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​SHEMETOV  YEVGENY.  STUDIES OF PHASE TRANSITIONS  IN THE UNITED A₂BX₄ STRUCTURE β-K₂SO₄ METHOD NUCLEAR QUADRUPOLE RESONANCE 

 English abstract    § 1.1    § 1.2    §1.3    § 1.4    § 2.1    § 2.2    §2.3   §2.4    § 3.1    § 3.2    § 3.3    § 3.4    § 4.1    § 4.2    § 4.3    § 4.4    Reference    Template    pdf abstract -download  

RUSSIAN ACADEMY OF SCIENCES Institute of Chemical Physics. Semenov

A manuscript UDC 536,913,405 +539.89

Shemetov V. Yevgeny 

Investigation of the sequence of phase transitions in compounds with the structure A₂BX₄ β-K₂SO₄ NQR halogens at atmospheric and high pressure.

   01.04.07. - Solid state physics

Author's abstract.

thesis for the degree of Candidate of Physical and Mathematical Sciences

Moscow 1995

Work performed at the Institute of Physics SB RAS. L.V.Kirenskogo Krasnoyarsk branch of the Republican and Technology Center of the Russian government.

Supervisor Candidate of physico-mathematical sciences MOSKALEV AK

    Opponents: Dr. tehnicheskin, Professor T.A.Babushkina; Candidate of Sciences matematicheskik D.L.Zagorsky.

Lead Institute of Organoelement Compounds, AN Nesmeyanova

Thesis held ____ 1995, at ___ hours ___ minutes. At a meeting of the Specialized Council D002-26.04. at the Institute of Chemical Physics. N.I.Semenova RAS address: 117334, Moscow, ul. Kosygin 4.

With the thesis can be found in the library of the Institute of Chemical Physics

sent to 1995.

Scientific Secretary of the Specialized Council Ph.D., professor V.Ya.Rochev

 

 

Study of phase transitions in compounds such A₂BX₄ STRUCTURE β-K₂SO₄ NQR halogens at high and atmospheric pressure.

General characteristics of the work.

Relevance of the topic.

   Method of nuclear quadrupole resonance (NQR) is widely used as a method to study the structure of substances including the phase transitions in crystals. Recently, a special place in such studies has been paid partially disordered state, one of the classes which are dielectric crystals with a structure of β-K₂SO₄, were unearthed disproportionate (Inc) phase. Under disproportionate means special structural state of the crystal, which is formed by the interaction of crystalline functions with differing periods. Theoretical models disproportionate state in dielectrics show that the period of the structure of the dipoles can be both multiple and non-multiple grating period of the original high-temperature phase. When converting to the low-symmetric phase, the ordering depending on the nature of atomic interactions may be effected in various ways. On the one hand through a series of long-period phase states by transformations of the first kind, and the other, a continuous way: according to the actual change of the wave vector q_δ, corresponding to the minimum mode (or the maximum coupling constants) responsible for the stepwise behavior, called simple devil's staircase 'Devil `s Starcase», was recorded and investigated in magnets. After opening the disproportionate state dielectrics diffraction. radiospectroscopic and experimental data indicated the importance of continuous change q_δ.

    By the beginning of our studies and, there were only a few compounds A₂BX₄, from the family of β-K₂SO₄, where it was possible to detect incommensurate phases and study them in detail. Due to the fact that the NQR method is very sensitive to the gradient of the intracrystalline electric fields, its use for research disproportionate state in the dipole system is not in doubt. However, when initially applying it to this class of crystals detected very weak signals of radio frequency absorption mainly in the low temperature region of the incommensurate phase. In this regard, the intermediate objective was to increase the sensitivity of the method, from which depended the success of the study compounds with β-K₂SO₄ type and finds in them a thorough study of incommensurate phases.

Objective.

    Study of nuclear quadrupole resonance of halogens (Cl, Br, J), and other methods, the family of crystals with β-K₂SO₄ structural disproportion. Elucidation of changes in the spectral features of the resonance parameters in phase transitions; Analysis investigated the local environment of the nuclei, and its transformation into compounds of the series when the high temperature and hydrostatic pressure. Study processes of spin dynamics in combination with other methods of analysis of the crystal symmetry changes at structural phase transitions.

New scientific results.

First NQR using the methods of X-ray diffraction, calorimetry, electro-optics and dielectric measurements studied sequence of transitions in the compounds Cs₂CdJ₄, Cs₂ZnJ₄. and (NH)₂ZnJ₄ with an incommensurate phase.

   The spatial point symmetry groups and commensurate phases.

In compounds Rb₂ZnBr₄ , Cs₂ZnJ₄ and found two polymorphs α and β. While β-modification, where realized sequence of phase transitions with incommensurate structure is unsustainable and stabilization of metastable conditions, such as moisture, becomes α-modification.

In a first for Cs₂ZnJ₄ А₂ВX₄ family connections with cesium cation registered full range ¹²⁷J   NQR frequency scale throughout the incommensurate phase below Ti. Before TC found wide temperature range before ordering transition. Change NQR spectrum in this area can be interpreted in terms of phase transitions with clusters predecessors.

      In β-Rb₂ZnBr₄ updated temperature frequency dependence of the NQR spectrum   ^79,81Br    at atmospheric pressure and obtained navye data. Established that the phase transition at Ti decreases fluctuations before the first race. At low temperatures, the incommensurate phase at TC, along with the basic structure according to a tripling of the unit cell of the crystal (q_δ ~1/3), in the form of the NQR spectrum indicated a different structure. In studies under a pressure established that the latter refers to the state of the crystal, characterized wavevector q_δ~1/4   and the other branch fashion ∑ .    ^79,81Br   NQR under high pressure is detected and investigated in detail the sequence of highly symmetric and disproportionate-ordered phases in β-Rb₂ZnBr₄. The phase P-T diagram in the region of 170-360⁰K at pressures up to 1 GPa. Recorded sequence of phase transitions in the Inc P-T field Rb₂ZnBr₄, interpreted in the framework of the "'devil's staircase." Analysis of transformation of the NQR spectrum indicates that the low temperature portion of the incommensurate phase observed behavior of the type simple stairs (complete D'S), and in the medium temperature is possible mixed (incomplete) view "satanic stairs" A comparison-with the theoretical phase diagrams of pseudo-spin model with phonon coupling.

Scientific and practical value.

  1. Realizovany Methods and devices that increase the sensitivity of pulsed NQR spectrometer for the purpose of detection of weak signals of radio frequency absorption and precision temperature measurements under high hydrostatic pressure.

  2. Construct a high pressure, which is used with a standard NQR spectrometer in the study of the structural features of the phase transitions in disordered crystalline systems,

  3. New experimental data on the structure transformation in compounds with symmetry Pnma A₂BX₄ temperature changes. Not detected previously observed phase transitions,

  4. NQR built P-T phase diagram of β-Rb₂ZnBr₄. Existing data can be used to construct a molecular model of the transformation of the family structure in the compounds β-K₂SO₄,

  5. First, in some connection A₂BX₄, revealed the existence of another polymorph - α. We propose to search for new members of the family A₂BX₄ with a metastable structure type β-K₂SO₄

 6. The spin-spin and spin-lattice relaxation of nuclei halogen NQR incommensurate phases. Shown that the new level of quality research is necessary to extend the lower range of equipment for temporary permission microsecond limit.

 7. NQR analysis of spectral data indicated the need to revise the previously accepted model to describe the shape of the resonance lines in the incommensurate phases.

Personal contribution of the author.

   Developed and implemented a device for temperature and hydrostatic pressure vysokog research. Synthesized compounds Cs₂ZnJ₄ and (NH) ₂ZnJ₄. Conducted all the experimental NQR measurements and their analysis. Developed and used applications for numerical simulation of the NQR line shape in the modulated phases. The author participated in the preparation and implementation of other experimental studies of teaching and learning, supported by research quadrupole resonance.

Testing work.

    The main part of the dissertation presented and discussed at the All-Union scientific conferences:

   -on rf, Kazan, 1984.

   -on the physics of ferroelectrics: Chernivtsi, 1986., Rostov-on-Don, 1989.;

   - International conferences and symposia; -XI conference MAVRID .. High Pressure Science and Technology. Kiev. 1987.:

-X Symposium on NQR, Takayama, Japan, 1989.;

   Crystallographic-European conferences:

XI-Poznan (Poland), 1987. , XII-Moskva, 1989.; Symposium - Sat 23rd Congress Ampere, Novosibirsk Shushenskoe 198g.;

-IV Japan-Soviet Symposium on ferroelectrics, 1988;

   -7th Symposium on ferroelectricity Sarbryuker, Germany, 1989;

   -Soviet-Polish seminar on ferroelectrics, Lvov, 1990;

   -XII Symposium on NQR, Zurich, Switzerland, 1993.

Publications: On the topic of the dissertation is published 22 print jobs.

Structure and scope of work:

The work consists of an introduction, four chapters, conclusions, list of references of 135 titles.

Dissertation presented in 172 pages of text, 5 tables and 70 figures.

• HIGHLIGHTS OF WORK.

Introduction. The place of research topics in the general range of problems in solid state physics. The features of dielectric crystals, where the first place stands the ordering of chemical bonds via dipole-dipole interaction. Is the objective of research.

Chapter 1.

   Disproportionately - modulated phases in dielectrics with a structure of β-K₂SO₄.

A brief overview of the theory of phase transitions in the concept of the soft mode and Ising models, taking into account for the nearest neighbors interaction cluster picture of phase transitions of the displacement type and order-disorder.

The theoretical principles of the NQR method and its application to the study of critical phenomena and phase transitions. Presented a phenomenological model of the line shape of the nuclear quadrupole resonance in the plane-wave approximation and the soliton. The theoretical data on the processes of nuclear relaxation in the incommensurate phase.

   Discussed the experimental data on the study at atmospheric pressure A₂BX₄ compounds with tetrahedral coordination of the anions and the high-temperature phase with the space group Pnma: Rb₂ZnJ₄, Cs₂HgBr₄ and others.

   Given P-T phase diagram. investigated in dielectric crystals of various experimental methods. At the end of an overview chapter summarizes the state of research in dielectrics structure β-K₂SO₄, and formulated the problem of work.

CHAPTER 2. Equipment and measurement technique.

2.1 equiv ... remote sensor NQR and conditions optimum adaptation to the measuring chamber. The ways to increase the sensitivity of pulse NQR experiments in conditions of prolonged tracking of temperature and pressure dependences. As follows from the dissertation in electrodynamic calculations, signal to noise ratio (SN) radio spectrometer increases with radius r and length l of the receiving sensor, r * as the square root of l. However, there is an optimal ratio between the size of the sensor and the internal dimensions of the measurement chamber, in which a balance is achieved between the reduction in the ratio of SN, because of the inductive-capacitive coupling and receiving radio frequency measurement and hardware circuits, and an increase in SN   due to increased take-up spool. Taking account of this and the use of suitable materials design allows ultimately increase the sensitivity of the method. The fruitfulness of this approach was confirmed after manufacturing thermal tops and high-pressure cells for measuring weak signals in the ³⁵Cl   NQR K₂ZnCl₄, where the author of the absorption line not to register earlier.

   2.2. Temperature consoles to the spectrometer ISSH research phase transitions. Set out details of the construction of thermo-gradient-top boxes for the study of phase transitions NQR. The basic idea underlying the action of liquid thermal camera is a replacement for liquid gas environment, providing better heat transfer properties. The design of this thermal camera was not realized by the author based on the standard thermo-top box to the spectrometer ISSH. As coolants used N-pentane (a liquid-state 138-308⁰K), hexane, high oil (Т_кип   <600⁰C). With vigorous stirring, the temperature gradient in the fluid volume measuring chamber out (with an accuracy of ±0.01⁰K/sm).

    Due to the fact that the storage of liquids at room temperature with a melting point <110⁰K requires special measures that impede the use of liquid thermal cameras below these temperatures was developed another design that has a heat transfer fluid circulation mode and can be used in an embodiment of the gas conditioning. The design of this camera is significantly different from the presentation. In a liquid coolant mixing mode is carried Circulator. To equalize the gradients at the gas conditioning (blowing), a system of grids and channels contributing self-mixing   hydrodynamic gas streams. In this case the temperature gradient on the sample was 0.01-0.02⁰K/mm.

   Distance of the sensor to the NQR solderless socket sensor NQR spectrometer and using the principles of optimal maTChing possible to increase the ratio of SN to 10 times.

      2.3. Synthesis of samples made in A₂BX₄ labopatory crystallophysics Institute of Physics SB RAS, and I.T.Kokov, V.A.Grankina based on chemical АX   and   ВX₂   as Bridgman method from the melt and from aqueous solutions.

    Connection of A₂BX₄ type structure β-K₂SO₄ usually crystallized in the form of single-crystal plate splices sizes up to 40mm and a thickness of 5mm. Some samples, to remove impurities, two; recrystallized three times. Before NQR measurements and samples were ground in a thin-walled container placed in NQR sensor. Been tested influence operations preconditioning on the measurement results, and found that the samples grown and (or) inhomogeneously deformed in the temperature range of the incommensurate phase have different spectral characteristics in comparison with the samples or are born in the high-temperature phase. Detailed information on this, the example Rb₂ZnBr₄ given in paragraph 4.1 of the dissertation,

   2.4. Apparatus for NQR studies under high hydrostatic pressure. Described, created with the direct participation of the author and requires the highest level of the experiment, setting the research in atmospheric and high hydrostatic pressure P-T phase diagrams and critical behavior near singular P-T states of crystals. Instrumental complex includes NQR pulse spectrometer ISSH1-13 (Made in the USSR) , electronic assembly changes, control and maintain the temperature and pressure, high-pressure apparatus (multiplier,   pumping station, a high-pressure bomb) or thermo-console , precision graphic recorder.

    Particular attention is paid to the design of the dissertation high-pressure chambers (HPC) for studies at low temperatures. One of them - HPC1 having a lumen diameter of 16mm, allowed to observe NQR signals at pressures up to 1.5 GPa, Seal performed indentation chopper with a set of rings on the chamfer channel bomb. This design allows you to work with the sample to 3sm3 has radiospectroscopic sensitivity and a half orders of magnitude lower than the standard dimensions and is designed for intensive observation NQR signals. Another cell (HPC2) specially developed by the author for registration absorption signals from quadrupolar nuclei in incommensurate phases at pressures up to 400MPa. Diameter of the channel, where the coil NQR increased to 40mm. Construction of the obturator and seal have fundamental differences from the standard and are to reduce its axial size; increasing the diameter of the channel under electric high voltage input; in the hydraulic seal cavity bombs flattening bronze ring of triangular section. To reduce RFI (Radio Frequency Interference) and delinking conductors and high frequency P-T sensors, the latter is supplied through an additional seal. This design has achieved the conditions of observation NQR signals approximate to standard conditions.

Chapter 3. Sequence of phase transitions in compounds A₂BX₄ (X = J). Experimental data on the study of phase transitions in compounds not previously studied A₂BX₄ NQR: Cs₂CdJ₄ (CCJ), Cs₂ZnJ₄ (CZJ) and (NH₄)₂ZnJ₄ (AZJ).

   3.1. Phase transitions in Cs₂CdJ₄. In 1986,   by NQR method in Cs₂CdJ₄ author found two modifications - α and β. Modification of α is stable in conditions of high humidity and changes in β modification by drying the crystal, for example, after heating the sample in vacuum above 100^OC When thermally induced α   ↔ β transition, there is no reduction of the sample weight and absent (with an accuracy of ~ 2%) NMR signals from protons associated crystal. On this basis, it is concluded that the α-phase is the phase of the polymorphic compounds β a metastable phase, since in the presence of atmospheric moisture rapidly into α modification.

 

 

 

 

 

 

 

In Cs₂CdJ₄ NQR in the range 420¸77⁰K found three phase transitions at T_C1=335; T_C2=260 and T _C3 = 184⁰K with changes multiplicity range from three to four lines of equal intensity 16 (Fig. l). In the temperature range 335 ¸ 260⁰K absorption recorded in the narrow neighborhood before T_C2, where the view of the spectrum typical for Inc phase. As was explained, the absence of signals due to the fact that the spin-spin T_Q2 and spin- lattice T_Q1   relaxation are below the resolution limit of the temporary pulse spectrometer (Т_Q < 10^-8s).   Below   T_C2 is fixed below 4 absorption signal abruptly transforming at T_C3 in 16 singlet NQR lines. In this case the temperature dependence of the frequency, critical slowing T_Q1 and phase coexistence ~ 2⁰K clearly indicates close to the tricritical point phase transition at T _C3. Multidisciplinary studies involving methods calorimetry, electro-optics and X-ray diffraction allowed to establish a framework for the transformation of β-modified tetra-iodo- cadmiat cesium

       Pnma (Z=4)   ↔   Jc   ↔ P2₁/n (Z=4)   ↔    (Z=8) 

Fig. 2. Temperature dependence of the NQR frequencies   ¹²⁷J    Cs₂ZnJ₄.

 In the incommensurate phase detected X-ray satellites in the position qδ ≈0.24¸0.25a*.   Symmetry of α-modification identified by us as P2₁/m   (Z=2). A relaxation of the NQR data indicate the nature of its slow spin dynamics   (Т _Q1   ≥   l ms;   Т_Q2   ≈ 200µks)   with respect to the β-phase.

 

       3.2. Investigation of the sequence of phase transitions in Cs₂ZnJ₄.

    In CZJ change the multiplicity of the spectrum of the complex spectrum through three to eight, and again to eight lines, and   the temperature dependence of the frequency quadrupole transition 1/2   ↔ 3/2 isotope ¹²⁷J observed three phase transitions at Ti = 118; T_С1 =108, and T_С2 =95⁰K (Fig. 2). In the temperature range 118¸108⁰K throughout the frequency scale for the first time with cesium cation of compounds A₂XB₄ registered NQR spectrum of the detected author of the incommensurate phase. With the use of complementary techniques were established symmetry phases: Pnma(Z=4)    ↔ Jc, q_δ = (1+d)a*/2   ↔   P2₁/n(Z=8)   ↔ P1 (Z=4), and X-ray studies recorded below T_i satellite reflections with the value q_δ ≈ 0.15a* at T = 112⁰K.

 According to the quadrupole transition   1/2↔3/2 times the temperature dependence tracked T _Q1 and T _Q2   in 380-77⁰K from all cores ¹²⁷J   nonequivalent positions structure Pnma.

   Feature relaxation characteristics CZI is a distinct two-scale character associated with structural features. In a wide temperature range ~100⁰K   before T_i   signals recorded as the decay of free precession and asymmetric inhomogeneous broadening of the spin-echo signals nuclear response. Changes in the relative intensity of the longitudinal and transverse contributions absorption and asymmetry last indicates abnormal change in the nature of atomic motions at 320⁰K, below which develop cluster-type fluctuations structure of the incommensurate phase.

.

 

  

3.3. NQR study of phase transitions in (NH₄)₂ZnJ₄.

   Author first synthesized and studied by NQR ¹²⁷J   compound (NH₄)₂ZnJ₄. In the temperature range   77-400⁰K   found three phase transitions at Т_i=165⁰К, Т_С1 = 126⁰K   and Т_С2= 87⁰K (Fig. 3).

Above Ti observed three intense absorption line (1/2 ↔ 3/2) with the character of the dependence Bayer ν(T). When Ti marked decrease in the intensity of lines and race frequencies. Shape of the spectrum, fixed below T _C1 indicates disparity observed structure. Below T_C1   multiplicity spectrum changes to seven broadened lines are not of equal intensity, and at the boiling point of nitrogen absorption signals recorded 12 NQR. Stroke frequency ν(T), intensities, and shapes NQR signals are comparable with those observed in Rb₂ZnCl₄ and Rb₂ZnBr₄, confirms the assumption that the author of symmetry transformation in tetra-iodo ammonium zincate under the scheme:

D_2h^l6(Pnma)   ↔       Inc1       ↔    Inc2    ↔   C_2v⁹(Pn2₁a)   ,

while the phase between 125 and 87⁰K may be interpreted as incommensurate rather arbitrary and is rather long-periodic (q_d ≈ 2/7) with symmetry Pn2₁a. Noted the existence of another structure of the compound with the same observation conditions for modifying α - Cs₂CdJ₄.

  

   Fig. 3. Temperature dependence of the frequency of the NQR spectrum (NH₄)₂ZnJ₄.

 

At the end of the third chapter of the comparison data for NQR А₂ВHа1₄ compounds, which exhibit phase Inc. The greatest amount of frequency decoupling Dn_I-III , noted for ammonium compounds (31%) and the smallest for cesium (up 7%). It is poorly explained from the viewpoint of crystal chemical relationships, because radii of the cations NH₄ and Cs are close in value, and indicates significant chemical (electronic) contribution to the nature of this splitting. Temperature dependence of ν(T) below T_i unusual and poorly explained by known types of molecular motions. Significant line widths in the initial phases Inc and at extremely short time values   T_Q1 and T_Q2 indicate structural dynamic disordering of the molecular fragments. Largest Δν_I-III   and especially in the form of components n_I   celebrated a special position compound Cs₂ZnJ₄.

       Significant impact on the correctness of the nuclear dynamics registering frequency spectral characteristics NQR involves revising the phenomenological model describing the shape of the resonance line Inc phases.

Comparison of the data with the results of NQR spectroscopy in other crystal systems (with charge density waves, one-dimensional organic conductors, octahedral crystals with Jahn-Teller complexes) allows to plan a way to explain the data NQR through electron-nuclear movements in the two-scale timeline.

3.4. Polymorphism in the family A₂BX₄. After the discovery of the author in 1986. α-polymorphic modifications Cs₂CdJ₄ and later Rb₂ZnBr₄, K₂ZnBr₄   etс. , with symmetry group P2₁/m and the establishment of the fact that in these compounds is metastable β-form,   the question arises about   the generality of this phenomenon in the family A₂BX₄. The classification of compounds A₂BX₄ on experimentally known for their space groups: Pnma and P2₁/m and the method of implementation of the structure type β-K₂SO₄ in other compounds of the family.

    On the basis of experimental data the mechanism of the α↔β transition: through surface orientation ordering β structure to α, the dipoles of water molecules. The reverse process is carried out by removing traces of water and thermal induction of metastable modification with the accelerated nature of the spin dynamics.

    At the end of this section discusses the cause of the observed phenomena in terms of mathematical theory for systems with long-range interatomic interactions.

Chapter 4. Sequence of phase transitions in Rb₂ZnBr₄ at atmospheric and high hydrostatic pressure. The results of studies of phase transitions in tetrabromzincates rubidium NQR. High pressure is a factor that allows to obtain fundamentally new information on the transformation of the structure in the compounds A₂BX₄.

   4.1. ^79,81Br   absorption spectra in the incommensurate phase at atmospheric pressure.

   At the initial stage of the study in Rb₂ZnBr₄ using NQR found: 1) the dependence of signal intensity on the number of cycles in the cooling phase, Inc; 2) that the majority of freshly grown or present during wet (≈ 50%) atmosphere crystals are fully or partially, or are moving to another structure with symmetry P2₁/m. Conditions for its stabilization and transition to the metastable β-modification with an incommensurate phase completely analogous α ↔ β transformation in Cs₂CdJ₄ and differs only significantly longer duration of α ↔ β process; 3) induction of non-hydrostatic stress, residual growth and thermal stresses   nonequilibrium state structure β-Rb₂ZnBr₄, disappearing after annealing.

   Precision study of the temperature dependence of the NQR frequencies Rb₂ZnBr₄ in T_i transition at 1 atm revealed races frequency spectral lines from Br nuclei in positions II and III structure Pnma, as well as unusual splitting frequency spectrum component Т_i   in the incommensurate phase. On these grounds it has been found that a phase transition at a T_i close to first-order. In the transition to the ferroelectric phase F was registered racing frequencies and additional absorption lines not previously observed. The complex shape of the spectrum was numerically explained: 1) manifestation in the crystal structure of another type (different from the symmetry of the soft mode S submission) or another branch (except S₂) symmetry; 2) the coexistence of multiple modulation sequences of "devil's staircase." To clarify these issues, the study was used at high pressure.

  

    4.2. P-T phase diagram Rb₂ZnBr₄.

   In the first part of this section examines the structure of the phase P-T diagram RZB obtained using HPC1. In this first tracked P-T region existence paraelectric (P), F , Inc   phases   and detected   symmetrical high-pressure phase G and H (Fig. 5). The multiplicity of the NQR spectrum of twelve F-phase lines, through a complex range of intermediate N-phase and four to fourteen powerful singlet phase lines H and G, respectively (Fig. 4). A detailed study of the transformations of the NQR lines isotopes ⁸¹Br   and ⁷⁹Br   in the incommensurate phase performed using bombs HPC2.

Fig. 4. Baric move ⁸¹Br NQR frequencies in Rb₂ZnBr₄.

 

Fig. 5. Rb₂ZnBr₄ phase diagram according to the NQR.

20   iso-thermal and isobaric P-T sections, in which high-resolution spectra were recorded from all NQR nonequivalent positions ⁸¹Br , scanned the temperature and pressure dependences of the frequency ν(P, T), the intensities I (P, T) and linewidths Dn(P).. Tracked items transformation spectra of phase F through N to H phase, and in the middle region of the incommensurate phase through a sequence of disordered phases Inc↔М₁ ↔ М₂ ↔ М₃   to G phase. On the anomalies in the dependencies ν(P), I(P), Δν(P)   P-T installed P-T lines of phase transitions.   All of these transitions have the first order with the values of the pressure hysteresis DР from 20 to 160MPa, and DР increases   with   increase decreasing T and decrease P, and essentially depends on the direction of change of P-T parameters. Changing pressure phase transition temperature of the paraelectric phase to the incommensurate obtained involving DTA method.

 

4.3. Features transformations spectrum near Т_C. Comparison with the diffraction data.

    Considered parts of a complex transformation of the NQR spectrum and analyzed its performance in the low P-T region N incommensurate phase near Т_C, where we assume a step change in the structure. Analysis of non-monotonic changes in the intensities and spectral distributions of asymmetry possible to detect characteristic mild anomalies between different structurally related states N₁, N₂, N₃, N₄ modulated phase. In an embodiment of the NQR data interpretation in the framework of "devil's staircase" structure N_j correspond to states with different values of a step change in the wave vector q_δ, and observed P-T overlapping area. This interpretation was confirmed by NQR active on the same samples by neutron diffraction in the international scientific center of Orsay (France). Satellites have been detected, the values of a* in units which can be placed on an upward steps of (5/17 → 3/10 → 7/23 → 1/3) and down (3/10 → 8/27→ 5/17→ 7/24→ 2/7→ 1/4 ) of stairs. At this pressure the following sequence of transforming the structure β-Rb₂ZnBr₄. Fig. 6.

 

 

 

 

 

 

4.4. Discussion The phase diagram in the framework of the "devil's staircase."

   A comparison of the experimental phase P-T diagram RZB with the theoretical phase diagrams "Ds" S» and found that phase Inc М₁, М₂, М₃, meet stepwise reduction of the vector q_δ   when approaching a Lifshitz point.

    At the end of paragraph NQR data discussed based on the theory of the existence of Baka random phase is an intrinsic property of systems with frustrating interactions. In this case, incommensurate phases with wave vectors, for example, q_δ≈1/4+d и   q_δ≈1/3 can have different states with topologically overlapped structure, different phase shift. On the basis of this comparison indicated that in the M phase diagram observed smoothed (incomplete) mode "Ds" S ", in Inc - classical (smooth), and N region close to the step (complete) behavior.

    Based on this discussion, involving the analysis of solutions φ-model, which is applicable to describe compounds A₂BX₄ with β-form elements proposed model explaining changes in the structure Rb₂ZnBr₄ no diffraction data and microwave spectroscopy.

   In concluding the discussion, based on research conducted by the author, analyzed the experimental data on the temperature change of the wave vector of all known at the time of writing the work, A₂BX₄ compounds with the structure of β-K₂SO₄, and proposed a generalized P-T diagram predicts the possible phase diagrams of these crystals. While cesium compound was isolated subclass, characterized by a particular sequence of symmetry transformations associated with the softening of the phonon spectrum near G point of the Brillouin zone, and the rest - in the subclass - S along the line.

MAIN RESULTS the operation.

1. NQR for precision studies of phase transitions created gradientless thermo-consoles   and high-pressure chamber having an optimal agreement with the receiving part of the standard quadrupole resonance spectrometer.

2. Studied the sequence of phase transitions in Cs₂CdJ₄. First detected NQR incommensurate phase in β-modification of the crystal.

Complementary experimental methods given scheme symmetry changes. X-ray diffraction analysis confirmed the presence of the incommensurate phase.

3. The sequence of phase transitions in the crystal Cs₂ZnJ₄. NQR ¹²⁷J registered incommensurate phase, found a wide temperature range subtransient ordering. The assumption is made and confirmed the change of symmetry in phase transitions in this compound.

4. Measurements T_Q1 and T_Q2 wide range of temperatures show a significant effect of halogen nuclear spin relaxation on the shape of the NQR lines in the incommensurate phase.

5. Аuthor synthesized and studied by NQR compound (NH₄)₂ZnJ₄. Nuclear quadrupole absorption between levels 1/2↔3/2 isotope ¹²⁷J found three phase transitions crystal structure. Form lines and features of the temperature behavior of the frequency suggests that the intermediate phase in β-crystal state are disproportionate.

6. Polymorphic modifications crystal Rb₂ZnBr₄ and Cs₂CdJ₄ α↔β .   Found that β -modification in compounds Cs₂CdJ₄ and Rb₂ZnBr₄ is metast able. The conditions for the implementation of   transition.

7. Method of nuclear quadrupole resonance investigated   P-T phase diagram Rb₂ZnBr₄. Revealed two high-symmetry phase high pressure. Change in the spectra in the intermediate P-T field indicates a step change of the wave vector, which is consistent with the diffraction measurements, and does not contradict the theoretical pseudo-spin model of incommensurate phases.

 

 

Publications on the topic of the dissertation.

1. Serebrennikov VL No Comments EV Belobrova IA Investigation NQR incommensurate and commensurate phases in Rb₂ZnCl₄ ,Rb₂ZnBr₄ and under pressure; Union Conference on Magnetic Resonance in Condensed cpedah, Kazan, Abstracts, Part 1, 1984.

   2. Aleksandrova IP, No Comments EV. Serebrennikov VL, New effects in NQR spectra disproportionate Rb₂ZnBr₄, XI All-Union Conference on the physics of ferroelectrics, Chernivtsi, abstracts v.2, page 157-B, Kiev, 1986.,

   3. Aleksandrova IP, No Comments EV Serebrennikov B. L., Research P-T phase diagram Rb₂ZnBr₄ NQR; X1 MAVRID International Conference, Kiev, 1987, theses B24, p.20-21,

   4. Aleksandrova IP, No Comments EV Serebrennikov VL,, neighborhood study ferroelectric phase transition in Rb₂ZnBr₄ NQR under high pressure, Phys, 1987, v.29, 9, 2651-57.

5. Shemetov E.V., Aleksandrov K.S..Aleksandrova I.P.,Primak S.V., The ^1Z7J NQR study of the phase transition in Cs₂CdJ₄., Xth Ampere summer school, Abstracts, Novosibirsk, 1987, p.247.

 6. Aleksandrov K.S., Flerov I.N., Kruglik A. I., Melmkova S., Shemetov E.V., Phase transitions in Cs₂CdJ₄ crystal. XI Europress   meeting on ferroelectricity, Poland, 1987, Abstracts n-i.p.99.

 7. Shemetov E.V., Aleksandrov K.S., Aleksandrova I.P., Primak S.V. ^1Z7J NQR study of the phase transition sequence In Cs₂CdJ₄, Phys.Stat.Sol.(a), 1987, v.104, p.K89-92.

   8. Aleksandrov K.S., Flerov I.N., Kokov I.Т., Kruglik   A.I., Melnlkova S.V., Shemetov E.V., Successive phase transitions in Cs₂CdJ₄ crystal, Ferroelectrics, 1988, v.79, p. 137-140. 

 9. Aleksandrova I.P., Shemetov E.V., Serebrennikov   V.L., Primak S.V., Pressure-Temperature Phase Diagram of Rb₂ZnBr₄;   IV Japan-Soviet symposium on ferroelectricity, 1988, Abstracts.

 10. Shemetov E.V., Aleksandrova I.P., Primak S.V., J-127 NQR Study   of   Phase   Transition   in   Cs₂CdJ₄ and Cs₂ZnJ₄; Xth International Symposium on NQR, Takayma, Japan, 1989, Abstracts, III , P2.

 11. Shemetov E.V., Aleksandrova I.P., Primak S.V., J-127 NQR Study   of   Phase   Transition   in   Cs₂CdJ₄ and Cs₂ZnJ₄; Xth International Symposium on NQR, Takayma, Japan, 1989, Abstracts , III , P2.

 12. Aleksandrova   I.P., Shemetov E.V., Serebrennikov V.L., Pressure-Temperature Phase Diagram of Incommensurate Rb₂ZnBr₄, XII-th European Crystallografisal Meeting, Moscow, 1989, Abstracts.

 13. Aleksandrova I.P..Shemetov E.V,,Primak S.V., Krugiik A.I., Sucoesive phase transitions in Cs₂CdJ₄ and Cs₂ZnJ₄, 12th European Crystallographies meeting, Moscow, 1989, v.l, p.526.

14. Aleksandrova I.P., Shemetov E.V., Serebrennikov V.L., High Pressuren Sience   and   Technology,   Procedings   XI   AIRAPT International Conference, Kiev, Naukovs Dumka, 1989,v.1 p. 208-211 , A Study of Phase P-T Diagram of Rb₂ZnBr₄ by NQR Mettod.

15. Aleksandrova IP, No Comments EV, SV Primak, Kruglic AI, Study of phase transitions in Cs₂ZnJ₄ and Cs₂CdJ₄, XII All-Union Conference on the Physics of Ferroelectrics, Rostov / Don, 1989, v.1 , p.123.

   16. Aleksandrova IP, No Comments EV Serebrennikov V.L., Research proportionate and incommensurate phases Rb₂ZnBr₄ NQR under pressure, XII All-Union Conference on the Physics of Ferroelectrics, Rostov-Don, 1989, Vol.1, pp. 122.

17. Aleksandrova I.P., Prlmak S.V., Shemetov E.V. ,Krusrlik Successive   phase   transition   in   Cs₂CdI₄ and Cs₂ZnI₄. 7th   International Meeting   of Ferroelectricity, Saarhruoker, Germany,1989, Abstracts, P139.

18. Aleksandrova IP Primak NE., Elizariev Y.G . Denovue F. Kruglic AI, No Comments E.V., incommensurate phase in the crystal Cs₂ZnJ₄, Soviet-Polish seminar on ferroelectricity, Lviv, 1990, Proceedings, pp. 219-220.

19. Aleksandrova I.P., Primak S.V., Shemetov E.V., Krugiik A.I., Elizaryev Yu. G., Successive phase transitions in Cs₂CdJ₄ and Cs₂ZnJ₄. Ferroelectrics, 1990, v.105, p.177-181

20. Aleksandrova I.P., S.V. Primak, No Comments EV Kruglic AI sequence of phase transitions in Cs₂ZnJ₄, FTT, 1991, v.33, 5, str.1344-1349..

21. Semin G.K., Shemetov E.V., Moskalev А.К., ¹²⁷J NQR Study of phase Transitions in (NH₄)aZnJ₄, XII th International Symposiym on NQR Spectroscopy, Zurich, Shwezerland, 1993, Abstracts.

 22. Shemetov E.V., Moskalev A.K., Semin G.K., Pressure-Ntmperature Phase Diagram of   Rb₂ZnBr₄ by ^81,79Br   NQR Method, XII th International Symposiym on NQR Spectroscopy, Zurich, Shwezerland, 1993, Abstracts.