all the compositions would have high piezoelectric coefficient at room temperature. The results confirmed all compositions exhibit well developed hysteresis loops and a large piezoelectric coefficient at room temperature. This is due to the coexistence of several phases where the major phase is likely to be orthorhombic and the second phase is proposed to be tetragonal. The phase transition was found to occur over a broad temperature range instead of at a specific temperature only. A relationship between the tetragonal–orthorhombic phase transition temperature and Ca2+ and Zr4+ content was proposed. This enables clear determination of BCTZ compositions with high piezoelectric coefficient at a desired operation temperature.

Critical behavior of the spin density wave transition in underdopedBa(Fe1−xCox)2As2(x≤0.05):As75NMR investigation

We investigate the nature of the SDW (spin density wave) transition in the underdoped regime of an iron-based high-${T}_{c}$ superconductor ${\mathrm{Ba(Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{)}_{2}{\mathrm{As}}_{2}$ by $^{75}\mathrm{As}$ NMR, with primary focus on a composition with $x=0.02$ (${T}_{\mathrm{SDW}}=99$ K). We demonstrate that critical slowing down toward the three-dimensional SDW transition sets in at the tetragonal to orthorhombic structural phase transition ${T}_{s}=105$ K, suggesting strong interplay between structural distortion and spin correlations. In the critical regime between ${T}_{s}$ and ${T}_{\mathrm{SDW}}$, the dynamical structure factor of electron spins $S(\mathbf{q},{\ensuremath{\omega}}_{n})$ measured with the longitudinal NMR relaxation rate $1/{T}_{1}$ exhibits a divergent behavior obeying a power law $1/{T}_{1}\ensuremath{\propto}{\ensuremath{\Sigma}}_{\mathbf{q}}S(\mathbf{q},{\ensuremath{\omega}}_{n})\ensuremath{\sim}{(T/{T}_{\mathrm{SDW}}\ensuremath{-}1)}^{\ensuremath{-}\ensuremath{\delta}}$ with the critical exponent $\ensuremath{\delta}\ensuremath{\sim}0.33$.

In Situ Domain Structure Observation and Giant Magnetoelectric Coupling for PMN‐PT/Terfenol‐D Multiferroic Composites

In situ observations of ferroelectric domain structure evolution, and magnetoelectric (ME) coupling are investigated for PMN‐28PT/Terfenol‐D (abbreviation of Pb(Mg1/3Nb2/3)O3‐28PbTiO3/Tb0.3Dy0.7Fe2) and PMN‐33PT/Terfenol‐D composites under the magnetic loadings. The composite of PMN‐33PT/Terfenol‐D shows stronger ME coupling than that in PMN‐28PT/Terfenol‐D. At a thickness of 0.10–0.12 mm for the single crystal plate, a giant magnetoelectric coefficient (αME) up to 2 V/cm·Oe is obtained for PMN‐33PT/Terfenol‐D at a static magnetic field of 200 Oe and 1 kHz of the alternating magnetic field. In situ domain structure observations reveal the domain morphology change during the applied magnetic loadings. In PMN‐28PT, the domains are of predominantly rhombohedral (R) phase and they change into monoclinic MA phase upon the magnetic loading via the strain transferred between Terfenol‐D plate and PMN‐PT single crystal. In PMN‐33PT, domains of orthorhombic (O), R, and monoclinic MC coexist and phase transitions from O to MC and further to R phase occur upon the magnetic loading. The undulation and diversity of the domain structure makes the domains more susceptible to the magnetic loading via strain transferred between Terfenol‐D plate and PMN‐PT single crystal, and consequently, a strong ME coupling in the composites.

Phase boundary of pressure-induced I4mm to Cmc21 transition in ammonia borane at elevated temperature determined using Raman spectroscopy

The pressure-induced tetragonal (I4mm) to orthorhombic (Cmc21) phase transition in ammonia borane has been studied in a diamond anvil cell (DAC) using in situ Raman spectroscopy. At near ambient pressure in DAC, ammonia borane started decomposing as low as ∼88 °C, but at higher pressure, ammonia borane is stable up to 100 °C above 0.5 GPa. In the pressure–temperature phase diagram, the boundary between tetragonal (I4mm) and orthorhombic (Cmc21) phase was determined by the characteristic change in Raman spectra during compression and decompression at 3 constant temperatures: 20, 60 and 100 °C, respectively. The result shows that the phase transition is reversible with a small hysteresis, while the phase boundary shifts towards higher pressure with increasing temperature and a positive Clapeyron-slope. The positive Clapeyron-slope indicates that the phase transition is exothermic and it is predicted that the rehydrogenation of the high pressure orthorhombic phase would be easier due to its lower enthalpy.

Complex structures of different CaFe2As2 samples.

The interplay between magnetism and crystal structures in three CaFe2As2 samples is studied. For the nonmagnetic quenched crystals, different crystalline domains with varying lattice parameters are found, and three phases (orthorhombic, tetragonal, and collapsed tetragonal) coexist between TS = 95 K and 45 K. Annealing of the quenched crystals at 350°C leads to a strain relief through a large (~1.3%) expansion of the c-parameter and a small (~0.2%) contraction of the a-parameter, and to local ~0.2 Å displacements at the atomic-level. This annealing procedure results in the most homogeneous crystals for which the antiferromagnetic and orthorhombic phase transitions occur at TN/TS = 168(1) K. In the 700°C-annealed crystal, an intermediate strain regime takes place, with tetragonal and orthorhombic structural phases coexisting between 80 to 120 K. The origin of such strong shifts in the transition temperatures are tied to structural parameters. Importantly, with annealing, an increase in the Fe-As length leads to more localized Fe electrons and higher local magnetic moments on Fe ions. Synergistic contribution of other structural parameters, including a decrease in the Fe-Fe distance, and a dramatic increase of the c-parameter, which enhances the Fermi surface nesting in CaFe2As2, are also discussed.

Effect of calcium on the structural properties of Ba(1−x)Ca x TiO3 particles synthesized by complex polymerization method

Ferroelectric materials, such as barium titanate (BaTiO3), have been extensively studied for application in electronic and optical devices. The substitution of Ba by Ca is an effective method to improve the piezoelectricity temperature stability, as it can greatly lower the tetragonal–orthorhombic phase transition temperature, whereas the change of the Curie point is negligible. Ba(1−x)Ca x TiO3 (x = 0, 0.05, 0.10, 0.15, and 0.20) powders were prepared by complex polymerization method. The effect of calcium on the tetragonality of the BaTiO3 system was monitored using basic characterization techniques: X-ray diffraction, differential scanning calorimetry, and Raman spectroscopy. The results indicate that increased calcium contents raise the Curie temperature (T c) and that the addition of calcium in the BT matrix reduces tetragonality.

Elastically controlled magnetic phase transition in Ga-FeRh/BaTiO3(001) heterostructure

We demonstrate elastically induced ferromagnetic to antiferromagnetic phase transition of Ga-substituted FeRh thin films on BaTiO3(001). It is found that two abrupt changes of magnetization occur at the successive phase transitions from the tetragonal to orthorhombic and the orthorhombic to rhombohedral phases of BaTiO3. Magnetization and magnetoresistance together clearly reveal that a ferromagnetic to antiferromagnetic phase transition is induced due to the compressive lattice strain accompanied by the orthorhombic to rhombohedral structural phase transition, while the tetragonal to orthorhombic phase transition causes a change in the symmetry of the magnetic anisotropy in the ferromagnetic phase of FeRh.

Structure, dielectric and piezoelectric properties of Ba0.90Ca0.10Ti1−xSnxO3 lead-free ceramics

Lead-free piezoelectric ceramics Ba0.90Ca0.10Ti1−xSnxO3 have been prepared by a conventional ceramic fabrication technique and the effects of Sn4+ on the structure, dielectric and piezoelectric properties of the ceramics have been investigated. All the ceramics exhibit a pure perovskite structure. After the substitution of Sn4+, the crystal structure of ceramics is transformed gradually from a tetragonal to an orthorhombic phase, and becomes a pseudo-cubic phase at x≥0.14. The substitution also decreases the Curie temperature greatly from 138°C at x=0 to 33°C at x=0.12, and shifts the orthorhombic–tetragonal phase transition to higher temperatures. Coexistence of the orthorhombic and tetragonal phases is formed in the ceramic at x=0.10, leading to significant improvements in the piezoelectric properties: d33=521pC/N and kp=45.5%. Our results also reveal that the ceramics sintered at higher temperatures contain larger grains, and thus exhibit more noticeable tetragonal–orthorhombic phase transition and enhanced ferroelectric and piezoelectric properties.