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RUSSIAN ACADEMY OF SCIENCES INSTITUTE OF CHEMICAL PHYSICS. SEMENOV 

A manuscript UDC 536,913,405 +539.89
Shemetov Evgeny V.
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
Synopsis
dissertation for the degree of candidate of physical and mathematical sciences
Moscow 1995
The work is done and the Institute of Physics SB RAS im.L.V.Kirenskogo and Krasnoyarsk branch of the Republican Technology Center of the Russian government.
Supervisor Candidate of physico-mathematical sciences MOSKALEV A.K.
 OPPONENTS: tehnicheskin doctor, professor T.A.Babushkina; Doctor of Physical and Mathematical Sciences  D.L.Zagorski
Lead Institute of Organoelement Compounds, A.N. Nesmeyanova
 dissertation will be held ____ 1995, at ___ hours ___ minutes. At a meeting of the Specialized Council D0O2-26.04. at the Institute of Chemical Physics. N. N. Semenova RAS address: 117334, Moscow, ul. Kosygin 4.
With the thesis can be found in the library of the Institute of Chemical Physics sent
 1990.
Scientific Secretary of the Specialized Council Ph.D., professor V.Ya.Rochev
 

STUDY OF PHASE TRANSITIONS IN COMPOUNDS SUCH A₂BX₄ STRUCTURE  β-K₂SO₄ HALOGEN NQR AT HIGH AND ATMOSPHERIC PRESSURE
 

General characteristics of 
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 these studies was Uday partially disordered states 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 the crystal with different functions 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 the long-period sequence of phase states by transformations of the first kind, and the other, a continuous way: depending on the actual ienenenid wave vector q_s , 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 radiospectroscopic diffraction 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 the class kristallav recorded 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 tila to detect and study them thoroughly disproportionate phases.

Objective.
  Study of nuclear quadrupole resonance of halogens (Cl, Br, J), and other methods, the family of crystals with β-K₂SO₄ structural disproportion. Figuring features variations in the spectral parameters fazovyk resonance transitions; Analysis environment lkalnogo the nuclei and its transformation into a number of ooedineniyah changing temperature and high hydrostatic pressure. Study processes of spin dynamics. In combination with other methods of analysis of changes in the crystal symmetry at structural phase transitions.
New scientific results.
First NQR using the methods of X-ray diffraction, calorimetry, electro-optics and dielectric measurements investigated posledavatelnosti transitions in compounds Cs₂CdJ₄,  Cs₂ZnJ₄ and (NH)₂ZnJ₄ with an incommensurate phase.
 Prostrananstvennye defined and point group symmetry commensurate phases.

In compounds Rb₂ZnBr₄  and  Cs₂ZnJ₄ 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 А₂ВХ₄ compounds А₂ВХ₄ family with cesium cation registered full range of ¹²⁷J NQR frequency scale throughout the incommensurate phase below Т_i . Before Т_c found wide temperature range subtransient ordering. 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 skills obtained data. Established that the phase transition at Ti decreases fluctuations before the first race.  At low temperatures, the incommensurate phase at Т_с , along with the main structure corresponding to a tripling of the unit cell of the crystal (q_δ ~L/3), in the form of the NQR spectrum indicated a different structure. In studies under pressure revealed that the latter refers to ssstoyaniyu crystal with wave vector (q_δ ~L/4) and another branch of fashion Σ.


 ^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 170-360K at pressures up to 1 GPa. Recorded sequence of phase transitions in the Inc P-T field Rb₂ZnBr₄, interpreted within the 'devil's staircase'. Analysis of transformation of the NQR spectrum indicates that the low temperature portion of the incommensurate phase behavior of the type observed simple stairs (complete D'S), and in the medium-temperature

Chance of mixed (incomplete) view "satanic stairs" A comparison with theoretical-phase diagrams of pseudo-spin model with phonon coupling.

Scientific and practical value.
1. Methods and apparatus that increase  sensitizing pulse NQR spectrometer for the purpose of detection of weak signals of radio frequency absorption at precision and 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 А₂ВХ₄: Pnma symmetry with temperature. 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 compound β-K₂SO₄

 5. First, in some connection А₂ВХ₄, revealed the existence of another polymorph - α. We propose to search for new members of the family А₂ВХ₄ netastabilnoy type structure with β-K₂SO₄

 6. The spin-spin and spin-lattice relaxation NQR relaxation of halogens in the 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.
 Times apparatus realized under the high temperature and hydrostatic pressure . Science. Synthesized compounds Cs₂ZnI₄ and (NH)₂ZnI₄ . Conducted all the experimental NQR measurements and their processing. Developed and used applications for numerical simulation of the line shape in the NQR moduliravannyh phases. The author participated in the preparation and osuschesvelenii other experimental teaching and learning work is supported by research quadrupole resonance.

Testing work -
  The main part of the dissertation presented and discussed at
All-Union scientific conferences
 -By radiospektrosyaopii, Kazan 1984.
 -On physics segnetoelekrikov: 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 IE introduction, four chapters, conclusions, list of references of 135 titles.
Dissertation presented in 172 pages of text, 5 tables and 70 figures.

 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