The solid solution $0.5Ba(Ti_{0.8}Zr_{0.2})O_3$-$0.5(Ba_{0.7}Ca_{0.3})TiO_3$ (BCZT) has become a promising member of the lead-free piezoelectric materials because of its exceptionally high piezoelectric properties. In this study, we focus on studying pressure-dependent Raman spectroscopy, powder x-ray diffraction and dielectric constant measurements on BCZT. The data show several structural transitions are present, where the system from ambient mixed phase (tetragonal, {\it {P4mm}}+ orthorhombic {\it {Amm2}}) transforms into single phase ({\it {P4mm}}) at 0.26 GPa, then converts into cubic phase ({\it {Pm3m}}) at 4.7 GPa followed by another possible structural re-ordering around 10 GPa. Although there have been a lot of unanimity with the ambient crystallographic state of BCZT, our analysis justifies the presence of an intermediate orthorhombic phase in the Morphological Phase Boundary (MPB) of BCZT phase diagram. The transformation tetragonal to cubic is indicated by the Raman mode softening, unit cell volume change and the $(Ti/Zr)O_6$ octahedra distortion, which coincides with the well-known ferroelectric-paraelectric transition of the system. The sudden drop in the dielectric constant value at 4.7 GPa also confirms the loss of ferroelectric nature of the BCZT ceramic.
The solid solution 0.5Ba(Ti0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)TiO3 (BCZT) is a promising lead-free piezoelectric material with exceptionally high piezoelectric coefficients. The strong response is related to structural instabilities close to ambient temperature. We report here on temperature-induced anomalies in the dielectric, piezoelectric, and elastic coefficients and Raman spectroscopy of ceramic BCZT. The data indicate ferroelectric-ferroelectric structural phase transitions in this material in addition to those previously reported. An anomaly is also observed above the Curie temperature TC and i…
The oxygen octahedral tilted domains in 0.90(Bi1/2Na1/2)TiO3-0.5(Bi1/2K1/2)TiO3-0.5BaTiO3 lead-free perovskite piezoelectric ceramic have been studied by transmission electron microscopy (TEM). Selected-area electron diffraction patterns shows the 1/2ooo and 1/2ooe reflections, indicating the presence of antiphase (a-a-a-) and in-phase (aoaoc+) octahedral tilting, respectively. The morphology and distributions of these tilted domains are shown in the centered dark-field images. Further, the Bragg-filtered high-resolution TEM image reveals that the size of the in-phase tilted domains varies fro…
The structural phase diagram of the Pb-free ferroelectric (Na1/2Bi1/2)1-xBaxTiO3 (NBT-BT), x<0.1, has been explored by Raman spectroscopy at temperatures from 10 to 470 K. The data provide clear evidence for a proposed temperature-independent morphotropic phase boundary at x \approx 0.055. However, there is no evidence for a structural phase transition across T \approx 370 K for x > 0.055, where bulk-property anomalies appear to signal a transition to a nonpolar or antiferroelectric phase. The results identify that the phase above 370 K shows short-range ionic displacements that are identical …
We measured transverse and longitudinal dielectric permittivities of KDP and DKDP in the piezoelectric resonance frequency region under the hydrostatic pressure. The transverse permittivity decreases with pressure in the paraelectric phase and increases in the ferroelectric phase. The pressure dependence of the transverse permittivity of deuterated DKDP is well described by the presented theory. The longitudinal permittivity of a pure KDP exhibits several peaks in the vicinity of the transition point. Upon increasing hydrostatic pressure, the peaks get closer and may merge.
We find an unexpected tetragonal-to-monoclinic-to-rhombohedral-to-cubic phase transition sequence induced by pressure, and a morphotropic phase boundary in a pure compound using first-principles calculations. Huge dielectric and piezoelectric coupling constants occur in the transition regions, comparable to those observed in the new complex single-crystal solid-solution piezoelectrics such as Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$-PbTiO$_{3}$, which are expected to revolutionize electromechanical applications. Our results show that morphotropic phase boundaries and giant piezoelectric effects do not …
Dielectric tensor and lattice dynamics of alpha-PbO2-type TiO2 have been investigated using the density functional perturbation theory, with a focus on responses of the vibrational frequencies to pressure. The calculated Raman spectra under different pressures are in good agreement with available experimental results and the symmetry assignments of the Raman peaks of alpha-PbO2-type TiO2 are given for the first time. In addition, we identified two anomalously IR-active soft phonon modes, B1u and B3u, respectively, around 200 cm-1 which have not been observed in high pressure experiments. Compa…
Ab initio quasi-harmonic thermoelasticity, piezoelectricity, and thermoelectricity of polar solids at finite temperature and pressure: Application to wurtzite ZnO
School of Physics and Technology, Xinjiang University（新疆大学物理科学与技术学院）；International School for Advanced Studies (SISSA)（国际高等研究院）；IOM - CNR（意大利国家研究委员会材料研究所）
Titanium dioxide (TiO2) is a wide band gap semiconducting material which is promising for photocatalysis. Here we present first-principles calculations to study the pressure dependence of structural and electronic properties of two TiO2 phases: the cotunnite-type and the Fe2P-type structure. The band gaps are calculated using density functional theory (DFT) with the generalized gradient approximation (GGA), as well as the many-body perturbation theory with the GW approximation. The band gaps of both phases are found to be unexpectedly robust across a broad range pressures. The corresponding pr…
The physical mechanisms behind the reduction of the bulk modulus of a high-pressure cubic TiO2 phase are confirmed by first-principles calculations. An unusual and abrupt change occurs in the dependence of energy on pressure at 43 GPa, indicating a pressure-induced phase transition from columbite TiO2 to a newly-identified modified fluorite TiO2 with a Pca21 symmetry. Oxygen atom displacement in Pca21 TiO2 unexpectedly reduces the bulk modulus by 34% relative to fluorite TiO2. This discovering provides a direct evidence for understanding the compressive properties of such groups of homologous …
Pt catalyst particles on reducible oxide supports often change their activity significantly at elevated temperatures due to the strong metal-support interaction (SMSI), which induces the formation of an encapsulation layer around the noble metal particles. However, the impact of oxidizing and reducing treatments at elevated pressures on this encapsulation layer remains controversial, partly due to the 'pressure gap' between surface science studies and applied catalysis. In the present work, we employ synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study th…
Impact of high-pressure columbite phase of titanium dioxide (TiO2) on catalytic photoconversion of plastic waste and Simultaneous hydrogen (H2) production
This paper reports on the structural and optical properties of Co-doped TiO2 thin films grown onto (0001) Al2O3 substrates by non-reactive pulsed laser deposition (PLD) using argon as buffer gas. It is shown that by keeping constant the substrate temperature at as low as 310 C and varying only the background gas pressure between 7 Pa and 70 Pa, it is possible to grow either epitaxial rutile or pure anatase thin films, as well as films with a mixture of both polymorphs. The optical band gaps of the films are red shifted in comparison to the values usually reported for undoped TiO2, which is con…
Since recently, some interests appeared in TiO as a thin film coating material due to its promising physical properties. Moreover, because of the existence of intrinsic vacancies, TiO demonstrates memristive properties. To use these compounds in active region for memristive switching, the investigation of the dynamical stability of these compounds is of importance. Therefore, the structural, electronic, and dynamical properties of TiO in two different phases, namely cubic and monoclinic structures along with vacancies, are debated from the framework of density functional theory. The structural…
In piezoelectric ceramics the changes in the phase stabilities versus stress and temperature in the vicinity of the phase boundary play a central role. The present study was dedicated to the classical piezoelectric, lead-zirconate-titanate (PZT) ceramic with composition Pb(Zr$_{0.54}$Ti$_{0.46}$)O$_3$ at the Zr-rich side of the morphotropic phase boundary at which both intrinsic and extrinsic contributions to piezoelectricity are significant. The pressure-induced changes in this two-phase (rhombohedral $R3c$+monoclinic $Cm$ at room temperature and $R3c+P4mm$ above 1 GPa pressures) system were …
The effect of epitaxial strain on the phonon spectra, crystal structure, spontaneous polarization, dielectric, piezoelectric, and elastic properties of (001)-oriented ferroelectric (BaTiO$_3$)$_m$/(SrTiO$_3$)$_n$ superlattices ($m = n = {}$1-4) was studied using the first-principles density-functional theory. The ground state of free-standing superlattices is the monoclinic $Cm$ polar phase. Under the in-plane biaxial compressive strain, it transforms to tetragonal $P4mm$ polar phase, and under the in-plane biaxial tensile strain, it transforms to orthorhombic $Amm2$ polar phase. When changing…
Multifunctional capability, flexible design, rugged lightweight construction, and self-powered operation are desired attributes for electronics that directly interface with the human body or with advanced robotic systems. For these and related applications, piezoelectric materials, in forms that offer the ability to bend and stretch, are attractive for pressure/force sensors and mechanical energy harvesters. In this paper we introduce a large area, flexible piezoelectric material that consists of sheets of electrospun fibers of the polymer poly[(vinylidenefluoride-co-trifluoroethylene]. The fl…
This paper reports the application of zinc oxide (ZnO) in the pressure sensors that can be integrated with a microelectromechanical system (MEMS). ZnO is one of the materials that has received a great deal of attention due to its unique properties of being a semiconductor with wide bandgap and piezoelectric effects. The simpler crystal growth mechanisms of ZnO have resulted in a lower cost of ZnO-based sensors. Different types of pressure sensors based on ZnO sensing elements have also been explored. A thin circular ZnO film was simulated as a piezoelectric sensor employing the finite element …
The origin of the very large piezoelectric response observed in the vicinity of the morphotropic phase boundary (MPB) in perovskite lead zirconate titanate and related systems has been under intensive studies. Polarization rotation ideas are frequently invoked to explain the piezoelectric properties. It was recently reported that lead titanate undergoes a phase transformation sequence $P4mm\to Pm\to Cm\to R\bar{3}c$ at 10 K as a function of hydrostatic pressure [M. Ahart et al. Nature Letters. \textbf{451}, 545 (2008)]. We demonstrate that this interpretation is not correct by (i) simulating t…
Thin films of Co-TiO2 are deposited on silicon and quartz substrates using Pulse Laser Deposition (PLD) process at various oxygen partial pressures ranging from 6.6 x 10-3 Pascals (Pa) to 53 Pa. Crystal structure, transport and magnetic properties of reduced CoxTi(1-x)O2 (0 <x< 0.03) thin films are investigated and are found to have a strong dependence on the oxygen partial pressure. X-ray diffraction (XRD) data reveals the presence of mixed phase material containing both anatase and rutile. However, these phases intertransform with the change in the oxygen partial pressure in the chamber duri…
The relative energies of the low-pressure rutile, anatase, and brookite polymorphs and the high-pressure columbite polymorph of TiO$_2$ have been calculated as a function of temperature using the diffusion quantum Monte Carlo (DMC) method and density functional theory (DFT). The vibrational energies are found to be important on the scale of interest and significant quartic anharmonicity is found in the rutile phase. Static-lattice DFT calculations predict that anatase is lower in energy than rutile, in disagreement with experiment. The accurate description of electronic correlations afforded b…
Titanium dioxide has been extensively studied in the rutile or anatase phases, while its high-pressure phases are less well understood, despite that many are thought to have interesting optical, mechanical and electrochemical properties. First-principles methods such as density functional theory (DFT) are often used to compute the enthalpies of TiO$_2$ phases at 0~K, but they are expensive and thus impractical for long time-scale and large system-size simulations at finite temperatures. On the other hand, cheap empirical potentials fail to capture the relative stablities of the various polymor…
A comparative study of the electrical and optical properties has been done on 3d-doped TiO$_2$. Ti$_{1-x}$M$_x$O$_2$ (M= Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) powder and its corresponding pellets, with doping concentration $x= 0.05$. The samples were prepared using the solid-state route. Optical and electrical measurements have been performed for all prepared samples and interestingly, it is observed that due to external pressure (i.e. strain) both the properties change significantly. A rigorous theoretical calculation has also been carried out to verify the experimental band gap obtained from op…
Piezoelectric PZT solid solutions were studied as a function of pressure and temperature. Moderate pressure is found to induce phase transitions to monoclinic phases (Cm,Cc) for Ti-rich PZT, which are the same ferroelectric phases responsible for the high piezoelectric properties of the MPB. The following transformation sequence P4mm -> Cm -> Cc -> F1 -> F-1 was identified upon increasing pressure. Compression behaviour is dominated by: (1) a rotation and a reduction of the spontaneouspolarization and (2) octahedral tilting leading to unit cell doubling. Future studies as a function of pressur…
Here we present a study of magnetism in \CTO\ anatase films grown by pulsed laser deposition under a variety of oxygen partial pressures and deposition rates. Energy-dispersive spectrometry and transition electron microscopy analyses indicate that a high deposition rate leads to a homogeneous microstructure, while very low rate or postannealing results in cobalt clustering. Depth resolved low-energy muon spin rotation experiments show that films grown at a low oxygen partial pressure ($\approx 10^{-6}$ torr) with a uniform structure are fully magnetic, indicating intrinsic ferromagnetism. Firs…
The model of NH$_3$CH$_2$COOH$\cdot$H$_2$PO$_3$, modified by taking into account the piezoelectric coupling between the ordering structure elements and the strains $\varepsilon_i$, $\varepsilon_j$, is used for investigation of the effects that appear under external pressures. Within two-particle cluster approximation, the components of polarization vector and static dielectric permittivity tensor of the mechanically clamped and free crystals, their piezoelectric and thermal characteristics are calculated. The effect of hydrostatic pressure on the phase transition and the calculated physical ch…
The structural phase transitions of single crystal TiO2-B nanoribbons were investigated in-situ at high-pressure using the synchrotron X-ray diffraction and the Raman scattering. Our results have shown a pressure-induced amorphization (PIA) occurred in TiO2-B nanoribbons upon compression, resulting in a high density amorphous (HDA) form related to the baddeleyite structure. Upon decompression, the HDA form transforms to a low density amorphous (LDA) form while the samples still maintain their pristine nanoribbon shape. HRTEM imaging reveals that the LDA phase has an α-PbO2 structure with short…
Using Car-Parrinello molecular dynamics calculations we investigate the behavior of the low-dimensional multiorbital Mott insulator TiOCl under pressure. We show that the system undergoes {\it two} consecutive phase transitions, first at $P_\text{c}$ from a Mott-insulator to a metallic phase in the $ab$ plane with a strong Ti-Ti dimerization along $b$. At a pressure $P'_\text{c} > P_\text{c}$ the dimerization disappears and the system behaves as a uniform metal. This second transition has not yet been reported experimentally. We show that the insulator-to-metal transition at $P_\text{c}$ is dr…
We present a design for a piezoelectric-driven uniaxial stress cell suitable for use at ambient and cryogenic temperatures, and that incorporates both a displacement and a force sensor. The cell has a diameter of 46 mm and a height of 13 mm. It can apply a zero-load displacement of up to ~45 $μ$m, and a zero-displacement force of up to ~245 N. With combined knowledge of the displacement and force applied to the sample, it can quickly be determined whether the sample and its mounts remain within their elastic limits. In tests on the oxide metal Sr$_2$RuO$_4$, we found that at room temperature s…
We report our Raman studies of a new lead-free relaxor ferroelectrics, Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCTZO). The Raman modes of BCTZO are compared with those of BaTi0.8Zr0.2O3 (BTZO), and BaTiO3 (BTO). Also, they are compared with the eigenmodes of BTO calculated by using an ab-initio quantum-mechanical frozen-phonon method. The sharp mode at 321 cm-1 of BTO, reported as a coupled mode showing an interference effect, becomes progressively broader in BTZO and BCTZO. This behavior, together with a broadening of the 527-cm-1 mode, suggests that the mode-coupling is weakened in BTZO and BCTZO. The str…
We calculate from first principles the nonlinear piezoelectric response of ferroelectric PbTiO3 for the case of a polarization-enhancing electric field applied along the tetragonal axis. We focus mainly on the case of fixed in-plane lattice constants, corresponding to epitaxially constrained thin films. We find that the dependence of the c/a ratio on electric field is almost linear in the range up to 500 MV/m, with little saturation. This result contrasts with expectations from Landau-Devonshire approaches based on experimental results obtained at lower fields, but is in qualitative agreement …
We present a piezoelectric-driven uniaxial pressure cell that is optimized for muon spin relaxation and neutron scattering experiments, and that is operable over a wide temperature range including cryogenic temperatures. To accommodate the large samples required for these measurement techniques, the cell is designed to generate forces up to 1000 N, and to minimize the background signal the space around the sample is kept as open as possible. We demonstrate here that by mounting plate-like samples with epoxy, a uniaxial stress exceeding 1 GPa can be achieved in an active volume of 5 mm3. We sho…
Enhanced domain mobility is considered as one of the important characteristics of morphotropic phase boundary (MPB) ferroelectrics exhibiting a very large piezoelectric response. For nearly two decades, Rayleigh analysis of dielectric and piezoelectric response at sub-coercive fields has been used as a phenomenological tool to determine the relative contribution of domain walls in influencing polar properties in these systems. The high value of the extrinsic Rayleigh coefficient at the MPB is generally attributed to enhanced domain mobility. Using coercive field as a measure of hindrance to do…
We predict a new candidate high-temperature high-pressure structure of FeSiO$_3$ with space-group symmetry Cmmm by applying an evolutionary algorithm within DFT+U that we call post-perovskite II (PPv-II). An exhaustive search found no other competitive candidate structures with ABO$_3$ composition. We compared the X-ray diffraction (XRD) pattern of FeSiO$_3$ PPv-II with experimental results of the recently reported "H-phase" of (Fe,Mg)SiO$_3$. The intensities and positions of two main X-ray diffraction peaks of PPv-II FeSiO$_3$ compare well with those of the H-phase. We also calculated the sta…
Enhanced room-temperature electromechanical coupling in the lead-free ferroelectric system $(1-x)$BaZr$_{0.2}$Ti$_{0.8}$O$_{3}$ - $x$Ba$_{0.7}$Ca$_{0.3}$TiO$_{3}$ (abbreviated as BZCT) at $x=0.5$ is attributed to the existence of a morphotropic phase region (MPR) containing an intermediate orthorhombic ($O$) phase between terminal rhombohedral ($R$) BZT and tetragonal ($T$) BCT phases. However, there is ambiguity regarding the morphotropic phase transition in BZCT at room temperature - while some experiments suggest a single $O$ phase within the MPR, others indicate coexistence of three polar …
A detailed temperature and pressure investigation on BiGdO$_{3}$ is carried out by means of dielectric constant, piezoelectric current, polarization-electric field loop, Raman scattering and x-ray diffraction measurements. Temperature dependent dielectric constant and dielectric loss show two anomalies at about 290 K (T$_r$) and 720 K (T$_C$). The later anomaly is most likely due to antiferroelectric to paraelectric transition as hinted by piezoelectric current and polarization-electric field loop measurements at room temperature, while the former anomaly suggests reorientation of polarization…
We investigate the high-pressure behaviour of beryllium, magnesium and calcium difluorides using ab initio random structure searching and density functional theory (DFT) calculations, over the pressure range 0-70 GPa. Beryllium fluoride exhibits extensive polymorphism at low pressures, and we find two new phases for this compound - the silica moganite and CaCl2 structures - which are stable over the wide pressure range 12-57 GPa. For magnesium fluoride, our searching results show that the orthorhombic `O-I' TiO2 structure (Pbca, Z=8) is stable for this compound between 40 and 44 GPa. Our searc…
On Tailoring Structural and Optoelectronic Properties of TiO2 Thin Films Synthesized via 'Room' Temperature High Power Impulse Magnetron Sputtering (HiPIMS)
A metal oxide support and a catalytically active metal are the two main ingredients for complex catalysts used in heterogeneous catalysis. The gas environment can change the catalyst during the reaction, modifying its structural and electronic properties. Here, we use monochromated electron energy loss spectroscopy (EELS) to reveal hydrogen-pressure-dependent changes of the electronic structure at the Pd/rutile-TiO$_2$ interface in an environmental transmission electron microscope (ETEM). Hydrogen-induced changes are observed in rutile-TiO$_2$ within $2$~nm from the interface at $10$~Pa of hyd…
X-ray diffraction studies to ~90 GPa at room temperature show that Fe$_{2}$TiO$_{4}$ ferrous inverse spinel undergoes the following sequence of structural transitions : cubic (Fd3m) to tetragonal (I41/amd)to orthorhombic(Cmcm) to orthorhombic(Pmma),at the indicated onset transition pressures. Within the Cmcm phase, site-specific spin crossover is initiated and involves only highly distorted octahedral sites constituting ~25% of all Fe locations. This is manifest as a steeper volume decrease of dV/V0 ~ 3.5% beyond ~40 GPa and an emergent diamagnetic component discerned in 57Fe Mössbauer spectro…
The piezoelectric nanomaterial ZnO exhibits an excellent piezoelectric response that can transduce mechanical energy into electrical signals by applying pressure. The piezoelectric behavior of ZnO nanostructures (especially nanorods or microrods) is getting considerable attention in the fabrications of piezo tactile sensors, energy harvesting devices, and other self-powering implantable devices. Especially vertically aligned ZnO nanorods are of high interest due to their higher value of piezoelectric coefficient along the z-direction. In this report, various geometries and alignments of ZnO na…
The stability of MgH$_2$ has been studied up to 20~GPa using density-functional total-energy calculations. At ambient pressure $α$-MgH${_2}$ takes a TiO$_2$-rutile-type structure. $α$-MgH$_2$ is predicted to transform into $γ$-MgH${_2}$ at 0.39~GPa. The calculated structural data for $α$- and $γ$-MgH${_2}$ are in very good agreement with experimental values. At equilibrium the energy difference between these modifications is very small, and as a result both phases coexist in a certain volume and pressure field. Above 3.84~GPa $γ$-MgH${_2}$ transforms into $β$-MgH${_2}$; consistent with experim…
Our first-principles computations show that the ground state of PbTiO$_3$ under hydrostatic pressure transforms discontinuously from $P4mm$ to $R3c$ at 9 GPa. Spontaneous polarization decreases with increasing pressure so that the $R3c$ phase transforms to the centrosymmetric $R\bar{3}c$ phase at around 30 GPa. The first-order phase transition between tetragonal and rhombohedral phase is exceptional since there is no evidence for a bridging phase. The essential feature of the $R3c$ and $R\bar{3}c$ phases is that they allow the oxygen octahedron to increase its volume $V_B$ at the expense of cu…
We investigate the influence of quantum effects on the dielectric and piezoelectric properties of barium titanate in its (low-temperature) rhombohedral phase, and show the strongly anharmonic character of this system even at low temperature. For this purpose, we perform path-integral molecular-dynamics simulations under fixed pressure and fixed temperature, using an efficient Langevin thermostat-barostat, and an effective hamiltonian derived from first-principles calculations. The quantum fluctuations are shown to significantly enhance the static dielectric susceptibility (~ by a factor 2) and…
Studying the effect of high pressure (exceeding 10 kbar) on the structure of solids allows to gain deeper insight in the mechanism governing crystal structure stability. Here we report a study on the high-pressure behaviour of zinc difluoride (ZnF2) - an archetypical ionic compound which at ambient pressure adopts the rutile (TiO2) structure. Previous investigations, limited to a pressure of 15 GPa, revealed that this compound undergoes two pressure-induced phase transitions: TiO2 - CaCl2 at 4.5 GPa, and CaCl2 - HP-PdF2 at 10 GPa. Within this joint experimental-theoretical study we extend the …
There have existed for a long time a paradigm that TiO phases at ambient conditions are stable only if structural vacancies are available. Using an evolutionary algorithm, we perform an ab initio search of possible zero-temperature polymorphs of TiO in wide pressure interval. We obtain the Gibbs energy of the competing phases taking into account entropy via quasiharmonic approximation and build the pressure-temperature diagram of the system. We reveal that two vacancy-free hexagonal phases are the most stable at relatively low temperatures in a wide range of pressures. The transition between t…
Pb(Mg1/3Nb2/3)O3-PbTiO3 solid solution (PMN(1-x)-PTx) ceramics with 0 <x<1 have been prepared by homo-epitaxial templated grain growth (HTGG) using cubic PMN-PT single crystal seeds as templates and nanoparticles for thes ceramic matrix. Samples were prepared using the doctor Blade method and sintered in a controlled atmosphere. They were then analysed by DTA, TGA, optical/scanning electron microscopy, Raman spectroscopy and imaging. Relationships between Raman parameters such as band component or area versus composition (x) are proposed. Wavenumber and peak area shifts from the single crystal…
Substrate temperature dependent dielectric and ferroelectric properties of (100) oriented lead-free Na$_{0.4}$K$_{0.1}$Bi$_{0.5}$TiO$_3$ thin films grown by pulsed laser deposition
We report on a thermal-expansion study of the ferroelectric phase transition in insulating Sr$_{1-x}$Ca$_x$TiO$_3$ ($x=0.009$) and its evolution upon increasing charge-carrier concentration up to $n\simeq 60 \times 10^{19}$cm$^{-3}$. Although electric polarization is screened by mobile charge carriers, we find clear signatures of the ferroelectric phase transition in the thermal-expansion coefficient $α$ of the weakly doped metallic samples. Upon increasing $n$, the transition temperature $T_\mathrm{C}(n)$ and the magnitude of the anomalies in $α$ rapidly decrease up to a threshold carrier den…
A comprehensive phase diagram of lowest-energy structures and compositions of the rutile TiO_2(110) surface in equilibrium with a surrounding gas phase at finite temperatures and pressures has been determined using density functional theory in combination with a thermodynamic formalism. The exchange of oxygen, hydrogen, and water molecules with the gas phase is considered. Particular attention is given to the convergence of all calculations with respect to lateral system size and slab thickness. In addition, the reliability of semilocal density functionals to describing the energetics of the r…
Despite the technological significance of the tetragonal PbTiO3 for the piezoelectric transducer industry, its high pressure behaviour is quite controversial as two entirely different scenarios, involving pressure induced (1) morphotropic phase boundary (MPB) like structural transition with concomitant rotation of the ferroelectric polarization vector and (2) antiferrodistortive (AFD) phase transition followed by emergence of a reentrant ferroelectric phase, have been proposed in recent theoretical and experimental studies. We have attempted to address these controversies through a high resolu…
