It is known that boron containing compounds exhibit interesting chemical and physical properties. In the past 50 years modern preparative methods have led to an overwhelming number of different structures of novel and often unexpected boron–sulfur and boron–selenium compounds. Among all these new compounds, there was only one which comprises rare earth metal (RE), boron and heavier chalcogen, namely sulfur, the europium thioborate Eu[B2S4] [1]. Selenoborates of rare earth metals are hitherto unknown. On the other hand, rare earth oxoborates represent a well-known class of compounds [2] with a wide range of applications, especially in the field of optical materials. In addition, well-defined boron compounds containing the heavier group 16 elements are fairly difficult to prepare due to the high reactivity of in situ formed boron chalcogenides towards most container materials at elevated temperatures. The chalcogenoborates of the heavier chalcogens are sensitive against oxidation and hydrolysis and therefore have to be handled in an inert environment. Therefore, developing and optimization of preparative routes for the syntheses of pure and crystalline RE thio- and selenoborates was needed.
In the course of this study, the application of different preparation routes, such as optimized high-temperature routes (HT), metathesis reactions and high-pressure high-temperature routes (Hp – HT), led to sixteen new rare earth thioborates. Their crystal structures were solved and/or refined from powder and single crystal X-ray diffraction data, while the local structure around rare earth metal was confirmed from the results of the EXAFS analyses. Quantum mechanical calculations were used within this work in order to investigate the arrangement of intrinsic vacancies on the boron sites in the crystal structures of rare earth thioborates. Thermal, magnetic and optical properties of these compounds are also discussed.
The rare earth thioborates discovered during this work are the first examples of ternary thioborates containing trivalent cations. These compounds can be divided into two groups of isotypic compounds: the rare earth orthothioborates with general formula REIII[BS3] (RE = La – Nd, Sm, Gd and Tb) [3] and the rare earth thioborate sulfides with general formula REIII¦9B5S21, (RE = Gd – Lu, and Y) [4].
In the crystal structure of RE[BS3] (orthorhombic, space group Pna21, Z = 4), the sulfur atoms form the vertices of corrugated kagome nets, within which every second triangle is occupied by boron and the large hexagons are centered by RE cations. The structural features of the isotypic RE[BS3] phases show great similarities to those of rare earth oxoborates RE[BO3] and orthothioborates of alkali and alkaline earth metals as well as to thallium orthothioborate, yet pronounced differences are also observed: the [BS3]3– groups in the crystal structures of RE[BS3] are more distorted, where the distortion decreases with the decreasing size of the RE element, and the coordination environments of the [BS3]3– groups in the crystal structures of RE[BS3] are different in comparison with the coordination environments of the [BO3]3– groups in the crystal structures of λ-Nd[BO3] [5] and of o-Ce[BO3] [6].
The results of the IR and Raman investigations are in agreement with the presence of [BS3]3– anions in the crystal structure of RE[BS3]. Thermal analyses revealed the thermal stability of these compounds under inert conditions up to ~ 1200 K. Analyses of the magnetic properties of the Sm, Gd and Tb thioborates showed that both Gd and Tb phases order antiferromagnetically. The magnetic susceptibility for Sm orthothioborate approximately follows the Van-Vleck theory for Sm3+. Between 50 K and 62 K a transition appears which is independent of the magnetic field: the magnetic susceptibility becomes lower. This effect might indicate a discontinuous valence transition of Sm which was further investigated by means of XANES and X-ray diffraction using synchrotron radiation, both at low temperatures.
The series of isotypic RE thioborate sulfides with composition RE9B5S21, was obtained by the application of Hp – HT conditions to starting mixtures with the initial chemical composition “REB3S6“, after careful optimization of the pressure, temperature and treatment time, as well as the composition of the starting mixtures. Their crystal structures adopt the Ce6Al3.33S14 [7] structure type (hexagonal, space group P63, Z = 2/3). The special features of the RE9B5S21 crystal structures, concerning boron site occupancies and different coordination environments of the two crystallographically independent boron sites, were investigated in more detail by means of quantum chemical calculations, electron diffraction methods, optical and X-ray absorption spectroscopy as well as by 11B NMR spectroscopy. The results obtained from these different experimental and computational methods are in good mutual agreement. The crystal structures of the RE9B5S21 compounds are characterized by two types of anions: tetrahedral [BS4]5– and trigonal planar [BS3]3– as well as [(S2–)3] units. Isolated [BS4]5– tetrahedra (all pointing with one of their apices along the polar [001] direction) represent a unique feature of the crystal structure which is observed for the first time in a thioborate compound. These tetrahedra are stacked along the three-fold rotation axes. Vacancies are located at the trigonal-planar coordinated boron site with preferred ordering –B–B––B–B–– along [001]. No superstructure is observed by means of electron diffraction methods as adjacent columns are shuffled along the c axis, giving rise to a randomly distributed vacancy pattern. Positions of the sulfur atoms within the [(S2–)3] substructure as well as planarity of the [BS3]3– units were investigated in more detail by means of quantum mechanical calculations.
Results of the IR and Raman spectroscopy, as well as of the 11B NMR spectroscopy are in agreement with the presence of the boron atoms in two different coordination environments. Thermal analyses showed that compounds RE9B5S21 are stable under inert conditions up to ~ 1200 K. In accordance with the combined results of experimental and computational investigations, the chemical formula of the RE9B5S21 compounds is consistent with RE3[BS3]2[BS4]3S3.
A short overview of investigations towards rare earth selenoborates, where in most of the cases only known binary rare earth selenides could be identified, is presented as well in this work. Investigations in the RE–B–Se systems were conducted by the application of different preparation routes by varying the experimental parameters and the initial compositions of the starting mixtures. Although no crystal structure of a ternary phase in these systems could be solved, there are indications that such phases exist, but further investigations are needed.
[1] M. Döch, A. Hammerschmidt, B. Krebs, Z. Anorg. Allg. Chem., 2004, 630, 519.
[2] H. Huppertz, Chem. Commun., 2011, 47, 131; and references therein.
[3] J. Hunger, M. Borna, R. Kniep, J. Solid State Chem., 2010, 182, 702; J. Hunger, M. Borna, R. Kniep, Z. Kristallogr. NCS, 2010, 225, 217; M. Borna, J. Hunger, R. Kniep, Z. Kristallogr. NCS, 2010, 225, 223; M. Borna, J. Hunger, R. Kniep, Z. Kristallogr. NCS, 2010, 225, 225.
[4] M. Borna, J. Hunger, A. Ormeci, D. Zahn, U. Burkhardt, W. Carrillo-Cabrera, R. Cardoso-Gil, R. Kniep, J. Solid State Chem., 2011, 184, 296;
[5] H. Müller-Bunz, T. Nikelski, Th. Schleid, Z. Naturforsch. B, 2003, 58, 375.
[6] H. U. Bambauer, J. Weidelt, J.-St. Ysker, Z. Kristallogr., 1969, 130, 207.
[7] D. de Saint-Giniez, P. Laruelle, J. Flahaut, C. R. Séances, Acad. Sci. Ser. C, 1968, 267, 1029.:I INTRODUCTION ......................................................................... 7
1. Motivation and scope of the work .............................................. 9
2. Literature overview .................................................................. 11
2.1. The binary subsystems of the ternary systems RE–B–Q (RE = rare earth metals, Y; Q = S, Se) ......................................................... 12
2.1.1. RE–Q ............................................................................... 12
2.1.2. RE–B ............................................................................... 19
2.1.3. B–Q ................................................................................. 22
2.2. Related ternary compounds ................................................... 25
2.2.1. RE oxoborates .................................................................. 25
2.2.2. Thio- and selenoborates of alkaline, alkaline earth, transition and post
transition metals ......................................................................... 33
2.2.3. The RE thioborate Eu[B2S4]................................................ 45
II PREPARATIVE METHODS AND EXPERIMENTAL TECHNIQUES .......... 47
1. Starting materials and their characterization ............................... 49
2. Synthetic approaches and optimizations .................................... 51
2.1. High-temperature routes ...................................................... 52
2.2. Metathesis reactions ............................................................ 53
2.3. Spark Plasma Sintering (SPS) ............................................... 54
2.4. High-Pressure High-Temperature (Hp – HT) Syntheses ........... 55
3. Analytical methods and samples characterization ....................... 55
3.1. Powder X-ray diffraction ...................................................... 55
3.2. Crystal structure investigations using synchrotron radiation .... 57
3.3. Single crystal X-ray diffraction analysis .................................. 57
3.4. Metallographic investigations ................................................ 58
3.5. Electron microscopy ............................................................ 58
3.5.1. Scanning electron microscopy and energy dispersive X-ray spectroscopy ............................................................................ 58
3.5.2. Transmission electron microscopy ...................................... 59
3.6. Optical spectroscopy ........................................................... 59
3.6.1. Infra-Red spectroscopy .................................................... 59
3.6.2. Raman spectroscopy ........................................................ 60
3.7. X-ray absorption spectroscopy ............................................ 60
3.8. Thermal analysis ................................................................. 62
3.9. Magnetic susceptibility measurements ................................... 63
3.10. 11B NMR spectroscopy ..................................................... 63
3.11. Quantum chemical calculations ........................................... 64
3.11.1. Total energy calculations ................................................ 64
3.11.2. Charge transfer analysis ................................................ 64
3.11.3. Chemical bonding........................................................... 64
III RARE EARTH THIOBORATES ................................................. 67
1. Reinvestigation of the only reported rare earth thioborate – EuB2S4 ....69
2. RE[BS3] (RE = La – Nd, Sm, Gd, Tb) .................................... 69
2.1. Syntheses and phase analyses .......................................... 70
2.2. Crystal structure determinations ........................................ 74
2.3. X-ray absorption spectroscopy: EXAFS data analysis for Pr[BS3] ..... 79
2.4. Crystal chemistry .............................................................. 80
2.5. Optical spectroscopy ......................................................... 83
2.6. Thermal analysis ............................................................... 86
2.7. Magnetic susceptibility ....................................................... 88
2.8. X-ray absorption spectroscopy: XANES data analysis for Sm[BS3] .. 91
2.9. Crystal structure investigation at low temperature using synchrotron radiation ................................................................................... 91
2.10. Summary ......................................................................... 95
3. Gd[BS3] : Ce, Eu, Tb ............................................................. 97
3.1. Syntheses and phase analyses ............................................. 97
3.2. Crystal structure determinations ......................................... 101
3.3. Crystal chemistry .............................................................. 103
3.4. Optical spectroscopy ......................................................... 104
3.5. Thermal analysis ............................................................... 106
3.6. Summary ......................................................................... 107
4. RE9B5S21 (RE = Tb – Lu, Y) ................................................ 107
4.1. Syntheses and phase analyses ........................................... 108
4.2. Crystal structure determinations ........................................ 109
4.3. Crystal chemistry .............................................................. 112
4.4. Electronic structure, charge transfer and chemical bonding .... 115
4.5. X-ray absorption spectroscopy: EXAFS data analysis for Lu9B5S21 .............................................................................. 119
4.6. Thermal analysis ............................................................... 121
4.7. 11B NMR investigations ..................................................... 122
4.8. Optical spectroscopy ......................................................... 123
4.9. Summary ......................................................................... 126
IV ON THE WAY TO RARE EARTH SELENOBORATES .................... 127
1. Towards ternary phases in the systems RE–B–Se, with RE = Sm, Tb – Lu.......................................................................................... 129
2. The system La–B–Se ........................................................... 134
3. The system Gd–B–Se .......................................................... 136
4. The system Y–B–Se ............................................................ 137
5. Summary ........................................................................... 139
V SUMMARY AND OUTLOOK ..................................................... 141
VI APPENDIX .......................................................................... 149
VII REFERENCES .................................................................... 163
VIII LIST OF FIGURES ............................................................. 181
IX LIST OF TABLES ................................................................ 193
X CURRICULUM VITAE ........................................................... 199
XI VERSICHERUNG ............................................................... 203
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:26118 |
| Date | 13 August 2012 |
| Creators | Borna, Marija |
| Contributors | Kniep, Rüdiger, Ruck, Michael, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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