Ryōkou Ito, Y. Masuda, Yoshio N. Ito
May 16, 1988
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0
Influential Citations
9
Citations
Journal
Thermochimica Acta
Abstract
Abstract Thermal analyses of rubidium and cesium oxalate 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 Rb 2 C 2 O 4 · H 2 O and Cs 2 C 2 O 4 · H 2 O belong to the monoclinic crystal system with cell dimensions of a = 9.68, b = 6.35, c = 11.09 A and β = 109.4 ° at 20 ° C and a = 10.19, b = 6.67, c = 11.44 A 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 Rb 2 C 2 O 4 and Cs 2 C 2 O 4 in phase I have an orthorhombic unit cell with a = 3.69, b = 6.60 and c = 13.94 A at 330 ° C and a = 3.82, b = 6.84 and c = 14.44 A at 330 ° C, respectively. Heating the crystals of Rb 2 C 2 O 4 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 A 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 A 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 Cs 2 C 2 O 4 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 A at 440 ° C, and in phase III a tetragonal unit cell with a = 4.66 and c = 7.04 A 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 Rb 2 C 2 O 4 also proceeded as a phase boundary reaction. However, the decomposition behavior of Cs 2 C 2 O 4 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.