Jin S. Zhang

A new high-pressure phase transition in natural Fe-bearing orthoenstatite

Abstract Single-crystal X-ray structure refinements have been carried out on natural Fe-bearing orthoenstatite (OEN) at pressures up to 14.53 GPa. We report a new high-pressure phase transition from OEN to a monoclinic phase (HPCEN2) with space group P21/c, with a density change of ~1.9(3)%. The HPCEN2 phase is crystallographically different from low-pressure clinoenstatite (LPCEN), which also has P21/c symmetry. Upon release of pressure HPCEN2 reverts to OEN, and the transition pressure is bracketed between 9.96 and 14.26 GPa at room temperature. We find no evidence for a C2/c phase at high pressure. The lattice constants for the new phase at 14.26 GPa are a = 17.87(2), b = 8.526(9), c = 4.9485(10) Å, β = 92.88(4)° [ρ = 3.658(9) g/cm3]. Refinement of the new structure indicates rotation of tetrahedral chain as the key characteristic of this transition. This experiment points to the possibility of OEN and HPCEN2 as the stable phases in Earth’s upper mantle.

Enhanced dielectric properties from barium strontium titanate films with strontium titanate buffer layers

In order to enhance the permittivity and tunability of the dielectric component, a thin film dielectric composite consisting of a radio frequency sputtered SrTiO3 (STO) buffer layer and metalorganic solution deposited Mg-doped BaxSr1−xTiO3 (Mg-BST) thin film overgrowth was developed using affordable industry standard processes and materials. The effect of the STO buffer layer thickness on the dielectric response of the heterostructure was investigated. Our results demonstrate that the composite film heterostructure, evaluated in the metal-insulator-metal configuration Pt/STO/Mg-BST/Pt on sapphire substrate, with the thinner (9–17 nm) STO buffer layers possessed enhanced permittivity (er ∼ 491) with respect to the thicker 41 nm buffer layer (er ∼ 360) and that of a control Mg-BST film without a STO buffer layer (er ∼ 380). Additionally, the composite film with the thinner buffer layers were shown to have low losses (tan δ ∼ 0.02), low leakage characteristics (J = 7.0 × 10−9 A/cm2), high breakdown voltage (...

Pyroelectric response of lead zirconate titanate thin films on silicon: Effect of thermal stresses

Ferroelectric lead zirconate titanate [Pb(ZrxTi1-xO)3, (PZT x:1-x)] has received considerable interest for applications related to uncooled infrared devices due to its large pyroelectric figures of merit near room temperature, and the fact that such devices are inherently ac coupled, allowing for simplified image post processing. For ferroelectric films made by industry-standard deposition techniques, stresses develop in the PZT layer upon cooling from the processing/growth temperature due to thermal mismatch between the film and the substrate. In this study, we use a non-linear thermodynamic model to investigate the pyroelectric properties of polycrystalline PZT thin films for five different compositions (PZT 40:60, PZT 30:70, PZT 20:80, PZT 10:90, PZT 0:100) on silicon as a function of processing temperature (25–800 °C). It is shown that the in-plane thermal stresses in PZT thin films alter the out-of-plane polarization and the ferroelectric phase transformation temperature, with profound effect on the ...

Elasticity of cubic boron nitride under ambient conditions

As a superhard material with properties similar to diamond including chemical inertness, cubic boron nitride (cBN) is an excellent candidate as a pressure calibration standard for high-temperature high-pressure research using its pressure-volume-temperature (P-V-T) equation of state. However, the elastic properties of cBN at ambient conditions reported in the literature vary by up to 8%, which can likely be attributed in part to variability in the cBN composition and defect structure and measurement uncertainties. We have measured the single-crystal elastic moduli of high-purity cBN with high precision by Brillouin scattering measurements, making an effort to minimize experimental uncertainties. We obtain values of C11 = 798.4 ± 1.7 GPa, C44 = 469.0 ± 1.0 GPa, and C12 = 172.4 ± 1.1 GPa, from which the isotropic aggregate bulk modulus Ks = 381.1 ± 1.3 GPa and shear modulus G = 398.8 ± 1.2 GPa (the Hill average) were calculated. Our results improve the precision and reduce the uncertainties in the elastic m...

Enhanced electrocaloric and pyroelectric response from ferroelectric multilayers

Room temperature pyroelectric properties and adiabatic temperature change of (001)-textured ferroelectric multilayers on Si are computed by taking into account electrostatic interlayer interactions and thermal strains. We show that by adjusting internal electrical fields through changing relative thicknesses in a multilayer ferroelectric construct, electrothermal properties can be significantly enhanced. A quantitative analysis is provided for BaTiO3-PbZr0.2Ti0.8O3 (BTO-PZT) and SrTiO3-PbZr0.2Ti0.8O3 (STO-PZT) multilayers. For instance, 0.74 × BTO-0.26 × PZT and 0.35 × STO-0.65 × PZT bilayers show ∼120% and 65% increase in electrocaloric response, respectively, compared to PZT films on Si for ΔE = 500 kV/cm.

Bulk-like dielectric properties from metallo-organic solution–deposited SrTiO3 films on Pt-coated Si substrates

SrTiO3 (STO) thin films were grown onto Pt/Ti/TiO2/SiO2/Si and bare Si substrates via metallo-organic solution deposition (MOSD). X ray diffraction, scanning electron microscopy, atomic force microscopy, spectroscopic ellipsometry (SE), and dielectric/insulating measurements were utilized to characterize the STO thin films. By optimizing the solution molarity, it is shown that it is possible to obtain low-frequency dielectric properties in STO thin films on Si substrates that approach those of bulk STO: a dielectric permittivity of 325 at 100 kHz with a tunability of ∼28% at ∼650 kV/cm, a low loss (<2%), and a leakage current density of 1.0 × 10−6 A/cm2 at 100 kV/cm. The bulk-like dielectric response of these films is attributed to the elimination of the relatively large in-plane tensile thermal stresses, which arise due to the thermal expansion mismatch between the STO film and the Si substrate. These tensile thermal stresses may be relaxed or completely eliminated through modification of the post-coales...

Tailoring dielectric properties of ferroelectric-dielectric multilayers

We develop a nonlinear thermodynamic model for multilayer ferroelectric heterostructures that takes into account electrostatic and electromechanical interactions between layers. We concentrate on the effect of relative layer fractions and in-plane thermal stresses on dielectric properties of Ba0.6Sr0.4TiO3-, BaTiO3-, and PbZr0.2Ti0.8O3 (PZT)-SrTiO3 (STO) multilayers on Si and c-sapphire. We show that dielectric properties of such multilayers can be significantly enhanced by tailoring the growth/processing temperature and the STO layer fraction. Our computations show that large tunabilities (∼90% at 400 kV/cm) are possible in carefully designed barium strontium titanate-STO and PZT-STO even on Si for which there exist substantially large in-plane strains.

Pressure-induced Pbca-P21/c phase transition of natural orthoenstatite: Compositional effect and its geophysical implications

Abstract Raman spectroscopy has been employed to investigate possible compositional effects on the high-pressure phase transition of Mg-rich orthoenstatite to a newly discovered P21/c phase. Three natural orthoenstatite (OEN) samples were used in this study: near end-member Mg orthoenstatite (Zabargad Island, Egypt), Al-free, Fe-bearing orthoenstatite (Morogoro, Tanzania) and Al-rich, Fe,Ca-bearing orthoenstatite (Kilbourne Hole, New Mexico). Experiments were carried out at room temperature. For all samples, the high-pressure phase transition is characterized by a splitting of the 660-680 cm-1 doublet in the Raman spectrum into a triplet, with a corresponding change of peak intensities. These spectral changes are caused by the lowered symmetry of the high-pressure phase, as indicated by structural refinement from single-crystal X-ray diffraction results. The high-pressure phase of all samples appears to have space group P21/c. No evidence for a C2/c phase was observed. Our results indicate that upon compression, the presence of 10 mol% Fe decreases the onset pressure of formation of the high-pressure P21/c phase by about 1 GPa. Results for the Kilbourne Hole OEN show that upon compression, a combined enrichment of Al and Ca contents increases the onset pressure of formation of high-pressure clinoesntatite (HPCEN2) by over 3 GPa relative to Tanzania OEN. Upon decompression, all samples revert to single crystals of the orthoenstatite starting phase. Our measurements suggest that orthoenstatite is the prevalent phase of Mg-rich pyroxene throughout the uppermost mantle, whereas the newly discovered P21/c phase might be present near the bottom of uppermost mantle, slightly shallower than the top of the transition zone.

Sound velocities of olivine at high pressures and temperatures and the composition of Earth's upper mantle

We present the elastic properties of San Carlos olivine up to P = 12.8(8) GPa and T = 1300(200) K using Brillouin spectroscopy with CO2 laser-heating. A comparison of our results with the global seismic model AK135 yields average olivine content near 410 km depth of about 37% and 43% in a dry and wet (1.9%wt H2O) upper mantle, respectively. These olivine contents are far less than in the pyrolite model. However, comparisons of our results with regional seismic models lead to very different conclusions. High olivine contents of up to 87% are implied by seismic models of the western U.S. and eastern Pacific regions. In contrast, we infer less than 35% olivine under the Central Pacific. Strong variations of olivine content and upper mantle lithologies near the 410 km discontinuity are suggested by regional seismic models.

Electrocaloric properties of epitaxial strontium titanate films

The electrocaloric (EC) response of strontium titanate thin films is computed as a function of misfit strain, temperature, electric field strength, and electrode configuration using a nonlinear thermodynamic theory. For films in a capacitor configuration on compressive substrates, the transition between paraelectric and strain-induced ferroelectric tetragonal phases produces a large adiabatic temperature change, ΔT = 5 K, at room temperature for electric field changes ΔE = 1200 kV/cm. For films on tensile substrates, the transition between the paraelectric and strain-induced ferroelectric orthorhombic phases can also be accessed using inter-digitated electrodes (IDEs). The maximum EC response occurs for IDEs with a [110] orientation.