ABSTRACT

Enthalpy of formation (DH f) of single phase orthophosphate solid solutions La1-xLnxPO4 (Ln = Eu, Gd; 0 less than x less than 1) with monazite structure has been determined by high temperature oxide melt solution calorimetry. The results show a non-ideal behavior of the excess enthalpy of mixing of the solid solutions. This effect is more pronounced for the La1-xGdxPO4 solid solution series and can be attributed to the larger ion size mismatch between La and Gd. The resulting excess enthalpies of mixing given by the Margules interaction parameter of 2.5 +- 2.6 kJ mol-1 and 11.4 +- 3.1 kJ mol-1 for La:Eu and La:Gd cases respectively are in good agreement with data from previous modelling studies. The data are consistent with the formation of thermodynamically stable solid solution in the entire compositional range as is seen by the XRD and Raman measurements. These results are essential for validation of the thermodynamic models that are applied for characterization of the thermodynamic parameters and for the assessment of the long-term stability of monazite solid solution matrices as ceramics for immobilization of radionuclides for nuclear waste disposal.

Compounds

#	Formula	Name
1	Gd	gadolinium
2	P	phosphorus
3	O2	oxygen
4	La	lanthanum
5	Eu	europium
6	GdO4P	gadolinium orthophosphate
7	LaO4P	lanthanum orthophosphate
8	EuO4P	europium phosphate

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	6	Mass density, kg/m3 ; Crystal	Temperature, K; Crystal Pressure, kPa; Crystal		Crystal	X-ray diffraction	1
POMD	7	Mass density, kg/m3 ; Crystal	Temperature, K; Crystal Pressure, kPa; Crystal		Crystal	X-ray diffraction	1
POMD	8	Mass density, kg/m3 ; Crystal	Temperature, K; Crystal Pressure, kPa; Crystal		Crystal	X-ray diffraction	1
POMD	8 7	Excess molar enthalpy (molar enthalpy of mixing), kJ/mol ; Crystal	Mole fraction - 8; Crystal	Temperature, K; Crystal Pressure, kPa; Crystal	Crystal	Calvet calorimetry	5
POMD	8 7	Mass density, kg/m3 ; Crystal	Mole fraction - 8; Crystal	Temperature, K; Crystal Pressure, kPa; Crystal	Crystal	X-ray diffraction	9
POMD	6 7	Excess molar enthalpy (molar enthalpy of mixing), kJ/mol ; Crystal	Mole fraction - 6; Crystal	Temperature, K; Crystal Pressure, kPa; Crystal	Crystal	Calvet calorimetry	4
POMD	6 7	Mass density, kg/m3 ; Crystal	Mole fraction - 6; Crystal	Temperature, K; Crystal Pressure, kPa; Crystal	Crystal	X-ray diffraction	11
RXND	1 2 6 3	Molar enthalpy of reaction, kJ/mol				Solution calorimetry	1
RXND	4 2 7 3	Molar enthalpy of reaction, kJ/mol				Solution calorimetry	1
RXND	5 2 8 3	Molar enthalpy of reaction, kJ/mol				Solution calorimetry	1

Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

Thermochemistry of La1-xLnxPO4-monazites (Ln = Gd, Eu)

Neumeier, S.[Stefan], Kegler, P.[Philip], Arinicheva, Y.[Yulia], Shelyug, A.[Anna], Kowalski, P. M.[Piotr M.], Schreinemachers, C.[Christian], Navrotsky, A.[Alexandra], Bosbach, D.[Dirk]

ABSTRACT