Spinodal ordering in Cu3Au

Results from, X-ray diffuse scattering and inelastic neutron scattering demonstrate the presence of long-lived fluctuations in order up to 5.0°C above the first-order phase transition, T_c = 383.2°C. The temperature for continuous ordering has been determined to be 358.2°C. The coherent phase boundary is 3–13°C below T_c.     

Inelastic neutron scattering study in cubic NaNbO3

Inelastic neutron scattering measurements performed on NaNbO₃ close to the cubic to tetragonal phase transition show that the 2D fluctuations earlier revealed by X-ray diffuse scattering correspond to an overdamped type dynamics. The sharp decrease in the spectral width observed at T-T_c =10° is interpreted in terms of the increase in correlation lengths and the cross-over between 2D and 3D behaviour of the fluctuations.     

Electron hopping and optic phonons in Eu3S4

Raman scattering on single crystals of Eu₃S₄ does not show the allowed q=o phonon modes in the cubic phase and exhibits no new modes in the distorted low temperature phase (T<186 K). Above the Curie temperature T_c=3.8 K the scattering is dominated by a spin-disorder induced one-phonon density of states allowing for the observation of the zone boundary phonon breathing mode of the S^2?ions. This mode does not show any anomaly near the charge order -disorder phase transition T_t=186 K. Temperature tunable spin fluctuations associated with the temperature activated Eu²⁺→Eu³⁺ electron hopping are detected in the scattering intensity, superimposed on the usual thermal spin disorder.     

Soft modes below the incommensurate transition in K2SeO4

A coordinated series of Raman and Brillouin scattering measurements were carried out in K₂SeO₄ with emphasis on temperatures near and below the incommensurate structural phase transition (T_i=129 K). Below the lock-in phase transition (T_c=99 K) Raman scattering from both the amplitude mode and the phase mode analog was observed. However, only the amplitude mode could be followed into the incommensurate phase. Details of the amplitude mode frequency and damping are reported for temperatures considerably closer to T_i than previously possible. Our results do not confirm the diverging linewidth extrapolated from earlier Raman work. We also report here the first Brillouin scattering results in K₂SeO₄. Although substancial interaction effects between the acoustic phonons and the soft amplitude mode are evident near T_i, the coupled mode lineshapes can be well described without the introduction of a relaxing self energy for the soft mode. Finally, limits on the characteristics of the phason are drawn from our failure to observe it directly in the incommensurate phase.     

Observation of soft-modes in a single crystal of Rb2ZnBr4 by Raman scattering

The Raman spectra of an oriented single crystal of Rb₂ZnBr₄ shows three soft modes. One of them is interpreted as the amplitude mode of the incommensurate phase. The second one, seen well below the lock-in transition temperature appears as the phase mode. Moreover, an additional soft mode is observed between T=120 K and T=20 K, in the b(c, c)a scattering geometry. By extrapolation it is found that its frequency would vanish around T₃=140 K ± 10 K. This soft mode is likely to be related to a new phase transition.     

The magnetic phase diagram of CsNiF3 as determined by neutron diffraction

By neutron diffraction we investigated the magnetic properties of CsNiF₃ around and below T_N with and without magnetic field. We measured the phase diagram for 1.9 K ≤ T ≤ T_N = 2.7 K in an external field. The phase transition in this temperature-range is always continuous. We were able to measure the critical scattering connected with the magnetic phase transition. This critical scattering is centered around the position of the magnetic Bragg reflections. The critical scattering was found to be similar everywhere on the phase-separation line (H ≠ )0 and at T_N(H = 0). When applying a small field only processes were found, which correspond to a growing or diminishing of certain domains.     

Ferroelectricity and phase transition from incommensurate phase into commensurate one in (NH4)2ZnCl4

Phase transition has been found in (NH₄)₂ZnCl₄ at T = 266 ± 0.5 K by NQR method. There is a ferroelectric phase below T_c with a space group P2₁cn and with the trebling of the elementary lattice parameter along the axis c. Above the phase transition temperature in the crystal (NH₄)₂ZnCl₄ an incommensurate phase is realized.     

Dielectric properties of CsH2PO4 and CsD2PO4

Orthorhombic CsH₂PO₄ undergoes a ferroelectric transition at T_c = ?119.5°C, whereas the ferroelectric transition temperature in isomorphous CsD₂PO₄ is T_c = ?5.55°C. The transitions are of first order in both cases. The rather large isotope effect demonstrates the importance of the OHO bonds in the transition mechanism.     

1H chemical shift tensor in squaric acid: C4O4H2

The proton chemical shift tensor in squaric acid, which undergoes an antiferrodistortive phase transition at T_c = 97°C, has been determined by means of a multiple-pulse NMR experiment. No change of the shift tensor is observed, when going through the phase transition, indicating a purely order—disorder type transition. The principal values of the shift tensor are: — 26.5, —20.2 and 1.0 ppm with respect ti TMS.     

Dielectric properties and phase transitions of (Pb0.87La0.02Ba0.1)(Zr0.6Sn0.4−xTix)O3 ceramics with compositions near AFE/RFE phase boundary

The electrical properties and phase transition behavior of (Pb_0.87La_0.02Ba_0.1)(Zr_0.6Sn_0.4−xTi_x)O₃ solid solutions (PLBZST, 0.04≤x0.2) were investigated by the X-ray diffraction, permittivity, pyroelectric current, and P-E electric hysterisis loops. As the composition x increased from 0.04 to 0.2, the antiferroelectric ceramics (x≤0.07, AFE) with tetragonal phase changed to the ferroelectric relaxors (RFE, 0.09≤x). AFE ceramics showed a peculiar diffuse phase transition and dielectric relaxation at the low temperature (down to −100 °C) due to a frustration between AFE and FE state. With an increase in composition x, electrically field-induced AFE-FE switching field (E_AFE-FE) and AFE-paraelectric (PE) phase transition temperature (T_c) are depressed in the temperature (T)-Ti composition (x) phase diagram, a FE-AFE-PE triple phase point (T_tr) with the lowest transition temperature occurred at x=0.09. The pyroelectric currents under an application of various external electric field (E) were measured to identify a T-E phase diagram of the PLBZST compound.     

The heat capacity of the layer compounds (CH3CH2CH2NH3)2MnCl4 and (CH3CH2CH2NH3)2CdCl4 from 10 K TO 300 K

The heat capacity of the layer compounds tetrachlorobis (n-propylammonium) manganese II and tetrachlorobis (n-propylammonium) cadmium II, (CH₃CH₂CH₂NH₃)₂MnCl₄ and (CH₃CH₂CH₂NH₃)₂CdCl₄ respectively, has been measured over the temperature range 10 K ?T ? 300 K.Two known structural phase transitions were observed for the Mn compound in this temperature region: at T = 112.8 ± 0.1 K (ΔH_t= 586 ± 2 J mol^?1; ΔS_t = 5.47 ± 0.02 J K^?1mol^?1) and at T =164.3 ± (ΔH_t = 496 ± 7 J mol^?1; ΔS_t =3.29 ± 0.05 J K^?1mol^?1). The lower transition is known to be from a monoclinic structure to a tetragonal structure, while the upper is from the tetragonal phase to an orthorhombic one. From comparison with the results for the corresponding methyl Mn compound it is deduced that the lower transition primarily involves changes in H-bonding while the upper transition involves motion in the propyl chain.A new structural phase transition was observed in the Cd compound at T= 105.5 ± 0.1 K (ΔH_t= 1472.3 ± 0.1 J mol^?1; ΔS_t = 13.956 ± 0.001 J K^?1mol^?1), in addition to two transitions that have been observed previously by other techniques. The higher of these transitions(T = 178.7 ± 0.3 K; ΔH_t = 982 ± 4 J mol^?1 ΔS_t = 6.16 ± 0.02 J K^? mol^?1) is known to be between two orthorhombic structures, while the structural changes at the lower transition (T= 156.8 ± 0.2 K; ΔH_t = 598 ± 5 J mol^?1, ΔS_t = 3.85 ± 0.03 J K^?1 mol^?1) and at the new transition are not known. It is proposed that these two transitions correspond respectively to the tetragonal to orthorhombic and monoclinic to tetragonal transitions in the propyl Mn compounds.In addition to the structural phase transitions (CH₃CH₂CH₂NH₃)₂MnCl₄ magnetically orders at t? 130 K. The magnetic contribution to the heat capacity is deduced from the heat capacity of the corresponding diamagnetic Cd compound and is of the form expected for a quasi 2-dimensional Heisenberg antiferromagnet.     

51V nuclear magnetic resonance in polycrystalline ferroelastic BiVO4

Nuclear magnetic resonance studies on polycrystalline ferroelastic BiVO₄ indicate that the ⁵¹V electric field gradient asymmetry parameter is an order parameter in the ferroelastic transition. Using ∩ = A(T?T_c)^B, B is found to be 0.48(5), in good agreement with earlier studies of this material. Near the phase transition above and below, the vanadium nuclear quadrupole coupling is constant with a value of 4.8(1) MHz.     

Two-dimensional critical behaviour in antiferrodistortive Al ur6(CIO4)3 and Ga ur6(ClO4)3

Critical behaviour with dimensionality d = 2 has been observed for the 300 K antiferrodistortive phase transition in Al ur₆(ClO₄)₃ and Ga ur₆(ClO₄)₃ by means of the temperature dependence of the ESR parameter D. The systems exhibited d = 2 behaviour in the static critical behaviour for T<T_c?40 K for T>T_c + 40 K. From the ESR data including line width measurements the local order parameter relaxation rate ω₁ has been obtained for various temperatures above T_c, with a lowest value of ω₁ = 150 MHz at T_c + 15 K     

Polarisation remanente dans les varietes de basse temperature de (NH4) 3AlF6 ET (NH4) 3FeF6

Dielectric and thermocurrent measurements have been carried out on (NH₄) ₃AlF₆ and (NH₄) ₃FeF₆ ceramic samples. A maximum of permittivity is observed close to the transition temperature (T_{T(NH4) 3AlF6} = 217K; T_T(NH43FeF6 = 264K. In the low-temperature phase a polarization current of about 10^-9A is obtained and can be reversed when the sign of the polarization field is changed, a property which could correspond to a ferroelectric behavior. However, no pyroelectric current is detected when the temperature decreases from T_T. Another hypothesis, based on a field-induced polarization, has been considered : the depolarization current could be due to charge displacements from potential minima favored by rising temperature. In any way, the low-temperature phase is characterized by a remanent polarization.     

Phase transitions in the atomic, electronic, and magnetic subsystems of LaSrMnO films versus the synthesis temperature

The evolution of the cluster structure in amorphous LaSrMnO films as synthesis temperature T _s increases from 20 to 300°C is considered. Two order-disorder phase transitions with different scale parameters are observed. One of them, the aggregation of disordered atoms into small (～20 Å) amorphous clusters at T _s = 100°C, shows up as a sharp increase in the intensity of diffuse X-ray scattering (diffuse halo 1) with a simultaneous suppression of incoherent (background) scattering. At T _s > 150°C, disordering dominates (I _incoh = I _max) until the next stage of ordering sets in at T _s = 250?300°C. At this stage, the crystalline phase forms from large (>100 Å) crystalline clusters. This amorphous-crystalline phase transition is characterized by the appearance of Debye lines and a reduction of the halo intensity. The structural phase transition to long-range order is accompanied by a decrease in the LaSrMnO resistivity from 10¹⁰ to 10 Ω cm and a change from the tunneling mechanism of conductivity involving metallic clusters (which is typical of granulated systems) to the hopping mechanism with a hop variable length following the Mott law ρ ～ exp(T ^?1/4). In the magnetic subsystem, the paramagnetic-ferromagnetic phase transition occurs.     

Magnetic phase transitions in the 1D ising antiferromagnet CsCoCl3

We report on Raman scattering from the magnons in CsCoCl₃ with particular emphasis on two magnetic phase transitions occurring at T_N = 20.82 K and at 8 K. The T → 0 magnon spectrum is fitted by a $\text{S =}\text{1}\text{2}$ anisotropic Heisenberg model. With a simple cluster model we can explain the temperature behaviour of the magnon frequencies and intensities. The physics of the 8 K phase transition is also discussed.     

Evidence for a new phase transition in Rb2ZnCl4 by Raman scattering

The Raman spectra of an oriented single crystal of Rb₂ZnCl₄ give evidence for the existence of a phse mode at low temperature, as predicted by Wada et al. A new soft mode whose frequency decreases as the temperature T₃?72 K is approached from below, was observed in the a(cc)b scattering orientation, strongly supporting the onset of a new phase transition.     

Brillouin scattering investigation of paraelectric KH2PO4 under applied uniaxial stress

The Brillouin scattering spectra of KH₂PO₄ under an applied uniaxial stress have been studied in the neighborhood of the ferroelectric transition. Dependence of shear acoustic phonon on the uniaxial stress is similar to dependence on electric field. The acoustic phonon is overdamped near T_c, but become underdamped under the applied shear stress. In the case of ΔT(= T — T_c) < ～ 0.05 K, application of stress caused a transition not to have any soft acoustic phonon.     

X-ray and neutron investigations of the phase instabilities in K2SnCl6

From X-ray and neutron diffraction work with single crystals and powders we found two phase transitions at T_c1 = 260.5 ± 1 K and at T_c2 = 255 ± 1 K. The transition at T_c1 is characterised by the occurence of superstructure reflections breaking the f.c.c. translational symmetry, whereas the second one at T_c2 is indicated by a monoclinic lattice distortion without any further loss of translational symmetry. Both transitions can be explained by the softening of optical branches at the X- and the Γ-point of the Brillouin zone, corresponding to librations of the SnCl₆-octahedra. Evidence for this low energy phonon branch is offered from the observation of diffuse streaks with X-rats and from their inelastic nature as revealed by the neutron triple axes technique.     

Pressure induced phase transition in the highly anisotropic compound: Y2[Pt(CN)4]3·21H2O

For the linear chain system Y₂[Pt(CN)₄]₃·21H₂O a pressure induced phase transition is observed by emission spectroscopy. At p_trans=(5±0.5) kbar and T=295 K the compound undergoes a first order phase transition, in the course of which the intra-chain Pt-Pt distance R shrinks by $\text{ΔR≈}\text{-0.03}\text{A}\text{?}$. An approximate value had already been found at standard pressure for a temperature induced phase transition (T_trans=218 K).