Syntheses, Crystal Structure and Optical Properties of the Polymorphs of Potassium Ferrisilicate (KFeSiO4) & Effect of Calcium Doping on the Phase Stability

Ghosh, Arindam and Jai, Prakash (2018) Syntheses, Crystal Structure and Optical Properties of the Polymorphs of Potassium Ferrisilicate (KFeSiO4) & Effect of Calcium Doping on the Phase Stability. Masters thesis, Indian Institute of Technology Hyderabad.

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Abstract

The two polymorphs of KFeSiO4 namely α-KFeSiO4 and β-KFeSiO4 have been synthesized using solid-state methods at 1000ºC and 830ºC, respectively. The low temperature polymorph, β-KFeSiO4 crystalizes in the P6322 space group of the hexagonal crystal system (a = 5.260(1) Å, b = 5.260(1) Å, c = 8.801(1) Å) whereas the high temperature α-KFeSiO4 phase adopts orthorhombic crystal structure (a = 5.470(1) Å, b = 9.190(1) Å, c = 8.580(1) Å). The Fe and Si atoms are statistically disordered at the same site in the β-KFeSiO4 structure. Each Fe/Si atom in this structure is coordinated to four oxygen atoms in a tetrahedral fashion. The red colored β-KFeSiO4 phase starts transforming to yellowish colored α-KFeSiO4 phase on heating in the air above 910ºC. Yellow colored crystals of α-KFeSiO4 have been prepared by the flux method using KI as the molten flux. The stability of hexagonal β-KFeSiO4 phase on the substitution of calcium cations at the potassium sites has been investigated in detail. Our studies show that the β-phase could be stabilized even at 1000ºC by Ca-doping. Five different compounds having formula K1-xCaxFeSiO4 with different “x” were synthesized. From optical measurement, the band gap of β-KFeSiO4 and α-KFeSiO4 are 1.9(1) eV and 2.2(1) eV, respectively. The melting point of the α-KFeSiO4 phase has been determined by heating the sample in an alumina crucible at various temperatures. It has been found that the melting point of the alpha phase is above 1090ºC. The disordered distribution of Fe/Si in β-KFeSiO4 is also supported by the FTIR data. The Si—O—Si symmetric stretching frequency and Fe—O—Fe symmetric stretching frequencies in FTIR data of α-KFeSiO4 suggest the possibility of ordered Fe and Si sites in the orthorhombic crystal structure. However, crystallographic studies are required to support this conjecture.

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