HT Phase Research Articles

Phase Transition in Cs2KMnF6: Crystal Structures of Low- and High-Temperature Modifications

Crystalline Cs2KMnF6, when prepared below 500°C, adopts a tetragonal elpasolite structure type. Differential scanning calorimetric investigations indicated that Cs2KMnF6 undergoes a phase transition from the low-temperature tetragonal phase (LT) to a high-temperature phase (HT) at about 530°C. Single crystals of the new HT phase could be obtained by annealing a crystalline LT specimen at 600°C followed by rapid quenching to room temperature. In the present study the structures of both phases have been studied by single-crystal X-ray diffraction techniques. The LT phase has the tetragonal space group symmetry I4/mmm, with unit-cell parameters a=6.319(1) (a·2=8.936) and c=9.257(2) Å, and Z=2. The HT phase has the cubic symmetry Fm3m, with the cell parameter a=9.067 Å and Z=4. Structural models of the LT and HT phases have been refined vs collected single-crystal X-ray reflection data to R values of 0.034 and 0.022, respectively. The uneven Mn–F bond distance distribution in the LT form, four bonds of 1.860(6) two of 2.034(9) Å, are typical for an octahedrally coordinated high-spin Mn3+ ion affected by Jahn–Teller effects. Due to symmetry constraints, all six octahedral Mn–F bonds in the HT form are equal to 1.931(5) Å. However, the mean square atomic displacement parameters of the fluorine atoms increases significantly from about 0.022 Å2 for the LT phase to 0.042 Å2 for the HT phase. The increased displacement parameters indicate that the phase transition from the LT to the HT form is associated with a directional disorder of the Jahn–Teller distortions around the Mn3+ ions.

The formation of (0 0 1) oriented Ag 2Se films on amorphous and polycrystalline substrates via polymorphic-phase transition

Silver selenide thin films have been prepared on amorphous and polycrystalline substrates by the solid–vapour-phase reaction of vacuum deposited polycrystalline silver films and selenium. We have found earlier that at given conditions (0 0 1) oriented giant (100–300 μm) grained Ag 2Se layers develop. The details of formation of these peculiar films have been now studied by ex situ and in situ transmission electron microscopy. By quenching the samples from the reaction temperatures to room temperature earlier unknown intermediate stages of the formation of oriented giant grains were revealed. We observed, that during the reaction of polycrystalline Ag and selenium polycrystalline Ag 2Se film develops, which can then transform to a (0 0 1) oriented giant grained one. The formation of these oriented films occurred always only on slowly cooling during the polymorphic transition from the high-temperature cubic — to the low-temperature orthorhombic phase as a result of the selective appearance and fast growth of (0 0 1) nuclei of the orthorhombic Ag 2Se at the free upper film surface. The enthalpy difference between the HT and LT phases and the low surface energy of the (0 0 1)Ag 2Se is supposed to be the driving force of this process. The fundamental role of the polymorphic-phase transition in the formation of (0 0 1) oriented, giant grained Ag 2Se films will for the first time be shown in the present paper.

Phase transition and crystal structures of BaZn 2Si 2O 7

The phase transition and crystal structures of the low (LT) and high (HT) temperature phases of BaZn 2Si 2O 7 were studied by neutron and X-ray diffraction techniques. BaZn 2Si 2O 7 undergoes a first-order phase transition at about 250°C. The LT phase of BaZn 2Si 2O 7 is iso-structural with BaCo 2Si 2O 7, crystallizing in the monoclinic space group C2/c with the lattice constants, a=7.2782(4) Å, b=12.8009(7) Å, c=13.6869(7) Å and β=90.093(6)° at room temperature. The structure of the HT phase, which was established by analyzing the X-ray powder-diffraction data using the direct method, crystallizes in the orthorhombic space group Ccm2 1, with the lattice constants, a=7.6199(4) Å, b=13.0265(6) Å and c=6.7374(2) Å at 280°C. The underlying frameworks for both structures are similar and can be regarded as an extensive three-dimensional tetrahedral system [Zn 2Si 2O 7], in which both silicon and zinc atoms are coordinated tetrahedrally by oxygen atoms. The barium atoms are located within the channels of the framework, but their coordination polyhedra are different for the LT and HT structures. The lower coordination number of Ba atoms and the breaking of the Ba–O bonds in the HT phase may account for the endothermal behavior of the phase transition. The coefficient of volume expansion of the HT phase is smaller than that of the LT phase, and over the 280°C–900°C temperature range the cell constant along the a-axis decreases with increasing temperature.

Pyroelectric and sensor properties of ferroelectric thin films for energy conversion

High pyroelectric coefficients and improved conversion effects were successfully demonstrated in highly (111)-oriented ferroelectric PZT 90/10 films on Pt/Si substrates. The strong preferred orientation exhibited by these films was obtained through controlled heat-treatment using a specially developed MOD process on Pt/Ti/SiO 2/Si substrates. Within the FE LT–FE HT phase transition range of 80∼100°C, the films exhibited significant changes in spontaneous polarization, by roughly a factor of 2 (∼ 7 μC cm −2). A pyroelectric current, correlated to a pyroelectric coefficient of 30±10 nC cm 2 K, was obtained around room temperature without poling treatment. A prototype pyroelectric transformer based on these films gave a voltage output of 0·8 volt from a 1 mm 2 area on a 1·0 μm thick film. Compared to bulk ceramics and thick films, these thin films also exhibited at least one order of magnitude higher working frequency.

Structured habituation training for movement provoked vertigo after severe traumatic brain injury: a single case experiment

The purpose of this study was to evaluate the effect of structured habituation training HT for movement provoked vertigo PV secondary to unilateral peripheral hypofunction in a 16 year old patient who had sustained a severe TBI. Treatment of PV with severe TBI patients can be quite different from other patients with vestibular deficits because of the physiological, behavioural and cognitive sequelae of brain trauma. A single subject experimental paradigm using an ABA protocol was used to assess efficacy of HT. The data were submitted to C statistic analysis. The transformed data were submitted to combined visual and statistical analysis by the celeration line with a directional one tailed test and the two standard deviation band method. Significant change in duration from sitting to supine without triggering vertigo was found between baseline phase A and structured HT phase B and was maintained for 1 month after the end of treatment. This single case experiment demonstrates successful structured HT for PV for a 16 year old severe TBI patient. Important clinical decisions time of introduction of treatment, type of activity, pacing, frequency, intensity, repetition, support and education necessary to achieve optimal resolution of PV with HT in severe TBI patients are discussed.

Weak localization effects in Ga-intercalated layered compound NbS 2

Ga x NbS 2( x = 0 − 0.33) was prepared in two batches-≈ 1000°C (LT) and ≈ 1125°C (HT). X-ray diffraction shows random stacking of 2H-NbS 2 layers in the HT phase. Resistivity and thermopower measurements from 15 to 300 K show metallic behaviour and anomalies aroung 45 K. The disordered phases indicate weak localisation effects in conductivity above 45 K and a log T variation in the x = 0.25 compound indicating 2-D conduction. The thermopower is unaffected by the disorder. The 45 K transition suppresses this localisation.

The structural phase transition in anthracene—TCNB

Abstract The problem of the orientational disorder in mixed stack charge transfer crystals has been addressed. X-ray diffraction data in the HT phase of A-TCNB are reported. They were analysed using conventional structure-refinement procedures and utilizing symmetry-adapted functions. For the HT phase, the orientational probability function allowed us to characterize as dynamic the disorder of the anthracene molecules. The soft mode was measured by Raman and coherent neutron scattering. Measurement by Brillouin scattering of the elastic constant C11 led to another determination of the soft mode. The behaviour of this soft mode allows one to conclude the existence of a mixed regime (between displacive and order-disorder) for the LT phase of A-TCNB.

Stabilization of the high temperature phase in Bi-Ca-Sr-Cu-O system

Since the discovery of Bi-containing high-T c superconductors, difficulties in obtaining monophase samples have been known. The study of phase formation in the system Bi2Sr(3−x)Ca x Cu2O y , where 0.5<x<2.5 and conditions for raising HT phase (110 K) content by loading a superstoichiometric excess of Ca and Cu ions into the initial composition as well as by a substitution of lead for a part of bismuth is the object of the present paper.

Structural aspects of the α transition in stoichiometric FeS: Identification of the high-temperature phase

Compounds of the Fe 1− x S system ((0 ≤ x ≤ 0.125) are of NiAs type ( a and c being the substructure lattice constants). For0 ≤ x ≤ 0.05, the temperature-induced metal-nonmetal α transition is associated with a structural change. Powder studies identify the influence of x on the transition and especially on the superstructure reflection intensities of the hexagonal√3 a,2 c low-temperature ( T < T α ) phase. Crystal growth for compositions in the transition range is detailed. The structural study of a stoichiometric FeS crystal at various temperatures gives evidence for disorder phenomena in the pretransition range and for the exact nature of the hexagonal HT phase (T > T α) 2a,c (SG P6 3mc ). Its structure refinement ( R = 6.0%) rules out the previous tritwinned model of MnP type. The cationic lattice of this 2a,c modification is intermediate between the lattices of the LT √3a,2c phase and those of a pure NiAs phase.

Observation of an inverse spin-peierls phase transition from transport and magnetic properties of 1:2 salts of ethyl and methyldipropylamine with tetracyanoquinodimethane

We report a sharp discontinuity of the electrical resistivity, the thermoelectric power (TEP) and the ESR intensity of ethyl and methyldypropylamine TCNQ 2 around 212 K. In the high temperature phase (HT), the TEP tends to saturate to a value of ∼ - 70 μV K -1 indicating that the conducting electrons are strongly correlated and can be modelled as a strong coupling Hubbard model. In the low temperature phase (LT), in contrast with the normal behaviour of TCNQ salts, the conductivity reported here is larger than in the HT phase. The TEP is also larger in the LT phase. These results can be rationalized as coming from an increase of charge transfer ϱ as a consequence of the phase transition. Below T c the exchange interaction J is also stronger which indicates that this “inverse spin Peierls” transition involves a strong spin-phonon coupling.

Mixed valency in the high-temperature phases of transition metal molybdates,AMoO4 (A=Fe, Co, Ni)

Transition metal molybdates of the formulaAMoO4 whereA=Fe, Co or Ni exhibit a first-order phase transition between 670K–970K. An investigation of the lowtemperature (lt) and high-temperature (ht) phases by x-ray photoelectron spectroscopy, x-ray absorption spectroscopy, magnetic susceptibility and other physical methods shows that the phase transition is associated with a valence change of the typeA 2++Mo6+αA 3++Mo5+ in the cases of iron and cobalt molybdates.

Specific heat and bandstructure of two polymorphs of LaIr 2Si 2

LaIr 2Si 2 shows polymorphism between a high-temperature CaBe 2Ge 2-type and a low-temperature ThCr 2Si 2-type modification. Both forms represent ordered structures, distinguished only by a site exchange of half the Ir and Si populations. Specific heat measurements in combination with band structure results show that the Ir/Si site exchange does cause important changes of electronic structure, in particular near E F. The measured electronic γ and calculated DOS give both approximately a factor of two increased number of available states near E F in the high-temperature (ht) phase. Superconductivity of the observed magnitude is calculated for the ht phase, using semi-theoretical λ-values.

Les materiaux a proprietes non lineaires du systeme Sr 3WO 6 - Ba 3WO 6

Two solid solutions have been obtained in the Sr 3−xBa xWO 6 system. One of them with 0 ⪕ x ⪕ 0, 75, shows non-linear behavior. Four transitions have been detected by X-ray, μ-D.T.A., and dielectric measurements. The HT phase has the structure of cubic (NH 4) 3FeF 6, the other ones are related. Rising x decreases the transition temperatures. At room temperature the obtained phase is piezoelectric and ferroelastic, as discussed ferroelectric behavior may be expected.