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High-Pressure And High-Temperature Synthesis And Characterization Of Silicates With Special Structures

Posted on:2007-05-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:C WangFull Text:PDF
GTID:1101360185954734Subject:Inorganic Chemistry
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Silicate, including ceram, refractory material, glass material, cement,concrete and hydrothermal synthesized silicate building material, is oneof the three basic man-made materials and acts as an important role in ourlife. Our research mainly concentrated on high-pressure andhigh-temperature (HP-HT) synthesis of rare earth element (REE)disilicate, forsterite and REE silicate apatite and their characterization.The disilicates, especially the REE disilicates REE2Si2O7, have beenwidely studied for their unique magnetic, electrical and optical propertiesand their applications to optimize the properties of ceramic. Elevendifferent structure types (A-L) have been reported for the single rare earthelement disilicates, which exhibit a complex structural chemistry atambient conditions, due largely to the monotonic decrease in size ofREE3+ cations through the lanthanide series. Forsterite is a low-pressurephase of magnesium orthosilicate with chemical formula of Mg2SiO4. Thelarge family of forsterite based minerals is the major component of theearth's upper mantle. The low-pressure phase olivine [(Mg1-xFex)2SiO4,x≈0.1] is the dominant phase in this family of minerals, which includesmany other compounds and displays a surprising variety of chemical andphysical properties. The variability of their properties should be mainlyattributed to the Mg2SiO4 component, which is at the origin of severalimportant phenomena of geophysical interests. On the other hand,forsterite has become the focus of much recent researches, because of thedifficult of its crystal growth. Rare earth (REE)-based apatites haverecently attracted considerable attention for their high oxide ionconductivities which make them potentially useful as electrolytes forintermediate-temperature solid oxide fuel cells.Single crystal of lutetium disilicate Lu2Si2O7 has been synthesized atthe pressure of 4GPa and the temperature of 1200?C, using aPiston-Cylinder type high pressure apparatus;it consists of a series ofhelical chains, which are parallel to each other and possess the samechirality.A series of double REE disilicates, including HoGdSi2O7, HoErSi2O7with type B structure and GdDySi2O7 with type B and E structures, havebeen synthesized under the pressure of 2.0-3.0GPa and temperature of1200-1600?C, by using a piston-cylinder apparatus. Although we have notobtained their structure because of the poorly crystal qualities, webelieved that we will get the high quality single crystal in future.The crystal structure of forsterite was first determined in the year of1926 by Brown and Bragg. Up to now, however, only powder samples offorsterite could be prepared. High quality single crystal of the forsterite(Mg2SiO4), with different sizes, has been synthesized for the first time,under pressure of 0.2-2GPa and temperature of 900-1400?C, using apiston-cylinder HP-HT apparatus and a LECO HR-1B Geological &Mineralogical HP-HT research system, respectively. Single crystal X-raydiffraction analysis shows that it crystallizes in a orthorhombic spacegroup Pnma (No. 62) with a = 10.2073(11)?, b = 5.9863(5)?, c =4.7611(4)? and Z =4.Single crystals of gadolinium holmium silicate hydroxyapatiteGd4.33Ho4.33(SiO4)6(OH)2 have been synthesized at 2.0 GPa and 1450?Cusing a piston-cylinder type high pressure apparatus. The crystalsymmetry by single crystal X-ray diffraction analysis is hexagonal, spacegroup P63/m (No. 176), with a = 9.3142(5)?, c = 6.7010(4)?, Z = 1.Gadolinium and holmium are disordered over the two large cationpositions, A(1) and A(2), and charge balance in this silicate apatite ismaintained by cation vacancies in A(1). Two other apatite-structurecrystals investigated have P 3 and Imma symmetry, and represent eitherpartially ordered Gd-Ho distributions or crystal strain induced duringquenching.
Keywords/Search Tags:High-Pressure Synthesis, Silicate, Apatite Structure, Rear Earth Element Disilicate, Forsterite, Single Crystal X-ray Diffraction
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