Meera Keskar

Solid state reactions of CeO2, ThO2 and PuO2 with ammonium sulphate

Abstract Solid state reactions of CeO 2 , ThO 2 and PuO 2 with (NH 4 ) 2 SO 4 were investigated by chemical, IR, thermal and X-ray diffraction methods. At 250°C, CeO 2 , ThO 2 and PuO 2 formed tetrasulphate compounds i.e., (NH 4 ) 4 Ce(SO 4 ) 4 , (NH 4 ) 4 Th(SO 4 ) 4 and (NH 4 ) 4 Pu(SO 4 ) 4 , respectively. On further heating (NH 4 ) 4 Ce(SO 4 ) 4 decomposed to give (NH 4 ) 2 Ce(SO 4 ) 3 , (NH 4 )Ce(SO 4 ) 2 and Ce 2 (SO 4 ) 3 ; where, cerium undergoes change in oxidation state from +4 to +3. The decomposition of (NH 4 ) 4 M(SO 4 ) 4 (M = Th or Pu) led to the formation of (NH 4 ) 2 M(SO 4 ) 3 and M(SO 4 ) 2 . Crystal data for solid reaction products were determined from their X-ray powder diffraction data. Dissolution of the products formed at 350–360°C during the reactions of CeO 2 , ThO 2 and PuO 2 with (NH 4 ) 2 SO 4 was also investigated.

Solid state reactions of uranium oxides with ammonium sulphate

Abstract Solid state reactions of uranium oxides of O/U ratio from 2.0 to 3.0 with ammonium sulphate were studied in an attempt to understand the nature of the oxidation state of uranium in them. The two end members, UO 2 and UO 3 , react at 250°C to form (NH 4 ) 4 U(SO 4 ) 4 and (NH 4 ) 2 UO 2 (SO 4 ) 2 and at 350°C to form (NH 4 ) 2 U(SO 4 ) 3 and (NH 4 ) 2 (UO 2 ) 2 (SO 4 ) 3 , thereby retaining the oxidation state of uranium in the parent oxide. The intermediate oxides UO 2.17 , U 4 O 9 , U 3 O 7 and U 3 O 8 react at the above two temperatures to form a mixture of ammonium uranium double sulphates containing both U(IV) and U(VI) in the ratios satisfying overall O/U values in the oxides. Chemical, X-ray, thermal and spectral methods were used to identify and characterise the products.

The effect of vanadium substitution on photoluminescent properties of KSrLa(PO4)x(VO4)2−x:Eu3+ phosphors, a new variant of phosphovanadates

Eu3+ doped red emitting phosphors of formulae KSrLa1−y(PO4)x(VO4)2−x:yEu3+ (where x = 0, 1 and 2 and y = 0, 0.02, 0.05, 0.10 and 0.015) were synthesized by a high temperature solid state reaction route. The compounds were characterized by a powder XRD method, and a complete structure determination for KSrLa(PO4)(VO4) was carried out using the Rietveld refinement method. The compound crystallized in the rhombohedral system with space group (Rm). The structure of the compound shows that La3+ is occupied at two positions, namely, trigonal antiprism (six coordinated) and truncated hexagonal bipyramidal (10 coordinated) centres, with occupancy of 0.94 and 0.06, respectively. The effect of vanadium substitution on excitation and emission spectra of the compounds was investigated using time resolved photoluminescence spectroscopy (TRPLS). The lifetimes of 5D0 → 7F2 transition were determined for all the compounds, which are in agreement with emission spectroscopic results. Judd–Ofelt analysis of the compounds clearly shows that the more polarized environment around Eu3+ in the case of the vanadate and phosphovanadate in comparison to the phosphate leads to more colour purity of the emission, when excited by UV light.

Studies on the dissolution behaviour of ThO2 and (U,Th)O2 by a solid state reaction method

Summary A simple and fast method for the dissolution of sintered ThO2 and (U,Th)O2 was developed. The products formed when these oxides were fused with ammonium sulphate at various temperatures were found to be readily soluble in dilute nitric acid. Among all the products obtained at various temperatures, (NH4)2 Th (SO4)3 and (NH4)2(Ux,Th1-x)(SO4)3, obtained at 365 °C, were found highly soluble (>99%). After dissolution thorium was separated selectively by oxalate precipitation method. More than 95% of uranium was recovered from the solution by ammonium diuranate precipitation method. The products formed at different temperatures were characterised by X-ray, thermal and chemical analysis techniques.

Preparation and structure of K8Ca2U6O24, K8Sr2U6O24, and K8Ba2U6O24

Abstract K 8 Ca 2 U 6 O 24 , K 8 Sr 2 U 6 O 24 , and K 8 Ba 2 U 6 O 24 were synthesized by solid-state reactions of their respective oxides at 850°C. The structure of the compounds was derived from powder X-ray diffraction data, and structural parameters were refined by the Rietveld profile method. All the compounds are isostructural and belong to the cubic space group Im 3 m . The structure consists of uranyl octahedra and regular octahedra of oxygen atoms around Ca, Sr, or Ba atoms. Each potassium ion is surrounded by two types of oxygen atoms in 12-coordination geometry.

Phase studies in the Rb-Sr-U-O system: characterisation of new phases

The subsolidus phase relations in the Rb–Sr–U–O system were determined at 900 °C. Two new quaternary phases Rb2Sr2U4O15 and Rb8Sr2U6O24 in the Rb–Sr–U–O system were synthesized by heating the respective oxides at 900 °C in air. A pseudo-ternary phase diagram of Rb2O–SrO–UO3 was drawn using the new quaternary compounds, and various phase fields were established by X-ray powder diffraction analysis. X-ray powder diffraction (XRD) data of Rb2Sr2U4O15 were indexed by a monoclinic system with cell parameters, a=0.7875(4) nm, b=1.3199(4) nm, c=0.6667(5) nm and β=104.93(8)°, whereas XRD data of Rb8Sr2U6O24 were indexed by a cubic cell with a=0.8743(1) nm. The structure of Rb8Sr2U6O24 was derived from the powder data and structural parameters were refined by the Rietveld profile method.

Solid state reactions of UO2,ThO2 and their mixed oxides with sulphates of potassium

Abstract Solid state reactions of UO 2 ,ThO 2 and their mixed oxides (U x ,Th 1− x )O 2 , where x =0.05, 0.25 and 0.50, were carried out with KHSO 4 ,K 2 S 2 O 8 and K 2 S 2 O 7 at 400°C, 600°C and 900°C with an aim to study the formation of various products and to investigate their dissolution behaviour. The products obtained at 400°C by heating UO 2 ,ThO 2 and their mixed oxides with potassium salts were identified as K 4 U(SO 4 ) 4 ,K 4 Th(SO 4 ) 4 and K 4 (U x ,Th 1− x )(SO 4 ) 4 , respectively. The products formed at different temperatures were found to be soluble in 2 M HNO 3 or H 2 SO 4 . From the solution of the dissolved products of the mixed oxides, thorium was separated quantitatively by precipitating it as thorium oxalate. X-ray, thermal and chemical methods were used to characterise the products formed at various temperatures.

Preparation and characterization of new phases in Na-(U0.5Pu0.5)-Mo-O andNa-(ThxU1−x)-Mo-O (x = 0.5 and 0.8) systems by X-ray and thermal methods

Abstract Na2(ThxU1−x)(MoO4)3 and Na4(ThxU1−x)(MoO4)4 (x = 0.5 and 0.8) were synthesized, by reacting (ThxU1−x)(MoO4)2 with Na2MoO4 in appropriate molar ratios at 873 K in an evacuated quartz ampoule for 25 h. While Na2(U0.5Pu0.5)(MoO4)3 and Na4(U0.5Pu0.5)(MoO4)4 were synthesized, by reacting (U0.5Pu0.5)(MoO4)2 with Na2MoO4 in appropriate molar ratios at 873 K in argon atmosphere for 45 h. The crystal structure of these compounds were derived from powder XRD data in a tetragonal system (space group: I41/a) by Rietveld profile method. Thermogravimetric curves of Na2(ThxU1−x)(MoO4)3 and Na4(ThxU1−x)(MoO4)4 did not show any weight changes up to 973 K in helium atmosphere. High temperature X-ray diffraction (HTXRD) studies of Na2(ThxU1−x)(MoO4)3 and Na4(ThxU1−x)(MoO4)4 in vacuum showed positive thermal expansion in the temperature range of 298 to 873 K.

Structural studies of Na4(ThxU1−x)(MoO4)4(x = 0.5,0.8)

Na4(ThxU1−x)(MoO4)4(x = 0.5,0.8) type of compounds were synthesized by reacting (ThxU1−x)(MoO4)2 and Na2MoO4 in appropriate molar ratio at 873 K in an evacuated quartz ampoule for 25 h. The crystal structure of Na4(ThxU1−x)(MoO4)4 was refined from X-ray powder diffraction data in the tetragonal system (space group: I41/a) by Rietveld method and has super scheelite structure.