Antiferroelectric Instability Research Articles

Possible antiferroelectric-to-ferroelectric transition and metallic antiferroelectricity caused by charge doping in PbZrO3

Using linear-response density functional calculations we study how the soft modes at both the zone center and the zone boundary are influenced by charge doping in cubic $\mathrm{Pb}\mathrm{Zr}{\mathrm{O}}_{3}$. We find (i) upon the electron doping of $n=\ensuremath{-}0.5$ per unit cell, the soft-mode frequency at zone-center $\mathrm{\ensuremath{\Gamma}}$ becomes considerably lower than that at the zone-boundary $R$, showing that electron doping may turn antiferroelectric $\mathrm{Pb}\mathrm{Zr}{\mathrm{O}}_{3}$ into ferroelectric. (ii) On the other hand, when the hole is doped into $\mathrm{Pb}\mathrm{Zr}{\mathrm{O}}_{3}$, the soft mode at $R$ or $M$ remains to be the lowest in frequency, and therefore antiferroelectric instability is robust and persists after hole doping. Since hole doping makes $\mathrm{Pb}\mathrm{Zr}{\mathrm{O}}_{3}$ metallic, the coexistence of metallicity and antiferroelectricity demonstrates the feasibility of metallic antiferroelectricity. (iii) Electron doping and hole doping are revealed to impact the atom-atom interaction very differently. The origin responsible for these charge-doping phenomena is provided.

Competing structural instabilities inBi2SiO5

Bi${}_{2}$SiO${}_{5}$ recently emerged as a promising ambient-temperature lead-free ferroelectric with a Curie temperature of 663 K, where the ferroelectricity arises from the tilt of quasi-one-dimensional SiO${}_{4}$ chains. Here, the authors analyze the competition between ferroelectric and antiferroelectric instabilities by investigating the characteristic temperature-dependent phonon energies at the center and the edge of the Brillouin zone, respectively. The direct evidence for this competition opens the way towards tuning the properties of Bi${}_{2}$SiO${}_{5}$ by application of pressure or electrical field.

Effect of B -site randomness on the antiferroelectric/relaxor nature of the ground state: Diffuse and inelastic x-ray scattering study of Pb(In1/2Nb1/2)O3

We have investigated the effect of $B$-site randomness on the antiferroelectric/relaxor nature of the ground state by studying diffuse and inelastic x-ray scattering from ordered and disordered $\mathrm{Pb}({\mathrm{In}}_{1/2}{\mathrm{Nb}}_{1/2}){\mathrm{O}}_{3}$ (O- and D-PIN) single crystals. The diffuse scattering measurement of O-PIN, which is antiferroelectric at low temperatures, shows that the ferroelectrically interactive local polarization exists in the cubic phase, above the transition temperature ${T}_{\mathrm{N}}$. Inelastic x-ray scattering analysis of the diffuse scattering shows that the transverse-acoustic (TA) and transverse-optic (TO) modes are dominant at high temperatures ($\ensuremath{\sim}800$ K), while the central peak (CP) and TA modes, which are coupled, contribute majorly to diffuse scattering near ${T}_{{}_{\mathrm{N}}}$ and show critical behavior at temperature close to ${T}_{{}_{\mathrm{N}}}$. The TO mode shows no anomaly at temperature close to ${T}_{{}_{\mathrm{N}}}$. Furthermore, the phonon spectra are broad, indicating that a strong damping mechanism exists even in the sample with weak chemical disordering, O-PIN. No clear difference between O- and D-PIN is observed at temperatures above $\ensuremath{\sim}500$ K. Here, the major difference between them is the property of the CP mode, which shows no drop and increases rapidly with decreasing temperatures in D-PIN. The CP mode is thought to be directly related to the local polarization and to originate in a combination of Pb flipping and the TO mode. The $B$ site is considered to control the antiferroelectric/ferroelectric instability of lead-based perovskite materials. Finally, the $B$-site randomness is discussed in terms of suppressing the antiferroelectric instability and enhancing the polarization fluctuation.

Antiferroelectric instability in the kagome francisites Cu3Bi(SeO3)2O2X ( X=Cl,Br )

Density-functional calculations of lattice dynamics and high-resolution synchrotron powder diffraction uncover antiferroelectric distortion in the kagome francisite Cu$_3$Bi(SeO$_3$)$_2$O$_2$Cl below 115K. Its Br-containing analogue is stable in the room-temperature crystal structure down to at least 10K, although the Br compound is on the verge of a similar antiferroelectric instability and reveals local displacements of Cu and Br atoms. The I-containing compound is stable in its room-temperature structure according to density-functional calculations. We show that the distortion involves cooperative displacements of Cu and Cl atoms, and originates from the optimization of interatomic distances for weakly bonded halogen atoms. The distortion introduces a tangible deformation of the kagome spin lattice and may be responsible for the reduced net magnetization of the Cl compound compared to the Br one. The polar structure of Cu$_3$Bi(SeO$_3$)$_2$O$_2$Cl is only slightly higher in energy than the non-polar antiferroelectric structure, but no convincing evidence of its formation could be obtained.

First-principles study of competing ferroelectric and antiferroelectric instabilities inBaTiO3/BaOsuperlattices

We report a first-principles study of (BaTiO${_3}$)$_m$/(BaO)$_n$ superlattices for a wide range of periodicities $m/n$. We show that such a system develops a polar zone-center instability for sufficiently large \textit{m}/\textit{n} ratio, which can be understood, at least qualitatively, from a simple electrostatic model and should lead to a ferroelectric ground-state. However, the analysis of the phonon dispersion curves also points out the appearance of stronger antiferroelectric instabilities at the zone boundaries around $m=4$, before the critical ratio for ferroelectricity is reached and which still dominate beyond it. The dominant character of the anti-ferroelectric instability is explained from the depolarizing field which hardens the ferroelectric mode. This analysis allows us to predict that, (BaTiO${_3}$)$_m$/(BaO)$_n$ superlattices should present an antiferroelectric ground state for $m$ larger than 4, which should smoothly evolve to a multidomain structure for increasing $m$ values and only become ferroelectric for large $m$.

The effect of Pb2+ substitution on the quantum paraelectric behaviour ofCaTiO3

Temperature dependent dielectric, x-ray diffraction and‘1/f’ conductancenoise studies on (Ca1−xPbx)TiO3 ceramics provide evidence for a new phase transition in this mixed system for the composition range0.10≤x≤0.40. The dielectric anomaly corresponding to this transition issuperposed on the quantum paraelectric background of the hostCaTiO3 matrix. This anomaly is not associated with a ferroelectric or relaxor ferroelectrictransition, as confirmed by the absence of a polarization hysteresis loop. TheCurie–Weiss temperature for this transition is found to be negative which,using Kittel’s model, is proposed to be due to an antiferroelectric transition.These results are discussed in the light of recently reported soft modes inCaTiO3. It is alsoshown that the nature of phase transition changes abruptly from antiferroelectric to relaxor ferroelectrictype at 0.40<x<0.45 and finally to a normal ferroelectric type for0.60<x≤1.0. The relaxor behaviour in the intermediate composition range is attributed tofrustration caused by competing ferroelectric and antiferroelectric instabilities.

Effect of quenching on structure and antiferroelectric instability of La (2− x)/3 Li xTiO 3 compounds: a Raman study

A Raman study of the La (2− x)/3 Li x TiO 3 ionic conductors has been performed as a function of temperature, lithium content and synthesis cooling rate. Irrespective of thermal treatment or lithium content, the spectrum can always be explained within a tetragonal symmetry (space group P4/mmm), which suggests that, at a local scale of several unit cells, some cation ordering persists even in samples submitted to quenching conditions. The mode involving mostly in-plane titanium vibration softens on lowering temperature, though a phase transition never occurs. The effect is attributed to the tendency of these double-perovskite systems to antiferroelectric ordering, frustrated by structural disorder. Quenching the samples from high temperature increases cation disorder and suppresses the tendency of these systems toward antiferroelectric instability. A correlation is found between the degree of instability and the parameter η=La1−La2−x, where La1 and La2 stand for the site occupancy factors of lanthanum ions in sites La1 and La2, respectively.

Charge-density-wave formation in TiSe2 driven by an incipient antiferroelectricinstability

Recently reported temperature-dependent synchrotron x-ray thermal diffusescattering data (Holt M, Zschack P, Hong H, Chou M Y and Chiang T C 2001Phys. Rev. Lett. 86 3799) are analysed within an anharmonic electron–phononinteraction model, where the observed zone boundary phonon softening is relatedto an incipient antiferroelectric instability. The parameter-free model reproducesthe temperature dependence of the soft phonon mode in excellent agreement withthe experimental data. Predictions for the charge-density-wave-induced gap aremade and consequences for the electronic structure changes are discussed.

Long-range order effects in Pb(Zr1/2Ti1/2)O3

Total energies were calculated for Pb(Zr1/2Ti1/2)O3 (PZT) superlattices with B site cations ordered along the [001] and [111] directions within the local density approximation. The local orbital extension of the Linearized Augmented Planewave (LAPW + LO) method was used to obtain energies for the unrelaxed structures and the ab initio Potential-Induced Breathing (PIB) method was applied to establish relaxation energies. We compare the relative stabilities for the PZT superlattices, pure PbTiO3 (PT) and PbZrO3 (PZ). B site cation ordering along the [111] direction results in a lower energy superstructure than that of the [001] ordered structure. The unrelaxed [111] ordered structure results in lower energy than the average energy of the unrelaxed separate PZ and PT phases. If we expect relaxation energies of the same magnitude accompanying ferroelectric and antiferroelectric instabilities in PZ, PT and PZT, we can anticipate simultaneous competition between various processes, such as phase separation, long-range order and disorder. This richness of available structures can contribute significantly to the extraordinary properties of PZT with compositions near the morphotropic phase boundary.

Evidence of High Critical Temperature Charge Density Wave Transitions in the (PO2)4(WO3)2m Family of Low Dimensional Conductors for m ${\bf\geq}$ 8

The monophosphate tungsten bronzes with pentagonal tunnels (MPTBp) of general formula (PO 2 ) 4 (WO 3 ) 2m form a family of layered conductors where the average number of conduction electrons per tungsten atom 2/m can be changed while keeping the same structural array. Previous investigation of the low m = 4, 6 and 7 members of this family have shown that this series is subject to several successive incommensurate charge density wave (CDW) long range orders below room temperature (RT). Here we present an X-ray scattering study of the m = 8, 9, 10, 11, 12, 13 and 14 members of this family. A very rich and diverse structural phase diagram is observed. The m = 8 member shows only short range order below RT at two different incommensurate wave vectors while commensurate and/or incommensurate long range order is observed above RT for m ≥ 9. Incommensurate modulations are observed for the m = 11 and 13 members and commensurate ones for the other members. In most of the 7 ≥ m ≥ 13 members the observation of several harmonics suggests that the CDW modulation is non-sinusoidal, which could be the fingerprint of electron localization phenomena due either to strong electron-phonon or electron-electron interactions. These CDW instabilities have been tentatively attributed to chains a and a ± b of WO 6 octahedra into which the ReO3 type layers of (PO 2 ) 4 (WO 3 ) 2m can be decomposed. In addition, the observation for m ≥ 9 of a commensurate (1/2,0,0) or (1/2, 0, 1/2) modulation or (1/2, 0, l) diffuse scattering suggests the occurrence of an incipient WO 3 type antiferroelectric instability which interacts with the CDW for m ≤ 13. We further discuss the division between high m value and low m value members in relation to the z dependence of the physical properties of some M x WO 3 tungsten bronzes families and finally propose a dielectric to magnetic duality between CDW tungsten oxides and superconducting copper oxides.

Dynamic local lattice distortion in YBa2Cu4O8

Significant changes in the local atomic structure and lattice dynamics were observed by pulsed neutron inelastic scattering measurements on YBa2Cu4O8 both around the superconducting transition temperature and around the spin-gap temperature. These observations and earlier results of numerical calculations on the enhancement of electron-lattice interaction by electron correlation lead to a novel picture of unconventional lattice-induced superconductivity related to antiferroelectric instability.

Dynamical study of the antiferroelectric transition in a polymer crystal : pTS-D

Abstract We report a dynamical study of a structural antiferroelectric instability in a monocrystal of a conjugated polymer : pTS-D. The neutron and Raman scattering measurements reveal a very weakly first order transition with an intermediate character between order-disorder and displacive.

Phonon spectra of NH4H2PO4 in the para- and antiferroelectric phases obtained by infrared reflectivity

The polarized infrared reflectivity spectra of NH4H2PO 4 are reported for a wide spectral range (20-1 500 cm-1) in the para- and antiferroelectric phases. Measurements were performed despite the shattered character of the sample in the low-temperature phase. The phonon spectra are obtained by fitting the factorized form of the dielectric function to the experimental spectra. The most marked effect on the experimental spectra at the phase transition is the disappearance of a highly-damped low-frequency mode upon cooling below TN, in both polarizations. This mode originates from intersite proton motions, and is coupled with the Brillouin zone-corner antiferroelectric instability. The other modes are only slightly affected by the phase transition, especially the NH4 internal modes. The results are discussed in the light of a previous work on ferroelectric KH2PO4-type compounds, which enables to assign the lattice modes, and to confirm the role of an internal PO4 mode in the mechanism of ferroelectricity in these compounds.

Thermodynamic Features of the System of Packed Antiferroelectric Chains

AbstractThe system is considered of Ising‐like chains suffering the antiferroelectric instability. The interaction between the chains is taken into account within the framework of the molecular field approximation. Then summing over states of a single chain is carried out by the method making immediately use of the fact that the unit cell can consist of several atoms. Such an approach may sometimes be more convenient than the transfer‐matrix method. An appropriate phase diagram in an uniform external field is investigated. As a result, the electrical effects essentially must be small so that the corresponding measurements will be difficult in such systems. As an example the TlHF2 structure is discussed.