Ratikanta Mishra

Thermodynamic stability of barium thorate, BaThO3, from a Knudsen effusion study

Abstract The Gibbs energy of formation of barium thorate was determined using the Knudsen effusion forward collection technique. The evaporation process could be represented by the equation BaThO 3 (s)=ThO 2 (s)+BaO(g) The vapour pressure of BaO(g) over the two-phase mixture of BaThO3(s) and ThO2(s) was obtained from the rate of effusion of BaO(g) and could be represented as ln (p/ Pa ) (±0.39)=−50526.5/T/ K +26.95 (1770≤T/ K ≤2136) The Gibbs energy of formation of BaThO3(s) could be derived from this data and represented as Δ f G°( BaThO 3 (s) )/ kJ mol −1 ±8.0=−1801.75+0.276T/ K

Partial phase diagram of CaO–TeO2 system

The partial phase diagram of BaO-TeO2 system was experimentally determined in the composition range 50-100 mol% TeO2 employing differential thermal analysis and x-ray diffraction techniques. Only three intermediate compounds BaTeO3, BaTe2O5 and BaTe4O9 melting congruently at 1000, 658 and 599 °C respectively, were observed. Each of these three line compounds exhibited two crystallographic phase transitions with corresponding transition temperatures at 802 and 982 °C, 608 and 648 °C, 576 and 591 °C respectively. The system exhibited three eutectic reactions i.e. between BaTeO3 and BaTe2O5 at 602 °C and 60 mol% TeO2 [L = BaTeO3(s) + BaTe2O5(s)], BaTe2O5 and BaTe4O9 at 596 °C and 76 mol% TeO2 [L = BaTe2O5(s) + BaTe4O9(s)] and another one between BaTe4O9 and TeO2 at 592 °C and 84.5 mol% TeO2 [L = BaTe4O9(s) + TeO2(s)].

The Zintl Phase Eu2Si

The Zintl phase Eu2Si was synthesized from elemental europium and silicon in a sealed tantalum tube in a high-frequency furnace at 1270 K and subsequent annealing at 970 K. Investigation of the sample by X-ray powder and single crystal techniques revealed: Co2Si (anti-PbCl2) type, space group Pnma, a = 783.0(1), b = 504.71(9), c = 937.8(1) pm, wR2 = 0.1193, 459 F2 values and 20 variables. The structure contains two europium and one silicon site. 151Eu Mossbauer spectroscopic data show a single signal at an isomer shift of −9.63(3) mm/s, compatible with divalent europium. Within the Zintl concept electron counting can be written as (2Eu2+)4+Si4−, in agreement with the absence of Si-Si bonding. Each silicon atom has nine europium neighbors in the form of a tri-capped trigonal prism. The silicon coordinations of the Zintl phases Eu2Si, Eu5Si3, EuSi, and EuSi2 are compared. Die Zintl-Phase Eu2Si Die Zintl-Phase Eu2Si wurde aus elementarem Europium und Silicium in einer verschweisten Tantalampulle im Hochfrequenzofen bei 1270 K synthetisiert und anschliesend bei 970 K getempert. Untersuchungen der Probe mit Rontgen-Pulver- und Einkristalldaten ergab: Co2Si (anti-PbCl2)-Typ, Raumgruppe Pnma, a = 783, 0(1); b = 504, 71(9); c = 937, 8(1) pm; wR2 = 0, 1193; 459 F2-Werte und 20 variable Parameter. Die Struktur enthalt zwei kristallographisch unterschiedliche Europiumplatze. 151Eu Mossbauer-spektroskopische Daten zeigen nur ein Signal bei einer Isomerieverschiebung von −9.63(3) mm/s, was mit zweiwertigem Europium kompatibel ist. Im Rahmen des Zintl-Konzepts kann die Formel als (2Eu2+)4+Si4− geschrieben werden, in Ubereinstimmung mit der Abwesenheit von Si—Si-Bindungen. Jedes Siliciumatom hat neun Europiumnachbarn in Form eines dreifach uberkappten trigonalen Prismas. Die Siliciumkoordination in den Zintl-Phasen Eu2Si, Eu5Si3, EuSi, und EuSi2 wird verglichen.

Determination of the Gibb's energy of formation of CaTeO3 and CaTe2O5 by the transpiration technique

Abstract The vapour pressure of two compounds, 〈CaTeO 3 〉 and 〈CaTe 2 O 5 〉, in the pseudo-binary CaOTeO 2 system was measured employing the microthermogravimetric transpiration assembly built in our laboratory. Both compounds vaporized incongruently, giving TeO 2 vapour according to the reactions 〈CaTeO 3 〉 = 〈CaO〉 + (TeO 2 ) and 〈CaTe 2 O 5 〉 = 〈CaTeO 3 〉 + (TeO 2 ) respectively. The vapour pressure of (TeO 2 ) above the mixtures of 〈CaTeO 3 + CaO〉 was measured in the temperature range 1169 to 1247 K. The Gibbs energy of formation of CaTeO 3 derived from the vapour pressure data could be expressed as a function of temperature by the equation Δ f G° 〈 CaTeO 3 〉 (±12.83 kJ mol −1 ) = − 975.08 + 0.242T (1169 T K CaTe 2 O 5 exhibited the reversible crystallographic phase transition at 1077±1 K as recorded by DTA. The Gibbs energy of formation of CaTe 2 O 5 derived from its vapour pressure measured below and above this phase transition could be expressed as a function of temperature in terms of the following equations: Δ f G° 〈 CaTe 2 O 5 〉 (±13.11 kJ mol −1 ) = − 1357.3 + 0.464T (1007 T K Δ f G° 〈 CaTe 2 O 5 〉 (±13.26 kJ mol −1 ) = − 1313.9 + 0.423T (1082 T K The phase transition temperature T tr and the enthalpy of transition ( ΔH ° T tr ) deduced from these equations were found to be 1048±30 K and 43.4±10 kJ mol −1 respectively.

Vaporization behaviour and Gibbs' energy of formation of UTeO5 and UTe3O9 by transpiration

Abstract Vapour pressures of UTeO5 and UTe3O9 were determined by a transpiration technique employing their incongruent vaporization represented by the reactions: 3〈 UTeO 5 〉→〈 U 3 O 8 〉+1/2( O 2 )+3( TeO 2 ) and 〈 UTe 3 O 9 〉→〈 UTeO 5 〉+2( TeO 2 ). Standard Gibbs energies of formation of UTeO5 and UTe3O9 were derived from the vapour pressures of TeO2 measured in the temperature ranges of 1107–1217 and 947–1011 K, respectively, and could be represented by the equations Δ f G°〈 UTeO 5 〉/ kJ/mol (±0.72)=−1614.17+0.450 T (1107⩽ T/K ⩽1217) and Δ f G°〈 UTe 3 O 9 〉/ kJ/mol (±1.31)=−2313.12+0.868 T (947⩽ T/K ⩽1011)

Two‐ and Three‐Dimensional [IrSi] Networks in the Silicides Sm3Ir2Si2, HoIrSi, and YbIrSi

New intermetallic rare earth iridium silicides Sm3Ir2Si2, HoIrSi, and YbIrSi were synthesized by reaction of the elements in sealed tantalum tubes in a high-frequency furnace. The compounds were investigated by X-ray diffraction both on powders and single crystals. HoIrSi and YbIrSi crystallize in a TiNiSi type structure, space group Pnma: a = 677.1(1), b = 417.37(6), c = 745.1(1) pm, wR2 = 0.0930, 340 F2 values for HoIrSi, and a = 667.2(2), b = 414.16(8), c = 742.8(2) pm, wR2 = 0.0370, 262 F2 values for YbIrSi with 20 parameters for each refinement. The iridium and silicon atoms build a three-dimensional [IrSi] network in which the holmium(ytterbium) atoms are located in distorted hexagonal channels. Short Ir–Si distances (246–256 pm in YbIrSi) are indicative for strong Ir–Si bonding. Sm3Ir2Si2 crystallizes in a site occupancy variant of the W3CoB3 type: Cmcm, a = 409.69(2), b = 1059.32(7), c = 1327.53(8) pm, wR2 = 0.0995, 383 F2 values and 27 variables. The Ir1, Ir2, and Si atoms occupy the Co, B2, and B1 positions of W3CoB3, leading to eight-membered Ir4Si4 rings within the puckered two-dimensional [IrSi] network. The Ir–Si distances range from 245 to 251 pm. The [IrSi] networks are separated by the samarium atoms. Chemical bonding in HoIrSi, YbIrSi, and Sm3Ir2Si2 is briefly discussed. Zwei- und Dreidimensionale [IrSi]-Netzwerke in den Siliciden Sm3Ir2Si2, HoIrSi und YbIrSi Die neuen Seltenerd-Iridium-Silicide Sm3Ir2Si2, HoIrSi und YbIrSi wurden durch Reaktionen der Elemente in verschlossenen Tantalrohren in einem Hochfrequenzofen synthetisiert. Die Verbindungen wurden uber Rontgenbeugungsexperimente an Pulvern und Einkristallen charakterisiert. HoIrSi und YbIrSi kristallisieren in der TiNiSi-Struktur, Raumgruppe Pnma: a = 677,1(1); b = 417,37(6); c = 745,1(1) pm; wR2 = 0,0930; 340 F2-Werte fur HoIrSi und a = 667,2(2); b = 414,16(8); c = 742,8(2) pm; wR2 = 0,0370; 262 F2-Werte fur YbIrSi mit 20 Variablen je Verfeinerung. Die Iridium- und Siliciumatome bilden ein dreidimensionales [IrSi]-Netzwerk in dem die Holmium (Ytterbium)atome verzerrte hexagonale Kanale besetzen. Kurze Ir–Si-Abstande (246–256 pm in YbIrSi) weisen auf starke Ir–Si-Wechselwirkungen hin. Sm3Ir2Si2 kristallisiert in einer Besetzungsvariante des W3CoB3-Typs: Cmcm; a = 409,69(2); b = 1059,32(7); c = 1327,53(8) pm; wR2 = 0,0995; 383 F2-Werte und 27 Variable. Die Ir1-, Ir2- und Si-Atome besetzen die Co-, B2- und B1-Positionen der W3CoB3-Struktur, was zu Ir4Si4-Achterringen innerhalb des gewellten, zweidimensionalen [IrSi]-Netzwerkes fuhrt. Die Ir–Si-Abstande variieren von 245 bis 251 pm. Die [IrSi]-Netze werden durch die Samariumatome separiert. Die chemische Bindung in Sm3Ir2Si2, HoIrSi und YbIrSi wird kurz diskutiert.

X-ray powder diffraction investigation of new high temperature polymorphs of CaTeO 3 and CaTe 2 O 5

X-ray powder diffraction investigation of the new high temperature polymorphs beta- and gamma-CaTeO 3 and gamma- and delta-CaTe 2 O 5 and picnometric measurements of the room temperature phases of the two compounds have been carried out. The study led to the elucidation of their unit cell structures and assignment of entirely new lattice types and parameters to the room temperature phases of CaTeO 3 and CaTe 2 O 5 in contrast and supersession to the existing structural information. The results are as follows: CaTeO 3 has only one stable phase at room temperature and temperatures up to 882 °C, i.e., α- and has a triclinic unit cell with a =4.132±0.003 A, b =6.120±0.006 A, c =12.836±0.013 A, α=121.80°, β=99.72°, γ=97.26°. The first high temperature phase stable between 882 and 894 °C, i.e., β-CaTeO 3 , has a monoclinic lattice: a =20.577±0.007 A, b =21.857±0.009 A, c =4.111±0.002 A, β=96.15°, while the next phase stable above 894 °C, i.e., γ-CaTeO 3 , has a hexagonal unit cell with parameters: a =14.015±0.0001 A, c =9.783±0.001 A, c/a =0.698. CaTe 2 O 5 has one stable phase at temperatures up to 802 °C, i.e., α-CaTe 2 O 5 with a monoclinic lattice and parameters: a =9.069±0.002 A, b =25.175±0.007 A, c =3.366±0.001 A, β=98.29 °. The first high temperature phase stable in the range 802–845°, i.e., β-CaTe 2 O 5 , is monoclinic with unit cell parameters: a =4.146±0.001 A, b =5.334±0.002 A, c =6.105±0.002 A, β=98.362 °; the next higher temperature phase stable over 845–857 °C, i.e., γ-CaTe 2 O 5 , has an orthorhombic unit cell with: a =8.638±0.001 A, b =9.291±0.001 A, c =7.862±0.001 A and the highest temperature solid phase stable above 857 °C, i.e., δ-CaTe 2 O 5 has a tetragonal unit cell with a =5.764±0.000 A, c =32.074±0.020 A, c/a =5.5637.

Thermodynamic stability of Cs2ZrO3 by Knudsen effusion technique

Abstract Thermodynamic stability of cesium zirconate was determined by measuring the vapour pressure of Cs2O using Knudsen effusion forward collection technique. Cs2ZrO3(s) vaporized incongruently according to the reaction Cs 2 ZrO 3 (s)= ZrO 2 (s)+ Cs 2 O (g) The Gibbs energy of formation of Cs2ZrO3 obtained from the vapour pressure of Cs2O and other auxiliary data could be given by the equation Δ f G° ( Cs 2 ZrO 3 , s) (±18.0 kJ/mol )=−1671.6+0.440T (1142≤T/K≤1273)

A thermoanalytical study of solid state reactions between tellurium oxide and the oxides of zirconium and hafnium

Abstract The solid state reactions of tellurium oxide with zirconium and hafnium oxides were investigated employing thermogravimetry (TG), differential thermal analysis (DTA) and X-ray diffraction (XRD) techniques. The only compounds reported in these two pseudo-binary systems are ZrTe3O8 and HfTe3O8, respectively. From the results obtained in these studies it is concluded that these compounds can be synthesized in the solid state in the pure form from their binary component oxides by heating the stoichiometric mixtures to temperatures as low as 900 to 950K where the vaporization of the more volatile component TeO2, is insignificant.

The standard molar enthalpy of formation of HfTe3O8

Abstract The molar enthalpies of solution of HfTe 3 O 8 , TeO 2 and HfF 4 in 10 mol dm −3 HF(aq) have been measured using an isoperibol-type calorimeter. From these results and other auxiliary data, the standard molar enthalpy of formation of HfTe 3 O 8 (s) has been calculated to be Δ f H m 0 (298.15 K) = −2129.1 ± 10.1 kJ mol −1 . This value of enthalpy of formation of HfTe 3 O 8 is consistent with the free energy of formation of HfTe 3 O 8 determined in this laboratory by the transpiration technique.