I. P. Aleksandrova

Russian Academy of Sciences, Moskva, Moscow, Russia

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Publications (59)59.52 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: 23 Na NMR spectra have been measured in a very low magnetic field at temperatures between 203 K and 780 K. The orientation dependences of the second-order quadrupole shifts of the central component were studied. The achieved spectral resolution is sufficient to test various models of short –range order in the Na 1/2 Bi 1/2 TiO 3 (NBT) structure using computer simulations of the spectral line shape. The polar monoclinic areas were found to coexist with the tetragonal phase insertions till the lowest investigated temperature 203 K. In the temperature range 203 K – 640 K the NMR spectra also display the presence of areas with very small deviation from the cubic structure (matrix).
    Ferroelectrics 09/2010; 378(2009):16-22. · 0.42 Impact Factor
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    ABSTRACT: The orientation dependences of the second-order quadrupole shifts of the central component in the 23Na NMR spectrum were studied in the temperature range 293–760 K. The profile of the spectral distribution is calculated using various models of the Na1/2Bi1/2TiO3 structure. The calculations agree with the experimental data for the monoclinic structure of a polar cluster with two Na displacement components: a displacement along the [111] p direction and a small displacement statistically or dynamically disordered over six equally probable [100] p -type directions. Tetragonal-phase nuclei and monoclinic clusters with a very small displacement component along the [111] p direction are found to coexist and have close energies over the temperature range 580–610 K. The results obtained provide new information concerning the character of the diffuse phase transition at 610 K. PACS numbers76.60.-k–77.80.-e
    Physics of the Solid State 01/2008; 50(3):496-501. · 0.77 Impact Factor
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    ABSTRACT: A crystal of the Cs5H3(SO4)4 · xH2O (x ≈ 0.5) (PCHS) compound, which belongs to the family of proton conductors with a complex system of hydrogen bonds, is investigated by 2H NMR spectroscopy. The temperature and orientation dependences of the 2H NMR spectra are measured and analyzed. It is established that, upon transition to the glassy phase at the temperature T g = 260 K, the parameters characterizing the proton exchange between positions in hydrogen bonds remain unchanged to within the limits of experimental error. The protons in the two-dimensional network of hydrogen bonds in the (001) plane are dynamically disordered over possible positions down to temperatures considerably lower than the glass transition point T g . However, water molecules are fixed at particular structural positions in the phase transition range. In PCHS crystals with a nonstoichiometric water content, this circumstance can be responsible for the frustration that leads to the formation of the glassy state.
    Physics of the Solid State 01/2007; 49:1142-1148. · 0.77 Impact Factor
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    ABSTRACT: The 23Na NMR spectra of a Na1/2Bi1/2TiO3 crystal are studied at frequencies of 79.4 and 15.7 MHz in the temperature range 150–720 K. It is revealed that, at all temperatures, the crystal contains regions with a nearly cubic matrix and polar clusters. The temperature dependence of the local distortion of the Na environment in the clusters is determined. The dynamics of the reorientation of the local cluster polarization in the tetragonal and trigonal NBT phases is analyzed.
    Physics of the Solid State 01/2006; 48(6):1120-1123. · 0.77 Impact Factor
  • Yu. N. Ivanov, A. A. Sukhovsky, I. P. Aleksandrova, D. Michel
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    ABSTRACT: The proton exchange in a potassium hydrogen selenate crystal is investigated in detail. Partial deuteration of the crystal (approximately 80% of the protons are replaced by deuterons) makes it possible to use the method of nuclear magnetic resonance (NMR) on quadrupole nuclei to perform research into proton (deuteron) transport. In addition to conventional Fourier-transform NMR spectroscopy, elementary processes of deuteron chemical exchange are studied by two-dimensional NMR spectroscopy, which provides unique information regarding these processes. Slow exchange between protons of hydrogen bond chains and dimers consisting of two SeO4 groups is revealed. It is established that this process is responsible for the proton conduction in the potassium hydrogen selenate. The NMR data are compared with the results of dielectric measurements carried out at a frequency of 1 kHz.
    Physics of the Solid State 01/2006; 48(9):1698-1702. · 0.77 Impact Factor
  • Yu. N. Ivanov, A. A. Sukhovsky, I. P. Aleksandrova, D. Michel
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    ABSTRACT: The microscopic mechanism of proton transport in partially deuterated potassium hydrogen selenate (KHSe) and in partially deuterated ammonium hydrogen selenate (AHSe) were studied by means of one-dimensional Fourier transform2H nuclear magnetic resonance (NMR), two-dimensional2H NMR and dielectric measurements over a wide temperature range. In both systems, KHSe and AHSe, the slow chemical exchange processes of deuterons between different hydrogen bridges occur. It was established that the rates of exchange between deuteron sites, which are involved in infinite chains of hydrogen bonds, are approximately the same for both crystals. The rates of exchange between these positions and the deuterons in the dimer groups of KHSe are approximately hundred times more slowly. On the basis of our findings, we discuss the models of the microscopic mechanism of hydrogen transport for both substances.
    Applied Magnetic Resonance 09/2005; 28(3-4). · 0.83 Impact Factor
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    ABSTRACT: The ion mobility in a crystal of partially deuterated potassium hydroselenate (KHSe) was studied using nuclear spin resonance (NSR) of 1H, 2H, 77Se, and 39K in a wide temperature range. The temperature dependences of the deuterium NSR spectra above 360 K exhibit changes in the line shape characteristic of chemical exchange processes. These exchange processes were studied in detail using two-dimensional 2H NSR spectroscopy. It was ascertained that the exchange between deuterons of hydrogen bonds take place in the entire temperature range under study. However, the measured conductivity was approximately one-hundredfold lower than that estimated from the exchange frequencies. It was assumed that the low conductivity in the temperature range under study is caused by closed dimers of SeO4 groups in the KHSe structure.
    Physics of the Solid State 01/2004; 46(10):1845-1850. · 0.77 Impact Factor
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    ABSTRACT: The heat capacity, nuclear quadrupole resonance (NQR) and x-ray diffraction of (Cs1−xRbx)2ZnI4 single crystals have been measured, for x = 0, 0.001, 0.005, 0.01, 0.025 and 0.05. The normal to incommensurate (N–Inc) phase transition at TI, the incommensurate to commensurate (Inc–C) lock-in transition at TL and the structural commensurate monoclinic to triclinic transition at TLT, observed in the parent compound (x = 0), takes place for x = 0, 0.001, 0.005 and 0.01. For x = 0.025 only TI and TL are detected, while for x = 0.05 no transitions were observable. The values of TI and TL increase with x while TLT decreases and disappears at the concentration x = 0.025. The effect of defects, besides modifying the transition temperatures, is that of broadening and lowering the heat capacity anomaly at the lock-in transition until its total quenching for x = 0.05. No observable hysteresis is detected in this transition. NQR and x-ray diffraction data show the Inc–C transition up to the highest concentration. We conclude that this phenomenology is caused by weak interaction of the incommensurate modulation with point defects even in the region close to the Inc–C transition.
    Journal of Physics Condensed Matter 11/2002; 14(49):13623. · 2.22 Impact Factor
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    ABSTRACT: The trigonal (P-3 m1) modification of Cs3Bi2Br9 and Cs3Sb2I9 have been studied using NQR, X-ray single crystal and powder pattern methods. Moreover, the heat capacity was measured in a wide temperature interval: 4-300 K. In Cs3Bi2Br9 a second-order phase transition was found at T-C= 96 K. The low-temperature phase is monoclinic (C12/c1), with the unit cell doubled along the [001] direction. Cs3Sb2I9 has a sequence of phase transitions at T-C = 85 K, T-i = 78 K and T-L = 72.1 K. The mononclinic structure below 85 K is isomorphic with the low-temperature structure of Cs3Bi2Br9. According to calorimetric data the lock-in transition at 72.1 K is discontinuous.
    Phase Transitions 08/2002; 75(6). · 0.86 Impact Factor
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    ABSTRACT: The chemical exchange of deuterons in a partly deuterated ammonium hydrogen selenate crystal is investigated by deuteron magnetic resonance (2H NMR) spectroscopy over a wide range of temperatures. The changes observed in the line shape of the NMR spectra at temperatures above 350 K are characteristic of chemical exchange processes. The exchange processes are thoroughly examined by two-dimensional 2H NMR spectroscopy. It is established that, over the entire temperature range, only deuterons of hydrogen bonds are involved in the exchange and the rates of exchange between deuterons of all types are nearly identical. No deuteron exchange between the ND4 groups and hydrogen bonds is found. A new model of proton transport in ammonium hydrogen selenate is proposed on the basis of the experimental data. This model makes it possible, within a unified context, to explain all the available experimental data, including macroscopic measurements of the electrical conductivity.
    Physics of the Solid State 01/2002; 44(6):1077-1084. · 0.77 Impact Factor
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    ABSTRACT: The microscopic mechanisms of proton transport in partially deuterated ammonium hydrogen selenate [NH4HSeO4 (AHSe)] and partially deuterated mixed crystals of betaine phosphate (DBP) and betaine phosphite (DBPI) (DBP1−xDBPIx) were studied by means of one-dimensional Fourier transform 2H NMR, two-dimensional (2D) 2H NMR and dielectric measurements. Slow chemical exchange processes of deuterons between different hydrogen bridges occur which can be studied by means of the 2D 2H NMR technique over a wide temperature range. In both systems, AHSe and DBP1−xDBPIx, the ionic conductivity is probably realized via jumps of the protons or deuterons between different hydrogen bonds (inter-bond jumps). The importance of certain defects for the conductivity process is clearly revealed. In AHSe, jumps occur through non-occupied hydrogen positions. In the mixed crystals DBP1−xDBPIx, the creation of defects is in fact favoured by the substitution of DBP for DBPI molecules or vice versa. The creation of vacancies may be explained by the occurrence of defects in the lattice structure, e.g. on the basis of the known Grotthuis mechanism and a multi-dimensional conductivity model as suggested for KDP. Copyright © 2001 John Wiley & Sons, Ltd.
    Magnetic Resonance in Chemistry 11/2001; 39(S1):S50 - S58. · 1.53 Impact Factor
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    ABSTRACT: The phase transitions of Cs3Sb2I9, Cs3Bi2I9, and Cs3Bi2Br9crystals were investigated by 133Cs nuclear magnetic resonance. Cs3Bi2Br9was also studied by 87Br nuclear quadrupole resonance. The parameters of the Cs electric-field gradient tensors were determined. The phase transitions were shown to produce significant changes in the nearest neighbor environment of only one of two inequivalent Cs sites. In Cs3Sb2I9, the tensors remain axisymmetric below the transition at 85 K, in line with the assumption relying on powder x-ray diffraction data that, below the transition temperature, Cs3Sb2I9has a trigonal structure. The observed temperature variation in the shape of the 133Cs NMR and 127I NQR signals suggests that Cs3Bi2I9may undergo a transition to an incommensurate phase.
    Inorganic Materials 05/2001; 37(6):623-627. · 0.38 Impact Factor
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    ABSTRACT: The atomic structure of Cs3Sb2I9 single crystals was refined using X-ray diffraction data (sp. gr. P3m1; wR = 1.58% and R = 3.07%). The phase transitions revealed earlier were found to be accompanied by the appearance of superstructural reflections. At T c1 = 86 K, the reflections indicating doubling of the c parameter. At T c2 = 73 K, a first-order phase transition takes place accompanied by “freezing” of the satellites h, k + 1/2, l + 1/2 reflections. In the temperature range from 73 to 78 K, an incommensurate phase providing the satellites h, k + 1/2 + δ, l + 1/2 is formed.
    Crystallography Reports 12/2000; 46(1):26-29. · 0.52 Impact Factor
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    ABSTRACT: In incommensurate systems the NMR, NQR or EPR resonance frequency varies in space in a way which reflects the spatial variation of the incommensurate modulation wave. An evaluation of the magnetic resonance lineshape for the case of a multi-soliton lattice is presented for different ratios between the soliton width and the inter-soliton spacing as well as for different relations between the resonance frequency and the incommensurate order parameter. The results are compared with the experimental data in Rb2ZnCl4. It is found that in this system the soliton width is large compared with the inter-soliton spacing over most of the incommensurate phase. The temperature variation of the phase soliton density has been determined. The non-classical value of the critical exponent beta =0.35+or-0.03 for the amplitude of the order parameter demonstrates that the whole incommensurate phase is critical in this one-dimensionally modulated system.
    Journal of Physics C Solid State Physics 11/2000; 15(3):547.
  • R Blinc, V Rutar, B Topic, S Zumer, J Seliger, I P Aleksandrova, F Milia
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    ABSTRACT: The changes in the magnetic resonance spectra at the transition from the incommensurate plane wave to the incommensurate multi-soliton lattice modulation limits are analysed for different relations between the magnetic resonance frequency and the nuclear displacements. The temperature dependence of the soliton density in Rb2ZnCl4 is redetermined for different crystal orientations in the external magnetic field and the results for the 'linear' and the 'quadratic' case are found to be in good agreement. Whereas comparison of experimental and theoretical lineshapes allows for a rather precise determination of the soliton density, a determination based only on the intensity of the 'commensurate' peaks-as used in early studies-may lead to quantitatively inconsistent results.
    Journal of Physics C Solid State Physics 11/2000; 19(18):3421.
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    ABSTRACT: An X-ray study of three phase transitions (PTs) at 86, 78 and 72 K has been carried out for the trigonal modification of the Cs3Sb2I9 crystal (space group P-3m1, a = 8.460 Angstrom, c = 10.398 Angstrom). The T-c1 = 86 K PT preserves the trigonal symmetry and is associated with the doubling of the c-parameter. At T-c2 = 72 K a doubling of a and b is observed. An intermediate incommensurate phase with q(1) = 1/2c*, q(2) = (1/2 - delta)a* + 1/2c* has been detected in the temperature range of 78-72 K. The lock-in PT at 72 K has a first order character. (C) 1999 Published by Elsevier Science Ltd. All rights reserved. [References: 4]
    Solid State Communications 07/1999; 111(7):361–366. · 1.53 Impact Factor
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    ABSTRACT: (Cs1−x Rbx)2ZnI4 crystals were grown by two different methods with Rb concentrations varying from x=0 to 2.5%. 127I NQR and calorimetric measurements showed that crystals grown by the Bridgman technique contain residual impurities (∼0.5%) for all x. While x=0 crystals grown from solution do not contain detectable impurities, they allow incorporation only of a low Rb concentration, not above 0.5%. A transition-temperature-concentration (x) phase diagram has been constructed for Bridgman-grown crystals from NQR data. Rb doping shifts the normal-incommensurate and incommensurate-ferroelastic phase-transition points toward higher temperatures with different rates. The P21/m↔P1 first-order transition shifts toward lower temperatures. The region of low Rb concentrations lies closest to the critical point.
    Physics of the Solid State 01/1999; 41(1):128-131. · 0.77 Impact Factor
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    ABSTRACT: The two-dimensional (2-D) deuterium nuclear magnetic resonance (NMR) exchange spectroscopy is applied to two types of conducting ionic crystals for the study of hydrogen mobility and conductivity, viz. partially deuterated ammonium hydrogen selenate, NH4HSO4 (AHSe), and partially deuterated mixed crystals of betaine phosphate (DBP) and betaine phosphite (DBPI), DBP1−x DBPIx. In both crystals chemical exchange processes of deuterons between different hydrogen bridges occur which are studied by the 2-D-2H-NMR technique over a wide temperature range in the slow-exchange regime. For AHSe exchange only occurs between two sites which are involved in hydrogen bonds. Two Arrhenius-like exchange processes were found the activation energies of which could be determined. For the case of DBP1−x DBPIx, with several deuteron sites taking part in the exchange, the analysis of the quantitative exchange behavior required a combination of time-domain analysis of our 2-D NMR data with mixing-time- and temperature-dependent measurements. Different exchange rates for each two-site exchange, all showing Arrhenius behavior, were obtained for DBP0.3, DBPI0.7. For crystals with different phosphite concentrationx the differences towards DBP0.3, DBPI0.3 were established. With the help of conductivity data from dielectric measurements quantitative relations between the exchange and conductivity processes are obtained for AHSe as well as for DBP1−x DBPIx. Finally, an estimation of effective charge carrier densities is discussed in view of possible conductivity models for both crystals.
    Applied Magnetic Resonance 01/1999; 17(2):243-263. · 0.83 Impact Factor
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    ABSTRACT: Processes of chemical exchange of deuterons in partially deuterated ammonium hydrogen selenate, NH4HSeO4 (AHSe), crystals are investigated by means of 2H nuclear magnetic resonance (NMR) experiments over a wide temperature range. The temperature dependencies of the quadrupole line splittings in the one-dimensional spectra of AHSe above 350 K revealed line-shape changes which are characteristic for chemical exchange processes. A detailed study of these exchange processes in AHSe is achieved by means of two-dimensional 2H NMR experiments. In the temperature range investigated, a chemical exchange occurs only between those deuteron (proton) sites which are involved in hydrogen bonds (- and -positions). It was established that the rates of exchange between all types of hydrogen-bound deuteron are approximately the same. Exchange between these positions and the deuterons in the ND4 groups could not be detected. On the basis of our findings, we finally discuss a model for the microscopic mechanism of hydrogen transport in AHSe.
    Journal of Physics Condensed Matter 12/1998; 11(18):3751. · 2.22 Impact Factor
  • I.P. Aleksandrova, A.A. Sukhovsky, K.S. Aleksandrov
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    ABSTRACT: The incommensurate modulation has been found in the hexagonal Cs3Bi2I9, belonging to the A3B2X9 family of the layered compounds. The second order normal-incommensurate transition at Ti = 224 K is accompanied by the symmetry change . The transition into a ferroelastic incommensurate phase occurs at the zone center, according to the NQR and ultrasonic measurements.
    Solid State Communications 02/1998; 105(5):323-326. · 1.53 Impact Factor

Publication Stats

197 Citations
59.52 Total Impact Points

Institutions

  • 1984–2005
    • Russian Academy of Sciences
      Moskva, Moscow, Russia
  • 1980–2000
    • University of Ljubljana
      • Department of Physics
      Ljubljana, Ljubljana, Slovenia