Structural phase transitions in heteroepitaxial ferroelectric films

The changes in the structural—deformational characteristics occurring during the phase transitions in the crystal lattice of epitaxially grown thin (Ba,Sr)TiO₃ films on the (001) cleavage of MgO crystals have been studied. It is shown that microdeformations along the surface normal of the wall of a c-domain film increase with the temperature and attain maximum values in the vicinity of the phase transition. No decrease in the dimensions of the coherent-scattering regions was observed. The comparison of the deformation of the “average” lattice and microdeformations led to the assumption that the transition from the paraelectric to the ferroelectric phase is accompanied by the dislocation-induced formation of a dipole-glass-type intermediate phase at the temperatures exceeding that of the phase transition.     

Crystal Structures and Phase Transformations of the Fluorinated Fullerenes

Abstract

In situ x-ray diffraction experiments of the fluorinated fullerenes, C₆₀F _x (x = 0, 36 and 48) at high temperatures have been undertaken. The structural phase transformation of C₆₀F₄₈ was observed at about 358 K. It was also found that the lattice parameters of the low temperature phase of C₆₀F₄₈ show anomalous change at about 310 K. The thermal expansion coefficients of the fluorinated fullerenes were determined by using the change of the lattice parameters with temperature.     

A Structural Study of Aluminate Sodalite Ca8[Al12O24](CrO4)2(CACr)

The structure of the cubic high temperature phase of Ca₈[Al₁₂O₂₄] (CrO₄)₂, CACr, has been determined at 700 K by Rietveld analysis of neutron powder diffraction data. The structure is discussed with respect of those of its homologues. The phase transitions of the title compound have been investigated by means of synchrotron x-ray powder diffraction and differential thermo analysis. High precision lattice parameters have been determined in the temperature range between 350 and 750 K. Three phase transitions occurring at 610 K, 453 K, and 432 K could be verified. Observed anomalies in the lattice parameters versus temperature might indicate a yet unknown phase transition at about 555 K. This is supported by results of differential thermo analysis.     

Phase transitions in Cs5(HSO4)2(H2PO4)3 crystal

The symmetry (sp. gr.I $\bar 4$ 3d) and lattice parameters have been determined for the first time for Cs₅(H₂SO₄)₂(H₂PO₄)₃ crystals in the temperature range from 172 to 390 K. The thermal and optical properties of crystals, as well as their conductivity, have been investigated at elevated temperatures. It is shown that a crystal heated to T = 365 K undergoes a phase transition with symmetry lowering to the tetragonal phase (with the parameters a = 4.965(1) Å and c = 5.016(1) Å), while at T ≈ 390 K a phase transition to the cubic phase is presumably observed. With a decrease in temperature, a phase transition without a change in symmetry occurs at T = 240 K.     

Growth,composition, ferroelectric and magnetic properties of new multiferroic Pb3.3Mn4.8Ni1.1Ti0.56O15.3 single crystals

Multiferroic single crystals in the novel system Pb‐Mn‐Ni‐Ti‐O have been grown by the high temperature solution growth method. At room temperature the crystals are indexed in the hexagonal space group P6₃cm. The dielectric and magnetic properties along with the temperature dependence of the c‐lattice parameter have been studied in the temperature range 2 K ‐ 500 К. The magnetic measurements reveal a paramagnetic to antiferromagnetic phase transition around 48 K. The dielectric permittivity exhibits a maximum at 430 K, indicating ferroelectric to paraelectric phase transition. The temperature dependent Raman and XRD measurements around 430 K reveal an anomaly and abrupt change of the lattice parameter along the z‐axis respectively, thus confirming the ferroelectric‐to‐paraelectric phase transition.     

X-ray determination of thermal lattice expansion of CuSi2 + xP3 (x = 1, 2) at elevated temperatures

CuSi₂P₃ is a semiconductor having sphalerite structure with the space group F3 3m with random distribution of the copper and silicon atoms on the cation sites. Silicon is soluble in CuSi₂P₃ upto 3 moles to form CuSi_{2 + x}P₃ (x = 1, 2, 3) compounds in single phase. In continuation of our work on thermal expansion of ternary semiconductors, CuSi₃P₃ crystals have been grown by a modified Bridgman method. Using a Unicam high temperature camera, the precision lattice parameter and the coefficient of thermal expansion (CTE) of CuSi₃P₃ at various high temperatures have been evaluated from X-ray diffraction data. It has been found that the lattice parameter increases non-linearly while the coefficient of thermal expansion increases linearly with temperature. The results on thermal expansion of various semiconductors have been discussed in terms of their ionicities.     

Investigations of phase relations in the system CaO?Na2O?K2O?P2O5

The lattice constants of the compound Ca₂NaK(PO₄)₂ have been studied at room temperature using an X-ray powder diffractometer and also in the temperature range from 20 up to 1000 °C by Guinier-Lenné-technology. A hexagonal lattice with the parameters a₀ = 5.4367 Å and c₀ = 7.3125 Å and a cell volume of 187.18 ų has been determined for the high temperature phase, existing from 670 °C upward. At temperatures below 670 °C a superlattice structure is formed by tripling the axis a, a ′, and c so that it results in a hexagonal superlattice structure cell with the lattice constants of a₀ = 16.311 Å and c₀ = 21.939 Å and a cell volume of 5054 ų.     

X-Ray Studies of Some Europium Compounds at Elevated Temperatures

The temperature variation of the lattice constants of europium iron garnet (Eu₃Fe₅O₁₂), europium sulphide (EuS) and europium fluoride (EuF₂) has been studied using an X-ray powder diffractometer. The lattice constant of Eu₃Fe₅O₁₂ increases linearly upto 800 °C with an expansion coefficient of 10.4 × 10⁻⁶ °C⁻¹. In the case of EuS, the lattice constant increases non-linearly with temperature. At room temperature the expansion coefficient has a value of 14.3 × 10⁻⁶ °C⁻¹. In EuF₂ the lattice constant increases non-linearly upto 140 °C. At higher temperatures, the lattice constant decreases with increasing temperature with a negative expansion coefficient of −29 × 10⁻⁶ °C⁻¹ over the range 170–235 °C. The cause of the anomalous behaviour observed in EuF₂ is yet to be understood.     

Raman spectra and phase transitions in Rb<Subscript>2</Subscript>KInF<Subscript>6</Subscript> elpasolite

The Raman spectra of Rb₂KInF₆ elpasolite crystal have been studied in a wide temperature range, including two phase transitions: from the cubic phase to the tetragonal phase and then to the monoclinic phase. Several anomalies of internal modes of InF₆ octahedra and low-frequency lattice vibrations, which are related to the structural changes at the transition points, have been found and quantitatively analyzed. The results of a quantitative analysis of the temperature dependences of the parameters of spectral lines are in good agreement with the thermodynamic data on the phase transitions.     

In-situ size control and structural characterization of barium titanate nanocrystal prepared by crystallization of amorphous phase

We examined the influence of nanocrystalline particle size on the cubic-to-tetragonal phase transition of barium titanate (BaTiO₃). In-situ X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used in combination with electron microscopy to study the evolution of lattice structure and phase-transformation behavior with heat treatment and particle growth for BaTiO₃. The lattice constants were obtained for various polycrystal orientations in the diameter size range 10–90 nm. In-situ XRD results during the crystallization of amorphous 4BaTiO₃-SiO₂ revealed a critical size of ∼25 nm for the cubic-to-tetragonal phase transition of BaTiO₃ at room temperature.     

Characterization of nanocrystals in laser condensates deposited through vanadium evaporation in an oxygen atmosphere

The influence of the substrate temperature T _sub (20–360°C) and the oxygen pressure P(O₂) (5 × 10⁻³−0.13 Pa) in an evaporation chamber on the structure and phase composition of films prepared through laser sputtering of a vanadium target is investigated by electron diffraction and in situ transmission electron microscopy (with the use of the bend extinction contour technique for determining the bending of the crystal lattice). It is demonstrated that the oxygen content in the films increases with an increase in the oxygen pressure P(O₂) at a fixed substrate temperature T _sub and decreases with an increase in the substrate temperature T _sub at a fixed oxygen pressure P(O₂). The conditions responsible for the formation and composition of the crystalline (VO_0.9) and amorphous (V₂O₃) phases in the films are determined. It is established that the phase composition of the film depends on the angle of condensation of the vapor-plasma flow. The crystallization of the V₂O₃ amorphous phase is accompanied by an increase in the density by 9.2%. It is revealed that the V₂O₃ spherulites growing in the amorphous film have a bent crystal lattice. The bending of the crystal lattice can be as large as ∼42 deg/μm.

     

Superstructures in cubic A<Superscript>II</Superscript>B<Superscript>VI</Superscript> crystals heavily doped with Ni and V ions

Specific features of the crystal structure of bulk sphalerite-type Zn_0.9Ni_0.1S, Zn_0.9V_0.1Se, and Zn_0.997Ni_0.003Te crystals have been investigated in detail by thermal-neutron diffraction at room temperature. Fine effects (indicative of the existence of distortion microdomains, nucleation of long- and short-wavelength modulations, and tendencies toward local lowering of the symmetry based on the initial cubic structure) can be observed in the obtained scattering patterns. Various states preceding the fcc ? hcp phase transition have been revealed in these crystals. They depend on the elemental composition and are formed upon the reaction of the initial lattice to perturbations induced by foreign ions with an incomplete 3d shell.     

In‐situ investigation of the temperature dependent structural phase transition in CuInSe2 by synchrotron radiation

The temperature dependent structural phase transition from the tetragonal chalcopyrite like structure to the cubic sphalerite like structure in CuInSe₂ was investigated by in‐situ high temperature synchrotron radiation X‐ray diffraction. The data were collected in 1K steps during heating and cooling cycles (rate 38 K/h). The Rietveld analysis of the diffractograms led us to determine the temperature dependence of the lattice parameters, including the tetragonal deformation, |1‐η|, and distortion |u‐¼| (η=c/2a, a and c are the tetragonal lattice constant; u is the anion x‐coordinate). The thermal expansion coefficients α_a and α_c of the tetragonal lattice constant which are related to the linear thermal expansion coefficient α_L were obtained, as were α_a of the cubic lattice constant, also α_u and α_η. The transition temperature is clearly identified via a strong anomaly in α_L. The temperature dependence of the anion position parameter was found to be rather weak, nearly α_u∼0, whereas α_η increases slightly. However, both increase strongly when approaching to within 10 K of the transition temperature (the critical region) and |1‐η| as well as |u‐¼| go to zero with |T‐T_trans|^0.2 approaching the phase transition. The cation occupancy values, derived from the Rietveld analysis, remain constant below the critical region. Close to the transition temperature, the number of electrons at the Cu site increases with a dercrease in the number of electrons at the In site with increasing temperature, indicating a Cu‐In anti site occupancy, which is assumed to be the driving force of the phase transition. At the transition temperature 67% of Cu⁺ were found to occupy the Me1 site with a corresponding 67% of In³⁺ at the Me2 site. Although full disorder is reached with 50%, this level seems to be high enough that the phase transition takes place. The order parameter of the phase transition, goes with |T‐T_trans|^β to zero with the critical exponent β=0.35(7) which is in good agreement to the critical exponent β=0.332 calculated for order‐disorder transitions according to the Ising model. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic, magnetocaloric, and lattice properties of the La(Fe x Si1 − x )13 ferromagnets

The magnetic and lattice properties of a sample of La(Fe_0.86Si_0.14)₁₃ ferromagnet have been measured. The influence that neutron irradiation has on the physical properties of this ferromagnet is studied. It is shown that the irradiation of this sample by a fluence of 3 × 10¹⁹ n/cm² increases the lattice constant a and the Curie temperature (T _C ) as the volume magnetostriction decreases. A model of ferromagnet is proposed which satisfactorily describes the dependence a(T) of the initial and irradiated samples and their magnetic properties. The temperature dependence of the change in entropy when switching the magnetic field on and off is calculated. It is established that the change in both the magnetic and lattice parts of the total entropy at the magnetic phase transition must be taken into account for La(Fe _x Si_{1 ? x} )₁₃ compounds.     

Study of the Phase Transition in Hg<Subscript>2</Subscript>Cl<Subscript>2</Subscript> Crystals Using Anomalous X-Ray Transmission

The small-angle X-ray scattering in calomel (univalent mercury chloride Hg₂Cl₂) single crystals at low temperatures has been investigated. Data on the size of superstructural ferroelectric domains have been obtained for the first time. Lang measurements with anomalous X-ray transmission have made it possible to trace the temperature dynamics of crystal lattice of the high-temperature D_4h phase of Hg₂Cl₂ from 300 K to the Curie temperature (186 K) and the formation of ferroelastic domains in the temperature range of 186–100 K. The lowering of paraphase symmetry in the closest vicinity of the phase transition temperature is indicative of tricritical point transition.     

Evolution of crystal structure of Zn:N films under high temperature

Zinc‐nitrogen (Zn:N) compound thin film was prepared from a pure metallic Zn target by rf magnetron sputtering at ambient temperature under the mixture of nitrogen and argon gases with the ratio of 1:1. High temperature x‐ray diffraction (HTXRD) measurement under vacuum was used to examine the evolution of structural properties of the Zn:N film. At ambient temperature, the (002), (100), and (101) planes corresponding to Zn structure were observed while at higher temperature, the left shifts corresponding to the increase of lattice constants a and c of Zn were observed. At temperatures of 320 °C, 481 °C and 554 °C, the (222), (321) and (400) planes corresponding to Zn₃N₂ structure were observed with the decrease in the intensity amplitudes of the peaks belonging to the Zn structure. The results indicate the gradual transformation of the Zn₃N₂ phase in the Zn:N films at temperature greater than 320 °C. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Grain boundary faceting-roughening in Zn

The change in the grain-boundary (GB) shape with an increase in temperature near the GB faceting-roughening phase transition has been studied. The GB facet length decreases with an increase in temperature to complete facet disappearance. The facet orientation is determined by the constrained coincidence site lattice (CCSL). Facets are located along close-packed CCSL planes. Above the roughening temperature T _R, the tangents to the faceted and rough GB portions at the point of emergence of the first-order ridge are located along the close-packed CCSL planes (as facets below T _R). The faceting-roughening phase transition is reversible. The presence of temperature hysteresis is indicative of first-order phase transition.     

Dielectric properties of thallium gallium diselenide layered crystal in the incommensurate phase

The dielectric measurements of the layered crystal were studied in temperature range of successive phase transitions. The measurements revealed that the phase transition occurred in 242 K is an incommensurate phase transition. When the sample is annealed at a stabilized temperature in the incommensurate phase, a remarkable memory effect has been observed on cooling run. The mechanism of the memory effect in the incommensurate phase of the semiconducting ferroelectric TlGaSe₂ can be interpreted in the frame of the theory of defect density waves. This theory claims that the memory effect is the result of pinning of the incommensurate structure by the lattice inhomogeneities. With decreasing the annealing temperature the phase transition temperature shifts to lower temperatures gradually. Moreover, the peak intensities also increase gradually. In addition to these effects, the phase transition temperature shifts to lower temperatures with increasing annealing time. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural Phase Transition of an Intercalation Compound Mn1/4NbS2

Abstract

A structural phase transition of an intercalation compound Mn_1/4NbS₂ has been investigated by X-ray diffraction at high temperatures. The lattice parameter c exhibited a discontinuous change at 640K. The superlattice reflections observed below 640K disappeared suddenly above 640K. The phase transition at 640K took an aspect of the first-order phase transition. The precise structure analyses were performed at various temperatures above and below the phase-transition temperature. It was revealed that Mn atoms were arranged in disorder in the high-temperature phase, while the Mn atoms were ordered forming the 2a ₀ × 2a ₀ × c ₀ superlattice in the low-temperature phase. The Nb and S atoms around the ordered Mn atoms slightly shifted from the high-symmetry position in the low-temperature phase. The order parameters were the degree of order of the Mn atoms and the degree of displacement of the Nb and S atoms.     

Microstructural and magnetic characterization of Nd2Fe17 ball milled alloys

Microstructural and magnetic changes induced by ball milling in Nd₂Fe₁₇ alloy have been investigated. X-ray and neutron powder diffraction studies have shown that the main crystalline phase present in the as-cast Nd₂Fe₁₇ compound is the rhombohedral Th₂Zn₁₇-type crystal structure. Contrary to other materials, the crystal structure does not change after milling, and the crystal lattice parameters slightly increases while the induced strain is less than 0.1%. It has been observed from SEM and TEM images that the microstructure consists of agglomerates of nanoparticles with a mean size around 20 nm, and from magnetic measurements a broadening of the temperature range in which ferromagnetic to paramagnetic transition takes place.