Inorganic Chemistry
Article
Ge monochromator (111), a copper ceramic X-ray tube (40 kV, 40
mA), a Linx Eye position-sensitive detector, and equipped with an
Anton Paar HTK 2000 chamber. Measurements were conducted in a
600 mbar helium atmosphere from room temperature up to 1223 K,
with 100 K heating steps. The temperature, measured with a
thermocouple, was previously calibrated using the thermal expansion
data of MgO.17 The uncertainty on the temperature is estimated to be
20 K at 1223 K.
studied oxygen-deficient double perovskites Ba2BB′O5.5 (B =
Li, Na; B′ = Mo, W, Te), Ba2NaMoO5.5 has shown the highest
conductivity. However, thermophysical properties and chem-
ical behavior of the aforementioned compound at high
temperatures have not been determined to this date. These
data are key for the prediction of the behavior of the irradiated
fuel and the phase formation in case of an accident in the SFR,
as Ba-Mo-O phases are found in the irradiated fuel and are very
likely to react with the sodium coolant.
This work reports neutron diffraction and X-ray absorption
spectroscopy (XAS) measurements on Ba2NaMoO5.5 at room
temperature, which have allowed to assess and refine the
atomic oxygen positions and vacancies in the structure
precisely. Moreover, the thermal expansion and thermal
stability of the compounds were studied using high-temper-
ature X-ray diffraction (HT-XRD), high-temperature Raman
spectroscopy, and differential scanning calorimetry (DSC).
The standard enthalpy of formation of Ba2NaMoO5.5 at
298.15 K was measured via solution calorimetry. Using these
newly determined data, the likelihood of formation of this
quaternary compound in the irradiated fuel by reaction
between the sodium coolant and {BaMoO4 + BaO} or
{BaMoO3 + BaO} fission product phases was finally assessed.
2.5. X-ray Absorption Spectroscopy (XAS). XAS data were
collected at the BM26A-DUBBLE beamline of the European
Synchrotron Radiation Facility (ESRF) in Grenoble (France).18 For
the XAS measurements, ∼20 mg of powdered samples were mixed
with boron nitride (BN), pressed into pellets and encapsulated in
Kapton foils. The aforementioned operations were performed inside
an argon-filled glovebox, where oxygen and water levels were kept
below 1 ppm. The storage ring operating conditions were 6.0 GeV
and 170−200 mA. A double crystal monochromator mounted with a
Si(111) crystal coupled to collimating and focusing Pt coated mirrors
was used. Rejection of higher harmonics was achieved by three Si
mirrors at an angle of 2 mrad, relative to the incident beam.
XANES and EXAFS spectra were collected at room temperature, in
transmission mode at the Mo K-edge. A step size of 1 eV was used in
the edge region. The energy of the edge absorption threshold position
(E0) was taken at the inflection point of the spectrum by using the
first node of the second derivative. The position of the prepeak in the
edge was selected from the first node of the first derivative. Several
acquisitions were performed on the same sample and averaged to
improve the signal-to-noise ratio. Before averaging the scans, each
spectrum was aligned using the XANES spectrum of a metallic
molybdenum reference foil measured before and after the sample
under investigation. The ATHENA software19 was used to normalize
the spectra and extract the EXAFS signal from the raw data.
2. EXPERIMENTAL SECTION
2.1. Synthesis Method. Ba2NaMoO5.5 was synthesized by
reaction between barium molybdate (BaMoO4) and stoichiometric
amounts of barium carbonate (BaCO3) (Fluka, >99%) and anhydrous
sodium carbonate Na2CO3 (Sigma−Aldrich, >99.5%). The stoichio-
metric mixture was heated under oxygen flow at 973 K for 60 h with
intermediate regrinding steps. Samples were analyzed by X-ray and
neutron diffraction at room temperature.
The starting reagent BaMoO4 was synthesized by grinding barium
carbonate BaCO3 (Fluka, >99%) and molybdenum trioxide MoO3
(Alfa Aesar, >99.5%) together in stoichiometric ratio and heating
under dry oxygen at 1073 K for 45 h with intermediate regrinding
steps. Samples were analyzed by X-ray and neutron diffraction at
room temperature (see Figure S1 in the Supporting Information).
Na2Mo2O7 was synthesized for solution calorimetry measurement
purposes by heating a stoichiometric mixture of anhydrous Na2CO3
(Sigma−Aldrich, 99.5%) and Na2MoO4 (Sigma−Aldrich, 99.5%)
under dry oxygen at 773 K for 60 h with intermediary regrinding. The
sample then was checked by XRD and DSC measurements (see
In every analysis, no secondary phase was observed. Therefore, the
purity of every compound is expected to be better than 99.5%.
2.2. Neutron Diffraction. Neutron data were collected at the
Hoger Onderwijs Reactor at TU Delft15 at the PEARL beamline. The
sample was encapsulated under an argon atmosphere in a cylindrical
vanadium can (50 mm high, 6 mm inner diameter) closed with a
Viton O-ring. The data were collected at room temperature, at a fixed
wavelength λ = 1.667 Å for 6 h and λ = 1.33 Å for 12 h over a range of
11° ≤ 2θ ≤ 158°. The Rietveld method implemented in the
Fullprof2k suite16 was used for the structural analysis.
2.3. Powder X-ray Diffraction. X-ray diffraction (XRD) data at
room temperature were collected using a PANalytical X’Pert PRO X-
ray diffractometer mounted in the Bragg−Brentano configuration with
a Cu anode (Cu Kα1, λ = 1.541 Å, Cu Kα2, λ = 1.544 Å) (0.4 mm ×
12 mm line focus, 45 kV, 40 mA) and a real-time multi strip (RTMS)
detector (X’Celerator). Diffraction patterns were obtained by step
scanning in step sizes of 0.008° (2θ) in the angle range of 10° ≤ 2θ ≤
120° with an integration time of ∼8 h. The Rietveld method
implemented in the Fullprof2k suite16 was used for the structural
analysis.
The EXAFS data were collected in this work, up to 15 Å−1, and
were Fourier-transformed using the Hanning window over the k-range
of 3.5−13.5 Å−1 (dk = 1). Curve fitting was performed based on the
standard EXAFS equation using the ARTEMIS software19 in k-, k2-,
and k3-space. Phases and amplitudes for the interatomic scattering
paths were calculated with the ab initio code FEFF8.40.20 The shift in
the threshold energy (ΔE0) was varied as a global parameter. The
2
amplitude factor S0 was fixed for all paths to 0.9. The coordination
numbers, Debye−Waller factors, and interatomic distances parame-
ters, i.e., N, σ2 and R, respectively, were allowed to vary for each shell.
2.6. High-Temperature Raman Spectroscopy. High-temper-
ature Raman spectra were recorded using a Horiba Jobin-Yvon Aramis
spectrometer equipped with a Linkam TS-1500 heating device. The
Ba2NaMoO5.5 sample was placed in a platinum crucible and inserted
in the furnace. A rate of 10 K min−1 was applied upon heating, and 5
min of stabilization time was maintained at each temperature plateau
before acquisition of the spectra. The 632.8 nm line of a He−Ne laser
was used as the excitation wavelength and focused by means of an
Olympus BX41 (magnification factor of 50×), thus delivering ∼10
mW at the sample surface. Slits and a confocal hole were set to result
in a resolution of 1 cm−1. For each spectrum, an acquisition time of 4
s was considered with an average of four scans. Before analysis, the
apparatus was calibrated with a silicon wafer, using the first-order Si
line at 520.7 cm−1. Band component analysis of the different spectra
was performed with the Jandel Peakfit software, using pseudo-Voigt
functions with the minimum number of components. Correlation
coefficients of >0.997 were systematically obtained.
2.7. Differential Scanning Calorimetry. Three-dimensional
(3D)-heat-flow DSC measurements were performed from 303 K up
to 953 K for Na2Mo2O7 and up to 1473 K for Ba2NaMoO5.5, using a
Setaram Multi HTC module of the 96 Line calorimeter. The sample
(80.7 mg for Na2Mo2O7 and 80.3 mg for Ba2NaMoO5.5) was placed in
an alumina liner and encapsulated for the calorimetric measurements
inside a stainless steel crucible that was closed with a screwed bolt to
avoid vaporization at high temperatures.21 The measurement was
done as follows: four successive heating cycles with a heating rate of 5
K min−1, and cooling rates of 5, 8, 10, and 12 K min−1. The
temperature was monitored by a series of interconnected S-types
2.4. High-Temperature X-ray Diffraction (HT-XRD). The
thermal expansion of Ba2NaMoO5.5 was investigated by HT-XRD
using a Bruker Model D8 X-ray diffractometer mounted with a curved
6121
Inorg. Chem. 2020, 59, 6120−6130