T. Iglesias

The effect of uniaxial pressure on the ferroelectric phase transition of TGSe crystals

Isobaric studies of the influence of uniaxial pressure on the transitions of pure TGSe crystals for the three directions are reported. The critical exponents, and , from dielectric constant and hysteresis loop measurements, as well as the coefficients and of the fourth and sixth powers of P in the free-energy expansion under uniaxial pressure were determined. It has been found that uniaxial pressure applied along the -axis keeps the transition near a tricritical point for 0 < X < 200 bar, but that when it is applied along the b- and c-axes it strengthens the second-order phase transition behaviour. The tricritical behaviour of TGSe crystals is discussed and a possible driving mechanism for the change in order of the phase transition is proposed.

O3 tilt and the Pb/(Zr/Ti) displacement order parameters in Zr-rich PbZr1-xTixO3 from 20 to 500 K

Neutron diffraction and dielectric data have been collected from a polycrystalline sample of PbZr1-xTixO3 (x=0.035) to determine the temperature dependence of the two-order parameters associated with the oxygen octahedral tilt and the Pb/(Zr/Ti) displacement, respectively, through the sequence of phase transitions FRL-FRH-PC present in this mixed system. The weak coupling between tilt and polarization (displacements) is satisfactorily described within the framework of a two-order-parameters statistical theory in which the respective effective fields involve two higher-order terms in addition to a linear term.

Neutron difraction investigation of the FRL−FRH transition in Nb doped PbZr1−xTIxO3 with x=0.035

Abstract Neutron powder diffraction data have been collected in order to characterize the FRL-FRH transition of Nb-doped, Zr-rich PZT. Data for the first few diffraction peaks were obtained in the temperature range 270<T<350 K in order to make a detailed characterization of the FRL-FRH transition, which takes place for this composition at TLT≅323 K. The temperature dependence of the Pb and Zr/Ti(Nb) atomic shifts, and the rotation and distortion of the oxygen octahedra through the transition was derived. The results are compared with published data for PZT with x=0.10, and indicate that (a) the change in Zr/Ti(Nb) shift through TLH becomes more pronounced, and (b) the distortion and tilt of the oxygen octahedra below TLH become more important for x=0.035 than for x=0.10. It has been concluded that the increment in the variation of the spontaneous polarization through TLH with increasing Zr amount is mainly caused by the increase of the Zr/Ti(Nb) displacement in the low temperature rhombohedral phase.

Observations of two ferroelectric response times in TGSe at T ≤ Tc

Abstract Time dependent dielectric constant measurements in ferroelectric triglycine selenate reveal an existence of two characteristic response times with distinct temperature dependences near TC. Possible reasons connected with this behaviour are considered.

Field dependent temperature shift of dielectric losses peak in TGS

Abstract Dielectric constant and loss factor measurements have been performed in triglycine sulfate single crystals at the vicinity of the Curie temperature (Tc≈48.5°C) as a function of ac driving field (20≤v≤106 Hz) in a wide range of field amplitude (50≥Eo≥0.15V/cm). Well defined shifts of loss factor peak (Dmax), increasing monotonously with the field, not previously reported as far as we know, were observed and analyzed in terms of the energy dissipated within the vanishing hysteresis loops at T≈Tc corresponding to Eo≈Ec(T), coercive field.

Vertical boundary atx≈0.11in the structural phase diagram of theLa1−xSrxMnO3system(0.08<~x<~0.125)

The structural phase diagram of the La1-xSrxMnO3 system in the compositional range 0.08<= x<= 0.125 has been investigated by high-resolution synchrotron x-ray powder diffraction techniques between 20-600 K. Recent studies have reported that there is an unusual rentrant-type phase transition in this range involving an abrupt change in lattice parameters but no change in the crystal symmetry, which remains orthorhombic Pbnm. The transition to the reentrant phase is from a ferromagnetic metallic to a ferromagnetic insulating phase with some unusual properties. Our results demonstrate that for samples with x =0.11-0.125 there exist two lower-symmetry structural regions having monoclinic and triclinic symmetry respectively. There is a sharp first-order transition from the monoclinic to the triclinic phase coinciding with the transition to the ferromagnetic insulating phase, and an abrupt crossover from the orthorhombic Pbnm region with a near-vertical phase boundary just below x = 0.11. The new phases indicate the presence of some novel type of orbital ordering unlike that found in LaMnO3.

Temperature dependence of the order parameter at the quasi-tricritical phase transition in ferroelectric TGSe

Abstract The order parameter temperature dependence near Tc has been studied in TGSe crystals with two different experimental techniques: spontaneous polarization (hysteresis loops) and increment of birefringence by means of high resolution measurements. The values of g and h, characteristic dimensionless parameters in the equation of state, are obtained, confirming the previous result g ≈ 1/3, which corresponds to quasi-tricritical behavior.

Field and frequency dependence of dielectric losses in TGS near Tc

The field dependence of the dissipation factor peak in triglycine sulphate, previously investigated at 1 kHz, has been studied in a wide frequency range from 20 Hz to 1 MHz. The frequency dependenc...

Vertical Boundary at x {approx}0.11 in the Structural Phase Diagram of the La(1-x)Sr(x)MnO3 System (0.08 less than x less than 0.125

The structural phase diagram of the La1-xSrxMnO3 system in the compositional range 0.08<= x<= 0.125 has been investigated by high-resolution synchrotron x-ray powder diffraction techniques between 20-600 K. Recent studies have reported that there is an unusual rentrant-type phase transition in this range involving an abrupt change in lattice parameters but no change in the crystal symmetry, which remains orthorhombic Pbnm. The transition to the reentrant phase is from a ferromagnetic metallic to a ferromagnetic insulating phase with some unusual properties. Our results demonstrate that for samples with x =0.11-0.125 there exist two lower-symmetry structural regions having monoclinic and triclinic symmetry respectively. There is a sharp first-order transition from the monoclinic to the triclinic phase coinciding with the transition to the ferromagnetic insulating phase, and an abrupt crossover from the orthorhombic Pbnm region with a near-vertical phase boundary just below x = 0.11. The new phases indicate the presence of some novel type of orbital ordering unlike that found in LaMnO3.

Vertical boundary at x{approx}0.11 in the structural phase diagram of the La{sub 1-x}Sr{sub x}MnO{sub 3} system (0.08{<=}x{le}0.125)

The structural phase diagram of the La1-xSrxMnO3 system in the compositional range 0.08<= x<= 0.125 has been investigated by high-resolution synchrotron x-ray powder diffraction techniques between 20-600 K. Recent studies have reported that there is an unusual rentrant-type phase transition in this range involving an abrupt change in lattice parameters but no change in the crystal symmetry, which remains orthorhombic Pbnm. The transition to the reentrant phase is from a ferromagnetic metallic to a ferromagnetic insulating phase with some unusual properties. Our results demonstrate that for samples with x =0.11-0.125 there exist two lower-symmetry structural regions having monoclinic and triclinic symmetry respectively. There is a sharp first-order transition from the monoclinic to the triclinic phase coinciding with the transition to the ferromagnetic insulating phase, and an abrupt crossover from the orthorhombic Pbnm region with a near-vertical phase boundary just below x = 0.11. The new phases indicate the presence of some novel type of orbital ordering unlike that found in LaMnO3.