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Sveleba, S. A.; Zhmurko, V. S.; Polovinko, I. I.; Katerinchuk, I. N.; Semotyuk, O.; Fitsych, O. I.; Furgala, Y.

doi:10.1023/a:1004148824389

Journal of Applied Spectroscopy

2000

DOI: 10.1023/a:1004148824389

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S. A. Sveleba

V. S. Zhmurko

I. I. Polovinko

et al.

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Cited by 5 publications

(15 citation statements)

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“…1)) in the commensurate phase below the phase-transition temperature T c indicate that the defect density perturbations are timeindependent. Indeed, the temporal changes in δ(∆n i ) reported in [15,16,22] point to an increase in their lifetime as T c on the side of the commensurate phase is approached. And annealing in the initial phase leads to the disappearance of the band at λ = 585 nm [15].…”

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confidence: 95%

“…The interaction of the modulated structure with defects has been the subject of theoretical [7,26] and experimental studies [19,20,22]. The electric and mechanical stress violates the conditions of interaction of the modulated structure with defects, which leads to a change in the temperature regions of metastability and localization of the incommensurate wave vector q i on higher-order commensurate values, causing the appearance of anomalies of the physical quantities with temperature.…”

mentioning

confidence: 99%

“…Apparently, the spatial distribution of density defects leads to an increase in the force of interaction of the modulated structure with defects and, as a consequence, to an increase in the behavioral anomaly of the physical quantities. Therefore, in considering the influence of defects on the physical properties of the crystals in the IC phase, the following possibilities should be taken into account: 1) the IC modulation wave interacts with disordered immobile defects (the rate of motion of the modulated structure velocity u, for example, under the action of temperature is much higher than the rate of motion of defects u d (u >> u d )) [26]; 2) mobile defects interact with the static IC modulation wave, forming a defect density wave (u d >> u) [22]; 3) "viscous" interaction of mobile defects with the IC structure (u + u d ) [6,22].…”

mentioning

confidence: 99%

“…The theoretical [6,22] and experimental studies [22,27] have shown that at a diffusion rate of mobile defects close to the rate of motion of the soliton structure the excess concentration of impurities near DCs increases. It has been established that the nature of the effect of "viscous" interaction is associated with the excess concentration of defects around solitons and the influence of this excess concentration on the motion of solitons.…”

mentioning

confidence: 99%

“…Such behavior of the physical parameters is due to the pinning of solitons on the defects and impurities [22,26,27]. The existence of a TH in the IC phase is explained by the presence of a metastable chaotic state of solitons as a result of the pinning of solitons on the defects.…”

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Interaction of a Defect Density Wave With a Modulated Structure in Incommensurate-Phase Crystals

et al. 2005

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Experimental and theoretical studies of the optical birefringence increment in the transition and metastable states of the incommensurate phase of Rb 2 ZnBr 4 , [N(CH 3 ) 4 ] 2 MeCl 4 (Me = Zn, Cu, Fe) crystals have been made. It has been established that in the modulated structure field a defect density wave is formed. It has been shown that when the period of the defect density wave coincides with the period of the modulation wave, the region of existence of the metastable state expands; when the periods of these waves do not coincide, their superposition occurs, with the formation of a wave with a difference value of the modulation vector. The presence of defect density waves in the incommensurate phase leads to a temperature hysteresis and kinetics of the physical quantities in the vicinity of T c . Introduction. Incommensurate structures have common properties: hysteresis of temperature dependences of the wave vector [1] and dielectric [2-5] and optical parameters [6]; effects of thermal [7, 8], dielectric [9], and thermooptical memory [10, 11]; kinetic features [7, 8, 12]; dependence of the physical parameters of the crystal on the previous history [13], etc.It should be noted that anomalous hysteresis phenomena show up as the presence of a hysteresis not only near the incommensurate (IC)-commensurate phase transition, but also inside the IC phase [1,14], i.e., the temperature range of the hysteresis is abnormally wide and incorporates part of the commensurate and IC phases [14]. An important role in the manifestation of the above features is played by the interaction of the modulated structure with defects and impurities, which leads to the appearance of a metastable chaotic state of solitons [15,16]. The experimental studies of such an interaction were conducted by means of both purposeful introduction of impurities and defects into the crystals [17,18] and application to the crystal of external fields of different symmetries [19,20]. X-ray [1], optical [6,16], and dielectric [5,15,21] investigations confirmed the existence in crystals with an IC phase of a metastable chaotic state of solitons caused by the fixing (pinning) of solitons on defects. It has been established [22] that mobile and immobile defects differently influence the structure dynamics. Immobile defects predetermine the existence of a global hysteresis, temperature cycles of the "parallelogram" type, dielectric memory, and a hysteresis of the IC phase-commensurate phase transition.According to [23,24], under x-irradiation of [N(CH 3 ) 4 ] 2 MeCl 4 (Me = Zn, Cu, Fe) low-mobility defects whose concentration is proportional to the irradiation time arise. The nature of x-ray defects in these crystals is associated with the breakage of hydrogen bonds. Thus, the restriction on vibrations in the -CH 2 −2 and -CH −3 groups is lifted

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mentioning

confidence: 95%

mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Interaction of a Defect Density Wave With a Modulated Structure in Incommensurate-Phase Crystals

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Signs of phase transitions and a thermooptic memory effect in absorption spectra of (N(CH3)4)2Zn0.8Ni0.2Cl4 solid solutions

et al. 2008

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We have studied the signs of phase transitions and spatial modulation of the structure in the absorption spectra of an (NCH 3 ) 4 ) 2 Zn 0.8 Ni 0.2 Cl 4 crystal. We have observed the existence of phase transitions in the given solid solution at temperatures of 155 K, 168 K, 275 K, 280 K, and 296 K. We have established that the thermooptic memory effect observed in the absorption spectra is completely consistent with a model of defect ordering in the sample in the field of the modulated structure. According to this model, stabilization of the sample in an incommensurable phase leads to fixing of a certain symmetry in the crystal (usually a lower symmetry than the average symmetry of the incommensurable phase) and a metal-halogen complex corresponding to the defect wave. As a result, we observe an appreciable shift of the intra-ionic absorption bands and an increase in their intensity.Introduction. The scientific interest in tetramethylammonium tetrachlorozincate crystals (N(CH 3 ) 4 ) 2 ZnCl 4 is connected with the fact that they can be considered as model systems with incommensurable modulation. It is specifically the incommensurable modulation that is associated with such specific phenomena as global thermal hysteresis, special cycles of the "parallelogram" and "dielectric memory" type, and also the thermal memory effect which, in particular, can be observed as the thermooptic memory (TOM) effect in measurement of optical parameters [1,2]. The memory effect can be associated with localization of the incommensurable modulation vector at a higher order commensurable value [3,4].TMAZC crystals belong to the family [N(CH 3 ) 4 ] 2 XB 4 (X = Zn, Co, Cu, Mn, Fe, Cd, Ni; B = Cl, Br). Its characteristic feature is the presence of a complex series of ferroelectric, ferroelastic, and incommensurable phases [5]. The (N(CH 3 ) 4 ) 2 ZnCl 4 crystal is no exception, undergoing five phase transitions:

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Optical transmission coefficient of crystalline [N(CH3)4]2MeCl4 in the incommensurate phase

et al. 2011

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surate phase when a defect density wave is present. It is found that an anomalous reduction in the transmission coefficient is caused by scattering of light owing to a realignment of the superstructure during transitions between metastable states. When a defect density wave is present, the anomalous optical transmission of the crystal is related to the scattering of light on superstructure inhomogeneities produced by a superposition of existing modulation waves.Keywords: incommensurate phase, metastable state, optical transmission coefficient. Introduction.It has been shown [1] that the existence of superstructure in a crystal shows up through optical reflection and scattering. Light scattering in a dielectric is caused by optical inhomogeneities of the medium owing to fluctuations in its dielectric constant. The resulting incommensurate superstructure [1] disrupts the optical homogeneity of the crystal. Thus, the development of incommensurate modulation will be accompanied by the scattering of light.Phases with an incommensurate superstructure and periods greater than the interatomic distance, but less than the optical wavelength, have been discovered in a number of dielectrics. The shape of a modulation wave, which is mainly described as sinusoidal, has more complicated inflections, which are referred to as phase solitons. Impurities and defects affect the dynamics of the soliton system. Pinning on defects and impurities produces a number of universal properties in incommensurate phases, including hysteresis in the temperature dependences of the wave vector, thermal, dielectric, and thermo-optical memory effects, the kinetics of physical quantities, etc.The effect of defects on the modulated superstructure shows up most clearly when the soliton-defect interaction force becomes comparable to the soliton-soliton interaction force. This leads to the formation of multiwave states. Under these conditions the dynamics of the modulated superstructure is complicated and takes place through superposition of existing modulation waves.It is known that the optical effects owing to the superstructure are controlled by the ratio d/λ (d is the period of the superstructure wave and λ is the optical wavelength) [2, 3]. When d > λ, diffraction of a light beam is observed on the periodic superstructure formed by the superposition of existing spatial modulation waves [2, 4] (defect density waves and incommensurate modulation waves).Multiwave states of the superstructure in the incommensurate phase of a crystal originate in the spatial inhomogeneity of the crystal and may be accompanied by the scattering of light. Here we study the temperature and time behavior of the optical transmission coefficient of [N(CH 3 ) 4 ] 2 ZnCl 4 and [N(CH 3 ) 4 ] 2 CuCl 4 crystals with density defect waves (spatial structural deformation wave). It is known that when a crystal in the incommensurate phase is held at a constant temperature, the incommensurate superstructure relaxes to its equilibrium state. Since the time it spends in a metastable state ...

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Untitled

Cited by 5 publications

References 7 publications

Interaction of a Defect Density Wave With a Modulated Structure in Incommensurate-Phase Crystals

Interaction of a Defect Density Wave With a Modulated Structure in Incommensurate-Phase Crystals

Signs of phase transitions and a thermooptic memory effect in absorption spectra of (N(CH3)4)2Zn0.8Ni0.2Cl4 solid solutions

Optical transmission coefficient of crystalline [N(CH3)4]2MeCl4 in the incommensurate phase

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