Domain matching epitaxy stabilized metastable, tetragonal BiFeO3 on symmetry-mismatched c-plane ZnO

We report the stabilization of metastable tetragonal BiFeO3 epilayer on ZnO(0001) surface. X-ray reciprocal space map characterizations show that the BiFeO3 film is of true tetragonal symmetry, but not the commonly observed monoclinic structure. The critical thickness of the tetragonal BiFeO3 is higher than 140 nm, much larger than that reported previously. Despite the considerable lattice mismatch and symmetry mismatch, tetragonal BiFeO3 can be formed on ZnO(0001) though domain matching epitaxy which is featured by anisotropic growth. We show that by taking into account the elastic energy during the initial semi-coherent growth, the tetragonal phase is lower than the thermally stable rhombohedral phase in total energy by 70 meV per formula unit. Moreover, local piezoelectric characterizations reveal a coercive field of 360 kV/cm and a piezoelectric constant of 48 pm/V. The integration of tetragonal BiFeO3 with robust ferroelectricity on the platform of ZnO has potentials for all-oxide electronics applications.

Analysis of surface acoustic wave properties in CaYAl3O7 single crystal

The success on the growth of new piezoelectric materials allows sufficiently increase the operating temperature of the surface acoustic wave (SAW) devices from 300°C to 1000°C. A new calcium yttrium aluminate (CaYAl3O7) single crystal of the tetragonal symmetry has piezoelectric properties up to the temperature of 1000°C. The paper presents a numerical study of the surface acoustic wave properties in the crystal. The SAW velocity, electromechanical coupling coefficient and power flow angle are studied for different crystal cuts of CaYAl3O7. It is shown that the maximum value of SAW coupling coefficient (0.24%) is on the Z+60°-cut and wave propagation direction along the X-axis of the crystal. For the Z-cut and wave propagation direction along the X+45°-axis of crystal, the SAW coupling coefficient is equal to 0.2%. These two cuts of the crystal are potentially useful for SAW device applications.

Effect of chromium substitution on the dielectric, ferroelectric and piezoelectric properties of PLNZNT ceramics (P=Pb2+, L=La3+, N=Nd3+, Z=Zr4+, N=Nb5+, T=Ti4+)

Piezoelectricity is one of the renewable and clean electrical energy sources, as it is generated from materials specially manufactured for this purpose in proportions and scales called piezoelectric materials, that is, electricity resulting from mechanical work that produces the electric field, and this is known as the direct piezoelectric effect. An inverse mechanical effect can also be observed when an electric field is applied to the same piezoelectric material that deforms and returns to its original shape after the electric field is removed. In order to contribute to improving the properties of these materials that have been prepared from insulating ceramic materials having an equivalent formula: Pb0.975 La0.015 Nd0.01 [(Zr 0,524 Ti 0,476) 0.9875 -3/4 z Nb0.005 Crz]O3 abbreviated PLNZCNT (z = 0%, 0.5%, 0.75%, 1%, 1.25%, 1,5% and 2%). The formation of pure single-phase perovskite compounds of tetragonal symmetry for all samples was confirmed by X-ray diffraction (XRD) analyzes. The scanning electron micrographs show that the grains have melted, most of the separating walls have disappeared, and the average grain size has increased. Information about the effects of chromium concentration on the measured properties was obtained based on temperature and frequency measurements of the dielectric properties of PLNZNT ceramics, represented by an increase in Curie temperature with increasing Cr3+ concentration. The ferroelectric properties of materials are characterized by the presence of a polarization hysteresis cycle as a function of the applied electric field (P-E). To measure the piezoelectric and electromechanical properties at room temperature, was used by the standard resonance and anti-resonance method. It was found that the sample E2 (0.75%) sintered at 1200°C achieves excellent dielectric, ferroelectric and piezoelectric properties (𝜺r=24394.51, tan δ =0.072 and Tc = 378K). The values of saturated electric polarization (Ps=29.61 µC/cm²), remnant electric polarization (Pr=24.63 µC/cm²) and coercive electric field (Ec=0.905kV/mm), as well as piezoelectric charge coefficient (d33 = 435 pC/N) for sample E2 (0.75%).

Intriguing spin–lattice interactions in EuTiO3

During the last decade the cubic perovskite oxide EuTiO3 (ETO) has attracted enormous novel research activities due to possible multiferroicity, hidden magnetism far above its Néel temperature at TN = 5.5 K, structural instability at TS = 282 K, possible application as magneto-electric optic device, and strong spin–lattice coupling. Here we address a novel highlight of this compound by showing that well below TS a further structural phase transition occurs below 210 K without the application of an external magnetic field, and by questioning the assumed tetragonal symmetry of the structure below TS where tiny deviations from true tetragonality are observed by birefringence and XRD measurements. It is suggested that the competition in the second nearest neighbor spin–spin interaction modulated by the lattice dynamics is at the origin of these new observations.

CATALYTIC PROPERTIES OF SYNTHETIC HAUSMANNITE IN THE OZONE DECOMPOSITION REACTION

Samples of IS-Mn and IIS-Mn were synthesized and characterized by X-ray diffraction, IR spectroscopy and pH-metry. The X-ray diffraction method revealed both samples to be crystalline and single-phase. IS-Mn sample contains a phase of gausmannite (Mn3O4) with the parameters of the crystal lattice of tetragonal symmetry and the crystallites sized 43 nm. IIS-Mn sample, obtained by calcination of Mn3O4 sample in air under the condition of 600 oC for 4 hours, contains a phase of bixbyite Mn2O3 with a cubic crystal lattice and the crystallites sized 66 nm. The spectra of the Mn3O4 and Mn2O3 samples are particularly different in the region of Mn-OH and Mn-O valence vibrations. For gausmannite, the absorption band for Mn2+ in Td-coordination at 631 cm-1 and the absorption band for Mn3+ in Oh-coordination at 523 and 407cm-1 were clearly shown. In the IR spectrum of bixbyite, several intense absorption bands were observed at 674; 658; 606 and 542 cm-1, which correlate with the literature data and correspond to the valence oscillations of Mn-O in Mn2O3. The pH measurement of the aqueous suspension of the synthesized samples showed that Mn3O4 forms an alkaline medium, and Mn2O3 –weakly acidic upon contact with water molecules. Testing of Mn3O4 and Mn2O3 samples in the ozone decomposition reaction at the initial concentration of 100 mg/m3 showed that the kinetic curves profiles were determined by the phase composition. The testing of the samples was completed and it was established that the same degree of ozone decomposition, namely 50%, was achieved. For hausmannite, the values of the ozone half-life (t1/2) and the amount of ozone (Qexp), which reacted at the end of the experiment, are greater than Mn2O3 sample. The presence of manganese in two oxidation states Mn2+ and Mn3+ in the Mn3O4 structure promotes the redox process, and the alkaline medium contributes to the radical-chain reaction, which affects values t1/2 of and Qexp, which reacted.

Learning crystal field parameters using convolutional neural networks

We present a deep machine learning algorithm to extract crystal field (CF) Stevens parameters from thermodynamic data of rare-earth magnetic materials. The algorithm employs a two-dimensional convolutional neural network (CNN) that is trained on magnetization, magnetic susceptibility and specific heat data that is calculated theoretically within the single-ion approximation and further processed using a standard wavelet transformation. We apply the method to crystal fields of cubic, hexagonal and tetragonal symmetry and for both integer and half-integer total angular momentum values JJ of the ground state multiplet. We evaluate its performance on both theoretically generated synthetic and previously published experimental data on CeAgSb_22, PrAgSb_22 and PrMg_22Cu_99, and find that it can reliably and accurately extract the CF parameters for all site symmetries and values of JJ considered. This demonstrates that CNNs provide an unbiased approach to extracting CF parameters that avoids tedious multi-parameter fitting procedures.

Enhanced magnetoelastic performance in Pr/Mn-doped Laves phase (Tb,Ho)Fe2 compounds

Magnetostrictive TbxHo0.8−xPr0.2Fe1.8Mn0.1 (0 ⩽ x ⩽ 0.20) alloys are prepared by arc-melting and subsequent annealing. The dopant of Pr/Mn introduced into RFe2 compounds effectively stabilizes the forming of single C15 Laves phase at ambient pressure. The easy magnetization direction (EMD) varies when Tb content increases, which is accompanied by a crystal-structural transition. EMD lies along ‹1 0 0› axis for x ⩽ 0.05, rotating to ‹1 1 1› axis for x ⩽ 0.12, with a tetragonal symmetry changing to a rhombohedral one. Magnetocrystalline-anisotropy compensation is obtained with the optimized composition of x = 0.12, shifting to the Tb-poor side in comparison to Pr/Mn-free counterpart. An enhanced effect on magnetoelastic properties is achieved in Tb0.12Ho0.68Pr0.2Fe1.8Mn0.1, which simultaneously possesses a low anisotropy and high magnetostriction performance, i.e. λS ~ 420 ppm, λ111 ~ 970 ppm, and a large low-field λa ~ 390 ppm/2 kOe, being 30 % higher than that of Pr/Mn-free compound. Combining low-cost light rare earth Pr with the lower Tb content, Tb0.12Ho0.68Pr0.2Fe1.8Mn0.1 may make it promising solution in magnetostrictive applications.

High-pressure crystal chemistry of four natural REE(As,P)O4 minerals from Mt. Cervandone, Italy

<p>REE orthoarsenates and orthophosphates are common accessory minerals characterized by the general chemical formula REEXO<sub>4</sub>, where REE represents one of the lanthanides (La-Lu series), Y, Sc, Ca or Th, whereas X stands for As, P or Si. In the framework of a long-term project on the high-<em>T</em>/high-<em>P</em> crystal-chemistry and phase-stability of REE-bearing minerals, the high-pressure behavior of chernovite-(Y) (nominally YAsO<sub>4</sub>), xenotime-(Y) (nominally YPO<sub>4</sub>) gasparite-(Ce) (nominally CeAsO<sub>4</sub>) and monazite-(Ce) (nominally CePO<sub>4</sub>), has been studied. Chernovite-(Y) and xenotime-(Y) show a HREE- (Gd-Lu series) and Y-enrichment, and the same tetragonal symmetry (space group <em>I</em>4<sub>1</sub>/<em>amd</em>), whereas gasparite-(Ce) and monazite-(Ce) share the same LREE (La-Eu) enrichment and monoclinic cell (space group <em>P</em>2<sub>1</sub>/<em>n</em>). All these minerals occur at Mt. Cervandone (Western Alps, Italy), a renowned Alpine REE-bearing mineral deposit. The crystal chemistry of the four minerals has been studied via EPM-WDS analysis. Excluding gasparite-(Ce), which formation is bound to the replacement of the mineral synchisite-(Ce) (CaCe(CO<sub>3</sub>)<sub>2</sub>F), a sensible enrichment in Gd and Ho is observed. Moreover, the majority of the chernovite-(Y) show a variable amount of ThO<sub>2</sub>, up to 13 wt%, and phosphorous as substitute for arsenic in almost every proportion. In the case of the monoclinic series between monazite-(Ce) and gaparite-(Ce), no solid solution has been observed. Experiments at high-pressure were performed by in situ synchrotron X-ray diffraction using a diamond anvil cell. The high-pressure behavior of single crystals of xenotime-(Y), gasparite-(Ce) and monazite-(Ce) has been studied up to ~20 GPa, whereas that of chernovite-(Y) has been studied by powder diffraction up to 8.20(5) GPa. A II-order Birch-Murnaghan equation of state was fitted to the <em>V</em>-<em>P</em> data, within the phase stability field of the minerals, yielding the following bulk moduli: <em>K</em><sub>P0,T0</sub> = 125(3) GPa (β<sub>V</sub><sub>0 </sub>= 0.0080(2) GPa<sup>-1</sup>) for chernovite-(Y); <em>K</em><sub>P0,T0</sub> = 145(2) GPa (β<sub>V</sub><sub>0 </sub>= 0.0069(1) GPa<sup>-1</sup>) for xenotime-(Y);  <em>K</em><sub>P0,T0</sub> = 106.7(9) GPa (β<sub>V</sub><sub>0 </sub>= 0.0094(1) GPa<sup>-1</sup>) for gasparite-(Ce), <em>K</em><sub>P0,T0</sub> = 121(2) GPa (β<sub>V</sub><sub>0 </sub>= 0.0083(1) GPa<sup>-1</sup>) for monazite-(Ce). <em>K</em>’ = ∂<em>K</em><sub>V</sub>/∂<em>P</em> = 4 (fixed) for all the minerals. Deformation mechanisms, at the atomic scale, were described on the basis of structure refinements.  </p><p>Acknowledgments: This research was partly funded by the PRIN2017 project “Mineral reactivity, a key to understand large-scale processes” (2017L83S77).</p>

Structural, Morphological and Thermal Decomposition Studies of Calcium and Hafnium Modified BaTiO3 Electro Ceramics

Calcium and hafnium co-doped barium titanate could be used as a replacement for lead zirconate titanate, which is a lead-based ferroelectric material. Solid state reaction accompanied by the usual sintering technique is the classical ceramic-processing method, which demands a lot of time and effort. The present work aims to make the process a lot easier and quicker by employing a modified sol-gel combustion technique to synthesize polycrystalline Ba0.85Ca0.15Ti(1-x)HfxO3 (x=0.00, 0.05, 0.10, 0.15) electro ceramics . The molar ration is fixed at 1:1 for metal and citric acid at pH ~ 1. It was found that Ba0.85Ca0.15Ti(1-x)HfxO3 (where x=0.00, 0.05, 0.10, 0.15) crystallizes completely at around 1000 °C which is much lower than traditional methods. The structure supposedly displays a tetragonal symmetry with the P4mm space group as confirmed through x-ray diffraction (XRD) and Raman spectroscopy.