Phase transition effects in polycrystalline Hg<Subscript>2</Subscript>Br<Subscript>2</Subscript> samples

The effects accompanying the ferroelastic phase transition in Hg₂Br₂ polycrystalline samples are compared in an x-ray diffraction study with similar effects observed to occur in Hg₂Br₂ single crystals. In particular, an analysis is made of the “orthorhombic” splitting of the basal plane reflections and the behavior with temperature of the Bragg and diffuse reflections from the X points of the Brillouin zone, which characterize the behavior of the order parameter and its fluctuations, respectively. Polycrystalline samples exhibit strong smearing of the phase transition effects originating from the existence of damaged surface layers and elastic and plastic strain fields which induce order parameter fluctuations over a wide temperature range.     

Phase transition in a KPb<Subscript>2</Subscript>Br<Subscript>5</Subscript> crystal

Crystals of the KPb₂Br₅compound are investigated using polarized light microscopy and calorimetry. The birefringence and the angle of rotation of the optical indicatrix are measured in the temperature range 270–620 K. It is found that the KPb₂Br₅ crystal undergoes a first-order ferroelastic phase transition at temperatures T_0↑ = 519.5 K and T_0↓ = 518.5 K with a change in the enthalpy ΔH = 1300 ± 200 J/mol. This transition is accompanied by both twinning and the symmetry change mmm ? P2₁/c. It is revealed that the angle of rotation of the optical indicatrix exhibits an unusual behavior under variations in the temperature due to a strong temperature dependence of the birefringence.     

Superproton phase transition in a K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

The conductivity and heat capacity of tripotassium hydrogen trisulfate single crystals were studied. It was shown that these single crystals undergo a solid-solid phase transformation upon heating with the formation of new phases, and that their properties change considerably. Anomalies related to the sorption and desorption of water vapor were observed in the formed multiphase system. Such anomalies were observed for the first time in measurements of the true heat capacity.     

Pressure-induced metamagnetic transition in the Cd<Subscript>0.7</Subscript>Mn<Subscript>0.3</Subscript>GeAs<Subscript>2</Subscript> ferromagnetic semiconductor

The magnetic susceptibility χ/χ₀ and the longitudinal Δρ _zz /ρ₀ and transverse Δρ _xx /ρ₀ magnetoresistances have been measured as functions of the hydrostatic pressure P ≤ 7 GPa at room temperature in the high-temperature ferromagnetic semiconductor Cd_0.7Mn_0.3GeAs₂ with a chalcopyrite structure and the Curie temperature T _c = 355 K. A pressure-induced metamagnetic transition from the low-magnetization state to the high-magnetization state has been observed in Cd_0.7Mn_0.3GeAs₂ near the magnetic ordering temperature. This transition is accompanied by the hysteresis of the magnetic susceptibility and magnetoresistance.     

Electron energy structure of defect chalcopyrite CdGa<Subscript>2</Subscript>Se<Subscript>4</Subscript>

The CdGa₂Se₄ compound, with a crystalline lattice of defect chalcopyrite, shows birefringence. However, in contrast to the similar CdGa₂S₄ compound, CdGa₂Se₄ has no experimentally observed isotropic points in the optical spectral range. In this study, the electron energy structure and optical spectra of CdGa₂Se₄ are calculated for three different experimentally obtained sets of crystalline parameters. Only one of these sets shows the absence of isotropic points.     

Phase behaviour and superionic phase transition in K<Subscript>3</Subscript>H(SeO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The phase behaviour of K₃H(SeO₄)₂ (TKHSe) above room temperature has been studied by differential scanning calorimetric (DSC), thermogravimetric analysis (TGA), simultaneous thermogravimetric and mass spectroscopy analysis (TG-MS), impedance spectroscopy (IS) and X-ray powder diffraction (XRD). Our results show that the previously claimed superionic phase transition in TKHSe at around 388 K (114.85 °C) is also the onset temperature of a slow thermal dehydration that occurs at reaction sites distributed over the surface of the crystal. That is, we propose that the TKHSe undergoes simultaneously a superionic phase transition and a decomposition process with a very slow reaction rate that is evident when the sample is pulverized to fine powder, both starting at the same temperature. As a matter of fact, we observe a decrease of the magnitude of the dc-conductivity on successive thermal runs in powdered sample attributed to sample decomposition that starts at the surface of the TKHSe grains, but the jump in conductivity is only a consequence of the order–disorder transition in the TKHSe phase that remains inside the grains.     

Phase transition in YbAl<Subscript>3</Subscript>C<Subscript>3</Subscript>

¹⁷⁰Yb M?ssbauer spectroscopy, temperature dependent X-ray, magnetisation and specific heat data are presented in the hexagonal intermetallic YbAl₃C₃, in order to shed light on the isostructural transition occurring near 80 K and to investigate the electronic state of the Yb ion above and below the transition. In the low temperature phase, we find that there occurs an atomic rearrangement in the hexagonal unit cell, leading to a strong symmetry lowering at the Yb site. We show that no magnetic ordering of the Yb³⁺ moments occurs down to 0.04 K, and we discuss this finding in terms of 4f-conduction electron hybridisation and geometric frustration.     

Methane sensor based on SnO<Subscript>2</Subscript>/In<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> nanostructure

Two sets of samples of SnO₂/In₂O₃/TiO₂ system have been fabricated with different concentrations of component materials. In the first set TiO₂ with rutile structure was used, while in the second set it has the structure of anatase. Thin films (up to 50 nm) of obtained mixtures were deposited. Their sensitivity and selectivity with respect to methane (CH4) were studied. Nanostructure on the basis of 70%SnO₂ — 10%In₂O₃ — 20%TiO₂(anatase) exhibits sufficient sensitivity to methane.     

Electron transport and anisotropy of the upper critical magnetic field in Ba<Subscript>0.68</Subscript>K<Subscript>0.32</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> single crystals

Early work on the iron-arsenide compounds supported the view, that a reduced dimensionality might be a necessary prerequisite for high-T _c superconductivity. Later, however, it was found that the zero-temperature upper critical magnetic field, H _c2(0), for the 122 iron pnictides is in fact rather isotropic. Here, we report measurements of the temperature dependence of the electrical resistivity, ρ(T), in Ba_0.5K_0.5Fe₂As₂ and Ba_0.68K_0.32Fe₂As₂ single crystals in zero magnetic field and in Ba_0.68K_0.32Fe₂As₂ in static and pulsed magnetic fields up to 60 T. We find that the resistivity of both compounds in zero field is well described by an exponential term due to inter-sheet umklapp electron-phonon scattering between light electrons around the M point to heavy hole sheets at the Γ point in reciprocal space. From our data, we construct an H-T phase diagram for the inter-plane (H | c) and in-plane (H | ab) directions for Ba_0.68K_0.32Fe₂As₂. Contrary to published data for 122 underdoped FeAs compounds, we find that H _c2(T) is in fact anisotropic in optimally doped samples down to low temperatures. The anisotropy parameter, γ = H _c2 ^ab /H _c2 ^c , is about 2.2 at T _c . For both field orientations we find a concave curvature of the H _c2 lines with decreasing anisotropy and saturation towards lower temperature. Taking into account Pauli spin paramagnetism, we perfectly can describe H _c2 and its anisotropy.     

Polarization processes in BaBi<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> and SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramics in infralow-frequency fields

The dielectric nonlinearity in BaBi₂Nb₂O₉ and SrBi₂Ta₂O₉ layered ceramics was studied by measuring their polarization loops and reverse dependences of permittivity. It was shown that the features of the dielectric response of BaBi₂Nb₂O₉ and SrBi₂Ta₂O₉ in strong fields can be explained by glass-like properties and the contribution of the domain structure of the ferroelectric material to repolarization processes, respectively.     

Polarization modes in the Ba<Subscript>2</Subscript>Mg<Subscript>2</Subscript>Fe1<Subscript>2</Subscript>O<Subscript>22</Subscript> multiferroic

The spectra of complex permittivity of a Ba₂Mg₂Fe₁₂O₂₂ single crystal belonging to the family of Y-type hexaferrites have been measured over a wide temperature range (10–300 K) with the aim of determining the dynamic parameters of the phonon and magnetic subsystems in the terahertz and infrared frequency ranges (3–4500 cm⁻¹). A factor-group analysis of the vibrational modes has been performed, and the results obtained have been compared with the experimentally observed resonances. The oscillator parameters of all nineteen phonon modes of E _u symmetry, which are allowed by the symmetry of the Ba₂Mg₂Fe₁₂O₂₂ crystal lattice, have been calculated. It has been found that, at temperatures below 195 and 50 K, the spectral response exhibits new absorption lines due to magnetic excitations.     

The high-pressure-induced spin-reorientation transition in a ferromagnetic semiconductor Cd<Subscript>0.7</Subscript>Mn<Subscript>0.3</Subscript>GeAs<Subscript>2</Subscript>

The dependences of the longitudinal magnetoresistance (Δρ _zz /ρ₀)(P), transverse magnetoresistance (Δρ _xx /ρ₀)(P), and magnetic susceptibility (χ/χ₀(P)) on hydrostatic pressure P ≤ 7 GPa in the ferromagnetic semiconductor Cd_0.7Mn_0.3GeAs₂ at room temperature were investigated.     

Ferroelastic phase transition in crystalline K<Subscript>3</Subscript>Na(CrO<Subscript>4</Subscript>)<Subscript>2</Subscript>: Acoustic studies

Ultrasonic studies of the temperature behavior of the velocity and damping of sound for the xx and zz longitudinal and yx and zx transverse waves in K₃Na(CrO₄)₂ have been carried out in the temperature interval 185–295 K, which includes the region of the ferroelastic phase transition. The acoustic parameters for both shear and longitudinal waves were found to have anomalies in the region of the phase transition with a Curie temperature of 235.5 K. A theoretical analysis of the softening of the elastic moduli c₄₄ and c₆₆ was performed on the basis of the Landau expansion in terms of the strain tensor components ?₄ and (?₂-?₁)/2 considered as the linearly coupled primary and secondary order parameter, respectively. The absolute values of the elastic moduli c₁₁, c₃₃, c₄₄, c₆₆, c₁₂, and c₁₄ at 295 K were calculated.     

Electron momentum density in Cu<Subscript>0.9</Subscript>Al<Subscript>0.1</Subscript>

A reconstruction technique based on the solution of the Radon transform in terms of Jacobi polynomials is used to obtain the 3D electron momentum density, ϱ(p), from nine high-resolution Compton profiles (CPs) for a Cu_0.9Al_0.1 disordered alloy single crystal. The method was also applied to theoretical CPs computed within the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) first-principles scheme for the same nine orientations of the crystal. The experimental ϱ(p) is in satisfactory agreement with the theoretical ϱ(p), shows most details of the Fermi surface (FS) and exhibits electron correlation effects. We comment on the map of the FS obtained by folding the reconstructed ϱ(p) into the first Brillouin zone, which yields the occupation number density, ϱ(k). A test of the validity of data via a consistency condition (within our reconstruction algorithm) as well as the propagation of experimental noise in the reconstruction of both ϱ(p) and ϱ(k) are investigated. Received: 24 October 2001 / Accepted: 20 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +48-71/344-10-29, E-mail: samsel@int.pan.wroc.pl     

Phase transition in Bi<Subscript>8</Subscript>Fe<Subscript>6</Subscript>Ti<Subscript>3</Subscript>O<Subscript>27</Subscript> multiferroic ceramics

The polycrystalline Bi₈Fe₆Ti₃O₂₇ compound was prepared by a high-temperature solid-state reaction technique. Preliminary structural analysis by X-ray diffraction (XRD) confirms the formation of a single-phase compound in an orthorhombic crystal system at room temperature. The elemental content of the compound was analyzed by EDAX microanalysis. Microstructural analysis by scanning electron microscopy (SEM) shows that the compound has well defined grains, which are distributed uniformly throughout the surface of the pellet sample. Detailed studies of temperature-dependent dielectric response at various frequencies show dielectric anomalies at 380, 389 and 403°C for 10 kHz, 100 kHz, and 1 MHz respectively. The hysteresis loop observed by applying an electric field of 12 kV/cm on the poled sample with smaller remanent polarization supports the existence of ferroelectricity in this material. The value of d₃₃ of the compound was found to be 19 pC/N.      

Spin dynamics in LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript> and NaCu<Subscript>2</Subscript>O<Subscript>2</Subscript> low-dimensional helical magnets

Comprehensive NMR investigation of low-frequency spin dynamics of LiCu₂O₂ (LCO) and NaCu₂O₂ (NCO) low-dimensional helical magnets in the paramagnetic state has been carried out for the first time. Temperature dependences of the spin–lattice relaxation rate and anisotropy on various LCO/NCO nuclei have been determined at various orientations of single crystals in an external magnetic field. The spatial asymmetry of spin fluctuations in LCO multiferroic has been discovered. The quantitative analysis of the anisotropy of spin–lattice relaxation in LCO/NCO has allowed estimating the contributions of individual neighboring Cu²⁺ ions to the transferred hyperfine field on Li⁺(Na⁺) ions.     

Field-induced magnetic transition in a mixed rare-earth aluminum garnet Er<Subscript>2</Subscript>HoAl<Subscript>5</Subscript>O<Subscript>12</Subscript>

The temperature dependence of the ac magnetic susceptibility of a single-crystal mixed rare-earth garnet Er₂HoAl₅O₁₂ has been investigated within the range from 1.8 to 300 K in a zero constant field and in applied bias fields of up to 9 T. In the absence of a constant magnetic field the magnetic susceptibility followed the Curie–Weiss law. The application of a constant magnetic field caused a magnetic phase transition, the temperature of which increased with increasing magnetic field. The temperature of the maximum of the ac magnetic susceptibility, which is a characteristic of the phase transition, did not show a noticeable dependence on the frequency of the alternating magnetic field.     

Magnetic properties of the tetranitrosyl-iron complex Fe<Subscript>2</Subscript>(SC<Subscript>3</Subscript>H<Subscript>5</Subscript>N<Subscript>2</Subscript>)<Subscript>2</Subscript>(NO)<Subscript>4</Subscript>

The magnetic properties of the binuclear nitrosyl-iron complexes Fe₂(SC₃H₅N₂)₂(NO)₄ are investigated. It is demonstrated that several types of particles, such as dimers with a pair of spins 1/2, dimers with a pair of spins 5/2, and paramagnetic particles with spin 3/2, make a contribution to the magnetic properties of the complexes. A decrease in the temperature below 25 K leads to a change in the shape of the EPR spectra corresponding to these dimers, so that Lorentzian lines (homogeneous broadening) transform into Gaussian lines (inhomogeneous broadening). This is accompanied by a stepwise change in the EPR line width and g factors. The change in the line shape indicates that complexes become asymmetric at low temperatures, possibly, due to the decrease in the spin exchange frequency below the frequency of the microwave field of the spectrometer.