Abstract
The Raman spectra of three orthophosphates RPO₄ (R = Y, Tb, Tm) having the tetragonal zircon-type structure (D4h19 space group symmetry) are presented and studied at ambient conditions and under variable low-high temperature (20-800 K) and hydrostatic pressure (1 Atm-15.5 GPa). The primary objectives of the present thesis are: (i) a study of the lattice dynamics of the crystals through the temperature and pressure dependences of their Raman active modes and (ii) the observation of possible structural phase transitions in the crystals induced by either of the two thermodynamic parameters. Polarized Raman measurements performed on single, oriented crystalline samples have enabled to make assignments of symmetries of the Raman active modes (phonons) for the three crystals. On the other hand, based on a clear separation of phonons in two distinct groups which is observed in the phonon-frequency (or Grüneisen parameter) versus pressure plots, a character assignment of the Raman modes is mad ...
The Raman spectra of three orthophosphates RPO₄ (R = Y, Tb, Tm) having the tetragonal zircon-type structure (D4h19 space group symmetry) are presented and studied at ambient conditions and under variable low-high temperature (20-800 K) and hydrostatic pressure (1 Atm-15.5 GPa). The primary objectives of the present thesis are: (i) a study of the lattice dynamics of the crystals through the temperature and pressure dependences of their Raman active modes and (ii) the observation of possible structural phase transitions in the crystals induced by either of the two thermodynamic parameters. Polarized Raman measurements performed on single, oriented crystalline samples have enabled to make assignments of symmetries of the Raman active modes (phonons) for the three crystals. On the other hand, based on a clear separation of phonons in two distinct groups which is observed in the phonon-frequency (or Grüneisen parameter) versus pressure plots, a character assignment of the Raman modes is made which distinguishes them in internal and external ones, corresponding to atomic vibrations within the (ΡO₄)³⁻ tetrahedra and pure lattice vibrations (relative displacements of R³⁺ and (ΡO₄)³⁻ ions), respectively. One common feature in the spectra of the three crystals is the unusual temperature dependence of the B2g internal mode which corresponds to bending vibrations of oxygen atoms in the ab plane. The frequency of this phonon increases slightly with increasing temperature in the region 20-300 K, while for T>300 K decreases marginally, thus displaying (in the latter case) normal mode behaviour. The abnormal temperature dependence of the phonon for T<300 K may be related with an incipient phase transition at a very low temperature; however, there has been no evidence for such a transition in any of the three crystals of this work down to the lower limit of 20 K. Given the almost linear temperature dependence of the frequency and width of the Raman phonons in the high-temperature region (T>300 K) for all three crystals, it is implied that the process of splitting of each phonon to two different phonon species dominates over other anharmonic processes. In the Raman spectra under variable hydrostatic pressure, it is observed that all, but two, phonon species of the three crystals display normal mode dependence with pressure, that is, their frequency increases with increasing pressure. One of the exceptions concerns the B2g internal mode which exhibits soft mode behaviour and is likely related with an incipient phase transition at a higher pressure. Indeed, from the evolution of the Raman spectrum of the TbP04 crystal and particularly from the abrupt spectral changes (discontinuities and slope changes in the frequency-versus-pressure plots of phonons, disappearance of some and appearance of other phonon Raman peaks) at a pressure Pc ≈ 9.5 GPa, it is implied that this crystal undergoes a first-order structural phase transition to a lower symmetry structure. Various probable structures are considered for the high-pressure phase of TbPΟ₄, with the most likely appearing to be the monoclinic monazite-type one (C2h5 space group symmetry) in which the light rare-earth orthophosphates RPO₄ (R = La to Gd) are known to crystallize. Combining the Raman spectra under variable temperature and pressure, it is possible to separate and evaluate the contributions of (i) the volume thermal expansion and (ii) the pure temperature effect related to the occupation of phonon states, to the observed total shift of phonon frequencies with temperature. Such an anharmonic analysis presumes the knowledge of the thermal expansion coefficients and elastic constants of the crystals over the entire temperature range of each study. Since there are no relevant elastic constant data in the literature for the crystals of the present study, an anharmonic analysis of the Raman results has been carried out by adopting the elastic constant values of other related isomorphous crystals. This attempted study has not been successful as it has led to unexpected and incompatible conclusions concerning mainly the nature of the internal phosphorus-oxygen atomic bonds. This suggests that the elastic constant values adopted for the three crystals must diverge significantly from the actual ones.
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