Investigate the relationship between crystal structure of (CaxSr1-x)2SiO4 solid solutions as functions of composition and temperature using X-ray diffraction technique

Abstract

Ca2SiO4 could be found in the contact metamorphic rocks and played an important role in the field of industry since the middle of 18th century The solid solutions of composition (CaxSr1-x)2SiO4 were developed to be a material of phosphor The high temperature phase of Ca2SiO4 can be retained at room temperature based on previous observations However the reason of that remains controversial Only few studies focused on the solid solutions (CaxSr1-x)2SiO4 at room temperature Furthermore the phase transformation of solid solutions at high temperature was not investigated yet The solid-state reaction was applied to synthesize samples in this study To investigate the phase transformation of Ca2SiO4 upon cooling and the geological implication the X-ray diffraction was used to determine the phase of final products after different heat treatment for Ca2SiO4 The first-principle calculation result was also taken to compare the energy between phases To investigate the relationship between crystal structure of (CaxSr1-x)2SiO4 solid solutions and composition and establish the phase diagram the in-house X-ray diffraction high temperature and low temperature X-ray diffraction of synchrotron was used with Rietveld method The results showed that the crystal phases of final products were determined by the soaking temperature If the sintering temperature was set within the lower temperature range (the stability field of ?’L phase 1173-1373 K) the dominated phase of products was high temperature phase (β); otherwise at higher temperature range (the stability field of ?’H phase higher than 1373 K) the dominated phase was low temperature phase (γ) The first-principle calculation results showed that the energy of the γ phase was slightly lower than that of the β phase at 0 K and 0 GPa It indicated that the γ phase was a stable phase and the β phase was a metastable phase at ambient conditions However the energy difference of two phases was fairly small This study suggested that the energy barrier between β and γ phases should be high The transition temperature was about 770 K suggested by previous study These might be the reason why the β phase could be retained at room temperature More strains would be accumulated in the crystal when higher sintering temperature was applied The accumulated strains in the crystal would make the β phase unstable and then induce them transform into the γ phase The cooling rate might affect the ratio of mixtures of the β and γ phase The ?’L phase would be the only crystal phase in the series of solid solutions with the composition (CaxSr1-x)2SiO4 x=0 1-0 9 which was synthesized at room temperature The temperature of phase transformation ?’L→?’H in solid solutions was lower than end-members and would vary with compositions There was a lowest transition temperature at x=0 2 The results showed that the variation of cell parameters was non-linear with increasing Ca2+ concentration and might be related to the state of order-disorder The intensity of (111) diffraction peak would be affected by the distribution of cation in two occupancy sites with refinement Therefore the diffraction peak (111) might be an indicator of order-disorder refinement It was further observed that the wave vector was majorly influenced by the composition whereas impacted by the temperature less This study attempted to speculate the geological setting based on the experimental results and expected to determine the thermal history that the outcrop went through It was the first time to provide the high temperature phase diagram and to investigate the correlation between modulated structure as the functions of temperature and composition

Date of Award	2016 Aug 31
Original language	English
Supervisor	Hui-Chen Kung (Supervisor)