ABSTRACT

A calorimetric and thermodynamic investigation of two alkali-metal uranyl molybdates with general composition A2[(UO2)2(MoO4)O2], where A = K and Rb, was performed. Both phases were synthesized by solid-state sintering of a mixture of potassium or rubidium nitrate, molybdenum (VI) oxide and gamma-uranium (VI) oxide at high temperatures. The synthetic products were characterised by X-ray powder diffraction and X-ray fluorescence methods. The enthalpy of formation of K2[(UO2)2(MoO4)O2] was determined using HF-solution calorimetry giving DfH (T = 298 K, K2[(UO2)2(MoO4)O2], cr) = (4018 +- 8) kJ mol 1. The low-temperature heat capacity, Cp , was measured using adiabatic calorimetry from T = (7 to 335) K for K2[(UO2)2(MoO4)O2] and from T = (7 to 326) K for Rb2[(UO2)2(MoO4)O2]. Using these Cp values, the third law entropy at T = 298.15 K, S , is calculated as (374 +- 1) J K 1 mol 1 for K2[(UO2)2(MoO4)O2] and (390 +- 1) J K 1 mol 1 for Rb2[(UO2)2(MoO4)O2]. These new experimental results, together with literature data, are used to calculate the Gibbs energy of formation, DfG , for both phases giving: DfG (T = 298 K, K2[(UO2)2(MoO4)O2], cr) = ( 3747 +- 8) kJ mol 1 and DfG (T = 298 K, Rb2[(UO2)2(MoO4)], cr) = 3736 +- 5 kJ mol 1. Smoothed Cp (T) values between 0 K and 320 K are presented, along with values for S and the functions [H (T) H (0)] and [G (T) H (0)], for both phases. The stability behaviour of various solid phases and solution complexes in the (K2MoO4 + UO3 + H2O) system with and without CO2 at T = 298 K was investigated by thermodynamic model calculations using the Gibbs energy minimisation approach.

Compounds

#	Formula	Name
1	Mo	molybdenum
2	K	potassium
3	U	uranium
4	O2	oxygen
5	K2MoO10U2	molybdenum potassium uranium oxide (MoK2U2O10)
6	MoO10Rb2U2	molybdenum rubidium uranium oxide (MoRb2U2O10)

Datasets

The table above is generated from the ThermoML associated json file (link above). POMD and RXND refer to PureOrMixture and Reaction Datasets. The compound numbers are included in properties, variables, and phases, if specificied; the numbers refer to the table of compounds on the left.
Type	Compound-#	Property	Variable	Constraint	Phase	Method	#Points
POMD	5	Molar heat capacity at constant pressure, J/K/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	18
POMD	5	Molar enthalpy, kJ/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	18
POMD	5	Molar entropy, J/K/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	18
POMD	5	Molar heat capacity at constant pressure, J/K/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	128
POMD	6	Molar heat capacity at constant pressure, J/K/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	18
POMD	6	Molar enthalpy, kJ/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	18
POMD	6	Molar entropy, J/K/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	18
POMD	6	Molar heat capacity at constant pressure, J/K/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	115
RXND	1 2 3 5 4	Molar enthalpy of reaction, kJ/mol				Solution calorimetry	1

Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

A calorimetric and thermodynamic investigation of A2[(UO2)2(MoO4)O2] compounds with A = K and Rb and calculated phase relations in the system (K2MoO4 + UO3 + H2O)

Lelet, M. I.[Maxim I.], Suleimanov, E. V.[Evgeny V.], Golubev, A. V.[Aleksey V.], Geiger, C. A.[Charles A.], Bosbach, D.[Dirk], Alekseev, E. V.[Evgeny V.]

ABSTRACT