Rhombohedral Clusters Research Articles

Changes in spin and lattice dynamics induced by magnetic and structural phase transitions in multiferroic SrMn7O12

${\mathrm{SrMn}}_{7}{\mathrm{O}}_{12}$ is a recently synthesized homolog of multiferroic ${\mathrm{CaMn}}_{7}{\mathrm{O}}_{12}$. Upon cooling, ${\mathrm{SrMn}}_{7}{\mathrm{O}}_{12}$ undergoes a series of structural and magnetic phase transitions from cubic to rhombohedral symmetry, and to an incommensurately modulated crystal structure, which is connected with charge and orbital ordering of the Mn cations. We report infrared, terahertz, and Raman spectra of ${\mathrm{SrMn}}_{7}{\mathrm{O}}_{12}$ ceramics reflecting corresponding changes in phonon selection rules, including new phonons appearing in spin-order-induced ferroelectric phases. The observed phonon activities are compared with the predictions from the factor-group analysis. In the high-temperature phase, more phonons are observed than the number predicted for the cubic symmetry. This is explained by the presence of rhombohedral clusters in the cubic phase. The strongest variations occur in THz spectra near the two magnetic phase transitions, at ${T}_{\mathrm{N}1}=87\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}=63\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. These activate new modes in the spectra, with resonance frequencies and intensities changing with temperature and magnetic field. Below ${T}_{\mathrm{N}2}$, we observed a transfer of oscillator strengths from low-frequency phonons to these excitations, which we assign to electromagnons.

Quantum-fluctuation-stabilized orthorhombic ferroelectric ground state in lead-free piezoelectric (Ba,Ca)(Zr,Ti)O3

We numerically investigate the phase diagram of the giant-piezoelectric (1−x) Ba(Zr0.2Ti0.8) O3−x (Ba0.7Ca0.3) TiO3 system, treating the ions either as classical objects (via classical Monte Carlo or CMC simulations) or quantum mechanically (via path-integral quantum Monte Carlo or PI-QMC simulations). It is found that PI-QMC not only provides a better agreement with available experimental data for the temperature-composition phase diagram, but also leads to the existence of an orthorhombic ground state in a narrow range of composition, unlike CMC that “only” yields ground states of rhombohedral or tetragonal symmetry. X-ray powder diffraction experiments are further conducted at 20 K. They confirm the occurrence of a quantum-fluctuation-induced orthorhombic state for some compositions and therefore validate the PI-QMC prediction. The role of quantum effects on the local structure, such as the annihilation of a homogeneous rhombohedral system in favor of an inhomogeneous mixing of orthorhombic and rhombohedral clusters, is also documented and discussed.

Investigation of chromium oxide clusters grafted on SBA-15 using Cr-polycation sol

A comparative study of chromium oxide clusters grafted on mesoporous silica SBA-15 was carried out using samples synthesized by one-pot, impregnation and Cr-polycation sol grafting methods. The nature of CrOx species incorporated into SBA-15 by direct hydrothermal one-pot method as well as impregnation of Cr(NO3)3·9H2O and Cr-polycation precursors was characterized by XRD, BET isotherms, UV–Vis DRS, FTIR, TGA, O1s XPS, 29Si-MAS NMR, H2-TPR, NH3-TPD, SEM and TEM. Powder XRD did not show the presence of Cr2O3 in the calcined samples obtained by one-pot method. It, however, shows the rhombohedral clusters of α-Cr2O3 dispersed over SBA-15 for CrOx/SBA-15 samples prepared by impregnation and polycation sol grafting methods. The absorption band at 296 nm, observed for Cr3+ in solution, is absent for the aqueous Cr-polycation sol. There is evidence that the presence of chromium precursor in the reaction medium can influence the morphology of SBA-15 as seen in the SEM micrographs. Charge transfer transitions demonstrate the insertion of CrOx species on SBA-15 matrix synthesized by one-pot method. Cr-polycation grafted SBA-15 sample shows unique vibrational features at 573 and 624 cm−1 attributed to extra-framework CrOx species. The ratio of Cr6+/Cr3+ species present in CrOx/SBA-15 samples depends on the Cr-precursor employed for grafting on SBA-15. One-pot synthesized samples predominantly contain coordinated water (δH–O–H at 1,635 cm−1) on SBA-15 while impregnated samples show water molecules associated with CrOx species (δH–O–H at 1,594 cm−1).

Surface self-diffusion behavior of individual tungsten adatoms on rhombohedral clusters

The diffusion of single tungsten adatoms on the surfaces of rhombohedral clusters is studied by means of molecular dynamics and the embedded atom method. The energy barriers for the adatom diffusing across and along the step edge between a {110} facet and a neighboring {110} facet are calculated using the nudged elastic band method. We notice that the tungsten adatom diffusion across the step edge has a much higher barrier than that for face-centered cubic metal clusters. The result shows that diffusion from the {110} facet to a neighboring {110} facet could not take place at low temperatures. In addition, the calculated energy barrier for an adatom diffusing along the step edge is lower than that for an adatom on the flat (110) surface. The results show that the adatom could diffuse easily along the step edge, and could be trapped by the facet corner. Taking all of this evidence together, we infer that the {110} facet starts to grow from the facet corner, and then along the step edge, and finally toward the {110} facet center. So the tungsten rhombohedron can grow epitaxially along the {110} facet one facet at a time and the rhombohedron should be the stable structure for both large and small tungsten clusters.

Study of potassium-sodium-niobate alloys: A combined experimental and theoretical approach

Experimental and theoretical techniques are combined to investigate properties of the environmentally-friendly and technologically-promising (K 0.5 Na 0.5 )NbO 3 (KNN) solid solutions. Two long-range ferroelectric phases of orthorhombic and tetragonal symmetry are reported above 250K. Furthermore, our dielectric measurements, X-ray data and simulations suggest an unusual formation of rhombohedral clusters within an orthorhombic matrix at lower temperature.

Order-Disorder Component in the Phase Transition Mechanism ofO18Enriched Strontium Titanate

Ti and Sr nuclear magnetic resonance spectra of 18O enriched SrTiO3 (STO-18) provide direct evidence for Ti disorder already in the cubic phase and show that the ferroelectric transition at T(C)=24 K occurs in two steps. Below 70 K rhombohedral polar clusters are formed in the tetragonal matrix. These clusters subsequently grow in concentration, freeze out, and percolate, leading to an inhomogeneous ferroelectric state below T(C). This shows that the elusive ferroelectric transition in STO-18 is indeed connected with local symmetry lowering and implies the existence of an order-disorder component in addition to the displacive soft mode one. Rhombohedral clusters, Ti disorder, and a two-component state are found in the so-called quantum paraelectric state of STO-16 as well. The concentration of the rhombohedral clusters is, however, not high enough to allow for percolation.

Variety of LaSrMnO structures induced by growth conditions and laser irradiation

Crystallographic phase transitions in perovskite-like LaSrMnO metallic oxides are studied. The transitions are induced when internal stresses generated during film synthesis (at temperatures between 450 and 730°C) vary (decrease or increase) upon subsequent irradiation by a KrF laser emitting in the UV range. As the synthesis temperature Ts grows, the rhombohedral-to-orthorhombic phase transition occurs at 650–670°C. The resistivity is shown to be either temperature-independent, ρ(T)=const, at T<Tcrit, or varies and reaches a maximum, ρ(T)=ρmax, at the Curie temperature Tc. Optical transmission spectra taken at photon energies ℏω=0.5–2.5 eV exhibit both a high (0.8–0.9) and low (0.1–0.3) transmission coefficient t, depending on the synthesis temperature. As follows from X-ray diffraction data, the laser irradiation causes a phase transition only in LaSrMnO films grown at Ts<650°C. Phases of different size scales appear: the long-range-order orthorhombic matrix and mesoscopic-range-order rhombohedral clusters are observed in the films grown at Ts=450–550°C and the rhombohedral matrix with orthorhombic clusters, in the films grown at Ts=550–650°C.