Thermal Hysteresis Effects Research Articles

Catena-[μ-Tris(1,2-bis(tetrazol-1-yl)ethane-N4,N4′)iron(II)] bis(tetrafluoroborate): synthesis, structure, spectroscopic and magnetic characterization of a chain-type coordination polymer spin-crossover compound

In analogy to a common synthesis of 1-substituted 5-H tetrazoles (Tetrahedron Lett. 36 (1995)1759; Beloruss. Gos. Univ., Minsk, USSR. Khim. Geterotsikl. Soedin. 11 (1985) 1521; Beloruss. Gos. Univ., Minsk, USSR. Khim. Geterotsikl. Soedin. 1 (1991) 66; BGU, Belarus. Vestsi Akad. Navuk Belarusi, Ser. Khim. Navuk 1 (1992) 73), the new bidentate ligand 1,2-bis(tetrazol-1-yl)ethane [endi] was synthesized and characterized by X-ray diffraction, NMR, IR and UV–Vis spectroscopy. By using iron(II) tetrafluoroborate hexahydrate the complexation with this ligand yields a 1-dimensional linear coordination polymer similar to the recently published chain compound (Inorg. Chem. 39 (2000) 1891) exhibiting a thermally induced spin-crossover phenomenon. Similar to the 1,2-bis(tetrazol-1-yl)propane-bridged compound, our 1,2-bis(tetrazol-1-yl)ethane-bridged compound shows a gradual spin transition, but the spin-crossover temperature T1/2≈140 K is found to be 10 K above the other T1/2. The T1/2 was determined by temperature-dependent 57Fe-Mössbauer, far FT-IR and UV–Vis spectroscopy as well as by temperature-dependent magnetic susceptibility measurements. Single crystals of the complex were grown in situ from a solution of the ligand and iron(II) tetrafluoroborate. The X-ray structure determinations of both the high spin as well as the low spin state of the compound revealed a solid state structure, which is comparable to that of catena-[Fe(1,2-bis(tetrazole-1-yl)propane)3](ClO4)2 (Inorg. Chem. 39 (2000) 1891; 2nd TMR-TOSS Meeting, 4th Spin Crossover Family Meeting, Lufthansa Training Center, Seeheim/Germany, April 30–May 2, 1999). Both the 1,2-bis(tetrazol-1-yl)propane-bridged and our compound do not show a thermal hysteresis effect (J. Am. Chem. Soc. 115 (1993) 9810; Inorg. Chim. Acta 37 (1979) 169; Chem. Phys. Lett. 93 (1982) 567). The synthesis of the complex described in the experimental section yielded a fine powdered product being poorly soluble in most common solvents. The single crystal measurements were done with crystals obtained by various diffusion methods. Most of them yielded either thin needles or small hexagonal prism crystals depending on the specific conditions.

高温・低温の温度履歴を受ける岩盤空洞の熱的挙動

When high and low temperature materials, such as heated water, LNG, LPG and frozen food etc. actually are stored temporally in openings excavated in rock mountain, as the quantity of these changes continually, the rock mass around openings will receive the effects of thermal hysteresis of high and low temperatures. Therefore, obtaining the strength and deformation characteristics of rocks after receiving thermal hysteresis becomes important for discussing the stability of the openings. In this study, these characteristics of rocks were examined. From the results of the tests, it was found that strength and deformation characteristics of rocks decrease with the increasing amount of thermal hysteresis. However, the ratio of the decrease also decreases. From these facts, it is supposed that the strength and deformation characteristics of rocks will converge to a constant value. Then using the results of the tests, thermal behavior of rock mass around openings were analyzed and discussed.

Dynamical model for spin-crossover solids. II. Static and dynamic effects of light in the mean-field approach

We have introduced a photoexcitation contribution in the dynamical model we previously presented (see previous paper), as an additional term in the rate transition, in order to reproduce the light induced thermal hysteresis effect, experimentally observed in the spin-crossover materials. The set of motion equations governing the evolution of the high-spin (HS) fraction of cooperative spin-crossover systems under light, is obtained. The mean-field treatment leads to a macroscopic master equation previously given on a phenomenological basis. With both the $\mathrm{HS}\ensuremath{\rightarrow}$ LS (low-spin) and $\mathrm{LS}\ensuremath{\rightarrow}$ HS transition rates terms, a model of general use is derived. This model describes the static and dynamic properties under light. We have investigated this model analytically, as far as possible. The properties under light involve two instabilities different in nature: the high-temperature instability, chiefly entropy driven, and the low-temperature one, light induced. We have determined the conditions for these instabilities to occur, and have examined their possible collapse. The described phenomena are discussed with respect to the available data on the photoinduced bistable regime: thermal hysteresis loops, optical hysteresis loops, and transient regimes. The kinetic dependence of the photoinduced hysteresis loops is calculated.

Photomagnetic Properties in a Series of Spin Crossover Compounds [Fe(PM-L)2(NCX)2] (X=S, Se) with Substituted 2′-Pyridylmethylene- 4-amino Ligands

The spin crossover (SC) compounds [Fe(PM-AzA)2(NCX)2] and [Fe(PM-FIA)2(NCX)2] (with PM-AzA = N-2'-pyridylmethylene-4-(phe-nylazo)aniline, PM-FIA = N-2'-pyridylmethylene-4-(2-amino)fluorene, and X = S, Se) have been prepared. The SC regimes have been deduced from variable-temperature magnetic susceptibility data. The enthalpy and entropy changes associated with the SC have been evaluated from DSC measurements. A cooperativity factor, C, has been defined, and its values for the different compounds have been deduced from the spin crossover curves. At 10 K, the light-induced excited spin state trapping (LIESST) effect has been observed within the cavity of the SQUID magnetometer. The critical temperatures Tc(LIESST) have been determined for [Fe(PM-AzA)2(NCS)2] and [Fe(PM-F1A)2(NCX)2], and the role of cooperativity has been analyzed. A linear correlation has been found between the Tc(LIESST) and C values. The kinetics of HS-->LS relaxation have been investigated; a thermally activated mechanism at elevated temperatures and a nearly temperature independent relaxation behavior at low temperatures have been found. Finally, the magnetic behavior recorded under light irradiation in the warming and cooling modes has revealed the occurrence of the light-induced thermal hysteresis (LITH) effect.

Evidence for a bulk Meissner state in the ferromagnetic superconductorRuSr2GdCu2O8from dc magnetization

From magnetization measurements we provide evidence that the ferromagnetic superconductor ${\mathrm{RuSr}}_{2}{\mathrm{GdCu}}_{2}{\mathrm{O}}_{8}$ with ${T}_{c}\ensuremath{\approx}45\mathrm{K}$ and ${T}_{M}\ensuremath{\approx}135\mathrm{K}$ exhibits a sizeable diamagnetic signal at low temperature $(Tl{T}^{\mathrm{ms}}=30\mathrm{K})$ and low magnetic field ${(H}^{\mathrm{ext}}l30l30g\mathrm{Oe}),$ corresponding to a bulk Meissner phase. We argue that a recent negative report [C. W. Chu et al., cond-mat/9910056 (unpublished)] regarding the Meissner effect in Ru-1212 can be explained by impurity scattering or grain-size effects. At intermediate temperatures, ${T}^{\mathrm{ms}}lTl{T}_{c},$ we observe unique thermal hysteresis effects which are characteristic of a spontaneous vortex phase.

Synthesis of copper–magnesium–zinc ferrites and correlation of magnetic properties with microstructure

A novel route for the preparation of high density, high permeability Cu–Mg–Zn ferrites using oxalate precursors is reported, with various Mg 2+ contents represented as Cu (0.5− x) Mg x Zn 0.5Fe 2O 4 wherein x=0.00, 0.20, 0.25, 0.40. To investigate the ferritization temperature of this system, TG/DTG/DTA studies have been carried out varying from 599 to 743 K with increasing x. X-Ray diffraction (XRD) study revealed that all the samples had a single spinel phase and the cubic lattice parameter ‘ a’ decreased with an increase in x. Particle size distribution (PSD) and scanning electron microscopy (SEM) techniques were employed to determine the average particle size and to study the microstructure respectively. Initial permeability of all the samples increased with increase in temperature and Mg 2+ content ( x≤0.20). It also exhibited a thermal hysteresis effect. The magnetic properties are correlated with high density and grain structure. The magnetic properties of this ferrite system prepared by novel route are superior to those of commercial ETKMG ferrites.

Spin-crossover in the [Fe(abpt) 2(NCX) 2] (X=S, Se) system: structural, magnetic, calorimetric and photomagnetic studies

The compounds [Fe(abpt) 2(NCS) 2] ( 1) and [Fe(abpt) 2(NCSe) 2] ( 2) with abpt=4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole have been synthesized. The X-ray structures have been determined at 293 K. 1 and 2 are isostructural and crystallize in the monoclinic space group P2 1/ n with Z=2, a=8.538(8), b=10.246(8), c=16.45(2) Å, β=93.98(9)° for 1 and a=8.623(2), b=10.243(3), c=16.585(3) Å, β=93.19(2)° for 2. In both complexes, the coordination core has a similar pseudo-octahedral geometry with the NCS − ( 1) and NCSe − ( 2) groups in the trans-position. Variable-temperature magnetic susceptibility data give evidence for a low-spin (LS)↔high-spin (HS) conversion centered at T 1/2 around 180 and 224 K for 1 and 2, respectively. The spin conversion takes place gradually, without hysteresis. The enthalpy and entropy changes associated with the spin conversion are evaluated from the DSC measurements: Δ H=5.8±0.5 ( 1) and 8.6±0.8 kJ mol −1 ( 2); Δ S=32.5±3 ( 1) and 38±4 J mol −1 K −1 ( 2). At 10 K the light-induced excited spin state trapping (LIESST) effect has been observed within the SQUID magnetometer cavity, by irradiating powder samples with a Kr + laser coupled to an optical fiber. The critical LIESST temperatures T liesst are around 40 and 32 K for 1 and 2, respectively. The magnetic behavior recorded under light irradiation in the warming and cooling modes has revealed a light-induced thermal hysteresis (LITH) effect. The HS→LS relaxation kinetics have been investigated in the temperature range 6–40 K. A thermally activated mechanism at elevated temperatures and a nearly temperature independent relaxation behavior at low temperatures can be observed for 1. The very fast relaxation precludes the estimation of the kinetic parameters for 2 at temperatures higher than 10 K.

A chromatically addressed optical fibre sensor for non-contact temperature monitoring

A non-contact, optical fibre based temperature probe is described with potential applications in hostile environments where contact between the sensing element and the fibre tips may not be possible nor desirable. This enables the sensor addressing fibres to be located outside the hostile environment so, for instance, removing the need for high temperature tolerant fibres and providing a cost effective sensing solution. The probe is based upon the wavelength shift of the spectral absorption edge of semiconductor materials, the shift being addressed by chromatic modulation methodologies. Relevant aspects of the chromatic modulation methodology and the physical basis of the temperature shift in absorption spectra are briefly reviewed. Theoretical and experimental results for the performance of various differently configured chromatic monitoring systems in conjunction with a temperature sensitive element are described. The use of chromatic monitoring is shown to be advantageous when addressing such broad wavelength modulating sensors yielding a resolution of 0.5%, covering the temperature range 20–250°C. It is also shown that care is needed in choosing appropriate photo diode detectors so that the chromatic system's output is immune to unavoidable thermal hysteresis effects associated with the fibre tips.

Thermal Hysteresis in Ganglioside Micelles Investigated by Differential Scanning Calorimetry and Light-Scattering

Both light-scattering and differential scanning calorimetry (DSC) measurements account for the existence of a dramatic thermal hysteresis effect occurring in micellar solutions of gangliosides. Light-scattering measurements reveal that the mean aggregation number of ganglioside micelles assumes different values depending on the sample thermal history, a hysteresis phenomenon that completely disappears above a well-defined temperature. On the other side, DSC measurements assess that the heat-capacity variation versus temperature for the ganglioside micellar system depends as well on the thermal history of the sample. Although rather complicated, the calorigrams show a peak associated with the melting of the micelle hydrophobic core. The position of the peak undergoes changes consistent with the observed hysteresis of the mean aggregation number. The two parallel descriptions, geometric and calorimetric, can be clearly correlated as two aspects of the same phenomenon and give support to each other. The hysteresis behavior below a critical temperature is related to cooperative conformational variations of the oligosaccharide headgroup at the micellar surface. Such surface phenomena are coupled to the well-known order−disorder transition of the hydrophobic chains in the core via the geometric constraints imposed by the confinement of the amphiphiles in the micellar aggregate.

岩石力学 高温・低温の温度履歴を受けた花こう岩および凝灰岩の強度・変形特性について

The authors have been proposed and discussed temporary storage of high and low temperature materials in openings excavated in rock mountain. Here, high temperature materials means such as heated water which is produced by surplus heat from garbage burning plants, while low temperature materials means such as LNG, LPG, frozen food etc. In this case, as the storage quantity of heated water, LNG and LPG will change with these use continually, rock masses around openings to store these materials will receive effects of thermal hysteresis of high and low temperatures. Therefore, obtaining the strength and deformation characteristics of rock affected by the thermal hysteresis becomes important for discussing the stability of the openings. In this study, strength and deformation characteristics of granite and tuff were examined after undergoing thermal hysteresis of high and low temperatures. From the results, it was found that the decreasing ratio of strength and deformation of rocks decrease with the increasing number of thermal hysteresis, and it is supposed that the value will converge to a constant value.

An investigation of the phase transitions in bornite (Cu5FeS4) using neutron diffraction and differential scanning calorimetry

Other| December 01, 1998 An investigation of the phase transitions in bornite (Cu 5 FeS 4 ) using neutron diffraction and differential scanning calorimetry Benjamin A. Grguric; Benjamin A. Grguric University of Cambridge, Department of Earth Sciences, Cambridge, United Kingdom Search for other works by this author on: GSW Google Scholar Andrew Putnis; Andrew Putnis Search for other works by this author on: GSW Google Scholar Richard J. Harrison Richard J. Harrison Search for other works by this author on: GSW Google Scholar American Mineralogist (1998) 83 (11-12_Part_1): 1231–1239. https://doi.org/10.2138/am-1998-11-1211 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Benjamin A. Grguric, Andrew Putnis, Richard J. Harrison; An investigation of the phase transitions in bornite (Cu 5 FeS 4 ) using neutron diffraction and differential scanning calorimetry. American Mineralogist 1998;; 83 (11-12_Part_1): 1231–1239. doi: https://doi.org/10.2138/am-1998-11-1211 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Bornite (Cu 5 FeS 4 ) exists in three polymorphic forms related by superstructuring, with structural transitions at 200 and 265 degrees C. The phase transitions and structural behavior in two natural bornite samples were investigated as a function of temperature using differential scanning calorimetry (DSC) to characterize the thermal anomalies associated with each transition and in-situ high-resolution neutron powder diffraction to determine the variation in superlattice intensity and lattice parameters. These two methods, carried out over the temperature range 50-350 degrees C, provided insight into the short- and long-range interactions, respectively, and a comparison of measurements taken up-T and down-T enabled thermal hysteresis effects to be quantified. The high to intermediate transition at 265 degrees C involves long-range cation ordering and vacancy clustering, resulting in a doubling of the high-temperature cubic unit cell (m3m). The square of both the spontaneous strain and the 2a superlattice intensity varied linearly with temperature, indicating that the transition is tricritical in character. Loss of long- and short-range order occurred simultaneously during heating, whereas during cooling, the reappearance of 2a superlattice reflections was depressed some 50 degrees C below the short-range transition recorded by DSC due to the formation of antiphase domains. The presence of antiphase domains also caused the discontinuity in the strain associated with the transition to occur at a different temperature to the appearance of superlattice intensity. The intermediate-low transition associated with the development of a 2a4a2a orthorhombic superlattice (Pbca) is strongly first-order and exhibits a large intrinsic hysteresis (38 degrees C). The down-T transition temperature of the intermediate-low transition is shown to be dependent on the degree of order attained in the intermediate phase. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Thermal hysteresis and memory effects in TIInS2

The dielectric susceptibility of layered TlInS2 was studied in the temperature range of successive phase transitions. Thermal hysteresis was observed in the incommensurate phase. It was shown that after annealing the crystal at a fixed temperature within the incommensurate phase, the existing temperature interval of this phase reveals noticeable broadening. The thermal memory effect is discussed using a defect density wave model.

Structural, Magnetic, and Photomagnetic Studies of a Mononuclear Iron(II) Derivative Exhibiting an Exceptionally Abrupt Spin Transition. Light-Induced Thermal Hysteresis Phenomenon.

The new spin-crossover compound Fe(PM-BiA)(2)(NCS)(2) with PM-BiA = N-(2-pyridylmethylene)aminobiphenyl has been synthesized. The temperature dependence of chi(M)T (chi(M) = molar magnetic susceptibility and T = temperature) has revealed an exceptionally abrupt transition between low-spin (LS) (S = 0) and high-spin (HS) (S = 2) states with a well-reproducible hysteresis loop of 5 K (T(1/2) downward arrow = 168 K and T(1/2) upward arrow = 173 K). The crystal structure has been determined both at 298 K in the HS state and at 140 K in the LS state. The spin transition takes place without change of crystallographic space group (Pccn with Z = 4). The determination of the intermolecular contacts in the LS and HS forms has revealed a two-dimensional structural character. The enthalpy and entropy variations, DeltaH and DeltaS, associated with the spin transition have been deduced from heat capacity measurements. DeltaS (= 58 J K(-)(1) mol(-)(1)) is larger than for other spin transition bis(thiocyanato) iron(II) derivatives. At 10 K the well-known LIESST (light-induced excited spin state trapping) effect has been observed within the SQUID cavity, by irradiating a single crystal or a powder sample with a Kr(+) laser coupled to an optical fiber. The magnetic behavior recorded under light irradiation in the warming and cooling modes has revealed a light-induced thermal hysteresis (LITH) effect with 35 < T(1/2) < 77 K. The HS --> LS relaxation after LIESST has been found to deviate from first-order kinetics. The kinetics has been investigated between 10 and 78 K. A thermally activated relaxation behavior at elevated temperatures and a nearly temperature independent tunneling mechanism at low temperatures have been observed. The slow rate of tunneling from the metastable HS state toward the ground LS state may be explained by the unusually large change in Fe-N bond lengths between these two states.

Ignition, extinction, and thermal hysteresis of a heterogeneous exothermic reaction

Critical conditions of ignition and extinction are studied theoretically for the case of a heterogeneous exothermic reaction proceeding on the uniformly accessible surface of a channel or a pore. Particular emphasis is placed on the thermal-hysteresis effect of the reaction (an ignition temperature in excess of the extinction temperature), which ensures stability of the reaction against changes in the external conditions. A model for the process is proposed and substantiated. It permits one to analyze the main regularities of the thermal reaction regime with allowance for the distribution of heat flows from the reaction zone to the reaction medium and the channel wall. The dependence of the critical conditions of ignition and extinction on the rate constants of heat release, the reaction order, and the thermal and geometric characteristics of the system is established. A rule is proposed that permits evaluating the effect of heat losses on the critical phenomena and the thermal hysteresis of a reaction on a cooled surface in the case where the critical conditions for the reaction on an adiabatic surface are known.

Numerical studies of groundwater flow, grouting and solute transport in jointed rock mass

The authors have proposed and discussed temporary storage of high and low temperature materials in openings excavated in rock mountain from the view point of efficient utilization of land and preservation of the environment. In this case, high temperature materials means such as heated water which is produced by surplus heat from garbage burning plants, and low temperature materials means such as LNG, LPG, frozen food etc. When heated water, LNG and LPG actually are stored in openings, as the quantity of these materials changes continually, the rock mass around openings will receive the effects of thermal hysteresis of high and low temperatures. Therefore, obtaining the strength and deformation characteristics of rock after undergoing thermal hysteresis of high and low temperatures becomes important for discussing the stability of the openings. In this study, strength and deformation characteristics of rock which has low and high porosity were examined after undergoing thermal hysteresis of high and low temperatures using a thermal cycle apparatus. From the results, it was found that temperature, the number of hysteresis and porosity etc. were important factors which have an influence on strength and deformation characteristics.

Strength and deformation characteristics of rocks after undergoing thermal hysteresis of high and low temperatures

Abstract The authors have proposed and discussed temporary storage of high and low temperature materials in openings excavated in rock mountain from the view point of efficient utilization of land and preservation of the environment. In this case, high temperature materials means such as heated water which is produced by surplus heat from garbage burning plants, and low temperature materials means such as LNG, LPG, frozen food etc. When heated water, LNG and LPG actually are stored in openings, as the quantity of these materials changes continually, the rock mass around openings will receive the effects of thermal hysteresis of high and low temperatures. Therefore, obtaining the strength and deformation characteristics of rock after undergoing thermal hysteresis of high and low temperatures becomes important for discussing the stability of the openings. In this study, strength and deformation characteristics of rock which has low and high porosity were examined after undergoing thermal hysteresis of high and low temperatures using a thermal cycle apparatus. From the results, it was found that temperature, the number of hysteresis and porosity etc. were important factors which have an influence on strength and deformation characteristics.

96/04662 Effect of different drying methods on coal structure and reactivity toward liquefaction

In analogy to a common synthesis of 1-substituted 5-H tetrazoles (Tetrahedron Lett. 36 (1995)1759; Beloruss. Gos. Univ., Minsk, USSR. Khim. Geterotsikl. Soedin. 11 (1985) 1521; Beloruss. Gos. Univ., Minsk, USSR. Khim. Geterotsikl. Soedin. 1 (1991) 66; BGU, Belarus. Vestsi Akad. Navuk Belarusi, Ser. Khim. Navuk 1 (1992) 73), the new bidentate ligand 1,2-bis(tetrazol-1-yl)ethane [endi] was synthesized and characterized by X-ray diffraction, NMR, IR and UV–Vis spectroscopy. By using iron(II) tetrafluoroborate hexahydrate the complexation with this ligand yields a 1-dimensional linear coordination polymer similar to the recently published chain compound (Inorg. Chem. 39 (2000) 1891) exhibiting a thermally induced spin-crossover phenomenon. Similar to the 1,2-bis(tetrazol-1-yl)propane-bridged compound, our 1,2-bis(tetrazol-1-yl)ethane-bridged compound shows a gradual spin transition, but the spin-crossover temperature T1/2≈140 K is found to be 10 K above the other T1/2. The T1/2 was determined by temperature-dependent 57Fe-Mössbauer, far FT-IR and UV–Vis spectroscopy as well as by temperature-dependent magnetic susceptibility measurements. Single crystals of the complex were grown in situ from a solution of the ligand and iron(II) tetrafluoroborate. The X-ray structure determinations of both the high spin as well as the low spin state of the compound revealed a solid state structure, which is comparable to that of catena-[Fe(1,2-bis(tetrazole-1-yl)propane)3](ClO4)2 (Inorg. Chem. 39 (2000) 1891; 2nd TMR-TOSS Meeting, 4th Spin Crossover Family Meeting, Lufthansa Training Center, Seeheim/Germany, April 30–May 2, 1999). Both the 1,2-bis(tetrazol-1-yl)propane-bridged and our compound do not show a thermal hysteresis effect (J. Am. Chem. Soc. 115 (1993) 9810; Inorg. Chim. Acta 37 (1979) 169; Chem. Phys. Lett. 93 (1982) 567). The synthesis of the complex described in the experimental section yielded a fine powdered product being poorly soluble in most common solvents. The single crystal measurements were done with crystals obtained by various diffusion methods. Most of them yielded either thin needles or small hexagonal prism crystals depending on the specific conditions.

Iron (II) - 1,2,4,- triazole spin transition molecular materials

The phenomenon of spin transition is probably one of the most spectacular examples of molecular bistability. Our main goal along this line concerns the design of molecular materials exhibiting abrupt spin transitions accompanied by thermochromic and large thermal hysteresis effects. The ideal situation, in terms of possible application of these compounds as active elements of memory devices, is realized when room temperature falls in the middle of the thermal hysteresis loop. In this context, we review our work concerning iron(II)-1,2,4-triazole compounds. We first introduce the idea that the cooperativity should be more pronounced in polymeric than in mononuclear compounds. Then we present some results concerning trinuclear iron(II)-1,2,4-triazole species. The heart of the paper is devoted to the polymeric compounds of formulae \[Fe(Htrz)$\_{2}$(trz)\](BF$\_{4}$) and \[Fe(Htrz)$\_{3}$\](BF$\_{4}$)$\_{2}\cdot $ H$\_{2}$O. We report first on the magnetic, optical and calorimetric, then on the structural properties of these compounds. Afterwards, we introduce the concept of spin transition molecular alloy, and emphasize that it is possible to fine tune the spin transition regime of these alloys through their chemical composition. For some alloys, room temperature falls within the thermal hysteresis loop. In the conclusion, the mechanism of cooperativity in the spin transition polymeric compounds is briefly discussed.

Correlation of freezing dynamics and domain-like states in relaxor ferroelectrics

Abstract Tetragonal PLZT ceramics (Zr/Ti = 40/60) with lanthanum contents of approximately 12 atomic% posses a unique transformation. Dielectric constant measurements have revealed a spontaneous switching between relaxor and normal ferroelectric behaviors, in addition significant thermal hysteresis effects were observed near the switching temperature. Temperature dependent x-ray line broadening above the switching temperature was observed which revealed the development of significant internal strain indicating the presence of local cubic-tetragonal structural transformations which preserve the average cubic symmetry, while line splitting was observed below the switching temperature indicating the formation of a macroscopic tetragonal structure. Micropolar domain coarsening and a subsequent transformation to a normal micron-size domain configuration was observed by hot-stage transmission electron microscopy.

Comment on "Thermal hysteresis effects in the ferroelectric-ferroelastic phase transition in

Comment on "Thermal hysteresis effects in the ferroelectric-ferroelastic phase transition in