Add time:07/24/2019         Source:sciencedirect.com

Thermal analyses of rubidium and CESIUM OXALATE (cas 1068-63-9) monohydrates were carried out by means of thermogravimetry, differential thermal analysis and differential scanning calorimetry together with powder X-ray diffraction analysis. Both the oxalate monohydrates Rb2C2O4 · H2O and Cs2C2O4· H2O belong to the monoclinic crystal system with cell dimensions of a = 9.68, b = 6.35, c = 11.09 Å and β = 109.4 ° at 20 ° C and a = 10.19, b = 6.67, c = 11.44 Å and β = 108.0 ° at 20 ° C, respectively. Three crystallographic modifications (phases I, II and III) were identified for both dehydrated oxalates. The crystals of Rb2C2O4 and Cs2C2O4 in phase I have an orthorhombic unit cell with a = 3.69, b = 6.60 and c = 13.94 Å at 330 ° C and a = 3.82, b = 6.84 and c = 14.44 Å at 330 ° C, respectively.Heating the crystals of Rb2C2O4 in phase I to 380 ° C, transforms the specimen into phase II belonging to the orthorhombic crystal system with lattice parameters a = 4.56, b = 6.53 and c =9.49 Å at 385 ° C. At temperatures above 388 ° C. the crystals of phase II transform into phase III which belongs to the tetragonal crystal system with a = 4.59 and c = 6.67 Å at 405 ° C. Both the phase transitions I → II and II → III are reversible. The crystals in phase III begin to decompose at around 450 ° C.The crystals of Cs2C2O4 in phase I also transformed reversibly into phase II at 400 ° C, and into phase III at 451 ° C. The crystals in phase II have orthorhombic unit cell with a = 4.77, b =6.82 and c = 9.95 Å at 440 ° C, and in phase III a tetragonal unit cell with a = 4.66 and c = 7.04 Å at 465 ° C. The crystals in phase III decomposed above 470 ° C.The dehydration of both oxalate monohydrates proceeded as a phase boundary reaction. The decomposition of Rb2C2O4 also proceeded as a phase boundary reaction. However, the decomposition behavior of Cs2C2O4 was characteristic of a homogeneous first-order reaction since the specimen melted during the decomposition.Activation energies of the dehydration and the decomposition were determined for both oxalates.

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