therm0chimica acta ELSEVIER Thermochimica Acta 290 (1996) 85- 91 TG, DTA, DSC and X-ray powder diffraction studies on some nickel selenate hydrates D. Stoilova *, V. Koleva Institute ~)[" General and Inorganic Chemistry, Bulgarian Academy o[Sciences, 1113 Sofia, Bulgaria Received 13 December 1995; accepted 10 July 1996 Abstract The thermal dehydration of NiSeO 4 • 6H 2 O, NiSeO4'4H 20 and NiSeO 4- H 20 has been studied by TG, DTA and DSC. the dehydration of NiSeO4'6H20 depends on both the heating rate and the sample mass. The following intermediate hydrates are produced: NiSeO 4 •5H 2O; NiSeO~'4H z O; NiSeO 4. 2H 2O; NiSeO4- H 20 and NiSeO4.0.5H 20. The enthal- pies of dehydration of these stages have been determined. The enthalpies of formation of NiSeOg.6H 2O; NiSeO 4. 4H 20 and NiSeO 4' H 2O (amorphous and crystalline) as well as the enthalpy of transformation of amorphous NiSeO 4. H 20 into crystalline NiSeO4"H20 have been calculated from DSC data, The lattice parameters of NiSeO4"4H20 and NiSeOg.2H20 have been calculated. NiSeO4.4H20 crystallizes in the monoclinic system with lattice parameters: a = 5.959(3) ~; b = 13.771 (8) ~; c = 8.023 (5) ~;/3 = 90.66(6)°; V = 658.3 (5)/~3; SG P21/n. NiSeO4.2H 20 forms orthorhom- bic crystals with lattice parameters: a = 10.351 (4) ,~; b = 10.219(4) ,~; c = 9.017(5) ~; V = 953.9(6) ~3. Keywords: Dehydration; DSC; DTA; Heat of dehydration; Heat of formation; Lattice parameters; Nickel selenate hydrates 1. Introduction dehydration and decomposition of NiSeO~.6H 2 O: 300 C 390 C This paper continues our investigations of the NiSeO4.6H20 , NiSeO4.H2 O _ , thermal dehydration of divalent metal selenate hy- drates using TG, DTA and DSC as well as a X-ray 51o c 690 c powder diffraction at elevated temperatures [1,2,3]. NiSeO¢ , NiSeO 3 ~ NiO We report results on the thermal dehydration of some nickel selenate hydrates: NiSeO4.6H20; According to the above scheme no intermediate NiSeO4.4H20 and NiSeO4-H20. hydrates (for example, penta-, tetra- or dihydrates, Nabar et al. [4], using TG, DTG and DTA, some of which are known in the literature) were proposed the following mechanism for the thermal found. In our opinion, the transformation of NiSeO4-6H20 direct into NiSeO4.H20 is due to ..... the experimental conditions: i.e. the sample mass * Corresponding author, and heating rate. 0040-6031/96/$15.00 Copyright (c, 1996 Elsevier Science B.V. All rights reserved PII S0040-6031(96)03062-6