Reconstructing phase diagrams from local measurements via Gaussian processes: mapping the temperature-composition space to confidence

We show the ability to map the phase diagram of a relaxor-ferroelectric system as a function of temperature and composition through local hysteresis curve acquisition, with the voltage spectroscopy data being used as a proxy for the (unknown) microscopic state or thermodynamic parameters of materials. Given the discrete nature of the measurement points, we use Gaussian processes to reconstruct hysteresis loops in temperature and voltage space, and compare the results with the raw data and bulk dielectric spectroscopy measurements. The results indicate that the surface transition temperature is similar for all but one composition with respect to the bulk. Through clustering algorithms, we recreate the main features of the bulk diagram, and provide statistical confidence estimates for the reconstructed phase transition temperatures. We validate the method by using Gaussian processes to predict hysteresis loops for a given temperature for a composition unseen by the algorithm, and compare with measurements. These techniques can be used to map phase diagrams from functional materials in an automated fashion, and provide a method for uncertainty quantification and model selection

Phase transition and enhanced magneto-dielectric response in BiFeO3-DyMnO3 multiferroics

This work is partially supported by DST fast track Project No. SR/FTP/PS-16/2009. Dhiren K. Pradhan acknowledges IFN (NSF Grant No. EPS—01002410) for fellowship. The work at UPR was supported by National Science Foundation (NSF DMR 1410869) and Institute for Functional Nanomaterials (IFN).We report systematic studies on crystal structure and magneto-dielectric properties of (1 − x) BiFeO3-x DyMnO3 (0.0 ≤ x ≤ 0.2) nanoceramics synthesized by auto-combustion method. Rietveld refinement of X-ray diffraction data indicates a structural transition from R3c to R3c + Pn21a at x = 0.1. Field emission scanning electron micrographs display a decrease in grain size with increase in x. The presence of dielectric anomalies around antiferromagnetic transition temperature implies the magnetoelectric coupling. Dielectric measurements showed decrease in magnetic ordering temperature with increasing x in agreement with differential scanning calorimetry results. A significant increase in magnetization has been found with increasing DyMnO3 substitution. Magneto-impedance spectroscopy reveals a significant change (∼18%) in dielectricpermittivity at H = 2 T for x = 0.2.Peer reviewe

Effect of thickness on dielectric, ferroelectric, and optical properties of Ni substituted Pb(Zr0.2Ti0.8)O3 thin films

This work was supported by NSF Grant EPS-01002410. N. Ortega acknowledges support from the DoE Grant DE-FG02-08ER46526.We report thickness dependent dielectric,ferroelectric, and optical properties of Ni substituted Pb(Zr0.2Ti0.8)O3 thin films. The Pb(Zr0.2Ti0.8)0.70Ni0.30O3−δ (PZTNi30) thin films for various thicknesses, ranging from 5 nm to 400 nm, were fabricated by pulsed laser deposition technique. Giant dielectric dispersion, low dielectric loss, large dielectric constant  -1000–1500 from 100 Hz to 100 kHz, and diffused dielectric anomaly near 570–630 K were observed in PZTNi30 thin films. These films show well saturated ferroelectric hysteresis, with large remanent polarization. It also illustrated excellent optical transparency which decreased from 82 to 72% with increasing film thickness from 5 nm to 400 nm for the probe wavelengths ranging from 200 to 1100 nm. A decrease in direct bandgap (Eg) values from 4 eV to 3.4 eV and indirect-Eg values from 3.5 eV to 2.9 eV were observed for PZTNi30 thin films with increase in film thickness from 5 nm to 400 nm, respectively. The direct and indirect bandgaps were discussed in context of film thickness and grain size effects. Our investigations on optical properties of PZTNi30 thin films suggest that bandgap can be modified as a function of film thickness which may be useful for readers working to develop novel candidates for ferroelectric photovoltaic.Publisher PDFPeer reviewe

Palladium-based ferroelectrics and multiferroics : theory and experiment

Palladium normally does not easily substitute for Ti or Zr in perovskite oxides. Moreover, Pd is not normally magnetic (but becomes ferromagnetic under applied uniaxial stress or electric fields). Despite these two great obstacles, we have succeeded in fabricating lead zirconate titanate with 30% Pd substitution. For 20:80 Zr:Ti the ceramics are generally single-phase perovskite (>99%), but sometimes exhibit 1% PbPdO2, which is magnetic below T=90K. The resulting material is multiferroic (ferroelectric-ferromagnet) at room temperature. The processing is slightly unusual (>8 hrs in high-energy ball-milling in Zr balls), and the density functional theory provided shows that it occurs because of Pd+4 in the oversized Pb+2 site; if all Pd+4 were to go into the Ti+4 perovskite B-site, no magnetism would result.PostprintPeer reviewe

Enhanced ferroelectric and piezoelectric properties of BCT-BZT at the morphotropic phase boundary driven by the coexistence of phases with different symmetries

The discovery of lead-free piezoelectric materials is crucial for future information and energy storage applications. Enhanced piezoelectric and other physical properties are commonly observed near the morphotropic phase boundary (MPB) composition of ferroelectric solid solutions. The (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) system exhibits a large electromechanical response around its MPB region at x=0.5. We report experimental and theoretical results of BZT-xBCT over a wide composition range (0.3=x=1.0). X-ray diffraction and Raman spectroscopy studies indicate a composition-induced structural phase transition from a rhombohedral (R3m) phase at x=0.4 to a tetragonal (P4mm) phase at x=0.6 through a multiphase coexistence region at 0.45=x=0.55 involving orthorhombic + tetragonal (Amm2+P4mm) phases. First-principles calculations elucidate the phase competition in the coexistence region. The critical composition (x= 0.5) displays enhanced dielectric, ferroelectric and piezoelectric properties, where notably d33~ 320 pC/N. This work provides clear evidence of Amm2+P4mm crystallographic phases in the MPB region, which is responsible for the improved functional properties.C. C. acknowledges support from the Spanish Ministry of Science, Innovation, and Universities under the “Ramón y Cajal” fellowship RYC2018-024947-I.Peer ReviewedPostprint (author's final draft

Scalable and Stable Ferroelectric Non-Volatile Memory at > 500 ∘^\circC

Non-volatile memory (NVM) devices that reliably operate at temperatures above 300 $^\circ$C are currently non-existent and remains a critically unmet challenge in the development of high-temperature (T) resilient electronics, necessary for many emerging, complex computing and sensing in harsh environments. Ferroelectric Al$_x$Sc$_{1-x}$N exhibits strong potential for utilization in NVM devices operating at very high temperatures (> 500 $^\circ$C) given its stable and high remnant polarization (PR) above 100 $\mu$C/cm$^2$ with demonstrated ferroelectric transition temperature (TC) > 1000 $^\circ$C. Here, we demonstrate an Al$_{0.68}$Sc$_{0.32}$N ferroelectric diode based NVM device that can reliably operate with clear ferroelectric switching up to 600 $^\circ$C with distinguishable On and Off states. The coercive field (EC) from the Pulsed I-V measurements is found to be -5.84 (EC-) and +5.98 (EC+) (+/- 0.1) MV/cm at room temperature (RT) and found to decrease with increasing temperature up to 600 $^\circ$C. The devices exhibit high remnant polarizations (> 100 $\mu$C/cm$^2$) which are stable at high temperatures. At 500 $^\circ$C, our devices show 1 million read cycles and stable On-Off ratio above 1 for > 6 hours. Finally, the operating voltages of our AlScN ferrodiodes are < 15 V at 600 $^\circ$C which is well matched and compatible with Silicon Carbide (SiC) based high temperature logic technology, thereby making our demonstration a major step towards commercialization of NVM integrated high-T computers.Comment: MS and S

Studies of multiferroic palladium perovskites

Work at St. Andrews supported by EPSRC grant EP/P024637/1; at Queens University Belfast by EPSRC Grant No. EP/J017191/1 and EP/N018389/1, at Univ. Puerto Rico by DoD AFOSR Grant FA9550-16-1-0295 and at West Virginia University by DOE (DE-SC0016176).We have studied the atomic force microscopy (AFM), X-ray Bragg reflections, X-ray absorption spectra (XAS) of the Pd L-edge, Scanning electron microscopey (SEM) and Raman spectra, and direct magnetoelectric tensor of Pd-substituted lead titanate and lead zirconate-titanate. A primary aim is to determine the percentage of Pd+4 and Pd+2 substitutional at the Ti-sites (we find that it is almost fully substitutional). The atomic force microscopy data uniquely reveal a surprise: both threefold vertical (polarized out-of-plane) and fourfold in-plane domain vertices. This is discussed in terms of the general rules for Voronoi patterns (Dirichlet tessellations) in two and three dimensions. At high pressures Raman soft modes are observed, as in pure lead titanate, and X-ray diffraction (XRD) indicates a nearly second-order displacive phase transition. However, two or three transitions are involved: First, there are anomalies in c/a ratio and Raman spectra at low pressures (P = 1 − 2 GPa); and second, the c/a ratio reaches unity at ca. P = 10 GPa, where a monoclinic (Mc) but metrically cubic transition occurs from the ambient tetragonal P4 mm structure in pure PbTiO3; whereas the Raman lines (forbidden in the cubic phase) remain until ca. 17 GPa, where a monoclinic-cubic transition is known in lead titanate.Publisher PDFPeer reviewe