Low Symmetry Monoclinic Phase Research Articles

Emissive Dark Excitons in Monoclinic Two-Dimensional Hybrid Lead Iodide Perovskites.

Typically, bright excitons (XB) emit light in two-dimensional (2D) layered hybrid perovskites. There are also dark excitons (XD), for which radiative recombination is spin-forbidden. Application of a magnetic field can somewhat relax the spin-rule, yielding XD emission. Can we obtain XD light emission in the absence of a magnetic field? Indeed, we observe unusually intense XD emission at ∼7 K for (Rac-MBA)2PbI4, (Rac-4-Br-MBA)2PbI4, and (R-4-Br-MBA)2PbI4 (Rac-MBA: racemic methylbenzylammonium), which crystallize in a lower symmetry monoclinic phase. For comparison, orthorhombic (R-MBA)2PbI4 does not exhibit XD emission. XD has a lower energy than XB, with energy difference ΔE. In monoclinic samples, ΔE ∼ 20 meV is large enough to suppress the thermal excitation of XD to XB, at temperatures <30 K. Consequently, XD recombines by emitting light with a long lifetime (∼205 ns). At higher temperatures, the emission switches to the spin-allowed XB (lifetime < 1 ns).

Emergence of quasiparticle multiplets in curium

A combination of the density functional theory and the single-site dynamical mean-field theory is employed to study the electronic structures of various allotropes of elemental curium (Cm-I, Cm-II, and Cm-III). We find that the 5$f$ valence electrons in the high-symmetry Cm-I and Cm-II phases remain localized, while they turn into itinerancy in the low-symmetry monoclinic Cm-III phase. In addition, conspicuous quasiparticle multiplets are identified in the 5$f$ electronic density of states of the Cm-III phase. We believe that it is the many-body transition between $5f^{7}$ and $5f^{8}$ configurations that gives rise to these quasiparticle multiplets. Therefore, the Cm-III phase is probably a new realization of the so-called Racah metal.

Effect of Electric Field Orientation on Ferroelectric Phase Transition and Electrocaloric Effect

This paper demonstrates the influence of electric field orientation on the phase transition and the electrocaloric effect (ECE) in BaTiO3 single crystals by performing molecular dynamics simulation of first-principles-based effective Hamiltonian, where the ECE is directly characterized through the adiabatic temperature change (ΔT) of the microcanonical ensemble. Different orientation relationship between the electric field and the polarization direction of ferroelectric phases leads to abundant polarization states and various phase structures. If the electric field direction is collinear to the polarization direction of a ferroelectric phase, the stability of the corresponding phase will be improved, and its phase area in the phase diagram is also enlarged; whereas the noncollinear electric field has reverse impacts and distorts the lattice to a low symmetry monoclinic phase. The phase transitions produce large ECEs far beyond those caused by the domain switching. The tetragonal-cubic phase transition produces a positive ECE regardless of the electric field directions, while the orthorhombic-tetragonal and rhombohedral-orthorhombic phase transitions induce positive or negative ECE, or even their coexistence, depending on the field-induced phase transitions under different electric field directions. The signs of ECE are determined by the lattice symmetry before and after the field-induced phase transition. Our simulated results help reveal a more comprehensive physical understanding of ECE and offer an instruction for the design of the refrigeration cycle by combining positive and negative ECEs.

The nature of the intermediate phase in K3Na(SeO4)2 crystals: three possible transition paths of the trigonal-monoclinic phase transition

ABSTRACTThree different transition paths of the phase transition from the high-symmetry trigonal P m1 to the low-symmetry monoclinic phase C2/c in K3Na(SeO4)2 crystals have been analyzed using ab initio calculations based on the density functional theory (DFT) as well as programs and retrieval tools of the Bilbao Crystallographic server (BCS). We conclude that the intermediate state observed in experiments within 346–329 K temperature range consists of a mixture of two monoclinic phases C2/m and C2/c, both with the primitive cell doubled along the z-axis. Both monoclinic phases result from the softening of the double-degenerated zone-edge soft phonon .

Phase Instability and Thermal Properties of Multilayered Vanadium Diselenide: DAC-Based High Pressure Studies

Two-dimensional layered trichalcogenides are of current research interest because of their robust opto-electronics, and thermal properties. Mechanical and phonon vibrational properties of these materials are of significant importance in the field of stress and thermal management. Here we report the pressure dependence of structural and phonon behaviors of high quality few-layers vanadium diselenide nano-sheets grown by hydrothermal method employing synchrotron x-rays and micro-Raman scattering techniques. The high crystalline nature of the nanosheets was examined using transmission electron microscopy. Synchrotron x-ray studies at high pressure (up to 22 GPa) using a diamond-anvil cell identify a displacive type phase transition to a low symmetry monoclinic phase (C2/m) around 7 GPa corroborated with the P-dependent Raman scattering results. Raman spectroscopic studies at high pressure (up to 34 GPa) identify two prominent Raman bands with symmetry Eg and A1g. Eg band at 207 cm-1 shows normal hardening and A1g band at 236 cm-1 softens. The first order pressure derivatives of these phonons at ambient phase (P-3m1) are found to be -3.547(2) and 0.388(6) cm-1/ GPa, respectively. Using the experimental mode Grüneisen parameter of Eg Raman mode, the in-plane thermal expansion coefficient of the titular compound at the ambient phase is estimated. The observed results are expected to be useful in calibration and performance of next generation nano-electronics and optical devices under extreme stress conditions. These experimental results corroborated with our ab-initio calculations will be presented in the ECS meeting.

Tracking morphotropic phase boundary through total scattering technique

Morphotropic phase boundary (MPB) is an important theme in the research of ferroelectric materials since materials often exhibit enhanced physical properties at MPB. Although an established understanding of creating MPB behaviour relies on invoking instability of average crystal structure or a low-symmetry monoclinic phase in the system driven by composition [1], the actual mechanism is far more complex and heavily dependent on the local ordering of cations as evidences found in recent diffuse scattering studies [2] on both lead-based and lead-free systems. We have studied an important and popular ferroelectric system xBiScO3-(1-x) PbTiO3 (BS-PT), which was first reported in 2001 with MPB characteristics [3], through total scattering technique to understand its structural evolution as a function composition in the vicinity of the MPB. Total scattering technique which is essentially the analysis of the pair distribution function (PDF) of a system, provides crucial structural information at the microscopic level which are not easily available from conventional structural analysis like Rietveld refinement. Both x-ray and neutron powder diffraction experiments were carried out on six different compositions of BS-PT in the range 0.30 &lt; x &lt; 0.40 and neutron PDFs were analysed through RMC simulations to extract the behaviour of individual cations. It was observed that locally there was no abrupt change in any of the cation-cation correlations to assign the MPB unambiguously, however a trend was noticed in the sigmas of individual cationic displacements where the reported MPB composition show a minimum.

The crystal structure of p-type transparent conductive oxide CuBO2

Abstract

Phase transition and possible metallization in CeVO4 under pressure

Abstract Phase stability of CeVO 4 under pressure has been investigated using synchrotron based angle dispersive x-ray diffraction (ADXRD), electrical resistance and first principles calculations. The results indicate that the ambient zircon structure of the compound transforms to a low symmetry monoclinic monazite phase beyond 3.8 GPa with nearly 8.6% volume discontinuity. Beyond 11 GPa, the pattern could be fitted to a similar monazite structure which is about 12.7% denser and has a much larger monoclinic beta angle. On pressure release the first monoclinic phase is recovered. The electrical resistance data show a large drop in resistance with pressure indicating substantial narrowing down of the band gap. Electronic structure calculations support these observations and suggest possible pressure induced metallization in this material.

Low-symmetry monoclinic ferroelectric phase stabilized by oxygen octahedra rotations in strained EuxSr1−xTiO3thin films

Using Landau--Ginzburg--Devonshire theory and phase-field modeling, we explore the complex interplay between a structural order parameter (oxygen octahedron rotation) and polarization in Eu${}_{x}$Sr${}_{1\ensuremath{-}x}$TiO${}_{3}$ thin films. Under a biaxially tensile strain, a low-symmetry monoclinic phase with in-plane ferroelectric polarization is found to be stabilized by antiferrodistortive oxygen octahedra tilts. The monoclinic phase is stable over a wide temperature range. It is characterized by a large number of energetically equivalent polar and structural twin domains. This work demonstrates the development of a spontaneous polarization and piezo- and pyroelectricity in a ferroelastic twin boundary arising from flexoelectric coupling and rotostriction.

Structural phase transitions in yttrium under ultrahigh pressures

X-ray diffraction studies were carried out on the rare earth metal yttrium up to 177 GPa in a diamond anvil cell at room temperature. Yttrium was compressed to 37% of its initial volume at the highest pressure. The rare earth crystal structure sequence hcp → Sm type → dhcp → mixed(dhcp + fcc) → distorted fcc (dfcc) is observed in yttrium below 50 GPa. The dfcc (hR24) phase has been observed to persist in the pressure range of 50–95 GPa. A structural transition from dfcc to a low symmetry phase has been observed in yttrium at 99 ± 4 GPa with a volume change of − 2.6%. This low symmetry phase has been identified as a monoclinic C2/m phase, which has also been observed in other rare earth elements under high pressures. The appearance of this low symmetry monoclinic phase in yttrium shows that its electronic structure under extreme conditions resembles that of heavy rare earth metals, with a significant increase in d-band character of the valence electrons and possibly some f-electron states near the Fermi level.

A monoclinic-tetragonal ferroelectric phase transition in lead-free (K0.5Na0.5)NbO3-x%LiNbO3 solid solution

A monoclinic ferroelectric phase with space group Pm has been discovered in lead-free (K0.5Na0.5)NbO3-5%LiNbO3 solid solution ceramics by high energy synchrotron x-ray powder diffraction measurements. At ambient temperature, the lattice parameters of this monoclinic structure were (am, bm, cm; β) = (4.015 Å, 3.944 Å, 3.987 Å; 90.34°). This monoclinic phase transformed to a tetragonal (P4mm) one on heating between 340 K and 360 K. The results demonstrate the presence of structurally bridging low symmetry monoclinic phase in (K0.5Na0.5)NbO3-x%LiNbO3 solid solution system: indicating a means to achieve high piezoelectricity in Pb-free systems via domain engineering.

Shear-strain-induced low symmetry phase and domain ordering in epitaxial La0.7Sr0.3MnO3 thin films

The domain structure in epitaxial La0.7Sr0.3MnO3 films grown on (LaAlO3)0.3(Sr2AlTaO6)0.7(001) substrates has been investigated by x-ray reciprocal space maps on (003), (103), (1¯03), (013), and (01¯3) reflections in the high-resolution scattering zone containing the c* axis, and the transmission electron microscopy. We revealed that at above 20nm, the films show clearly a low symmetry monoclinic phase, and the shear strain relaxation can induce a patterned domain structure and in-plane superlattice in the films. The domain width versus the film thickness and the shear strain effect on the Curie temperature of the films were also discussed.

High Pressure Phase Transformations in Heavy Rare Earth Metals and Connections to Actinide Crystal Structures

AbstractHigh-pressure studies have been performed on heavy rare earth metals Terbium (Tb) to 155 GPa and Holmium (Ho) to 134 GPa in a diamond anvil cell at room temperature. The following crystal structure sequence was observed in both metals hcp ⟶ Sm-type ⟶ dhcp ⟶ distorted fcc (hR-24) ⟶ monoclinic (C2/m) with increasing pressure. The last transformation to a low symmetry monoclinic phase is accompanied by a volume collapse of 5 % for Tb at 51 GPa and a volume collapse of 3 % for Ho at 103 GPa. This volume collapse under high pressure is reminiscent of f-shell delocalization in light rare earth metal Cerium (Ce), Praseodymium (Pr), and heavy actinide metals Americium (Am) and Curium (Cm). The orthorhombic Pnma phase that has been reported in Am and Cm after f-shell delocalization is not observed in heavy rare earth metals under high pressures.

Low symmetry phase in Pb(Zr0.52Ti0.48)O3 epitaxial thin films with enhanced ferroelectric properties

The authors report the structural and ferroelectric properties of Pb(Zr0.52Ti0.48)O3 (PZT) epitaxial thin films grown on (001), (110), and (111) SrRuO3∕SrTiO3 substrates by pulsed laser deposition. A monoclinic C (Mc) phase has been found for (101) films, whereas (001) and (111) ones were tetragonal (T) and rhombohedral (R), respectively. The authors find that the ferroelectric polarization of the Mc phase is higher than that in either the T or R ones. These results are consistent with predictions (i) of epitaxial phase diagrams and (ii) that the enhanced ferroelectric properties of morphotropic phase boundary PZT are related to a low symmetry monoclinic phase.

Bridging phases at the morphotropic boundaries of lead oxide solid solutions

Ceramic solid solutions of PbZr1− x T x O3 (PZT) with compositions x≃ 0.50 are well-known for their extraordinarily large piezoelectric responses. The latter are highly anisotropic, and it was recently shown that, for the rhombohedral compositions (x ≲ 0.5), the piezoelectric coefficients were largest away from the polar direction, contrary to common belief. Shortly afterwards a low-symmetry monoclinic phase was observed by synchrotron X-ray diffraction at around x = 0.50. Similar behavior and features are also present in a number of related lead-based strongly-piezoelectric single crystals, such as Pb(Mg1/3Nb2/3)1− x Ti x O3, Pb(Zn1/3Nb2/3)1− x Ti x O3, and Pb(Sn1/2Nb1/2)1− x Ti x O3, with piezoelectric coefficients of about 2500 pm V−1, the highest values recorded to date. Recent experimental and theoretical work has greatly improved our understanding of these technologically important systems, but there are still some open questions. In this review, we summarize the most recent progress in the field.

Ferromagnetism Driven by Intrinsic Point Defects inHfO2

In view of recent experimental reports of unexpected ferromagnetism in HfO(2) thin films, we carried out first-principles investigations looking for magnetic order possibly brought about by the presence of small concentrations of intrinsic point defects. Ab initio electronic structure calculations using density functional theory show that isolated cation vacancy sites in HfO(2) lead to the formation of high-spin defect states. Furthermore these appear to be ferromagnetically coupled with a rather short range magnetic interaction, resulting in a ferromagnetic ground state for the whole system. More interestingly, the occurrence of these high-spin states and ferromagnetism is in the low symmetry monoclinic phase of HfO(2). This is radically different from other systems previously known to exhibit point defect ferromagnetism, warranting a closer look at the phenomenon.

Textural evolution of zirconia and yttria-doped zirconia on thermal treatment under different conditions

Abstract Pure zirconia and yttria-doped zirconia samples have been prepared from the corresponding microporous gels under different thermal treatment conditions. Doped materials show a delay in the high symmetry (cubic or tetragonal) to low symmetry (monoclinic) phase transformation, compared to pure ZrO 2 . However, as in pure zirconia, thermal treatment leads to the textural degradation of doped samples. Ageing of gels at 723 K in air yields mesoporous powders. On increasing the yttria content in the sample, adsorption capacity and mesopore surface area decrease and the most probable pore radii are slightly smaller. Differences observed in the porous texture of pure and doped zirconia could be justified in terms of the different packing of the primary particles in the gels. In both pure and doped gels, the evolution of the porous structure is highly affected by the conditions at which thermal dehydration is carried out. Under vacuum, the development of a mesoporous texture is hindered. Condensation and elimination of water produce the most important reduction of the micropore volume.