Centrosymmetric Phase Research Articles

Ferroelectricity in Pb1+δZrO3 Thin Films

Antiferroelectric PbZrO3 is being considered for a wide range of applications where the competition between centrosymmetric and noncentrosymmetric phases is important to the response. Here, we focus on the epitaxial growth of PbZrO3 thin films and understanding the chemistry–structure coupling in Pb1+δZrO3 (δ = 0, 0.1, 0.2). High-quality, single-phase Pb1+δZrO3 films are synthesized via pulsed-laser deposition. Although no significant lattice parameter change is observed in X-ray studies, electrical characterization reveals that while the PbZrO3 and Pb1.1ZrO3 heterostructures remain intrinsically antiferroelectric, the Pb1.2ZrO3 heterostructures exhibit a hysteresis loop indicative of ferroelectric response. Further X-ray scattering studies reveal strong quarter-order diffraction peaks in PbZrO3 and Pb1.1ZrO3 heterostructures indicative of antiferroelectricity, while no such peaks are observed for Pb1.2ZrO3 heterostructures. Density functional theory calculations suggest the large cation nonstoichiometry is accommodated by incorporation of antisite PbZr defects, which drive the Pb1.2ZrO3 heterostructures to a ferroelectric phase with R3c symmetry. In the end, stabilization of metastable phases in materials via chemical nonstoichiometry and defect engineering enables a novel route to manipulate the energy of the ground state of materials and the corresponding material properties.

New Intermediate Polymorph of 1-Fluoro-adamantane and Its Second-Order-like Transition toward the Low Temperature Phase

Phase transitions of 1-fluoro-adamantane have been thoroughly investigated by temperature-resolved second harmonic generation (TR-SHG) and X-ray powder diffraction (XRPD). A new polymorph—an intermediate centrosymmetric phase (MT)—between the known orientationally disordered high temperature phase (HT, Fm3m, Z = 4) and the low temperature phase (LT, P421c, Z = 2) was unveiled by TR-SHG. The crystal structure of MT was resolved by XRPD in the P42/nmc (Z = 2) space group, and it is related to the LT phase in a group–subgroup relation. No evidence of any solid–solid transition between these two phases by differential scanning calorimetry (DSC) or cold-stage microscopy could be obtained. Therefore, combing TR-SHG, XRPD, DSC, and cold-stage microscopy results, a second-order transition mechanism is proposed for the MT ↔ LT transition. Moreover, the critical exponent (β) of the order parameter was calculated by fitting TR-SHG data to a critical power law. The obtained β value (0.26) is close to the value from...

High-temperature phase transitions and domain structures of KLiSO4: studied by polarisation-optics, X-ray topography and liquid–crystal surface decoration

Abstract The transitions between the room temperature phase III (space group P63) and the two high-temperature phases II (Pcmn) and I (P63/mmc) of KLiSO4 and the domain structures generated by them were investigated by high-temperature polarisation optics (birefringence) and room-temperature X-ray topography, optical activity and nematic–liquid–crystal (NLC) surface decoration. The transition from the polar hexagonal phase III into the centrosymmetric orthorhombic phase II at 708 K leads, due to the loss of the trigonal axis and the radial temperature gradient of the optical heating chamber used, to a roughly hexagonal arrangement of three sets of thin orthorhombic {110} lamelleae with angles of 60° (120°) between them. The associated twin law “reflection m{110}orth” corresponds to the frequent growth twin m{101̅0}hex of phase III. The domains are easily ferroelastically switched. Upon further heating above 949 K into phase I (P63/mmc) all domains vanish. Upon cooling back into phase II the three domain states related by 60°(120°) reflections m{110}orth re-appear, however (due to the higher thermal agitation at 949 K) with a completely different domain structure consisting of many small, irregularly arranged {110}orth domains. Particular attention is paid to the domain structure of the hexagonal room temperature phase III generated during the re-transition from the orthorhombic phase II. Curiously, from the expected three twin laws inversion 1̅, rotation 2⊥[001]hex and reflection m{101̅0}hex only the latter, which corresponds to the frequent growth twinning, has been found. Finally a short treatise of the structural relations of the KLiSO4 high-temperature polymorphs is given.

Enantiopure and racemic radical-cation salts of bis(2'-hydroxylpropylthio)(ethylenedithio)TTF with polyiodide anions.

The chiral TTF-based donor molecule bis(2'-hydroxylpropylthio)(ethylenedithio)tetrathiafulvalene (BHPT-EDT-TTF) has produced enantiopure R,R and S,S radical-cation salts with polyiodide anions I3- and I82-. Enantiomorphic 6 : 6 donor : I3 phases grown from either the R,R or S,S donor are semiconducting with similar activation energies of 0.24-0.30 eV and 0.22-0.23 eV, respectively, and contain three unique face-to-face donor pairs whose relative orientation is determined by side chain conformations and hydrogen bonding. Racemic material under the same conditions gave an insulating centrosymmetric phase with R,R and S,S donor cations in a face-to-face pair partnered with an octaiodide dianion, and with a ca. 3 : 1 disorder between the enantiomers. Enantiopure BHPT-EDT-TTF yielded two further insulating crystalline phases of composition 2 : 2 with triiodide and 2 : 1 with octaiodide.

Antiferroelectric properties and site occupations of R3+ cations in Ca8MgR(PO4)7 luminescent host materials

Ca8MgR(PO4)7 (R = La, Pr, Nd, Sm−Lu, and Y) phosphates with a β-Ca3(PO4)2 related structure were prepared by a standard solid-state method in air. Second-harmonic generation, differential scanning calorimetry, and dielectric measurements led to the conclusion that all Ca8MgR(PO4)7 are centrosymmetric and go to another centrosymmetric phase in the course of a first-order antiferroelectric phase transition well above room temperature (RT). High-temperature electron diffraction showed that the symmetry changes from R3¯c to R3¯m during the phase transition. Structures of Ca8MgR(PO4)7 at RT were refined by the Rietveld method in centrosymmetric space group R3¯c. Mg2+ cations occupy the M5 site; the occupancy of the M1 site by R3+ cations increases monotonically from 0.0389 for R = La to 0.1667 for R = Er-Lu, whereas the occupancy of the M3 site by R3+ cations decreases monotonically from 0.1278 for R = La to 0 for R = Er-Lu. In the case of R = Er-Lu, the M3 site is occupied only by Ca2+ cations. P1O4 tetrahedra and cations at the M3 site are disordered in the R3¯c structure of Ca8MgR(PO4)7. Using synchrotron X-ray powder diffraction, we found that annealing conditions do not significantly affect the distribution of Ca2+ and Eu3+ cations between the structure positions of Ca8MgEu(PO4)7. Luminescent properties of Ca8MgEu(PO4)7 powder samples were investigated under near-ultraviolet (n-UV) light. Excitation spectra of Ca8MgEu(PO4)7 show the strongest absorption at about 395 nm that matches with commercially available n-UV-emitting GaN-based LED chips. Emission spectra show an intense red emission due to the 5D0 → 7F2 transition of Eu3+.

Electric relaxation and Mn3+/Mn4+ charge transfer in Fe-doped Bi12MnO20\u2013BiMn2O5 structural self-composite

Fe-doped Bi12MnO20–BiMn2O5 ceramics was sintered at 1130 K for 6 h in ambient air. Two centro-symmetric phases formed thermodynamically stable self-composite material that was deduced from X-ray pattern analysis. The lattice parameters were a = 10.147(8) Å—for the cubic I23 Bi12MnO20 phase; and a = 7.545(4) Å, b = 8.538(1) Å, c = 5.758(3) Å—for the orthorhombic Pbam BiMn2O5 phase. The 57Fe Mössbauer spectrum, recorded at room temperature, has shown pure electronic quadrupolar split. The major doublets reflected the occurrence of Fe3+ ions distributed in two sites, i.e., octahedral Fe4+O6 and square pyramidal Fe3+O5, with preferential occupation of the pyramidal sites, that was consistent with the Pbam phase symmetry. The third doublet resulted from the presence of iron Fe3+ in tetrahedral Fe3+O4 coordination and corresponded to a small admixture of the I23 phase. The DC resistivity ρDC(T) dependence on temperature has shown thermally activated features, and the value of Ea,DC varied in the range of 0.22–0.37 eV. The electric impedance was measured in the f = 20 Hz–1 MHz and 100–690 K range. Two electrical relaxations were determined using the electric modulus formalism M″(T). Low-temperature relaxation has shown the temperature-dependent activation energy EA,1 = 0.14–0.20 eV and characteristic time values of τ01 = 10−10–10−12 s in 100–200 K range. It was attributed to the charge transfer between Mn4+/Mn3+ sites. The other relaxation occurred in the 170–220 K range, and it exhibited the following values: τ02 = 10−11 s, and EA,2 = 0.27 eV. A disorder-related VRH polaron model was proposed for ρDC(T) and for electric relaxation processes.

Novel organic NLO material bis(N-phenylbiguanidium(1+)) oxalate – A combined X-ray diffraction, DSC and vibrational spectroscopic study of its unique polymorphism

Three polymorphic modifications of bis(N-phenylbiguanidium(1+)) oxalate are reported, and their characterization is discussed in this paper. The non-centrosymmetric bis(N-phenylbiguanidium(1+)) oxalate (I), which was obtained from an aqueous solution at 313K, belongs to the monoclinic space group Cc (a=6.2560(2)Å, b=18.6920(3)Å, c=18.2980(5)Å, β=96.249(1)°, V=2127.0(1)Å3, Z=4, R=0.0314 for 4738 observed reflections). The centrosymmetric bis(N-phenylbiguanidium(1+)) oxalate (II) was obtained from an aqueous solution at 298K and belongs to the monoclinic space group P21/n (a=6.1335(3)Å, b=11.7862(6)Å, c=14.5962(8)Å, β=95.728(2)°, V=1049.90(9)Å3, Z=4, R=0.0420 for 2396 observed reflections). The cooling of the centrosymmetric phase (II) leads to the formation of bis(N-phenylbiguanidium(1+)) oxalate (III) (a=6.1083(2)Å, b=11.3178(5)Å, c=14.9947(5)Å, β=93.151(2)°, V=1035.05(8)Å3, Z=4, R=0.0345 for 2367 observed reflections and a temperature of 110K), which also belongs to the monoclinic space group P21/n. The crystal structures of the three characterized phases are generally based on layers of isolated N-phenylbiguanidium(1+) cations separated by oxalate anions and interconnected with them by several types of N-H...O hydrogen bonds. The observed phases generally differ not only in their crystal packing but also in the lengths and characteristics of their hydrogen bonds. The thermal behaviour of the prepared compounds was studied using the DSC method in the temperature range from 90K up to a temperature near the melting point of each crystal. The bis(N-phenylbiguanidium(1+)) oxalate (II) crystals exhibit weak reversible thermal effects on the DSC curve at 147K (heating run). Further investigation of this effect, which was assigned to the isostructural phase transformation, was performed using FTIR, Raman spectroscopy and X-ray diffraction analysis in a wide temperature range.

First principles investigation of electronic and magnetic structures of centrosymmetric BiMnO3 using an improved approach

Recent temperature-dependent x-ray diffraction and Raman spectroscopy experiment proved that single-crystalline BiMnO3 assumes a centrosymmetric monoclinic structure (C2/c space group). Here we investigate magnetic structure and electronic structure of this centrosymmetric BiMnO3 phase by using the modified Becke-Johnson (mBJ) exchange functional within the density functional theory (DFT). Our mBJ calculated semiconductor gap, magnetic moment, and other aspects of the electronic structure, in contrast with previous DFT results, are in good agreement with recent experimental values. This satisfactory description of the electronic structure and magnetism of the BiMnO3 is because mBJ reasonably captures the kinetic property and correlation of electrons. Our calculated results with mBJ approach are both useful to study such Bi-based perovskite oxide materials for spintronics applications.

Competition between Polar and Centrosymmetric Packings in Molecular Crystals: Analysis of Actual and Virtual Structures

Imines obtained by condensation of 4-hydroxybenzohydrazide with aliphatic ketones are a rare example of a class of compounds showing a remarkable tendency to crystallize in acentric polar space groups (Pna21 or Cc). In fact, all of the (seven) compounds studied up to now show at least one polar polymorph. In some cases, polymorphism was detected, and a nonpolar centrosymmetric phase was also identified (P21/c or P21/n space group). With the aim to disclose the conditions that can favor the formation of acentric structures in molecular crystals, we report, in this paper, a theoretical analysis (ab initio density functional theory with periodic boundary) of the lattice energy and density of all the packing modes observed in the whole set of imines. The computational analysis has been performed by optimizing each compound in its own experimental packings (actual crystal structures) and also in the packings of the other compounds of the class (virtual structures). The experimental crystallographic data and the theoretical analysis suggest that two conformers, basically differing for the orientation of the phenolic H atom in the plane of the phenyl ring, compete, in solution, for the formation of polar or centrosymmetric packings. The transitions between polar and centrosymmetric polymorphs are of diffusive type, and single crystals are not preserved, while the transitions between different polar polymorphs can be of single-crystal-to-single-crystal type.

Structural analysis and IR-spectroscopy of a new anilinium hydrogenselenite hybrid compound: A subtle structural phase transition

An interesting structural behavior has been detected and characterized in a new anilinium based compound by single crystal X-ray diffraction measurements and infrared spectroscopy. The temperature dependent structural investigation reveals that the studied compound undergoes a subtle non-centrosymmetric to centrosymmetric structural phase transition. At room temperature the crystal structure is non-centrosymmetric and is characterized by an important disorder in the organic part where the independent aromatic rings are rotated around the C1—C4 and C7—C10 molecular axis by 52.06(6)° and 56.25(4)° respectively. By decreasing the temperature to 100K the organic cation is less disordered and the rotation angle of the aromatic rings changes to 54.73(2)°. As a consequence, the low temperature structure becomes centrosymmetric. The infrared spectra recorded on cooling and heating the sample in the temperature range of 300–9K support this analysis. The ammonium group (NH3) is significantly affected by the temperature change. This effect results, when cooling the sample below 140K, in the appearance of a new vibrational band corresponding to the (NH3)-wagging modes at 705cm−1. The phenomenon is completely reversible and the new vibrational band disappears upon heating the sample above 140K. Moreover, the ν(C–N)-stretching and the (NH3)-scissoring modes are also affected by the temperature variation as can be seen in the spectral ranges of 1250–1400cm−1 and 1610–1635cm−1 respectively. The spectroscopic results are complementary to the structural analysis and indicate that the structural phase transition is correlated with the molecular rotation of the aromatic moieties around the C1—C4 axis that affect the different vibrational modes of the ammonium group.

Raman phonons in the ferroelectric-like metalLiOsO3

The novel ferroelectric-like structural transition observed in metallic LiOsO$_3$ [Y. Shi et al., Nat. Mater. 12, 1024 (2013)], has invoked many theoretical and experimental interests. In this work, we have performed polarized and temperature-dependent Raman scattering measurements on high-quality single crystal LiOsO$_3$ and identified Raman-active modes in both centrosymmetric phase (300 K, R$\bar{3}$c) and non-centrosymmetric phase (10 K, R3c). Only four phonon peaks are observed in the former phase, while there are twelve peaks in the latter phase because of the reduction of crystal symmetry. With the help of careful symmetry analysis and first-principles calculations, we can make a systematic assignment for the observed Raman modes in both phases. The significant changes in line-width and the continuous evolution of Raman frequencies with temperatures were observed for the E$_g$ modes around the transition temperature, which suggests that the ferroelectric-like structural transition is a continuous order-disorder transition. The result sheds light on the coexistence of ferroelectricity and metallicity in the compound.

Improper Inversion Symmetry Breaking and Piezoelectricity through Oxygen Octahedral Rotations in Layered Perovskite Family, LiRTiO4 (R = Rare Earths)

An improper mechanism for breaking inversion symmetry is revealed and thus inducing piezoelectricity in the family of layered perovskites, LiRTiO4 (R = rare earths), which are previously reported as centrosymmetric. Noncentrosymmetry in this family of compounds arises from TiO6 octahedral rotation represented by a−boco/boa−co in the perovskite blocks between RO rock salt and LiO antifluorite layers. X‐ray diffraction and optical second harmonic generation complemented by density functional theory predictions are crucial in determining the new structures. High transition temperature (Tac) of up to 1200 K from noncentrosymmetric to centrosymmetric phase is observed. Piezoelectric coefficients (d36) of up to −15 pC/N are predicted, and piezoelectric force microscopy experiments confirm a piezoelectric response. The demonstrated improper mechanism in this and other layered oxide families, with a wide range of available topologies and chemistries, could aid in the search for high temperature piezoelectrics.

Electric Current Relaxation and Resistance Switching in Non-Homogeneous Bismuth Manganite

Bismuth manganite ceramics has been obtained by high temperature sintering in air. It was found from the X-ray diffraction pattern analysis that the ceramics consists of two centrosymmetric phases: the cubic I23 and the orthorhombic Pbam. Impedance has been measured at radiofrequencies in 85–500 K range. Two relaxation processes have been discerned. One process, which shows characteristic time value τ0 ≈ 10−12 s, occurs in 120–230 K range and it has been assigned to polaronic conductivity. The other process occurs in 270–310 K range and it exhibits longer characteristic time value τ0 ≈ 10−8 s. The resistance Rdc(T) temperature dependence shows semiconductor features with activation energy Ea = 0.28–0.40 eV for the as-sintered samples. The semiconductor to metallic-type transition has been induced after electroformation procedure carried out at 790 K in ambient air. We propose that extended defects, related to occurrence of the two phases, facilitate resistive switching.

ChemInform Abstract: Symmetry of Bioinspired Short Peptide Nanostructures and Their Basic Physical Properties

AbstractReview: 46 refs.

Weak C-H···O and C-H···π hydrogen bonds and π-π stacking interactions in a series of four N'-[(E)-(aryl)methylidene]-N-methyl-2-oxo-1,3-oxazolidine-4-carbohydrazides.

Oxazolidin-2-ones are widely used as protective groups for 1,2-amino alcohols and chiral derivatives are employed as chiral auxiliaries. The crystal structures of four differently substituted oxazolidinecarbohydrazides, namely N'-[(E)-benzylidene]-N-methyl-2-oxo-1,3-oxazolidine-4-carbohydrazide, C12H12N3O3, (I), N'-[(E)-2-chlorobenzylidene]-N-methyl-2-oxo-1,3-oxazolidine-4-carbohydrazide, C12H12ClN3O3, (II), (4S)-N'-[(E)-4-chlorobenzylidene]-N-methyl-2-oxo-1,3-oxazolidine-4-carbohydrazide, C12H12ClN3O3, (III), and (4S)-N'-[(E)-2,6-dichlorobenzylidene]-N,3-dimethyl-2-oxo-1,3-oxazolidine-4-carbohydrazide, C13H13Cl2N3O3, (IV), show that an unexpected mild-condition racemization from the chiral starting materials has occurred in (I) and (II). In the extended structures, the centrosymmetric phases, which each crystallize with two molecules (A and B) in the asymmetric unit, form A+B dimers linked by pairs of N-H···O hydrogen bonds, albeit with different O-atom acceptors. One dimer is composed of one molecule with an S configuration for its stereogenic centre and the other with an R configuration, and possesses approximate local inversion symmetry. The other dimer consists of either R,R or S,S pairs and possesses approximate local twofold symmetry. In the chiral structure, N-H···O hydrogen bonds link the molecules into C(5) chains, with adjacent molecules related by a 21 screw axis. A wide variety of weak interactions, including C-H···O, C-H···Cl, C-H···π and π-π stacking interactions, occur in these structures, but there is little conformity between them.

Symmetry of Bioinspired Short Peptide Nanostructures and Their Basic Physical Properties

AbstractSupramolecular bioinspired peptide nanostructures are considered as a new frontier in materials science and engineering. The nano‐crystalline packing of various peptide nanostructures, and especially those lacking a center of symmetry at the nanoscale, give rise to exceptional physical properties. Specifically, native aromatic diphenylalanine (FF) and aliphatic dileucine (LL) based nanotubes, which are related to hexagonal and orthorhombic non‐centrosymmetric crystalline groups respectively, exhibit fundamental physical phenomena, such as piezoelectricity and second harmonic generation (SHG). This review covers our latest findings on the physical properties of FF and LL nanostructures. We show that heat treatment at the temperature range of 140–180 °C induces irreversible phase transition in FF and LL nanotubes, wherein all their physical properties and structure at all levels (molecular, electronic, optical, space symmetry, morphology, wettability) change. Using high resolution microscopy tools, based on Kelvin probe force microscopy (KPFM), piezoresponse force microscopy (PFM), and SHG, as well as Raman spectroscopy, we demonstrate that the phase‐transition phenomena in FF and LL nanotubes leads to full reconstruction and reassembling of native open‐end nanotubes into new fiber‐like structures, followed by deep variation of non‐centrosymmetric to centrosymmetric space symmetry. As a result, the newly generated centrosymmetric phase in FF and LL nanostructures demonstrates neither piezoelectric effect nor nonlinear optical activity.

First-principles theory, coarse-grained models, and simulations of ferroelectrics.

CONSPECTUS: A ferroelectric crystal exhibits macroscopic electric dipole or polarization arising from spontaneous ordering of its atomic-scale dipoles that breaks inversion symmetry. Changes in applied pressure or electric field generate changes in electric polarization in a ferroelectric, defining its piezoelectric and dielectric properties, respectively, which make it useful as an electromechanical sensor and actuator in a number of applications. In addition, a characteristic of a ferroelectric is the presence of domains or states with different symmetry equivalent orientations of spontaneous polarization that are switchable with large enough applied electric field, a nonlinear property that makes it useful for applications in nonvolatile memory devices. Central to these properties of a ferroelectric are the phase transitions it undergoes as a function of temperature that involve lowering of the symmetry of its high temperature centrosymmetric paraelectric phase. Ferroelectricity arises from a delicate balance between short and long-range interatomic interactions, and hence the resulting properties are quite sensitive to chemistry, strains, and electric charges associated with its interface with substrate and electrodes. First-principles density functional theoretical (DFT) calculations have been very effective in capturing this and predicting material and environment specific properties of ferroelectrics, leading to fundamental insights into origins of ferroelectricity in oxides and chalcogenides uncovering a precise picture of electronic hybridization, topology, and mechanisms. However, use of DFT in molecular dynamics for detailed prediction of ferroelectric phase transitions and associated temperature dependent properties has been limited due to large length and time scales of the processes involved. To this end, it is quite appealing to start with input from DFT calculations and construct material-specific models that are realistic yet simple for use in large-scale simulations while capturing the relevant microscopic interactions quantitatively. In this Account, we first summarize the insights obtained into chemical mechanisms of ferroelectricity using first-principles DFT calculations. We then discuss the principles of construction of first-principles model Hamiltonians for ferroelectric phase transitions in perovskite oxides, which involve coarse-graining in time domain by integrating out high frequency phonons. Molecular dynamics simulations of the resulting model are shown to give quantitative predictions of material-specific ferroelectric transition behavior in bulk as well as nanoscale ferroelectric structures. A free energy landscape obtained through coarse-graining in real-space provides deeper understanding of ferroelectric transitions, domains, and states with inhomogeneous order and points out the key role of microscopic coupling between phonons and strain. We conclude with a discussion of the multiscale modeling strategy elucidated here and its application to other materials such as shape memory alloys.

Phase coexistence and the structure of the morphotropic phase boundary region in (1 − x)Bi(Mg1/2Zr1/2)O3–xPbTiO3 piezoceramics

The structures of the morphotropic phase and the phase coexistence region have been investigated in (1−x)Bi(Mg1/2Zr1/2)O3–xPbTiO3 ceramics. The structure is cubic with space group Pm3m for the compositions with x<0.57 and tetragonal with space group P4mm for the compositions with x>0.59. For the compositions with 0.56<x<0.60, both the tetragonal and cubic phases coexist, which suggests a very narrow morphotropic phase boundary (MPB) region of compositional width Δx≈0.03. Rietveld refinement of the structure using X-ray diffraction data confirms the coexistence of the tetragonal and cubic phases in the MPB region and rules out the coexistence of tetragonal and rhombohedral structures reported by earlier workers. After poling, the appearance of a significant electromechanical coupling coefficient for the cubic compositions (x=0.55, 0.56) suggests the presence of an electric-field-induced phase transition from centrosymmetric cubic phase to noncentrosymmetric phase. Polarization–electric field hysteresis loop measurement on the cubic composition with x=0.56 gives a well-saturated hysteresis loop similar to that reported the in Pb(Mg1/3Nb2/3)O3 relaxor below the freezing temperature. Our dielectric studies on the composition with x=0.56, in the temperature range 300–763K, reveals a relaxor ferroelectric transition at ∼530K with Vogel–Fulcher freezing temperature 482K.

New triclinic modification of lead iodate Pb(IO3)2: Synthesis and crystal structure

A new triclinic modification of lead iodate Pb(IO3)2 (space group \(C\bar 1\)) is obtained under hydrothermal conditions. The crystal structure is determined without preliminary knowledge of the chemical formula. [IO3]− groups, which are characteristic of pentavalent iodine, have a typical umbrella-like configuration. The new phase has a more complex structure than the known orthorhombic modification. The arrangement of heavy Pb and I atoms follows a’ = a/3 pseudoperiodicity. In the layers of two types that form sheets parallel to the ab plane, Pb and I atoms have different coordinations. A polytypic nature of the new modification is responsible for the one-dimensional disordering along the a axis as well as different variants of layer alternation. It is possible to obtain crystals of acentric and polar derivatives of the centrosymmetric phase that will be promising for application.

Concentration-phase transitions in solid solutions Li x Na1 − x Ta0.1Nb0.9O3, Li0.12Na0.88Ta y Nb1 − y O3, and NaTa y Nb1 − y O3 and their manifestations in Raman spectra

Using the Raman-spectroscopy method, we have studied concentration-phase transitions in the solid solutions Li x Na1 − x Ta0.1Nb0.9O3, Li0.12Na0.88Ta y Nb1 − y O3, and NaTa y Nb1 − y O3 (x = 0−0.16, y = 0–1). It has been revealed that, for the solid solutions Li x Na1 − x Ta0.1Nb0.9O3 and Li0.12Na0.88Ta y Nb1 − y O3, the concentration-phase transition is a transition between the ferroelectric and antiferroelectric states. It is accompanied by the disappearance from the spectrum of a line that corresponds to stretching bridge vibrations of oxygen atoms along the polar axis, which is forbidden by selection rules in the centrosymmetric phase, and by splitting into two components of a line that corresponds to librational vibrations of oxygen octahedra as a whole, which can be caused by doubling of the unit cell in the antiferroelectric phase. Manifestation and variation of intensities of lines with frequencies 860–873 and 900–905 cm−1 upon variation of the concentration of tantalum for solid solutions Li0.12Na0.88Ta y Nb1 − y O3, and NaTa y Nb1 − y O3 is caused by the formation of polar clusters in the medium, which is centrosymmetric in general due to disordering in the sublattice of niobium and tantalum.