Giant thermodynamical optical effect near ferroelastic phase transition point in Cs3Bi2I9 layered crystal

For the first time we have found a new giant thermodynamical optical effect near the ferroelastic phase transition point in Cs₃Bi₂I₉ layered crystal. The effect is appeared as periodical oscillations in time of the reflection coefficient. This phenomenon is caused by the small temperature deviations in thermodynamical system the appearance of which in the reflection spectra is strongly amplified in the ferroelastic phase transition point. The optical oscillations are explained on the base of a model that takes into account the temperature dependence of the refractive index through the order parameter (spontaneous strain) of the crystal.     

Nontraditional temperature shift of the fundamental absorption edge for layered substances and the ferroelastic phase transition in Cs3Bi2I9

We have carried out investigations of the exciton reflection spectra of Cs₃Bi₂I₉ layered crystals as a function of temperature. For the first time for the layered substances we have found nontraditional temperature shift of the energy gap E_g(T) described by the Varshni formula. We have registered a transition region in the temperature broadening of the half-width, H(T), of the exciton band with increasing of temperature in the interval between 150 and 220 K. It is shown that this region may be identified as the heterophase structure region where ferroelastic and paraelastic phases coexist. We have also found a surge in H(T) at the ferroelastic phase transition point      

Dependence of high electric-field-induced strain on the composition and orientation of Pb(Mg1/3Nb2/3)O3-PbTiO3 crystals

Electric-field-induced strain behavior of (1−x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMNT) crystals with different orientations and compositions was investigated for use as electromechanical actuators. Crystallographically, high strains with low hysteresis were achieved for 〈001〉 oriented rhombohedral crystals (29%≤x≤31%) near a morphotropic phase boundary, rather than 〈110〉 and 〈111〉. Domain instability could explain inferior strain levels and large hysteresis for 〈110〉 and 〈111〉 oriented crystals. Ultrahigh strain levels up to 1.8% could be achieved for 〈001〉 oriented PMNT crystals, being related to an E-field induced phase transition. −2 kV/cm negative E-field can be applied to PMNT ferroelectric material with low hysteresis. High strain with low hysteresis makes PMNT crystals promising candidates for high performance solid-state actuators.     

Ferroelectric properties of Pr and Nb co-substituted Sr0.8Bi2.2Ta2O9 thin films

Thin film of both A- and B-site co-substituted Sr_0.8Bi_2.2Ta₂O₉ (SBT) by Pr³⁺ and Nb⁵⁺, i.e. Sr_0.8Pr_0.1Bi_2.1Ta_1.5Nb_0.5O₉ (SPBTN) was fabricated on Pt/Ti/SiO₂/Si substrates by metalorganic decomposition method. The Nb⁵⁺ substitution at B-site and Pr³⁺ substitution at A-site enhanced the remanent polarization and reduced the coercive field of the films, respectively. The remanent polarization (2P_r) value of the SPBTN film was 22 μC/cm². The coercive field (2E_c) value of the SPBTN film was 102 kV/cm, which was much lower than that of SBTN (165 kV/cm). The effects of substitution on structural and ferroelectric properties of SBT were discussed in detail. As a result, the A- and B-sites co-substitution may be one of the promising ways to improve ferroelectric properties of SBT.     

A study on new phase transitions in Cs2CaCl4·2H2O single crystals by observation of Cs NMR

The ¹³³Cs 1/2→−1/2 spin-lattice relaxation rate, , and the spin-spin relaxation rate, , for a Cs₂CaCl₄·2H₂O single crystal have been measured in function of temperature. The dominant relaxation mechanism of this crystal over the whole temperature range investigated here proceeds via quadrupole interaction. The changes in the ¹³³Cs spin-lattice relaxation rate near 325 K (=T_c1) and 360 K (=T_c2) correspond to phase transitions in the crystal. The change in the spin-lattice relaxation rate at T_c1 is small because the crystal lattice does not change very much during this phase transition. The change in near T_c2 is due to the critical slowing down of the soft mode that typically occurs in structural phase transitions. The temperature dependence of the spin-lattice relaxation rate for this crystal has maximum values at about 240 K, which is attributable to the effect of molecular motion as described by Bloembergen-Purcell-Pound theory. The phase transition temperatures T_c1 and T_c2 obtained from the temperature dependence of the relaxation rate is also clear from data obtained using differential scanning calorimetry. Therefore, we know that previously unreported phase transitions occur at 325 and 360 K.     

Characterization of lead free (K0.5Na0.5)NbO3-LiSbO3 piezoceramic

(K_0.5Na_0.5)NbO₃ (KNN) based lead free ceramics have been fabricated by a solid state reaction. In this work, LiSbO₃ (LS) modified KNN based ceramics were sintered at atmospheric pressure and high density (>96% theoretical) was obtained. The detailed elastic, dielectric, piezoelectric and electromechanical properties were characterized by using the resonance technique combined with the ultrasonic method. The full set of material constants for the obtained polycrystalline ceramics were determined and compared to the pure hot pressed KNN counterpart. KNN-LS polycrystalline ceramic was found to have higher elastic compliance, dielectric permittivity and piezoelectric strain coefficients, but lower mechanical quality factor, when compared to pure KNN, exhibiting a “softening” behavior. However, a high coercive field (∼17 kV/cm) was found for the LS modified KNN material. The properties as a function of temperature were determined in the range of −50-250 ^°C, showing a polymorphic phase transition near room temperature, giving rise to improved piezoelectric behavior.     

无机非铅钙钛矿Cs3Bi2I9的电子和光学性质

有机-无机卤化钙钛矿材料因具有优异的光电性质而被广泛应用于太阳电池中,然而材料及器件的稳定性及含铅问题却严重制约其生产发展.与杂化钙钛矿相比,无机非铅钙钛矿Cs₃Bi₂I₉因具有更强的稳定性和环境友好性受到人们的广泛关注.Cs₃Bi₂I₉具有单斜、三角和六方3种晶型,目前,对Cs₃Bi₂I₉的理论和实验研究主要集中在六方相.本文基于密度泛函理论的第一性原理对Cs₃Bi₂I₉单斜、三角和六方相的电子性质、载流子有效质量(m~*)、稳定性和光学性质进行了理论研究.结果表明,3种晶相具有相近的稳定性,三角相因具有较小的直接带隙(1.21 eV)性质成为最具研究潜力的对象.3种晶相的m~*均具有沿a,b方向相同和沿c方向不同的特点,三角相的电子有效质量最小、且沿a方向的电子有效质量小于c方向.相比单斜和六方相,三角相Cs_3<...     

Proton activity and spontaneous strain of Cs3H(SeO4)2 in the phase transition at 369 K

We have investigated the origin of the change in proton activity in the phase transition at T_II-III (=369 K) in Cs₃H(SeO₄)₂ from the viewpoint of its ferroelasticity by using ¹H NMR and X-ray measurements. It is found that the second moment of the ¹H NMR absorption line rapidly decreases at T_II-III with increasing temperature. From this result, we conclude that the hopping motion of a proton, which is the precursor motion in the superprotonic phase, becomes more active above T_II-III. This result is consistent with the fact that the electrical conductivity in phase II is larger than that in phase III. Furthermore, it is also found that the spontaneous strain decreases abruptly at T_II-III. From these results, it is deduced that the decrease in the spontaneous strain at T_II-III causes the increase in the proton activity at T_II-III. In addition, it is deduced that the increase in proton activity and the decrease in the spontaneous strain at T_II-III are closely related with the appearance of the superprotonic phase transition at T_I-II (=456 K).     

Interrelation of antiferrodistortive and ferroelectric phase transitions in Sr1−xAxTiO3 (A=Ba, Pb)

A dielectric and ultrasonic velocity study of antiferrodistortive and ferroelectric phase transitions in Sr_1−xA_xTiO₃ (A=Ba, Pb) is reported. It is shown that both phase transitions co-exist at x<0.03 but at the higher concentration the ferroelectric phase transition entirely suppresses the antiferrodistortive phase transition. The experimentally obtained phase diagrams are discussed in the framework of the Landau phenomenological theory.     

Local structure of multiferroic RMn2O5: Important role of the R site

The temperature and magnetic field dependent local structure of RMn₂O₅ systems was examined. While no significant displacements of the Mn ions are observed, it is found that the R-O distribution exhibits changes at low temperature which are possibly related to the changes in the electric polarization. Density functional computations are used to explore the system dynamics and to link the local structural measurements with anomalous changes in the infrared absorption spectra. The anomalous R-O distribution and observed coupling to magnetic fields point to the need to properly treat the 4f electrons on the R sites in these systems.     

Investigations of structural, dielectric and ferroelectric behavior of europium substituted SrBi2Ta2O9 ferroelectric ceramics

Europium substituted samples of the compositions Sr_1−xEu_xBi₂Ta₂O₉ (x=0.0,0.025,0.050,0.10 and 0.20) were synthesized by solid state reaction method and studied for their structural, dielectric and ferroelectric properties. The X-ray diffractograms confirmed the formation of single phase layered perovskite structure in all the samples. The temperature variation of dielectric constant shows that the Curie temperature (T_c) decreases on increasing concentration of europium. The dielectric loss reduces significantly with europium addition. The P-E studies of the Eu-substituted SBT ceramics show that the remanent polarization increases with increasing concentration of europium.     

Domain structures in tetragonal Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals studied by piezoresponse force microscopy

The domain structures in (001) surface of Pb(Mg_1/3Nb_2/3)O₃-40% PbTiO₃ single crystals were investigated by piezoresponse force microscopy. Both micron-sized fingerprint 180° and parallel 90° domains were observed in the sample. Different sets of favourable {110} oriented domain patterns were found to meet, intersect or grow through each other. In addition, the piezoelectricity decreases sharply at the domain walls in 180° structures, but does not in the 90° domain structures.     

Phase transition and ferroelastic property studied by using the Cs nuclear magnetic resonance in a CsHSO4 single crystal

The ¹³³Cs spin-lattice relaxation time in a CsHSO₄ single crystal was measured in the temperature range from 300 to 450 K. The changes in the ¹³³Cs spin-lattice relaxation rate near T_c1 (=333 K) and T_c2 (=415 K) correspond to phase transitions in the crystal. The small change in the spin-lattice relaxation time across the phase transition from II to III is due to the fact that during the phase transition, the crystal lattice does not change very much; thus, this transition is a second-order phase transition. The abrupt change of T₁ around T_c2 (II-I phase transition) is due to a structural phase transition from the monoclinic to the tetragonal phase; this transition is a first-order transition. The temperature dependences of the relaxation rates in phases I, II, and III are indicative of a single-phonon process and can be represented by T₁⁻¹=A+BT. In addition, from the stress-strain hysteresis loop and the ¹³³Cs nuclear magnetic resonance, we know that the CsHSO₄ crystal has ferroelastic characteristics in phases II and III.     

Molecular dynamics of [(CH2OH)3CNH3]2SiF6 by phase transition of 178 K

We have investigated the molecular motions of TRIS⁺ ([(CH₂OH)₃CNH₃]⁺) and ions in the [(CH₂OH)₃CNH₃]₂SiF₆ crystal below room temperature from the measurements of the spin-lattice relaxation time T₁ and the NMR absorption line of ¹H and ¹⁹F nuclei, in order to elucidate the changes of the molecular motions by the phase transition of T_c=178 K. The narrowing of the ¹⁹F-NMR line was observed around T_c=178 K and the reorientation of the anion appears above T_c. Moreover, from the analysis of the temperature dependence of T₁, we have observed that the activation energy of the reorientational motion of ions changes from 0.168 eV (T>T_c) to 0.185 eV (T<T_c). Based on these results, we found that the reorientational motion of ions is closely related to the origin of the phase transition at T_c. In addition, from the measurement of the ¹H-NMR line, we also found that the reorientational motion of H₂ in the -CH₂OH group becomes active accompanied by the phase transition.     

First-Principles Prediction of High-Pressure Phase of CaC6

The lattice dynamics of rhombohedral GaG6 is studied as a function orpressure to probe Its high pressure phase with low superconducting transition temperature using the density functional liner-response theory. The pressureinduced phase transition in CaC6 is attributable to the softening transverse acoustic （TA） phonon mode at the zone boundary X （0.5, 0.0, 0.5） point. The high pressure phase is then explored by performing fully structural optimization in the supercell which accommodates the atomic displacements corresponding to the eigenvectors of the unstable mode of TA（X）. The high-pressure phase is predicted to be a monoclinic unit cell with space group P21/m.     

Thermal and optical properties of the ferroelectric (C3N2H5)5Bi2Cl11 crystal

Thermal (specific heat) and optical (linear birefringence) studies were performed for a new ferroelectric crystal (C₃N₂H₅)₅Bi₂Cl₁₁. Two phase transitions were confirmed and described. The first-order paraelastic-ferroelastic phase transition at 360 K was studied with a polarizing microscope. The continuous second-order phase transition at 165 K to the ferroelectric phase is described by the Landau model using specific heat and linear birefringence data. The Landau expansion coefficients B and C are of an order of magnitude higher than the closely related ferroelectric crystal; MAPCB — (CH₃NH₃)₅Bi₂Cl₁₁. Thermal parameters (such as the excess enthalpy and the excess entropy ) of the continuous transition were estimated and discussed. The ‘two-site’ model describing the motion of three of the five imidazolium cations, which is proposed from the structural studies, is fully confirmed by the data from the ac-calorimetric measurements.     

Emission characteristics of Er in vapor-transport-equilibrated Er/Zn-codoped LiNbO3 crystals

Polarized visible and infrared emission characteristics of Er³⁺ ions in vapor-transport-equilibration (VTE)-treated LiNbO₃ crystals codoped with different concentrations of Zn and Er were investigated in comparison with corresponding as-grown crystals. The results show that the VTE treatment leads to substantial spectral changes of Er³⁺ emissions at 0.65, 0.98 and 1.5 μm regions, and the spectral changes in the 0.98 and 1.5 μm regions appear to be Zn-concentration-dependent. It is concluded in combination with X-ray powder diffraction results and optical absorption characteristics reported previously that the VTE treatment resulted in crystalline phase transformation with respect to Er³⁺ ions from original LiNbO₃ to ErNbO₄ phase in all crystals studied. The formation of the ErNbO₄ phase and the Zn²⁺ codopants are responsible for the VTE-induced substantial spectral changes. The emission characteristics of the ErNbO₄ precipitates in the Zn/Er-codoped crystals are found to be very different from those of the ErNbO₄ precipitates in the only Er-doped crystal in the infrared region, and the difference is attributed to the influence of the Zn²⁺ codopant on the Er³⁺ ion environment. The mechanism of the crystalline phase transformation is qualitatively explained from the viewpoint of the declined solubility of Er³⁺ ion in a Li-rich LiNbO₃ crystal and from the phase diagram of Li₂O-Nb₂O₅ system.     

Piezoresponse imaging and local characterization of ferroelectric domains in Pb(Zn1/3Nb2/3)O3–7%PbTiO3 single crystals

Ferroelectric domain structures of (001)‐oriented Pb(Zn_1/3Nb_2/3)O₃–7%PbTiO₃ (PZN‐7%PT) single crystals were visualized and characterized by piezoresponse force microscopy (PFM). Locally regular domain configurations are found to be possibly related to the stable macroscopic properties in the PZN‐7%PT single crystals. Nanoscale piezoresponse hysteresis loops measured by PFM tip revealed no evidence of local domain switching behavior in the PZN‐7%PT single crystal. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sub-coercive field dynamic hysteresis in morphotropic phase boundary composition of Pb(Zr1/2Ti1/2)O3-Pb(Zn1/3Nb2/3)O3 ceramic and its scaling behavior

Sub-coercive field dynamic ferroelectric hysteresis of a morphotropic phase boundary composition of the PZT-PZN ceramic was investigated under influence of the compressive stress. The scaling relation of hysteresis area 〈A〉 against frequency f, field amplitude E₀, and stress σ took a form of , which is not different significantly to that of other PZT-PZN compositions with pure tetragonal or rhombohedral structure, as well as to that of soft and hard PZT bulk ceramics. This study suggested that the domain structures, not ceramic compositions, played a key role in controlling dynamic hysteresis behavior of ferroelectric materials.