Structural modulation and phase transitions in melilites [Elektronische Ressource] / vorgelegt von Zhihong Jia

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Informations

• Publié par: philipps-universitat_marburg
• Publié le: 01 janvier 2005
• Nombre de lectures: 14
• Langue: English
• Poids de l'ouvrage: 8 Mo

Extrait

Structural Modulation and Phase Transitions
in Melilites





Dissertation

zur
Erlangung des Doktorgrades
der Naturwissenschaften
(Dr. rer. nat.)


dem
Fachbereich Geowissenschaften
der
Philipps-Universität Marburg


vorgelegt von
Zhihong Jia
aus der V. R. China


Marburg/Lahn, Germany 2005



























Vom Fachbereich Geowissenschaften
der Philipps-Universität Marburg
als Dissertation am 17. 01. 2005 angenommen.
Erstgutachter: Dr. Helmut Rager
Zweitgutachter: Prof. Dr. Werner Massa
Tag der mündlichen Prüfung am 02. 02. 2005


Contents
Contents

1 Introduction......................................................................................................1
2 Structure modulation of melilites...................................................................3
2.1 General aspects of incommensurability.............................................................3
2.2 The melilite system............................................................................................4
2.2.1 Temperature dependence of the structure modulation.......................................5
2.2.2 The compositional dependence of the structure modulation .............................8
2.3 Mechanisms of the structure modulation in melilites........................................9
3 Experimental methods...................................................................................13
3.1 Single-crystal growth method and apparatus...................................................13
3.1.1 Double ellipsoid mirror furnace.......................................................................13
3.1.2 Factors affecting singe crystal quality .............................................................16
3.2 Electron microscopy ........................................................................................18
3.2.1 Transmission electron microscopy ..................................................................18
3.2.2 Scanning electron microscopy, energy-dispersive X-ray spectrometry and
wavelength-dispersive X-ray spectrometry .....................................................29
3.3 Electron spin resonance ...................................................................................32
3.4 X-ray diffraction ..............................................................................................34
3.5 Magnetic measurements36
3.6 Other methods..................................................................................................39
3.6.1 Differential scanning calorimeter ....................................................................39
3.6.2 Photoluminescence measurements...................................................................39
4 Results and discussions..................................................................................40
4.1 Synthesis of Ca MgSi O , Ca CoSi O and Ca ZnSi O melilites..................40 2 2 7 2 2 7 2 2 7
4.2 Single crystal growth .......................................................................................44
4.2.1 The preparation of a feed rod and a seed crystal .............................................44
4.2.2 Growth of Ca Mg Zn Si O , Ca Co Zn Si O , (Ca Sr ) CoSi O crystals2 1-x x 2 7 2 1-x x 2 7 1-x x 2 2 7
..........................................................................................................................46
4.3 Investigations of Ca Mg (Zn, Co) Si O with modulated structure..............55 2 1-x x 2 7
4.3.1 The Ca Mg Zn Si O system.........................................................................55 2 1-x x 2 7
iContents
4.3.2 The Ca Mg Co Si O system ........................................................................60 2 1-x x 2 7
4.3.3 Conclusions......................................................................................................65
4.4 Modulation and phase transitions of Ca Co Zn Si O in dependence on 2 1-x x 2 7
temperature and composition...........................................................................66
4.4.1 Crystal growth and characterization ................................................................66
4.4.2 The average structure observed by high-resolution transmission electron
microscopy and corresponding image simulation............................................71
4.4.3 Electron microscope studies of the modulated structure .................................74
4.4.4 The transition from the incommensurate phase to the commensurate lock-in
phase and to the normal phase .........................................................................78
4.4.5 Refinement of the commensurate lock-in structure of Ca Co Zn Si O .....88 2 0.9 0.1 2 7
4.4.6 Conclusions......................................................................................................92
4.5 Investigations of the structural modulation of (Ca Sr ) CoSi O ..................94 1-x x 2 2 7
4.5.1 Crystal characterization ...................................................................................94
4.5.2 The modulated structure and the transition from the incommensurate to the
normal phase with varying Sr-content .............................................................99
4.5.3 Conclusions....................................................................................................106
4.6 Investigations of Ca (Mg,Co)Si O doped with Cr and Eu ...........................107 2 2 7
4.6.1 Synthesis and characterization of Ca MgSi O :Cr solid solutions................107 2 2 7
4.6.2 Synthesis, characterization and optical properties of Ca Mg(Co)Si O :Eu solid 2 2 7
solutions .........................................................................................................110
4.6.3 Conclusions....................................................................................................116
5 Summary and outlook .................................................................................117
6 Zusammenfassung und Ausblick................................................................120
7 References.....................................................................................................123
Appendix A. Lock-in phase of Ca (Co Zn )Si O refined in s. g. P2 2 2 2 0.9 0.1 2 7 1 1
_
Appendix B. Lock-in phase of Ca (Co Zn )Si O refined in s. g. P 4 . 2 0.9 0.1 2 7
Appendix C. Abbreviations
Acknowledgements


ii1 Introduction
1 Introduction
A crystal consisting of atoms repeated regularly in three dimensions has a so-called
translational symmetry, which can be described by one of the 230 crystallographic
space groups. The lattice periodicity is apparent in X-ray, electron or neutron
diffraction patterns which consist of sharp spots located on points of the reciprocal
lattice. Often, additional spots besides the main reflections are observed, they can be
attributed to the existence of various kinds superlattice structures, lattice defects or
structure modulations. Incommensurately modulated crystals are known from, e.g.,
[1] [2] [3] [4, 5]quasi one-dimensional conductors , ferroelectrics , alloys , minerals ,
[6]composite crystals .
Extensive studies have been done on melilite-type compounds with the general
1 2formula X T T O to elucidate the nature of the observed incommensurate ordering. 2 2 7
1 2The melilite structure consists of layers formed by T and T tetrahedra and the larger
X cations located halfway between adjacent layers. The incommensurate modulation
[4] [5]was first described independently by Hemingway et al. and Seifert et al. in
synthetic Ca MgSi O åkermanite. It was supposed that the misfit between the large X 2 2 7
cations and the sheet-like tetrahedral framework might be responsible for the
modulation. Changing the structural misfit by substitution of other cations or by
temperature varied the amplitude of the modulation and the length of the modulation
[7-11] P 42 m1vector . X-ray refinement suggested that the modulated structure is P p4mg
according to the (3+2)-dimensional superspace formalism, and that the modulation is
caused by a displacive shift of the constituent atoms resulting in a rotation and
1 2 [12, 13]deformation of the T and T tetrahedra . These changes appear to be
accompanied by changes of the interlayer X-cation environment in a way that reduces
[14-17]the coordination number of X from eight to seven or even six . It was further
1concluded that the flattened T -tetrahedra surrounded by low-coordinated X cations
show the tendency to form octagonal clusters, and that the arrangement of these
[16, 18, 19]clusters determines the strength of the overall modulation .
In spite of the number of studies, the detailed structure of modulated melilite
crystals has not yet been clarified till now. This holds for the atomic configurations
and the correct symmetry relations of the five-dimensional structure as well as the
different aspects of superstructure ordering. The transition from the incommensurate
11 Introduction
to the commensurate lock-i