Phase Transition Of Diffuse Type Research Articles

Diffuse ferroelectrics phase transition in Pb5RTi3Nb7O30 (R=Eu and Gd)

The polycrystalline complex tungsten–bronze (TB)-type compounds Pb5RTi3Nb7O30 (R=Eu and Gd) were prepared by a high-temperature solid-state technique. X-ray diffraction (XRD) study confirmed the formation of single-phase compounds in an orthorhombic structure at room temperature. Detailed studies of dielectric constant (ε) and tangent loss (tanδ) of the above compounds as a function of temperature (room temperature (30 °C) to 350 °C) at 10 kHz exhibit a ferroelectric phase transition of diffuse type. Temperature variation of resistivity shows that these compounds have negative temperature coefficient of resistance (NTCR) at high temperature.

Structural and dielectric properties of Pb 0.9(La 1− ySb y) 0.1(Zr 0.65Ti 0.35) 0.975O 3 ferroelectric ceramics

A binary solid solution of Pb 0.9(La 1− y Sb y ) 0.1(Zr 0.65Ti 0.35) 0.975O 3 (PLSZT) was synthesized by a high-temperature solid-state reaction method, near the morphotropic phase boundary (MPB). Detailed studies of the dielectric permittivity ( ε) and loss tangent (tan δ) of PLSZT ceramics as a function of temperature (30–400 °C) at 10 kHz suggest that the compounds exhibit a phase transition of diffuse type. The transition temperature ( T c) is found to increase with an increase in Sb concentration. The activation energy (DC and AC) of PLSZT was found to be very small (<0.1 eV), which is comparable to other ferroelectric oxides of the family. The decrease of DC resistivity of PLSZT with an increase in temperature suggests a negative temperature coefficient resistance (NTCR) behavior of the compound.

Diffuse phase transition in Sr 5RTi 3Nb 7O 30 (R=La, Nd and Sm)

Polycrystalline samples of Sr 5RTi 3Nb 7O 30, (R=La, Nd and Sm) compounds were prepared by a high-temperature solid-state reaction technique. The formation and structure of the compounds were checked by preliminary X-ray studies. Detailed dielectric and electrical properties of the compounds as a function of frequency (1–100 kHz) and temperature (150–650 K) were studied. The temperature dependence of dielectric constant suggests that the compounds undergo ferro–paraelectric phase transition of diffuse type. Measurements of conductivity as a function of temperature suggest that the compounds have negative temperature coefficient of conductivity above 373 K.

Effect of tunneling frequency on relaxor behavior

A main characteristic of typical relaxor ferroelectrics distinguished from normal ferroelectric is the diffuse phase transition (DPT), i.e. the dielectric constant slowly varies nearby the temperature of the dielectric peak. DPT behavior can be well described by a modified Curie–Weiss formula. Dependence of the critical exponent on the tunneling frequency has been investigated with the eight-potential-well order–disorder ferroelectric model. For low tunneling frequency, the ferroelectric phase transition is of second-order and the system conforms to the Curie–Weiss law. The critical exponent departs significantly from the Curie–Weiss value of unity and increases with increasing tunneling frequency at the region of the first-order phase transition, which is a typical diffuse phase transition behavior. Our work is useful for understanding the phenomena that some normal ferroelectrics show pressure-induced crossover from normal ferroelectric to relaxor.

Dielectric properties of Pb (Zn1/3Nb2/3)O3-PbZrO3-PbTiO3 solid solution near morphotropic phase boundary

The dielectric properties of Pb(Zn1/3Nb2/3) O3-PbZrO3−PbTiO3 (PZN-PZ-PT) system near the rhombohedral/tetragonal morphotropic phase boundary (MPB) are carefully studied in this paper. It is found that, for all samples, the curves around the temperatures of dielectric permittivity peak show the characteristics of diffuse phase transition. The change in PbZrO3/PbTiO3 ratio has much influence on the dielectric properties of the samples. The extent of diffuse phase transition increases with the increasing Zr/Ti ratio. The samples in rhombohedral region have typical diffuse phase transition in the temperature range measured. However, for the samples with tetragonal symmetry, a spontaneous normal ferroelectric-relaxor phase transition exists at temperature lower than that of permittivity peak. This normal ferroelectric-relaxor phase transition is confirmed by the experiment of thermally driven current. The analysis of TEM reveals that the samples in tetragonal region show a 90% macrodomain structure, while the samples in rhombohedral region have the configuration of microdomain structure.

Dielectric Properties of the Perovskite System Pb(Mg1/3Nb2/3)O3‐PbTiO3 Modified by Pb(Mg1/3Ta2/3)O3 and Pb(Zn1/3Nb2/3)O3

Effects of Pb(Mg1/3Ta2/3)O3 and Pb(Zn1/3Nb2/3)O3 substitution in the Pb(Mg1/3Nb2/3)O3‐PbTiO3 ceramic system on structure formation, crystallographic aspects, and dielectric properties are discussed. Developed phases in the B‐site precursor and perovskite systems were studied by X‐ray diffraction. Crystal symmetries and dimensions of the perovskite unit cell of the two systems are compared. Changing rates of the lattice parameter with substituent PbTiO3 concentration in the two modified systems are virtually identical to that of the unmodified Pb(Mg1/3Nb2/3)O3‐PbTiO3 system. Weak‐field low‐frequency dielectric responses of the ceramics were investigated. The dielectric maximum temperatures of the two perovskite systems shifted almost linearly with compositional change. Dielectric constant spectra at low concentrations of PbTiO3 exhibited typical diffuse phase transition behavior, whereas those at high PbTiO3 concentrations were rather sharp. The phase transition modes reflected on the dielectric spectra were quantitatively analyzed in terms of diffuseness parameters.

DIFFUSED PHASE TRANSITION IN Pb 0.91(La 1− z/3 ,Li z) 0.09(Zr 0.65Ti 0.35) 0.9775O 3 CERAMICS

The sol–gel process was used to prepare lithium-modified polycrystalline PLZT, Pb 0.91(La 1− z/3 ,Li z ) 0.09(Zr 0.65Ti 0.35) 0.9775O 3 ( z=0.0, 0.1, 0.3, 0.5, 0.7] (PLLZT), near the morphotropic phase boundary. X-ray diffraction studies of these compounds suggest that they can be formed in single phase at relatively low temperatures (∼800°C). The green pellets of these compounds were sintered at 1100°C. Detailed studies of the dielectric constant (ϵ) and loss tangent (tan δ) of PLLZT ceramics as a function of temperature (room temperature to 350°C) at 1 and 10 kHz frequencies suggest that the compounds exhibit relaxor behaviour and undergo ferroelectric phase transition of diffuse type on increasing the lithium concentration. Analysis of the broadened dielectric peaks of these compounds provided values of diffusivity (γ) between 1 and 2, which indicates the variation in the degree of disordering with change in z. The transition temperature ( T C) of the compounds was found to increase with increasing lithium concentration. The variation in the values of dielectric constant (ϵ) and loss tangent (tan δ) with lithium concentration shows very interesting behaviour of the compounds. The electrical conductivity and the activation energy of PLLZT provided some interesting results.

Structural and dielectric properties of sol-gel prepared PZT(La, K) ferroelectric ceramics

Polycrystalline samples of Pb 0.9(La 1− z 3 K z ) 0.1(Zr 0.53Ti 0.47) 0.975O 3 where ( z = 0.0, 0.1, 0.3, 0.5, 0.7) complex perovskite compounds (abbreviated as PLKZT) have been synthesized by sol-gel technique. Preliminary X-ray powder diffraction studies of the compounds suggest the formation of single-phase PLKZT compounds of tetragonal structure at room temperature. Detailed studies of the dielectric constant and loss of PLKZT as a function of temperature (from room temperature to 350 °C) at 10 kHz exhibit a ferroelectric phase transition of diffuse type. The transition temperature ( T c) is found to increase with increasing K concentration. Electrical resistivity and its temperature variations are measured for all these compounds.

Ferroelectric phase transition in (La, Na) modified PZT ceramics

Polycrystalline samples of Pb0.9(La1-z/3Naz)0.1(Zr0.53Ti0.47)0.975O3 where (Z = 0.0, 0.1, 0.3, 0.5, 0.7) complex perovskite compounds (abbreviated as PLNZT) have been synthesized by sol-gel technique. Preliminary X-ray powder diffraction studies of the compounds suggest the formation of single-phase PLNZT compounds of tetragonal structure at room temperature. Detailed studies of the dielectric constant and loss of PLNZT as a function of temperature (from room temperature to 350°C) at 10 kHz exhibit a ferroelectric phase transition of diffuse type. The transition temperature (Tc) is found to decrease with increasing Na concentration upto 50% (z = 0.5), then it increases with the increase in Na concentration.

Structural and dielectric properties of Pb(Li1/4Dy1/4W1/2)O3

The polycrystalline samples of Pb(Li1/4Dy1/4W1/2)O3 have been synthesized by high-temperature solid-state reaction techniques. Room temperature X-ray diffraction (XRD) studies of the compound provided preliminary structural data, and hence formation of a single phase desired material was confirmed. Detailed studies of dielectric constant (e) and loss (tanδ) as a function of frequency (100 Hz to 10 kHz) at room temperature (298 K) and also as a function of temperature (liquid nitrogen to 403 K) at 10 kHz suggest that the compound undergoes a ferroelectric phase transition of diffuse type.

Preparation and Properties of Pb[(Sc1/2Nb1/2)0.575Ti0.425]O3 Ceramics

Pb[(Sc1/2Nb1/2)0.575Ti0.425]O3 (PSNT) was prepared by the two-step calcining method. The sintering conditions for PSNT ceramics were investigated in the temperature range of 1200 to 1300° C. The density of PSNT fired at 1300° C was 7.5 g/cm3, which was in the range of 93% of the theoretical value. The dielectric constant and tan δ were 1760 and 0.022, respectively. The pyroelectric coefficient of this ceramic sample was 50 nC/cm2 K, which is larger compared with other well-known pyroelectric materials. The spontaneous polarization P s calculated by the pyroelectric current method showed a typical diffuse phase transition and its value was 28 µ C/cm2 at room temperature. Large coupling factors of k p=51 % and k t=53 % were obtained.

Phenomenological and structural study of a low-temperature phase transition in the PbZrO 3PbTiO 3 system

The Landau-Ginsburg-Devonshire phenomenological theory has been applied to the PbZrO 3PbTiO 3 crystalline solid solution system to explore the behavior of the rhombohedral-tetragonal morphotropic phase boundary in the region below room temperature. The theory suggests that morphotropy is preserved, i.e., that the phase boundary occurs at nearly the same composition right down to 0 K. The rhombohedral ( R3 m)-rhombohedral ( R3 c) phase transition was investigated for a composition PbZr 0.6Ti 0.4O 3 using neutron diffraction. Structures in both phases were refined by the Rietveld profile fitting technique. The transition behavior in this composition was indicative of a diffuse-type phase transition, with a transition temperature somewhere between 250 and 300 K. The diffuse nature of this transition is perhaps due to short-range ordering of Zr and Ti. However, powder neutron diffraction is not ideal for determining critical behavior; therefore, it is difficult to make a quantitative conclusion in this respect. Values of the spontaneous polarization were obtained from the (Zr/Ti) shifts, and compared to those deduced from phenomenological theory.