arXiv subjects

Z. -G. Ye

Publications and source records attributed to Z. -G. Ye.

At least 19 recordsLinked to original sources

arXiv2025

Twinning Relationships in Coexisting Cubic and Tetragonal Phases of Ferroelectrics

Many ferroelectric materials undergo thermally driven transitions between cubic and tetragonal phases. Despite being well-known for decades, such transitions still pose unresolved questions, particularly about the role of domains in shaping the transition pathway. It is understood that tetragonal domain assemblages are crucial for mismatch-free coexistence of both tetragonal and cubic phases, as described by the Weschler-Lieberman-Read (WLR) crystallographic theory. However, direct experimental characterization of domain patterns during the phase transition remains challenging. Here, we enhance the single-crystal X-ray diffraction to investigate domain microstructures during cubic-tetragonal phase transition, leveraging the bulk-penetrating and non-destructive character of this method. We extend the WLR model by the analytical expressions for the orientation relationships between the real and reciprocal bases of the cubic phase and tetragonal domains. We systematically catalogue the Bragg peak separations for 24 possible coexistence variants and validate these predictions using 3D reciprocal space maps from a piezo-/ferroelectric PMN-35PT crystal at phase coexistence temperatures. Our results provide a framework for interpreting domain configurations during the phase transition and offer insights applicable to other systems involving orientation relationships in coexisting phases.

cond-mat.mtrl-sci↗

arXiv2019

Decoupled molecular and inorganic framework dynamics in CH$_3$NH$_3$PbCl$_3$

The organic-inorganic lead halide perovskites are composed of organic molecules imbedded in an inorganic framework. The compounds with general formula CH$_{3}$NH$_{3}$PbX$_{3}$ (MAPbX$_{2}$) display large photovoltaic efficiencies for halogens $X$=Cl, Br, and I in a wide variety of sample geometries and preparation methods. The organic cation and inorganic framework are bound by hydrogen bonds that tether the molecules to the halide anions, and this has been suggested to be important to the optoelectronic properties. We have studied the effects of this bonding using time-of-flight neutron spectroscopy to measure the molecular dynamics in CH$_3$NH$_3$PbCl$_3$ (MAPbCl$_3$). Low-energy/high-resolution neutron backscattering reveals thermally-activated molecular dynamics with a characteristic temperature of $\sim$ 95\,K. At this same temperature, higher-energy neutron spectroscopy indicates the presence of an anomalous broadening in energy (reduced lifetime) associated with the molecular vibrations. By contrast, neutron powder diffraction shows that a spatially long-range structural phase transitions occurs at 178\,K (cubic $\rightarrow$ tetragonal) and 173\,K (tetragonal $\rightarrow$ orthorhombic). The large difference between these two temperature scales suggests that the molecular and inorganic lattice dynamics in MAPbCl$_3$ are actually decoupled. With the assumption that underlying physical mechanisms do not change with differing halogens in the organic-inorganic perovskites, we speculate that the energy scale most relevant to the photovoltaic properties of the lead-halogen perovskites is set by the lead-halide bond, not by the hydrogen bond.

cond-mat.mtrl-sci↗

arXiv2019

Charge effects in donor doped perovskite ferroelectrics

Doping is a widely used method to tune physical properties of ferroelectric perovskites. Since doping can induce charges due to the substitution of certain elements, charge effects shall be considered in doped samples. To understand how charges can affect the system, we incorporate the dipole-charge interaction into our simulations, where the pinched hysteresis loops can well be reproduced. Two charge compensation models are proposed and numerically investigated to understand how lanthanum doping affect BaTiO$_{3}$'s ferroelectric phase transition temperature and hysteresis loop. The consequences of the two charge compensation models are compared and discussed.

cond-mat.mtrl-sci↗

arXiv2019

Common acoustic phonon lifetimes in inorganic and hybrid lead halide perovskites

The acoustic phonons in the organic-inorganic lead halide perovskites have been reported to have anomalously short lifetimes over a large part of the Brillouin zone. The resulting shortened mean free paths of the phonons have been implicated as the origin of the low thermal conductivity. We apply neutron spectroscopy to show that the same acoustic phonon energy linewidth broadening (corresponding to shortened lifetimes) occurs in the fully inorganic CsPbBr$_{3}$ by comparing the results on the organic-inorganic CH$_{3}$NH$_{3}$PbCl$_{3}$. We investigate the critical dynamics near the three zone boundaries of the cubic $Pm\overline{3}m$ Brillouin zone of CsPbBr$_{3}$ and find energy and momentum broadened dynamics at momentum points where the Cs-site ($A$-site) motions contribute to the cross section. Neutron diffraction is used to confirm that both the Cs and Br sites have unusually large thermal displacements with an anisotropy that mirrors the low temperature structural distortions. The presence of an organic molecule is not necessary to disrupt the low-energy acoustic phonons at momentum transfers located away from the zone center in the lead halide perovskites and such damping may be driven by the large displacements or possibly disorder on the $A$ site.

cond-mat.mtrl-sci↗

arXiv2019

Lifetime-shortened acoustic phonons and static order at the Brillouin zone boundary in the organic-inorganic perovskite CH$_3$NH$_3$PbCl$_3$

Lead halide hybrid perovskites consist of an inorganic framework hosting a molecular cation located in the interstitial space. These compounds have been extensively studied as they have been identified as promising materials for photovoltaic applications with the interaction between the molecular cation and the inorganic framework implicated as influential for the electronic properties. CH3NH3PbCl3 undergoes two structural transitions from a high temperature cubic unit cell to a tetragonal phase at 177 K and an orthorhombic transition at 170 K. We have measured the low-frequency lattice dynamics using neutron spectroscopy and observe an energy broadening in the acoustic phonon linewidth towards the symmetry point QX =(2,1/2,0) when approaching the transitions. Concomitant with these zone boundary anomalies is a hardening of the entire acoustic phonon branch measured near the (2, 0, 0) Bragg position with decreasing temperature. Measurements of the elastic scattering at the Brillouin zone edges QX = (2,1/2,0), QM = (3/2,1/2,0), and QR = (3/2,3/2,5/2) show Bragg peaks appearing below these structural transitions. Based on selection rules of neutron scattering, we suggest that the higher 177 K transition is displacive with a distortion of the local octahedral environment and the lower transition is a rigid tilt transition of the octahedra. We do not observe any critical broadening in energy or momentum, beyond resolution, of these peaks near the transitions. We compare these results to the critical properties reported near the structural transitions in other perovskites. We suggest that the simultaneous onset of static resolution-limited Bragg peaks at the zone boundaries and the changes in acoustic phonon energies near the zone center is evidence of a coupling between the inorganic framework and the molecular cation.

cond-mat.mtrl-sci↗

arXiv2016

Insight into dielectric response of ferroelectric relaxors by statistical modeling

Ferroelectric relaxors are complex materials with distinct properties. The understanding of their dielectric susceptibility, which strongly depends on both temperature and probing frequency, have interested researchers for many years. Here we report a macroscopic and phenomenological approach based on statistical modeling to investigate and better understand how the dielectric response of a relaxor depends on temperature. Employing the Maxwell-Boltzmann distribution and considering temperature dependent dipolar orientational polarizability, we propose a minimum statistical model and specific equations to understand and fit numerical and experimental dielectric responses versus temperature. We show that the proposed formula can successfully fit the dielectric response of typical relaxors, including Ba(Zr,Ti)O$_{3}$, Pb(Zn$_{1/3}$Nb$_{2/3}$)$_{0.87}$Ti$_{0.13}$O$_{3}$, and Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$-0.05Pb(Zr$_{0.53}$Ti$_{0.47}$)O$_{3}$, which demonstrates the general applicability of this approach.

cond-mat.mtrl-sci↗

arXiv2014

Phase transitions and thermal-stress-induced structural changes in a ferroelectric Pb(Zr0.80Ti0.20)O3 single crystal

A single crystal of lead-zirconate-titanate (PZT), composition Pb(Zr0.80Ti0.20)O3, was studied by polarized-Raman scattering as a function of temperature. Raman spectra reveal that the local structure deviates from the average structure in both ferroelectric and paraelectric phases. We show that the crystal possesses several, inequivalent complex domain boundaries which show no sign of instability even 200 K above the ferroelectric-to-paraelectric phase transition temperature TC. Two types of boundaries are addressed. The first boundary was formed between ferroelectric domains below TC. This boundary remained stable up to the highest measurement temperatures, and stabilized the domains so that they had the same orientation after repeated heating and cooling cycles. These domains transformed normally to the cubic paraelectric phase. Another type of boundary was formed at 673 K and exhibited no signs of instability up to 923 K. The boundary formation was reversible: it formed and vanished between 573 and 673 K during heating and cooling, respectively. A model in which the crystal is divided into thin slices with different Zr/Ti ratios is proposed. The physical mechanism behind the thermal-stress-induced structural changes is related to the different thermal expansion of the slices, which forces the domain to grow similarly after each heating and cooling cycle. The results are interesting for non-volatile memory development, as it implies that the original ferroelectric state can be restored after the material has been transformed to the paraelectric phase. It also suggests that a low-symmetry structure, stable up to high-temperatures, can be prepared through controlled deposition of layers with desired compositions.

cond-mat.mtrl-sci↗

arXiv2014

The role of random electric fields in relaxors

PbZr_{1-x}Ti_xO_3 (PZT) and Pb(Mg_{1/3}Nb_{2/3})_{1-x}Ti_xO_3 (PMN-$x$PT) are complex lead-oxide perovskites that display exceptional piezoelectric properties for pseudorhombohedral compositions near a tetragonal phase boundary. In PZT these compositions are ferroelectrics, but in PMN-xPT they are relaxors because the dielectric permittivity is frequency dependent and exhibits non-Arrhenius behavior. We show that the nanoscale structure unique to PMN-xPT and other lead-oxide perovskite relaxors is absent in PZT and correlates with a greater than 100% enhancement of the longitudinal piezoelectric coefficient in PMN-xPT relative to that in PZT. By comparing dielectric, structural, lattice dynamical, and piezoelectric measurements on PZT and PMN-xPT, two nearly identical compounds that represent weak and strong random electric field limits, we show that quenched (static) random fields establish the relaxor phase and identify the order parameter.

cond-mat.mtrl-sci↗

arXiv2013

A statistical model approximation for perovskite solid-solutions: a Raman study of lead-zirconate-titanate single crystal

Lead titanate (PbTiO3) is a classical example of a ferroelectric perovskite oxide illustrating a displacive phase transition accompanied by a softening of a symmetry-breaking mode. The underlying assumption justifying the soft-mode theory is that the crystal is macroscopically sufficiently uniform so that a meaningful free energy function can be formed. In contrast to PbTiO3, experimental studies show that the phase transition behaviour of lead-zirconate-titanate solid solution (PZT) is far more subtle. Most of the studies on the PZT system have been dedicated to ceramic or powder samples, in which case an unambiguous soft-mode study is not possible, as modes with different symmetries appear together. Our Raman scattering study on titanium-rich PZT single crystal shows that the phase transitions in PZT cannot be described by a simple soft-mode theory. In strong contrast to PbTiO3, splitting of transverse E-symmetry modes reveals that there are different locally-ordered regions. The role of crystal defects, random distribution of Ti and Zr at the B-cation site and Pb ions shifted away from their ideal positions, dictates the phase transition mechanism. A statistical model explaining the observed peak splitting and phase transformation to a complex state with spatially varying local order in the vicinity of the morphotropic phase boundary is given.

cond-mat.mtrl-sci↗

arXiv2012

Evidence for anisotropic polar nanoregions in relaxor PMN: A neutron study of the elastic constants and anomalous TA phonon damping

We use neutron scattering to characterize the acoustic phonons in the relaxor PMN and demonstrate the presence of an anisotropic damping mechanism directly related to short-range, polar correlations. For a large range of temperatures above Tc ~ 210, K, where dynamic polar correlations exist, acoustic phonons propagating along [1\bar{1}0] and polarized along [110] (TA2 phonons) are overdamped and softened across most of the Brillouin zone. By contrast, acoustic phonons propagating along [100] and polarized along [001] (TA1 phonons) are overdamped and softened for only a limited range of wavevectors. The anisotropy and temperature dependence of the acoustic phonon energy linewidth are directly correlated with the elastic diffuse scattering, indicating that polar nanoregions are the cause of the anomalous behavior. The damping and softening vanish for q -> 0, i.e. for long-wavelength acoustic phonons, which supports the notion that the anomalous damping is a result of the coupling between the relaxational component of the diffuse scattering and the harmonic TA phonons. Therefore, these effects are not due to large changes in the elastic constants with temperature because the elastic constants correspond to the long-wavelength limit. We compare the elastic constants we measure to those from Brillouin scattering and to values reported for pure PT. We show that while the values of C44 are quite similar, those for C11 and C12 are significantly less in PMN and result in a softening of (C11-C12) over PT. There is also an increased elastic anisotropy (2C44/(C11-C12)) versus that in PT. These results suggest an instability to TA2 acoustic fluctuations in relaxors. We discuss our results in the context of the debate over the "waterfall" effect and show that they are inconsistent with TA-TO phonon coupling or other models that invoke the presence of a second optic mode.

cond-mat.mtrl-sci↗

arXiv2012

Structural heterogeneity and diffuse scattering in morphotropic lead zirconate-titanate single crystals

Complementary diffuse and inelastic synchrotron X-ray scattering measurements of lead zirconate-titanate single crystals with composition near the morphotropic phase boundary (x=0.475) are reported. In the temperature range 293 K < T < 400 K a highly anisotropic quasielastic diffuse scattering is observed. Above 400 K this scattering disappears. Its main features can be reproduced by model of inhomogeneous lattice deformations caused by inclusions of a tetragonal phase into a rhombohedral or monoclinic phase. This observation supports the idea that PZT at its morphotropic phase boundary is essentially structurally inhomogeneous.

cond-mat.mtrl-sci↗

arXiv2010

Single Crystal Study of Competing Rhombohedral and Monoclinic Order in Lead Zirconate Titanate

Neutron diffraction data obtained on single crystals of PbZr1-xTixO3 with x = 0.325 and x = 0.460, which lie on the pseudorhombohedral side of the morphotropic phase boundary, suggest a coexistence of rhombohedral (R3m/R3c) and monoclinic (Cm) domains and that monoclinic order is enhanced by Ti substitution. A monoclinic phase with a doubled unit cell (Cc) is ruled out as the ground state.

cond-mat.mtrl-sci↗

arXiv2009

A reassessment of the Burns temperature and its relationship to the diffuse scattering, lattice dynamics, and thermal expansion in the relaxor PMN

We have used neutron scattering techniques to characterize the diffuse scattering and lattice dynamics in single crystals of the relaxor PMN from 10K to 900K. We observed two distinct types of diffuse scattering. The first is weak, relatively temperature independent, persists to at least 900 K, and forms bow-tie-shaped patterns in reciprocal space centered on (h00) Bragg peaks. We associate this primarily with chemical short-range order. The second is strong, temperature dependent, and forms butterfly-shaped patterns centered on (h00) Bragg peaks. This diffuse scattering has been attributed to the PNR because it responds to an electric field and vanishes near Td ~ 620K when measured with thermal neutrons. Surprisingly, it vanishes at 420K when measured with cold neutrons, which provide ~4 times superior energy resolution. That this onset temperature depends on the instrumental energy resolution demands a reassessment of the Burns temperature Td. Neutron backscattering measurements made with 300 times better energy resolution confirm the onset temperature of 420+/-20K. The energy width of the diffuse scattering is resolution limited, indicating that the PNR are static on timescales of at least 2 nsec. Transverse acoustic (TA) phonon lifetimes are temperature independent up to 900K for q < 0.2 1/A. This motivates a physical picture in which sufficiently long-wavelength TA phonons average over the PNR; only those TA phonons having wavelengths comparable to the size of the PNR are affected. The PMN lattice constant changes by less than 0.001 Angstroms below 300K, but expands rapidly at a rate of 2.5x10^-5 1/K at high temperature. The regimes of low and high thermal expansion bracket the revised value of Td, which suggests the anomalous thermal expansion results from the condensation of static PNR.

cond-mat.mtrl-sci↗

arXiv2008

Spatially Resolved Mapping of Local Polarization Dynamics in an Ergodic Phase of Ferroelectric Relaxor

Spatial variability of polarization relaxation kinetics in relaxor ferroelectric 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 is studied using time-resolved Piezoresponse Force Microscopy. Local relaxation attributed to the reorientation of polar nanoregions is shown to follow stretched exponential dependence, exp(-(t/tau)^beta), with beta~~0.4, much larger than the macroscopic value determined from dielectric spectra (beta~~0.09). The spatial inhomogeneity of relaxation time distributions with the presence of 100-200 nm "fast" and "slow" regions is observed. The results are analyzed to map the Vogel-Fulcher temperatures on the nanoscale.

cond-mat.mtrl-sci↗

arXiv2005

Relaxation and freezing of dielectric response in relaxor Pb(Mg1/3Nb2/3)O3 crystal

Dielectric investigations of the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 crystal were performed in the frequency domain at 10^(-4) 10^5 Hz. In the ergodic relaxor phase two overlapping relaxation processes obeying the Kohlrausch-Williams-Watts law, exp[-(t/tau)^beta], and the Curievon Schweidler law, t^(-n), respectively, were found and parameterized. The values of beta, n and 1/tau tend to zero at 211 K following the Vogel-Fulcher law, pointing to the glassy freezing of the dipole dynamics related to both relaxation processes.

cond-mat.mtrl-sci↗

arXiv2005

Deviations from the Summerfield scaling in the piezoelectric PbZr0.57Ti0.43O3

Extending our earlier observations of the deviations from Summerfield scaling in a crystalline ferroelectric system of SBCT, we have studied the scaling of conductivity in another ferro/piezoelectric perovskite crystalline oxide, PZT. The deviations from Summerfield scaling are found at T greater than TC. Correction to the Summerfield scaling is achieved by the application of the Random Barrier Model and master cures are constructed with a value of the scaling function alpha 3.2. As in the case of SBCT, deviations still occur at T greater TC. The master curves are constructed from the conductivity data sigma prime fitted to a modified Jonscher law in the frequency range 10 to 1e5 Hz over the temperature region 30 to 480 degrees C. The positive values of alpha indicate coulomb exchange between the interacting particles. This work provides an additional example which allows us to believe that crystalline systems as well may follow the scaling laws, not only amorphous glasses, as was previously believed.

cond-mat.mtrl-sci↗

arXiv2004

Raman spectroscopy study of internal dynamics in Pb(Zn_{1/3}Nb_{2/3})O_{3} and 95.55%Pb(Zn_{1/3}Nb_{2/3})O_{3}-4.5%PbTiO_{3} crystals

Pb(Zn$_{1/3}$Nb$_{2/3}$)O$_{3}$ is one of the simplest representatives of the lead ferroelectric relaxors. Its solid solution with PbTiO$_{3}$, at concentrations near the morphotropic phase boundary, is especially important. In this paper, we apply the tested earlier on Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ approach, to analyze the light scattering from both, nominally pure Pb(Zn$_{1/3}$Nb$_{2/3} $)O$_{3}$ crystal and a crystal containing 4.5% of PbTiO$_{3}$, measured in a broad temperature range from 1000 to 100 K. We propose a comprehensive picture of the temperature evolution of the lattice dynamics in these crystals and associated structural transformations. We show, that in PZN, like in PMN, short-lived dynamic lattice distortions exist even at the highest measured temperatures. With cooling down, these distortions develop in Fm$\bar{3}$m clusters and R3m polar nanoregions that are still capable of reorientational motion. From the Burns temperature T$_{d}$, this motion becomes progressively restricted. The freezing process starts at the temperature T$^{*}$ and continues down to T$_{d0}$. The major Raman lines in all these materials behave very similarly. However, the temperature behavior of a weak line E in each case is essentially different. Its splitting can serve as a measure of the order parameter of the system.

cond-mat.mtrl-sci↗

arXiv2004

Structural phase transition and dielectric relaxation in Pb(Zn1/3Nb2/3)O3 single crystals

The structure and the dielectric properties of Pb(Zn1/3Nb2/3)O3 (PZN) crystal have been investigated by means of high-resolution synchrotron x-ray diffraction (with an x-ray energy of 32 keV) and dielectric spectroscopy (in the frequency range of 100 Hz - 1 MHz). At high temperatures, the PZN crystal exhibits a cubic symmetry and polar nanoregions inherent to relaxor ferroelectrics are present, as evidenced by the single (222) Bragg peak and by the noticeable tails at the basis of the peak. At low temperatures, in addition to the well-known rhombohedral phase, another low-symmetry, probably ferroelectric, phase is found. The two phases coexist in the form of mesoscopic domains. The para- to ferroelectric phase transition is diffused and observed between 325 and 390 K, where the concentration of the low-temperature phases gradually increases and the cubic phase disappears upon cooling. However, no dielectric anomalies can be detected in the temperature range of diffuse phase transition. The temperature dependence of the dielectric constant show the maximum at higher temperature (Tm = 417 - 429 K, depending on frequency) with the typical relaxor dispersion at T < Tm and the frequency dependence of Tm fitted to the Vogel-Fulcher relation. Application of an electric field upon cooling from the cubic phase or poling the crystal in the ferroelectric phase gives rise to a sharp anomaly of the dielectric constant at T 390 K and diminishes greatly the dispersion at lower temperatures, but the dielectric relaxation process around Tm remains qualitatively unchanged. The results are discussed in the framework of the present models of relaxors and in comparison with the prototypical relaxor ferroelectric Pb(Mg1/3Nb2/3)O3.

cond-mat.mtrl-sci↗