Low-temperature Monoclinic Phase Research Articles

Structures and Polymorph-Sensitive Luminescence Properties of BiPO4/Eu Grown in Hydrothermal Conditions

In this work, BiPO4/Eu of different polymorphs was prepared using a simple hydrothermal method. It is found that a hexagonal phase (HP) was formed at 100 °C, which transformed to a low-temperature monoclinic phase (LTMP) when hydrothermal temperature was increased to 160 °C. This LTMP transformed reversely to HP when the temperature increased beyond 180 °C. This phase evolution is quite distinct from those when using high-temperature calcinations (CrystEngComm, 2011, 13, 6251–6257). Accompanying this phase evolution, sample morphology varied from homogeneous rodlike shape to prismlike shape, while the lattice strain features changed from tensile to compressive, and then to tensile, as followed by a decrease in symmetry of tetragonal PO4 groups from pseudo-Td to C1. These variations were closely related to the reaction mechanism in solution chemistry, in which two interfacial processes were indicated, differing from the ionic diffusion process during calcination. As a consequence, luminescence properties b...

Neutron diffraction study of spin and charge ordering in SrFeO3−δ

We report a comprehensive neutron diffraction study of the crystal structure and magnetic order in a series of single-crystal and powder samples of SrFeO$_{3-\delta}$ in the vacancy range $0 \leq \delta \leq 0.23$. The data provide detailed insights into the interplay between the oxygen vacancy order and the magnetic structure of this system. In particular, a crystallographic analysis of data on Sr8Fe8O23 revealed a structural transition between the high-temperature tetragonal and a low-temperature monoclinic phase with a critical temperature T = 75 K, which originates from charge ordering on the Fe sublattice and is associated with a metal-insulator transition. Our experiments also revealed a total of seven different magnetic structures of SrFeO$_{3-\delta}$ in this range of $\delta$, only two of which (namely an incommensurate helix state in SrFeO3 and a commensurate, collinear antiferromagnetic state in Sr4Fe4O11) had been identified previously. We present a detailed refinement of some of the magnetic ordering patterns and discuss the relationship between the magneto-transport properties of SrFeO$_{3-\delta}$ samples and their phase composition and magnetic microstructure.

Solvent-Driven Room-Temperature Synthesis of Nanoparticles BiPO4:Eu3+

In this work, a novel solvent-driven room-temperature synthesis of BiPO(4):Eu(3+) nanoparticles was presented. By virtue of 11 solvents with different properties and function groups, phase structure and composition of BiPO(4):Eu(3+) can be systematically tailored. Hexagonal phase (HP) of BiPO(4):Eu(3+) was obtained in water and hydrophobic organic solvents such as arenes and cyclohexane, while low-temperature monoclinic phase (LTMP) was prepared in hydrophilic alcohols. In other solvents (i.e., hydrophilic ethers, aldehydes, ketones, and carboxylic acids), a mixture of HP and LTMP was formed, in which the relative content of LTMP gradually increased following the above solvent sequence. It is also found that particle sizes of BiPO(4):Eu(3+) nanoparticles were closely related to the phase structure: HP exhibited a comparatively larger particle size. The phase evolution processes for both polymorphs with varying solvents were investigated in details. Photoluminescence (PL) properties were sensitive to the phase structure and compositions of the final products. With increasing the phase content of LTMP, the lifetimes and quantum yields both increased. The methodology reported here is fundamentally important, which may give a novel insight into the polymorph-controlled synthesis for further optimized materials performance.

Atomistic study of LaNbO4; surface properties and hydrogen adsorption

We have calculated fundamental properties of pure and hydrogen-covered (010), (101), (100) and (001) surfaces of the low temperature monoclinic phase of LaNbO4 (LN). The (010) surface was the most stable one, exhibiting electronic structure and local geometric configurations similar to bulk. As the first stage of proton migration into the electrolyte, the ability of LN surfaces to split H2 molecules was probed indirectly by calculating the adsorption energy of H atoms on two of the LN surfaces. H adsorption on the (010) surface was found to be strongly endothermic, and thus cannot contribute much in splitting H2. The adsorption energy on the relatively unstable (101) surface was on the other hand approximately −0.6 eV, in the right range for surface H2 to be catalyzed beneficially. H adsorption on this surface was induced by surface states in the band gap of the clean surface. Since the unstable (101) surface is not abundant, the rate of dissociative adsorption of H2 on the LN surface can be anticipated to be very low. Application of the energies to simple adsorption isotherm calculations for typical proton conducting fuel cells (PCFCs) operating temperatures correspondingly showed very low H coverage, and it is not expected that LaNbO4 surfaces can contribute much to the H2 activation reaction of a PCFC anode.

Elucidating the Influence of Local Structure Perturbations on the Metal–Insulator Transitions of V1–xMoxO2 Nanowires: Mechanistic Insights from an X-ray Absorption Spectroscopy Study

The substitutional doping of Mo within VO2 substantially alters the electronic and structural phase diagrams of the host lattice, most notably by bringing the technologically relevant metal–insulator phase transition temperature in closer proximity to room temperature. Here, we have used X-ray absorption fine structure (XAFS) spectroscopy at V and Mo K-edges to examine the local electronic and geometric structure of both the dopant atoms and the host lattice. A nominal Mo oxidation state of +5 has been determined, which implies electron doping of the VO2 band structure. In addition, XAFS studies suggest that doping with Mo creates locally symmetric domains that are more akin to the high-temperature rutile phase of VO2, thereby lowering activation energy barriers for structural transformation to the metallic phase. Substantial rectification of octahedral canting is also observed upon Mo doping, which has the effect of decreasing V 3d–O 2p hybridization and likely assists in closing the characteristic band gap of the low-temperature monoclinic phase. A correlated set of cationic interactions is seen to emerge with increasing Mo doping, which can be ascribed to local dimerization along the rutile c axis as has been proposed to be a characteristic structural feature of a correlated metallic phase with intermediate mass.

In-situ non-ambient X-ray diffraction studies of indium tungstate

In situ variable temperature and high pressure X-ray diffraction studies were carried out on indium tungstate (In2W3O12). This material displays positive volume expansion in both its low temperature monoclinic and high temperature orthorhombic phases, with negative thermal expansion along the a axis and positive thermal expansion along the b and c axes. Upon hydrostatic compression in a diamond anvil cell, one crystalline to crystalline phase transition is observed in the range 1.9 to 2.7GPa, and progressive irreversible amorphization occurs at pressures above 4.3GPa. The crystalline high pressure phase appears to be isostructural to previously observed high pressure phases in other A2M3O12 compounds.

PH-driven hydrothermal synthesis and formation mechanism of all BiPO4 polymorphs

Understanding of formation mechanism of inorganic solids by solution chemistry is always disturbed by the interference effect of the impurities from starting materials. Herein, we exploited a new two-step route to the selective synthesis of BiPO4 of different polymorphs with an aim to eliminate the impurity interference effect. The first step is the room-temperature solution synthesis of a hexagonal phase (HP), and the second step involves sufficient washing of HP and a subsequent hydrothermal treatment of HP under given conditions. The formation mechanism of BiPO4 nanocrystals of different polymorphs was studied by monitoring the reaction parameters like pH, reaction temperature, time, and impurity ions as well as by sample characterizations using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. It is found that the pH of the solution is the determinant parameter for the selective synthesis of BiPO4 polymorphs. Namely, at 240 °C and under strong acidic conditions (pH < 1), HP underwent a phase transformation to a low-temperature monoclinic phase (LTMP), while under neutral or weakly acidic conditions (3 < pH < 7), HP transferred to a high-temperature monoclinic phase (HTMP). Further increasing the solution pH value up to 14 led to the formation of P-doped Bi2O3, a phase which is never accessible by conventional solution chemistry but firing in air at T > 750 °C. Based on these observations, two kinds of phase transition mechanisms were discussed.

A comparative Raman study of 0.65(PbMg1/3Nb2/3O3) -0.35(PbTiO3) single crystal and thin film

We report a comparative Raman study of 0.65(PbMg1/3Nb2/3O3)-0.35(PbTiO3) (PMN-0.35PT) single crystal and thin film. Raman spectra investigation indicates a change in bulk from the high temperature cubic to the tetragonal phase and then to the low temperature Mc monoclinic phase. The transition temperatures are in good agreement with the ones previously observed by dielectric measurements on the same sample. In contrast, we observe no phase transition to the monoclinic phase in the PMN-0.35PT 4000 A thick film and only a cubic to tetragonal diffuse transition has been determined at high temperature. The enhanced stability of the tetragonal phase and the absence of low temperature monoclinic phase have been attributed to the in plane strain.

Thermal and Structural Properties of Ethylammonium Chloride and Its Mixture with Water

The temperature dependence of ethylammonium chloride structure has been investigated by in situ laboratory parallel-beam X-ray powder diffraction. A polymorphic transition from a monoclinic LT phase to a tetragonal HT phase has been observed at 358 K. Such transformation has a reconstructive character. The thermal expansion of both polymorphs is small and anisotropic as a consequence of their organization through an anisotropic interaction network. The high temperature (HT) phase (possible space group P4/n or P4/nmm, a = 5.05 Å, c = 9.99 Å) has an excess volume of ~11% as compared with the low temperature (LT) one. The HT polymorph's structure has been solved by direct methods using powder diffraction data. In the absence of clear indications, it has been refined in P4/nmm. The structural properties of an ethylammonium chloride/water mixture at ambient conditions were also studied by using an integrated approach, which combines X-ray diffraction measurements and molecular dynamics simulations carried out with both the SPC/E and TIP5P water models. By refining a single interaction potential, very good agreement between the theoretical and experimental diffraction patterns was obtained, especially in the case of the TIP5P simulation. A complex structural behavior in which cations and anions do not possess a completely closed hydration shell of their own has been highlighted. Conversely, "solvent-shared ion pairs" are formed, in which one or more water molecules act as a bridge between the chloride and ethylammonium ions. Moreover, a strong water-water correlation is found, indicating that the water molecules in the mixture tend to aggregate and form water clusters.

Facile Synthesis of Silver Chalcogenide (Ag2E; E = Se, S, Te) Semiconductor Nanocrystals

A general, one-pot, single-step method for producing colloidal silver chalcogenide (Ag(2)E; E = Se, S, Te) nanocrystals is presented, with an emphasis on Ag(2)Se. The method avoids exotic chemicals, high temperatures, and high pressures and requires only a few minutes of reaction time. While Ag(2)S and Ag(2)Te are formed in their low-temperature monoclinic phases, Ag(2)Se is obtained in a metastable tetragonal phase not observed in the bulk.

From High-Temperature Orientationally Disordered Mixed Crystals to Low-Temperature Complex Formation in the Two-Component System (CH3)3CBr + Cl3CBr

The phase diagram of the two-component systems (CH(3))(3)CBr + Cl(3)CBr has been experimentally determined by means of differential scanning calorimetry and X-ray powder diffraction techniques from the low-temperature ordered phases to the liquid state. Before melting, both components have the same orientationally disordered (OD) face-centered cubic (FCC) and rhombohedral (R) phases, and the two-phase equilibria [FCC + L] and [R + FCC] are accounted for by means of the existence of an isomorphic relationship between the OD phases of pure compounds. The thermodynamic assessment of such equilibria enables us to get the excess properties of the involved OD phases and to rationalize the existence of a maximum and a minimum in the [R + FCC] equilibrium on the basis of the contribution of the entropic term in the excess Gibbs energy function. At low temperature, two complexes, (CH(3))(3)CBr:Cl(3)CBr (1:1) and (CH(3))(3)CBr:2Cl(3)CBr (1:2), appear. The structures of 1:1 and 1:2 complexes have been determined to be monoclinic (P2(1)/n, c, Z = 4) and hexagonal (P6(3), Z = 6). Within both "ordered" structures, the Cl(3)CBr entities of the asymmetric unit were found to be disordered so that sites have fractional occupancies of 0.75 and 0.25 for Cl and Br atoms, in the same way that it occurs for the low-temperature monoclinic (C2/c, Z = 32) phase of Cl(3)CBr. Finally, the existence of complexes is connected with the special intermolecular interactions appearing between a methyl group and a halogen, as previously inferred by Calvet et al. [T. Calvet et al. J. Chem. Phys. 1999, 110, 4841].

Phase transition and thermal expansion of hexafluoroethane

X-ray studies of the lattice parameters of polycrystalline hexafluoroethane provide data on the linear and volume coefficients of thermal expansion in the low- and high-temperature phases for temperatures of 5–130K. The thermal expansion of the low-temperature monoclinic phase is found to be highly anisotropic. The expansion anisotropy is similar to that usually observed in laminar crystals. Strong damping of the diffraction pattern is observed in the high-temperature phase. It is proposed that this effect may be related to peculiarities in the lattice dynamics of C2F6 owing to a strong rotational-translational interaction. In this case, increased disorder associated with rising temperature, in both the orientational and translational subsystems of the crystal, becomes probable, as does the formation of a “dynamic glass” state at a certain time. A previously observed shift of the temperature of the orientational phase transition to 70K is explained. The contributions of the translational and rotational subsystems to the specific heat are analyzed thermodynamically. Indications of extremely strong disinhibition of torsional rotation of C2F6 are obtained, which ultimately leads to a structural phase transition. The temperature variation in the Grüneisen coefficient for the low-temperature phase of hexafluoroethane is found to be qualitatively similar to that observed for ethane and other simple molecular substances. This suggests that the lattice dynamics and scenarios for orientational disorder in the region of the phase transition are similar for these substances.

Preparation and polymorph-sensitive luminescence properties of BiPO4:Eu, Part I: room-temperature reaction followed by a heat treatment

In this work, BiPO4:Eu of three different polymorphs was prepared using a room-temperature reaction followed by a heat treatment. The formation mechanism, morphologies, structures, and luminescence properties of these three polymorphs were investigated in detail. It is found that a hexagonal phase (HP) was formed at room temperature, which transformed to a low-temperature monoclinic phase (LTMP) and then a high-temperature monoclinic phase (HTMP) when the treatment temperature was increased to 500 and 700 °C, respectively. With the phase transformation, the morphology changed from homogeneous rod-like shape to nearly spherical-like shape, while the lattice strain features varied from the compressive to tensile, and the symmetry of tetragonal PO43−groups decreased from pseudo-Td to C1. Such a change in the symmetry of PO43−groups also showed an impact on the local environment of the lattice sites for Eu3+ and moreover the luminescence properties. Judd–Ofelt parameters (Ω2) were estimated to understand the asymmetric nature of the dopant Eu3+ in these polymorphs. It is indicated that the Ω2 value for HP was 8.24 × 10−20 cm2, which is slightly increased to 8.42 × 10−20 cm2 for LTMP, and the quantum efficiency increased from 12.76% for HP to 70.77% for LTMP. These results demonstrate that rare-earth doped BiPO4 materials can be tailored to show optimum luminescence properties through kinetic control over the polymorphs.

Phase formations and tunable red luminescence of Na2CaMg1−xMnx(PO4)2 (x = 0.05–1.0)

Mn2+-doped phosphates Na2CaMg1−xMnx(PO4)2 (x = 0.05–1.0) were prepared by conventional solid-state reaction. X-ray powder diffraction (XRD), the emission and excitation spectra, and decay measurements were employed to characterize the synthesized phosphors. The XRD patterns show that Na2CaMg1−xMnx(PO4)2 (x = 0.05–0.3) forms the single low temperature monoclinic phase α-Na2CaMg(PO4)2 with the crystal group of P21/c (No.14). The heavily Mn2+-doped Na2CaMg1−xMnx(PO4)2 (x = 0.6–1.0) crystallizes in the single high-temperature trigonal phase of β-Na2CaMg(PO4)2 with space-groupPm1 (No. 164). The members with x = 0.3–0.5 form a series of solid solutions containing two distinct phases, α- and β-Na2CaMg(PO4)2. The dependence of luminescence spectra on the Mn2+-doping concentration in Na2CaMg1−xMnx(PO4)2 (x = 0.05–1.0) was investigated. The great red-shift of Mn2+ emission with increasing Mn2+-concentration in Na2CaMg(PO4)2 were observed. The results are discussed in relation with the detailed crystal structure and the spectral analyses. The CIE coordinates and the luminescence decay (lifetimes) of Mn2+ ions were discussed in order to further investigate the potential applications.

Ferroelastic phase transitions and anelastic dissipation in theLaAlO3-PrAlO3solid solution series

Resonant ultrasound spectroscopy has been used on single-crystal samples to observe pseudoproper ferroelastic softening across the $(\text{La},\text{Pr}){\text{AlO}}_{3}$ solid solution. It is suggested that softening is due to the presence of an intrinsic zone-center instability in addition to the small Jahn-Teller stabilization expected for ${\text{Pr}}^{3+}$. Softening increases as smaller ${\text{Pr}}^{3+}$ ions are substituted for larger ${\text{La}}^{3+}$ which is attributed to a simple size effect as well as the possibility of bilinear coupling of the intrinsic instability with the weak Jahn-Teller effect. Superattenuation is observed above 600 K for all samples, which is consistent with twin wall related dissipation behavior seen in other perovskites with octahedral tilting. Superattenuation is also observed in the low-temperature monoclinic phase, implying a high mobility also for the monoclinic twin walls.

A study of phase coexistence and temperature dependent monoclinic to tetragonal phase transition in the multiferroic (1−x)Pb(Fe1/2Nb1/2)O3−xPbTiO3 (x=0.08)

The origin of coexistence of tetragonal and monoclinic phases in the morphotropic phase boundary (MPB) region of (1−x)Pb(Fe1/2Nb1/2)O3−xPbTiO3 has been investigated for x=0.08 by synchrotron x-ray powder diffraction in the 10–550 K range. It is shown that the phase coexistence observed at room temperature in the MPB region is essentially due to a first order phase transition between the low temperature monoclinic and high temperature tetragonal phases. This transition is shown to be accompanied with a phase coexistence over a very wide temperature range (ΔT∼200 K) across the room temperature.

The phase transformations and cycling performance of copper–tin alloy anode materials synthesized by sputtering

A sputtering technique was adopted to synthesize Sn–Cu thin film electrodes. Island Sn particles were obtained on the copper foil. Cu 6Sn 5 was spontaneously generated at the interface between Sn and Cu foil. To further improve the cycling stability, Cu source was introduced to increase the formation of Cu 6Sn 5 and to serve as a buffer during cycling. Moreover, the phase and elemental ratio of Sn and Cu varied in the synthesized electrode by alternately adjusting sputtering time for Sn and Cu. The cell synthesized by sputtering Sn for 5 min and Cu for 9 s alternately exhibited the best cycling stability. The 1st charge capacity of cell was 635 mA hg −1, and the 1st efficiency was even higher than 97%. The capacity remained higher than 500 mA hg −1 after 15 cycles. The phase transformation of cell was investigated through voltage profile, CV curve and in situ XRD analysis. The in situ XRD analysis confirmed that Cu 6Sn 5 could react with lithium directly without the presence of Li 2CuSn during cycling. The reaction mechanism of Cu 6Sn 5 with lithium during cycling was demonstrated to be an alloying process, and the structure of Cu 6Sn 5 was thus a low-temperature monoclinic phase.

Growth of zirconia and yttria-stabilized zirconia nanorod arrays assisted by phase transition

Well-aligned, densely distributed ZrO2 nanorod arrays were fabricated using a non-catalytic, template-free metal–organic chemical vapour deposition process at a reaction temperature of 1000 °C. The reaction temperature was found to play a key role in product morphology, with particle thin films obtained at 550 °C and nanorod arrays produced at 1000 °C. Increasing the reaction temperature led to the emergence of the medium-temperature tetragonal phase from the dominant low-temperature monoclinic phase, which is advantageous for anisotropic growth necessary for the nanorod formation. With the same deposition process, yttria-stabilized zirconia nanorod arrays of polycrystalline cubic phase were fabricated by co-feeding the dopant precursor, YCl3, with the zirconia precursor, Zr(C5H7O2)4. The present work demonstrated the first example of monoclinic to tetragonal phase-transition assisted one-dimensional (1D) growth, and the concept can be extended to the formation of 1D nanostructures of materials possessing the monoclinic-tetragonal polymorphism.

Observation of anomalous magnetism in the low-temperature monoclinic phase of single-crystallinePrAlO3perovskite

We have investigated unusual magnetism of the ${\text{PrAlO}}_{3}$ perovskite system by performing measurements of the magnetization, magnetic susceptibility, and $^{27}\text{A}\text{l}$ NMR spectra on single crystals, prepared by the Czochralski method. Two kinds of samples were investigated, an as-grown crystal containing a mixture of ${\text{Pr}}^{3+}$ and ${\text{Pr}}^{4+}$ ions in the approximate ratio 4:1 and a crystal annealed in a reducing ${\text{H}}_{2}/{\text{N}}_{2}$ atmosphere, which contained the ${\text{Pr}}^{3+}$ ions only. Within the high-temperature rhombohedral and orthorhombic phases, the ${\text{PrAlO}}_{3}$ crystals behave as simple paramagnets, whereas in the low-temperature monoclinic phase below 152 K, anomalous magnetic properties were observed. The zero-field-cooled (zfc) magnetization vs the magnetic field $M(H)$ curves exhibit nonlinear behavior in the low-field part of the virgin field sweep, whereas they become linear paramagnetic at fields larger than the ``polarizing'' field ${H}_{\text{p}}$ and remain locked to the linear line for all subsequent field sweeps. The nonlinear part is absent in the field-cooled (fc) $M(H)$ curves. The zfc and fc susceptibilities exhibit very different temperature dependences as a function of the magnetic field. At low temperatures, the zfc and fc susceptibilities become temperature independent, consistent with the Van Vleck-type paramagnetism. The $^{27}\text{A}\text{l}$ NMR spectra show unusual tendency of increasing crystal symmetry toward cubic upon cooling. The experimental results can be explained by considering the monoclinic phase to contain tetragonally distorted domains whose fourfold axes can be reoriented by a magnetic field into the field direction. The tetragonal distortion is temperature dependent and decreases upon cooling.

Structure and electronic properties of iron oxide clusters: A first-principles study

In this study we present results of electronic structure calculations for some iron oxide clusters of the form ${\text{Fe}}_{n}{\text{O}}_{m}$ on the basis of the $\text{GGA}+U$ approximation. The cluster size ranged between 33 and 113 atoms corresponding to length scales between around $7\text{ }\text{\AA{}}$ and $12\text{ }\text{\AA{}}$ in diameter, respectively. Initial atomic configurations before relaxation were created by considering two different space groups corresponding to the cubic $Fd\overline{3}m$ and monoclinic $P2/c$ symmetries. The charge and the magnetization per atom were computed. In particular, the charge distribution of the cluster relaxed from cubic symmetry and containing 113 atoms reveals a well-defined periodic pattern of Fe pairs consistent with a partial charge-ordering scenario. Results evidence that the ground-state cohesive energy is smaller in the clusters originated from the $P2/c$ symmetry. This fact indicates that at least in the largest cluster, having more tendency to preserve the initial structure, the low-temperature monoclinic phase is energetically more stable. Clusters starting from monoclinic symmetry are characterized by an insulating state, whereas those optimized from cubic symmetry exhibit a very small electronic gap. Finally, radial and angular distribution functions reveal strong modifications of the starting crystalline structures after relaxation with a tendency of forming cagelike structures.