Evidence for a new,stress-induced phase transition in KMnF3

A new, stress-induced phase of KMnF₃ is found in emission, absorption and a.c.-susceptibility measurements. The phase transition is of first order and it occurs under uniaxial stress parallel to the [100] and to the [110] pseudocubic axes at 20.1 and 27 kp/mm² at 1.7 K. The experimental results are explained by a change in the amount of the tetragonal distortion in the new, stress-induced phase.     

Dynamic critical behaviour at the structural phase transition in SrTiO3

The soft mode frequency involved in the structural phase transition in SrTiO₃ is measured with high accuracy. Two regimes are observed: a Landau regime remote from T_c and a critical regime near T_c with exponents of 0.5 and 0.33, respectively. This gives evidence for a proportionality of this dynamic quantity with the static order parameter also in the critical regime. The results for the soft mode linewidth are close to the theoretical expectations.     

Pressure-induced structural phase transition in ReO3

We have investigated the pressure-induced structural phase transition in ReO₃ by neutron diffraction on a single crystal. We collected neutron diffraction intensities from the ambient and high pressure phases at P=7 kbar and refined the crystal structures. We have determined the stability of the high pressure phase as a function temperature down to T=2 K and have constructed the (P-T) phase diagram. The critical pressure is P_c=5.2 kbar at T=300 K and decreases almost linearly with decreasing temperature to become P_c=2.5 kbar at T=50 K. The phase transition is driven by the softening of the M₃ phonon mode. The high pressure phase is formed by the rigid rotation of almost undistorted ReO₆ octahedra and the Re-O-Re angle deviates from 180°. We do not see any evidence for the existence of the tetragonal (P4/mbm) intermediate pressure phase reported earlier.     

A comment on the phase transitions in KMnF3

The structural and magnetic phase transitions in KMnF₃ have been examined by means of neutron diffraction. The structural transition at 91 K accompanied with the M₃ soft-mode phonon condensation shows typical second-order character. The transition temperature and characterization of other structural and magnetic transitions are also given.     

Spin reorientation in the low temperature phase of KMnF3

The temperature dependence of anisotropy fields of the canted antiferromagnet KMnF₃ at the low temperature phase has been investigated by analysing the antiferromagnetic resonance. The results show that there exists a second-order magnetic transition associated with the spin reorientation at T₀ ～ 50°K in the absence of applied fields.     

Photoconductivity and the structural phase transition in SrTiO3

Detailed photoconductivity measurements have been performed in nominally pure SrTiO₃ in order to elucidate the effect of the antiferrodistorsive cubic-tetragonal phase transition. Small features in the photoconductivity’s temperature dependence in the phase transition region were found using low intensity interband UV or 514 nm light illumination. Such features are associated with a transformation of the defect system controlling the photoconductivity. At the same time, the temperature behavior of the photoconductivity spectral maximum reveals a rather unusual feature which is connected with changes in the absorption band edge structure in the phase transition region.     

Optically-detected antiferromagnetic resonance in KMnF3

Antiferromagnetic resonance has been detected optically by monitoring the intensity of light transmitted through a crystal of KMnF₃ at 2 K. At resonance, the intensity of the transmitted light decreases, implying that the absorption is enhanced by increases in magnon density.     

Brillouin scattering investigation of the structural phase transition of TlCdF3 and RbCaF3

The elastic constants of the fluoroperovskites TlCdF₃ and RbCaF₃ have been investigated by means of Brillouin scattering. Elastic and attenuation anomalies have been observed very near the phase transition temperature for longitudinal and transverse acoustic waves.     

β → γ structural phase transition in RbCrCl3 - crystals

A lattice vibration analysis of the β → γ phase transition in RbCrCl₃ - crystals is performed. Symmetry properties of the order parameter are also studied. The soft mode transforms according to a reducible representation belonging to the Γ and A points of the monoclinic Brillouin zone. A free-energy expansion that accounts for the symmetry properties of the order parameter is constructed and analysed.     

Cl35 NQR study of the structural phase transition in (CH3NH3)HgCl3

The Cl³⁵ nuclear quadrupole resonance spectra of (CH₃NH₃)HgCl₃ have been measured between -150°C and + 100°C. The spectra clearly show that a structural phase transition of first order takes place around T_c? 60°C. The transition may be related to a disordering of the CH₃NH₃ groups which are reorienting both above and below T_c. The positive temperature coefficient of the Cl NQR frequency, dv/dT may be also explained by the CH₃NH₃ motion.     

The influence of domains on the structural phase transition of RbCaF3

The temperature dependence of the tetragonal rotational displacement below the phase transition of RbCaF₃ is described by (T₀ - T)^β with T₀ = 199 K and β = 0.25 measured with single crystal X-ray diffraction. A method was developed to study the variability of the domains as a function of the temperature. The influence of the domains on the critical behaviour of the order parameter is pointed out.     

Investigation of the critical scattering at the structural phase transition in RbCaF3 using Mössbauer diffraction

The elastic and inelastic scattering of 14.4 keV Mössbauer gamma-rays from the 1/2(311) reflection of RbCaF₃ was investigated in the critical region. The temperature dependence of both intensities near the phase transition at 193 K is in good agreement with other experiments.     

On asymmetric propagation on transverse acoustic waves near the structural phase transition in RbH3(SeO3)2

Recently ultrasonic measurements of the RbH₃(SeO₃)₂ crystal have shown that a transverse acoustic wave propagating along the modulation axis of the incommensurate structure (c-axis) and polarized along the polar axis (b-axis) exhibits greater temperature anomalies of its velocity and damping than a wave propagating along the b-axis and polarized along the c-axis. The possible explanation of this experimental result is given on the basis of the Landau theory.     

Pressure-induced structural phase transition in rare-earth trihydrides. Part I. (GdH3, HoH3, LuH3)

High-pressure X-ray diffraction studies of gadolinum, holmium and lutetium trihydrides have been carried out in a diamond anvil cell up to 30 GPa at room temperature. A reversible structural phase transformation from the hexagonal to cubic phase has been observed for all the hydrides investigated. These results confirm our first discovery of the hexagonal to cubic phase transition in erbium trihydride published recently [T. Palasyuk, M. Tkacz, Solid State Commun. 130 (2004) 219. [1]]. The lattice parameters of the new cubic phases and the volume changes at transition points were determined for SmH₃, GdH₃, and HoH₃. The parameters of the equation of state for all the hexagonal and cubic phases of the investigated compounds have been determined.     

Pressure-induced phase transition in tysonite LaF3

We have studied the high-pressure phase stability of LaF₃ using full-potential linear augmented plane wave method. We have shown that experimentally observed orthorhombic phase is less stable compared to the theoretically predicted tetragonal structure above 25 GPa pressure. The structural transition is mainly due to the steric repulsion of ions and electrons to higher pressures.     

Attenuation et dispersion d'ondes ultrasonores dans KMnF3 au voisinage de la transition de structure

The attenuation and the velocity of sound waves have been measured along (1,0,0) and (1,1,1) axes, in KMnF₃ single crystal, at 184°K. From these measurements the coupling constant between the strain and the critical mode have been determined.     

Pressure-induced phase transition in Sb2Te3

An anomalous change in electrical resistance with pressure was observed in crystalline Sb₂Te₃ around 80 kbar at room temperature. This anomaly was found to be closely related with a pressure-induced structural phase transition which is discovered by the present x-ray diffraction experiment.     

Critical behavior near the paramagnetic to ferromagnetic phase transition temperature in La0.7Pb0.05Na0.25MnO3

Critical behavior in La_0.7Pb_0.05Na_0.25MnO₃ has been investigated by dc magnetization measurements. Magnetic data indicate that the compound exhibits a continuous (second-order) paramagnetic (PM) to ferromagnetic (FM) phase transition. Estimates of critical exponents yield δ=4.80±0.01, γ=1.296±0.002 and β=0.344±0.007 (consistent with both the predictions for the three-dimensional-Heisenberg model and with those reported for materials when the FM transition is ascribed to the double exchange (DE) mechanism as a major origin) with T_C=334.54±0.08. The critical exponent γ is slightly inferior than predicted from the 3D Heisenberg model. Such a difference may be due, within the context of the quenched disorder, to the presence of some alterations of short-range magnetic order of FM clusters in the PM phase. The temperature variation in the effective exponent (γ_eff) is similar to those for disordered ferromagnets.     

Spontaneous birefringence and order parameter of KMnF3 below the 186°K transition point

The birefringence of KMnF₃ was measured in a temperature range between 130 and 186°K. It is shown that the birefringence is proportional to the square of displacement of F^- ion and fits closely to $\text{(T}{}_{\mathrm{a}}\text{?T)}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}$ near the transition point, where T_a lies at about 1.5°K above the transition temperature. Values of the order parameter are determined at each temperature.     

Low temperature phase transition in the chain-like compounds NbSe3 and TaSe3

New transitions have been observed at low temperature by electrical measurements on the linear trichalcogenides NbSe₃ and TaSe₃. The resistivity of TaSe₃ drops near zero below 1.5 K which may indicate a superconducting transition. The resistivity of NbSe₃ drops below 2.2 K by 30 to 75% of its residual value at 4.2 K. However for TaSe₃ and NbSe₃ initial susceptibility measurements do not show any increase in diamagnetism indicating a flux expulsion. In addition the transitions below the critical temperatures are strongly non-linear with current density higher than 10^-3 A mm^-2.