Crystallographic Phase Transition Research Articles

Raman spectroscopy of the ammonium ion in NH4MnF3and NH4ZnF3perovskites: temperature dependence

The Raman spectra corresponding to the internal vibrations of the ammonium ion in NH4MnF3 and NH4ZnF3 single crystals have been measured from 10 to 300 K. The spectra clearly show the existence of a structural phase transition from cubic to tetragonal symmetry. The splitting of the internal modes is compatible with effective symmetries of the ammonium ion of D2d and Cs in the cubic and tetragonal phases, respectively. The temperature dependence of the symmetric stretching mode in the cubic phase is linear with a slope of 0.231 cm-1 K-1 for NH4MnF3. This has the opposite sign to that due only to the crystal expansion as obtained by applying hydrostatic pressure. A sharp band appearing only in the spectrum of NH4MnF3 just below the crystallographic phase transition originates in a strong symmetry-induced v2+v4:v1 Fermi resonance.

A remote laser system for ultrasonic velocity measurement at high temperatures

The temperature dependence of the longitudinal ultrasonic velocity in iron and Dural has been obtained using a totally remote laser technique. The ultrasound is generated by irradiation of one face of a sample with a Q-switched Nd:YAG laser pulse and is detected on the opposite face using a modified Michelson laser interferometer. The system has proved capable of measurements up to temperatures in excess of 1000 °C, with an absolute accuracy in velocity of ±1%, and relative accuracies of better than 0.1%. Anomalies in the data for iron have been discerned at the Curie temperature of ∼768 °C, due to the ferromagnetic to the paramagnetic phase transition, and at 910 °C due to the crystallographic phase transition from ferrite to austenite.

Ultrasonic Attenuation Peaks Near the Diffuse Transition Temperature in Solid Electrolytes with Fluorite Structure

AbstractMeasurements of the ultrasonic attenuation and velocity have been made in solid electrolytes with fluorite structure, PbF2, BaF2, and CdF2, from room temperature to temperature at or above the diffuse solid electrolyte transition temperature, Tc. The ultrasonic attenuation peaks observed in this class of materials are associated only with the ionic conductivity saturation rather than in combination with crystallographic phase transition found in many other solid electrolytes. The relaxation rates and Arrhenius activation energies for anion motion above the transition temperature were obtained from the temperature dependence of the ultrasonic attenuation and the theory of local site fluctuations. The ultrasonic attenuation peaks observed for the first time in CdF2was used to define the diffuse transition temperature in this material. An Anomalous peak is also observed in the linear thermal expansion coefficient of PbF2.

Mossbauer investigation of the crystallographic phase transition in Cs2FeCl5.H2O

The antiferromagnetic salt Cs2FeCl5.H2O is observed to undergo a crystallographic phase transition at TC=151.5+or-0.5 K in which the electric field gradient tensor becomes symmetric in the high-temperature phase. The order of this phase change is discussed. At temperatures below TC the quadrupole splitting has a saturation value of 0.295+or-0.005 mm s-1. Values of the Debye temperature for the material were obtained by relating the temperature variation of the spectrum centre shift and resonant absorption area to the second-order Doppler shift and the recoil-free fraction respectively.

Raman spectroscopy of the NH 4MnCl 3 crystal

The Raman spectra of the NH 4MnCl 3 crystal above and below the crystallographic phase transition are given for both the lattice and the ammonium modes. The results are discussed in terms of the structure of the crystal lattice and its influence on the symmetry of the ammonium ions.

The isomer shift of elemental iodine under high pressure

The Mossbauer isomer shift (IS) in elemental 129I was measured as a function of pressure to 30 GPa unsing diamond anvil cells. The IS increases gradually with pressure reaching a value of ΔIS = 0.56(3) mm sec −1 at P > 16 GPa. No discontinuity was observed at P = 21 GPa where a crystallographic phase transition has been reported. Using the experimental V(P) relation, the IS dependence on − ℓnV was found to be linear to the highest pressure. The dϱ(0) dℓnV value for elemental I 2 was found to be -6.6(2) a −3 0. The volume dependence of the p-holes is deduced resulting in a 5s 25p 4.62 configuration at the highest pressure. The systematics of dϱ(0) dℓnV for the 5sp elements from βSn to Xe are discussed.

Electron delocalization and magnetic behavior in a single crystal of ilvaite, a mixed valence iron silicate

Ilvaite, Ca(Fe2+, Fe3+)Fe2+Si2O8(OH), a black mixed valence iron silicate shows considerable Fe2+⇄Fe3+ electron delocalization above 400 K, reminiscent of magnetite. A crystallographic phase transition from orthorhombic (Pnam) to monoclinic (P21/a) symmetry takes place on cooling at 343 K induced by electron ordering. In both phases, Fe2+ and Fe3+ occur in double octahedral chains parallel to the c axis. The thermal characteristics of the magnetic susceptibilities and their anisotropies in different crystallographic planes have been measured in the temperature range 400−21 K. Below 343±1K, a continuous rotation of the molar susceptibility K∥ in the ab plane down to 90±2 K is observed, where the symmetry of the magnetic ellipsoid remains unchanged. Xa, Xb and Xc increase abruptly below 123±0.5 K, although antiferromagnetic ordering of Fe2+ and Fe3+ spins on A sites was suggested in previous Mossbauer and neutron powder diffraction studies. In addition, 1/Xa shows an antiferromagnetic maximum at 50±3 K, whereas 1/Xb and 1/Xc at first increase sharply below 123 K, followed by antiferromagnetic curvatures in the lowest temperature region. This behavior is consistent with the antiferromagnetic ordering of Fe2+ spins in the B sites. The observed magnetic phenomena suggest charge delocatization effects between adjacent Fe2+(A)-Fe3+(A) pairs not only along c, but also along a and b directions. The negative sign of the molar anisotropy (K∥-K⊥) suggests a singlet ground State 5A1 for the Fe2+ ions, in agreement with previous Mossbauer studies.

Superconductivity, localization and crystallographic phase transition in La(2-x)SrXCuO(4-y)

Superconductivity, localization and crystallographic phase transition in La(2-x)SrXCuO(4-y)

Pressure-induced changes in the electronic structure of solids

A variety of high-pressure metallization and metal-semimetal transitions, crystallographic phase transitions, and equation of state and lattice vibrational anomalies are reviewed in terms of the concepts of electronic transition and pressure-induced loss of covalency.

Crystal Structures at Megabar Pressures Determined by Use of the Cornell Synchrotron Source

X-ray diffraction studies have been carried out on alkali halide samples 10 micrometers in diameter (volume 10(-9) cubic centimeter) subjected to megabar pressures in the diamond anvil cell. Energy-dispersive techniques and a synchrotron source were used. These measurements can be used to detect crystallographic phase transitions. Cesium iodide was subjected to pressures of 95 gigapascals (fractional volume of 46 percent) and rubidium iodide to pressures of 89 gigapascals (fractional volume of 39 percent). Cesium iodide showed a transformation from the cubic B2 phase (cesium chloride structure) to a tetragonal phase and then to an orthorhombic phase, which was stable to 95 gigapascals. Rubidium iodide showed only a transition from the low-pressure cubic B1 phase (sodium chloride structure) to the B2 phase, which was stable up to 89 gigapascals.

A survey of domains and domain walls generated by crystallographic phase transitions causing a change of the lattice

physica status solidi (a)Volume 86, Issue 1 p. 11-42 Review Article A survey of domains and domain walls generated by crystallographic phase transitions causing a change of the lattice C. Boulesteix, C. Boulesteix Laboratoire de Microscopie Electronique Appliquée (ERA 545), Faculté des Sciences Saint-Jérǒme, Marseille Search for more papers by this author C. Boulesteix, C. Boulesteix Laboratoire de Microscopie Electronique Appliquée (ERA 545), Faculté des Sciences Saint-Jérǒme, Marseille Search for more papers by this author First published: 16 November 1984 https://doi.org/10.1002/pssa.2210860102Citations: 51 Rue H. Poincaré, 13397 Marseille Cédex 13, France. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume86, Issue116 November 1984Pages 11-42 RelatedInformation

Volume behaviour of CeSb and LaSb up to 25 GPa

The compression curves of CeSb and LaSb were measured up to 25 GPa by X-ray diffraction in a diamond anvil cell. For the two compounds the relative volume reductions are equal in the pressure range investigated. Below 11 GPa, CeSb and LaSb maintain the NaCl-type structure. Then there occurs a crystallographic phase transition. In both compounds, the high-pressure phases show similar diffraction patterns which can be indexed in a tetragonal structure (Ce or La:0, 0, 0; Sb: 1/2, 1/2, 1/2; space group P4/mmm). This structure is highly anisotropic with c/a approximately=0.82. The bulk moduli of CeSb and LaSb are equal and so a small negative electronic contribution in CeSb may be inferred. Comparison of CeAs with CeSb (or LaSb) shows an additional contribution to the bulk moduli in antimony compounds which could come from an increase of the covalent bonding.

Magnetic Susceptibility Studies on CsCrCl3and RbCrCl3

Magnetic susceptibility studies are made in 4.2 K≦ T ≦573 K on single crystals of linear chain Heisenberg antiferromagnets CsCrCl 3 and RbCrCl 3 . Anomalies in the temperature variation of the susceptibility are observed at temperatures where crystallographic phase transitions caused by cooperative Jahn-Teller effect take place. The anomaly is a discontinuous jump or a turning point according as the transition is of first or second order, respectively. The data are fitted to Fisher's classical theory, and temperature variations of the intrachain exchange interaction constants of the two magnets are evaluated. The constant changes discontinously at the first order transition temperature, and continuously in a temperature range below the second order transition temperature.

Neutron-diffraction evidence for structural phase transitions in the organic conductors bis-tetramethyltetraselenafulvalenium salts [(TMTSF)2X,X=ClO4 −andBF4−

Single-crystal neutron-diffraction experiments using the time-of-flight Laue technique establish that organic conductors, bis-tetramethyltetraselenafulvalenium salts [${(\mathrm{TMTSF})}_{2}X$, $X={\mathrm{ClO}}_{4}^{ \ensuremath{-}}$ and ${\mathrm{BF}}_{4}^{\ensuremath{-}}$], undergo crystallographic phase transitions at low temperatures. This is the first neutron-diffraction evidence for structural phase transitions in any TMTSF salts. The phase transition in the ${\mathrm{ClO}}_{4}^{\ensuremath{-}}$ salt leads to the creation of two crystallographically independent TMTSF columns with the possibility of different electrontransport properties for each column which can lead to independent low-temperature phase transitions.

Total energy and pressure in the Gaussian-orbitals technique. II. Pressure-induced crystallographic phase transition and equilibrium properties of aluminum

Total energy and pressure in the Gaussian-orbitals technique. II. Pressure-induced crystallographic phase transition and equilibrium properties of aluminum

Magnetic and crystallographic phase transitions in Dy p Gd 1- p VO 4

The compositional vanadate system Dy p Gd 1- p VO 4 (0 ≤ p ≤ 1) displays successive crystallographic and magnetic phase transitions below 14 K. An experimental investigation of these phase changes has been made with thermal expansion methods and Mössbauer spectroscopy. It is found that there is a surprising variation of the compositional dependence of the transition temperatures T D ( p ) and T N ( p ), which cannot be adequately explained by conventional pseudo-spin molecular field and pair model approximations. A detailed formalism has been developed that accounts for the observations in terms of a magneto-strictively induced quadrupole moment on the Gd 3+ ions in the mixed crsytal system and, concomitant quadrupole interactions between the Dy 3+ and Gd 3+ ions. The application of the formalism to the corresponding variation of T D ( p ) in the simple diluent system Dy p Y 1- p VO 4 (0 ≤ p ≤ 1) is also discussed.

Neutron diffraction camera method applied in the study of phase transitions

The neutron diffraction camera method is used for studying the crystallographic and magnetic phase transitions. Since this method shows a two-dimensional view of the reciprocal space at the same time, it is very useful for observing the pattern of satellite reflections or diffuse scatterings related to any kind of structural phase transitions. And also, since the instrument used is a Laue camera, an oscillating camera and a Weissenberg camera constructed for neutron diffraction, it is possible to determine crystallographic and magnetic structures as by an X-ray diffraction camera method. Neutron diffraction camera D12 at ILL was improved by Wright (1981) to remove a parasitic background, which comes from powder diffractions and from incoherent scatterings of the specimen-holder, radiation shields, and the specimen. The present experiments are done using D12-A and D12-B for studying the low-dimensionality of the crystallographic or magnetic phase transitions by observing the pattern of diffuse scatterings. The following themes are selected in order to test the application of the neutron diffraction camera method: (1) Ferroelectric transition: Since NaNO 2 has two structural transitions at 163 and 164° above room temperature, neutron photographs are taken around the principal axes at each phase by means of a Laue method. It is found that there are two kinds of diffuse scatterings above about 100°C. This fact suggests that the structural transition may be micro-structurally induced from about 100°C. Also, one of the diffuse scatterings is plane-like, in the (010) plane, and the other is rod-like, along [100]. The fine structure of these diffuse scatterings may be related to the two structural transitions by changing the pattern. (2) Low-dimensionality of magnetic short-range ordering: Although there is a short-range ordering of spin correlation above any kind of magnetic transitions, a uniaxial or layer structural compound often shows the low-dimensional one. Several experiments have been done for the specimens K 2CuF 4, K 2Cu x Zn 1− x F 4, KFeF 4, CsFeF 4, BaMnF 4 and TMMC by means of an oscillating camera and a Weissenberg camera. These compounds show a particular pattern, which is related to the short-range correlation among spins.

Structural phase transition study of NiCl 22H 2O by EPR

A single EPR line of isotropic and Lorentzian form was observed in a crystal of NiCl 22H 2O between 113 and 300 K. During a cooling run a sudden line broadening was observed at 200 K associated with a known crystallographic phase transition. On heating, the corresponding transition temperature was 220 K. The EPR spectra are interpreted in terms of a modified Anderson and Weiss model. The crystal field parameter is determined as D = 0.81 K and D = 1.07 K above and below the transition temperature, respectively.

Thermoanalytical studies on some tetrahedral A3AsO4-type compounds

Tetrahedral orthoarsenates, A3AsO4 (A=Li, Ag and Tl) have been prepared and characterized by chemical, thermal and X-ray diffraction analyses. Amongst these orthoarsenates, the thallous compound has the lowest thermal stability, undergoing melting before decomposition. While silver orthoarsenate undergoes a crystallographic phase transition from the low-temperature primitive cubic to a high-temperature face centered cubic form before decomposition, lithium orthoarsenate is highly stable at least up to 1600 K, though it also undergoes a polymorphic transition.

Low temperature magnetic properties of titanomagnetites

New measurements of high field magnetisation ( I s ), remanence ( I s ), and coercive force ( H c ) are presented between 4 and 300 K for x = 0.4, 0.5, 0.6 and 0.8 (Fe 3− x Ti x O 4). For x = 0.4 a pronounced minimum is found at T⋍100 K and for x = 0.5 and 0.6 broad minima occur around T⋍200 K , apparently coinciding with the temperature for K 1⋍0 . The magnetic properties below T⋍60 K ar complex and were found to be significantly changed by cooling in the presence of a high magnetic field. With no applied field during cooling, a distinct decrease in I s is observed for T⪅60 K, at which temperature there is a peak in the value of I rs . The effe cooling is to eliminate the sharp decrease in I s , reduce H c and to increase I rs below ∼60 K to a value the peak value, giving essentially a square hysteresis loop. The results are interpreted in terms of a form of crystallographic phase transition coupled to the magnetisation direction, possibly by the magnetostriction. Square hysteresis loops in ferrites have been explained by the presence of Jahn-Teller ions and, in the present case, the low temperature of the observed effect may be a consequence of the weak Fe 2+ Jahn-Teller ion coupled to other effects such as spin-lattice coupling. Details of this work can be found in Schmidbauer, E. and Readman, P.W., 1982. Low temperature magnetic properties of Ti-rich FeTi spinels. J. Magn. Magn. Mat., 27: 114–118. A paper reporting further work on Fe 2.4Ti 0.6O 0.4 is in preparation.