A study on the thermal conversion of scheelite-type ABO4 into perovskite-type AB(O,N)3

Article abstract of DOI:10.1039/c5dt00711a

Phase-pure scheelite AMoO₄ and AWO₄ (A = Ba, Sr, Ca) were thermally treated under an ammonia atmosphere at 400 to 900°C. SrMoO₄ and SrWO₄ were shown to convert into cubic perovskite SrMoO₂N and SrWO_1.5N_1.5, at 700°C and 900°C respectively, and to form metastable intermediate phases (scheelite SrMoO_4-xN_x and SrWO_4-xN_x), as revealed by X-ray diffraction (XRD), elemental analysis and FTIR spectroscopy. High-temperature oxide melt solution calorimetry reveals that the enthalpy of formation for SrM(O,N)₃ (M = Mo, W) perovskites is less negative than that of the corresponding scheelite oxides, though the conversion of the scheelite oxides into perovskite oxynitrides is thermodynamically favorable at moderate temperatures. The reaction of BaMO₄ with ammonia leads to the formation of rhombohedral Ba₃M₂(O,N)₈ and the corresponding binary metal nitrides Mo₃N₂ and W_4.6N₄; similar behavior was observed for CaMO₄, which converted upon ammonolysis into individual oxides and nitrides. Thus, BaMO₄ and CaMO₄ were shown to not provide access to perovskite oxynitrides. The influence of the starting scheelite oxide precursor, the structure distortion and the degree of covalency of the B-site-N bond are discussed within the context of the formability of perovskite oxynitrides.

Full text of DOI:10.1039/c5dt00711a

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DOI: 10.1039/C5DT00711A
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ARTICLE TYPE
Study on the thermal conversion of scheelite-type ABO4 into perovskite-
type AB(O,N)3
Wenjie Li,a Duan Li,b Xin Gao,b Aleksander Gurlo,c Stefan Zander,e Philip Jones,d Alexandra
Navrotsky,d Zhijian Shen,b Ralf Riedel,a and Emanuel Ionescu*, a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Phase-pure scheelite AMoO4 and AWO4 (A=Ba, Sr, Ca) were thermally treated in ammonia atmosphere at 400 to 900 °C. SrMoO4
and SrWO4 were shown to convert into cubic perovskite SrMoO2N and SrWO1.5N1.5, at 700 °C and 900 °C respectively, and to
form metastable intermediate phases (scheelite SrMoO4-xNx and SrWO4-xNx), as revealed by X-ray diffraction (XRD), elemental
10 analysis and FTIR spectroscopy. High-temperature oxide melt solution calorimetry reveals that the enthalpy of formation for
SrM(O,N)3 (M = Mo, W) perovskites is less negative than that of the corresponding scheelite oxides, though the conversion of the
scheelite oxides into perovskite oxynitrides is thermodynamically favorable at moderate temperatures. The reaction of BaMO4 with
ammonia leads to the formation of rhombohedral Ba3M2(O,N)8 and corresponding binary metal nitrides Mo3N2 and W4.6N4, Similar
behavior was observed for CaMO4, which converted upon ammonolysis into individual oxides and nitrides. Thus, BaMO4 and
15 CaMO4 were shown to not provide access to perovskite oxynitrides. The influence of the starting scheelite oxide precursor, the
structure distortion and the degree of covalency of the B-site-N bond are discussed within the context of the formability of
perovskite oxynitrides.
According to our previous work, only a limited number of
perovskite-type oxynitrides are formable.9 For instance,
SrMoO2N, SrWO2N, CaMoO2N and CaWO2N appear to be
40 feasible; while, BaMoO2N and BaWO2N are not stable in the
1 INTRODUCTION
Perovskite oxynitrides AB(O,N)3 are typically synthesized via
20 ammonolysis of oxide precursors; thus they can be formally
perovskite-type structure. Although perovskite-type SrMo(O,N)3,
2
represented as nitrogen-substituted perovskite-type oxides1,
,
6, 10-14
SrW(O,N)3 and CaMo(O,N)3
have been reported in the
which exhibit an emerging class of materials suitable for novel
applications in the fields of energy conversion, storage, non-toxic
pigments, dielectrics, etc. 3.
literature (consistent with our prediction based on tolerance and
octahedral factors)9, details of the structure evolution of the
45 oxides into perovskite-type oxynitrides are scarce. Furthermore,
25 Most perovskite-type oxynitrides are synthesized via conversion
of scheelite-type ABO4 and pyrochlore-type A2B2O7 upon
thermal treatment in ammonia atmosphere. However, not all
scheelite- and pyrochlore-type oxides are able to access
perovskite oxynitrides. For example, pyrochlore-type La2Zr2O7 as
30 well as scheelite-type EuMO4 (M = Nb and Ta) and SrMoO4
provide access to perovskite-type LaZrO2N4 as well as EuMO2N5
and SrMoO2N,6 respectively; whereas other precursor oxides
such as scheelite-type ATaO4 (A = Nd, Sm, Gd, Dy) and A2W2O9
(A = Pr, Nd, Sm, Gd, Dy) convert upon ammonolysis into
35 pyrochlore-type A2Ta2O5N2,7 and scheelite-type AWO3N,8
respectively.
the existence of perovskite-type BaMo(O,N)3 is questionable11, 12
;
whereas, perovskite-type BaW(O,N)3 and CaW(O,N)3 have not
yet been synthesized.
2 EXPERIMENTAL METHODS
50 2.1 Synthesis
Scheelite-type oxide precursors (i.e., SrMoO4, SrWO4, BaMoO4,
BaWO4, CaMoO4 and CaWO4) were synthesized via
solvothermal methods. Thus, Sr(NO3)2 (Sigma-Aldrich, ˃99.0%),
Ba(NO3)2 (Sigma-Aldrich, ˃99.0%) or Ca(NO3)2▪4H2O (Sigma-
55 Aldrich, ˃99.0%), was mixed in an equimolar ratio with
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Na2MoO4▪4H2O (Sigma-Aldrich, ˃99.5%) or Na2WO4▪4H2O
were performed using a neutron wavelength of λ=1.494 Å at
(Sigma-Aldrich, ˃99.5%) in ethylenediamine (FLUKA, ˃ 99.5 45 SINQ and λ=1.308 Å at HZB. Crystallographic parameters were
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%) under vigorous stirring. Subsequently, the reaction mixture
was transferred into an autoclave with Teflon lining and heated at
200 ⁰C for 24 h. The resulting mixture was rinsed 5 times with
deionized water and ethanol alternately. Centrifugation and
drying at 60 ⁰C overnight led to the powdered scheelite-type
oxides.
confirmed by the individual Rietveld refinemDenOtIs: 1o0f.1t0h3e9X/CR5DDTa0n0d711A
ND patterns. The peak shapes were modeled with pseudo-Voigt
function for XRD and Thompson-Cox-Hastings pseudo-Voigt
function27 for ND patterns. Isotropic thermal parameters of O/N
50 were constrained to the same value for the anions. All
refinements were performed with the Fullprof software28.
5
The resulting oxides were ground to fine powders (grain size
3 RESULTS AND DISCUSSION
10 < 500 nm) and placed in a silica crucible. The thermal treatments
were carried out in flowing ammonia at temperatures between
400 and 900 ⁰C for 4 – 24 h. The Schlenk system used for
thermal ammonolysis is specifically limited to small batch sizes
(about 0.3 – 0.5 g) to maximize exposure to flowing NH3 and
15 thus the product homogeneity.
3.1 Ammonolysis of scheelite-type oxides
BaMoO4 and BaWO4. The ammonolysis of the scheelite-type
55 BaMoO4 was performed at 600, 700 and 900 ⁰C for 6 h. The
sample treated at 600 ⁰C already formed small amounts of
Ba3Mo2(O,N)8 oxynitride phase (structure identical to
2.2 Sample characterization
31
Ba3Mo2N6N2 29, 30), as shown in Figure S1. At 700 °C, Mo3N2
and BaMoO3 32 were observed. Up to 900 °C, only small amounts
60 of Mo3N2 were detected besides the main phase Ba3Mo2(O,N)8.
The absence of the perovskite-type BaMo(O,N)3 is consistent
The crystalline phase composition of the as-synthesized samples
was analyzed by using powder X-ray diffraction (XRD, STOE
STADI P) with Mo Kα radiation (wavelength 0.7093 Å). The
20 oxygen and nitrogen contents of the synthesized samples were
determined by hot gas extraction using a LECO TC436 analyzer.
Fourier Transform Infrared (FT-IR) spectroscopy was performed
12
with the experimental work of Liu et al. and our previously
prediction9. The crystallographic data and phase compositions of
the samples obtained at 700 and 900 ⁰C were analyzed by
on a Varian 670-IR Spectrometer. Thermogravimetric analysis 65 Rietveld refinement (Figure S2 a and b). The refined lattice
(TGA 92, SETARAM) under ammonia atmosphere was done to
25 obtain the weight change of samples. High resolution
transmission electron microscope (HRTEM, JEOL JEM-2100F)
was used to assess the morphology and the local crystallinity of
the samples.
parameter of BaMoO3 was 4.0489 (6) Å, which is similar to
reported values11, 33. No cubic perovskite BaMo(O,N)3 formed.
The lattice parameter of the rhombohedral Ba3Mo2(O,N)8 were
5.9670 (3) and 21.4812 (10) Å (Table S1); these values are
70 smaller than those of Ba3Mo2O6N2 (5.9706 (5) and 21.5020 (6)
Å)34 probably because of the lower nitrogen content in our as-
synthesized oxynitride (however, we balanced the Equations (1) –
(3) based on Ba3Mo2O6N2 and Ba3W2O6N2).
High temperature oxidative -solution calorimetry was used to
30 determine the enthalpies of formation of the prepared oxynitride
samples. This method is well developed15-18 and has been applied
previously to study nitrides19-21 and oxynitrides22-24. Using this
A noticeable reaction between BaWO4 and NH3 occurs at 700
technique, ~ 5 mg pellets, made by pressing the powders into a 75 ⁰C (Figure S3). Compared to BaMoO4, BaWO4 seems to be
die with a diameter of 1mm, were dropped from room
35 temperature into molten sodium molybdate (3Na2O▪4MoO3)
rather more inert against ammonia, thus more than 50 wt% of
BaWO4 still remained after ammonolysis at temperatures up to
850 ⁰C (Table S2). Hence, the ammonolysis of BaWO4 at 700
solvent at 701 °C in
microcalorimeter15, 17
a custom made Tian-Calvet twin
.
and 850 °C leads to
a mixture consisting of BaWO4,
Neutron diffraction (ND) experiments were performed at the high
resolution powder diffractometer for thermal neutrons (HRPT)25
40 located at the Swiss Spallation Neutron Source (SINQ) of the
Paul Scherrer Institute in Switzerland and the Fine Resolution
Powder Diffractometer (FIREPOD, E9)26 at the BERII of the
Helmholtz-Zentrum Berlin (HZB), Germany. The measurements
80 Ba3W2(O,N)8 and W4.6N4. The lattice parameters of Ba3W2(O,N)8
and W4.6N4 assessed by Rietveld refinement of the XRD patterns
(Figure S4) are close to those reported [34] and [29] (see also the
Supporting Information, Table S2 and S4). As we recently
predicted 9, the perovskite-type BaWO2N cannot be formed.
2
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The samples obtained upon ammonolysis of BaMoO4 and 45 Thus, the O/N ratio decreases only slightly as the annealing time
BaWO4 were also investigated by FTIR spectroscopy; both show was extended from 4 h (SrWO1.50(6)N1.50(6); O/N ratiVoie1w.0Ar8ti)c,letoOn1lin2e
an absorption band around 975 cm-1 for oxynitride (Figure S5), h (SrWO1.42(2)N1.58(2); O/N 0.98) and 24DhOI:(1S0r.W10O391/.3C95(2D)NT01.0617(121)A
which was assigned to a stretching mode (ν(M-N)) in (WO3N)3-
;
/
O/N 0.86), respectively. Interestingly, the Sr,W-based system can
accommodate more nitrogen than its analogous Sr,Mo-based
50 system. Nevertheless the O/N ratio still cannot be pushed down to
0.5.
5
(MoO3N)3-, having W6+ / Mo6+ in tetrahedral coordination, as
reported by Herle et al.30
Thus ammonolysis of the scheelite-type oxides BaWO4 and
BaMoO4 leads to non-perovskite oxynitrides products following
the paths proposed in the equations (1)-(3):
Figure 2 XRD patterns of SrWO4 after heating at 400, 600, 700
and 900 ⁰C for different time under an ammonia flow.
ꢋꢌꢌ °ꢍ
10 10ꢀꢁꢂꢃꢄꢅ ꢆ ꢇꢈ ꢉꢊꢇ ꢎꢏꢏꢏꢐ 3ꢀꢁꢇꢂꢃꢑꢒO, Nꢓꢈ ꢆ ꢀꢁꢂꢃꢄꢇ ꢆ
ꢇ
55
ꢂꢃꢇꢉꢑ ꢆ 19ꢊꢑꢄ ꢆ ꢔꢅ ꢉꢑ
(1)
ꢇ
CaMoO4 and CaWO4. The ammonolysis of CaMoO4 was found
to proceed in a different way, leading to the formation of CaO
and various molybdenum nitrides (including Mo2N, Mo3N2 and
MoN, depending on the temperature, time and ammonia flow)
60 (Figure S6) and consequently CaMoO4 was not considered
further as a precursor for the corresponding perovskite-type
oxynitrides.
ꢕꢌꢌ °ꢍ
9ꢀꢁꢂꢃꢄꢅ ꢆ 12ꢉꢊꢇ ꢎꢏꢏꢐ 3ꢀꢁꢇꢂꢃꢑꢒO, Nꢓꢈ ꢆ ꢂꢃꢇꢉꢑ ꢆ
18ꢊꢑꢄ ꢆ 2ꢉꢑ
(2)
(3)
ꢈꢗꢌ °ꢍ
69ꢀꢁꢖꢄꢅ ꢆ 92ꢉꢊꢇ ꢎꢏꢏꢏꢐ 23ꢀꢁꢇꢖ ꢒO, Nꢓꢈ ꢆ 5ꢖꢅ.ꢘꢉꢅ ꢆ
ꢑ
15 138ꢊꢑꢄ ꢆ 13ꢉꢑ
SrMoO4 and SrWO4. A similar ammonolysis procedure was
applied to the scheelite-type SrMoO4 and SrWO4.
Ammonolysis of CaWO4 at 900 °C for 6 h leads to complete
decomposition into Ca3WO6 and W4.6N4 and no oxynitride phase
65 was observed (Figure S6). For both CaMoO4 and CaWO4, the
corresponding perovskite oxynitrides did not formed and thus
their conversion into oxide/nitride mixture is assumed to occur as
follows:
XRD measurements confirm that the ammonolysis of SrMoO4 at
20 700 °C for 4 h leads to the formation of SrMoO2N (Figure 1). The
change of O/N ratio with annealing time in SrMoO2N was shown
to decrease from 2.3 upon annealing time of 4 h (empirical
chemical composition of the oxynitride SrMoO2.09(1)N0.91(1)) to
1.89 after 12 h (SrMoO1.96(1)N1.04(1)) and to 1.54 after 24 h of
25 ammonolysis (SrMoO1.82(1)N1.18(1)). However, the nitrogen
incorporation seems to have limitations under the used
conditions, thus nitrogen-rich compositions (e.g., SrMoON2, with
a O/N ratio of 0.5) are not accessible in this way.
ꢋꢌꢌ ꢚ
70 6ꢙꢁꢂꢃꢄꢅ ꢆ 12ꢉꢊꢇ ꢛꢜꢜꢝ 6ꢙꢁꢄ ꢆ ꢂꢃꢑꢉ ꢆ ꢂꢃꢇꢉꢑ ꢆ ꢂꢃꢉ ꢆ
18ꢊꢑꢄ ꢆ 4ꢉꢑ
ꢞ4ꢟ
ꢕꢌꢌ ꢚ
6.9ꢙꢁꢖꢄꢅ ꢆ 9.2ꢉꢊꢇ ꢛꢜꢜꢝ 11.5ꢙꢁꢇꢖꢄꢘ ꢆ ꢖꢅ.ꢘꢉꢅ ꢆ
13.8ꢊꢑꢄ ꢆ 2.6ꢉꢑ
ꢞ5ꢟ
30 Figure 1 XRD patterns of SrMoO4 after heating at 400, 600 and
700 ⁰C for different times under an ammonia flow in forming gas
(mixture of 5 vol% H2 and 95 vol% N2). Arrow indicates the
diffraction pattern of the oxynitride obtained upon ammonolysis
of SrMoO3 which was synthesized by reducing SrMoO4 under
35 ammonia flow at 700 ⁰C.
75 3.2 Intermediate oxynitride phase during the conversion of
SrMoO4 into perovskite-type SrMoO2N
An interesting phenomenon during the ammonolysis of SrMoO4
at 600 °C relates to the incorporation of 2.23 wt % nitrogen
without the formation of any new crystalline phase; thus, the
80 color of the sample changed from white to light-grayish and the
FTIR spectrum showed a new absorption band at 978 cm-1 related
The ammonolysis of scheelite-type SrWO4 at 900 ⁰C leads to the
corresponding perovskite-type oxynitride as well (Figure 2).
However, the temperature required to obtain phase-pure Sr,W-
40 based oxynitride was higher than that used for SrMoO4. The
nitrogen content of the SrW(O,N)3 increases slightly with the
increasing temperature and holding time. Moreover, the O/N ratio
in SrW(O,N)3 seems to be more constant as compared to that in
SrMo(O,N)3 and appears independent of the annealing time.
30
to (MoO3N)3- units in tetrahedral coordination (Figure 3a), as
observed also in Ba3Mo2(O,N)8. Tetra-coordinated Mo6+ in
scheelite-type SrMoO4 can be identified by FTIR spectroscopy
85 via very broad band around 822 cm-1 representing the
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