- Email: [email protected]
Vanadium-doped hafnia: elaboration and structural characterization
Solid
State Sciences
t. 1, 1999, p. 3-13
Vanadium-doped elaboration and structural C. TURQUATa, a Materiaux b Physico-Chimie
C. LEROUXa*,
hafnia: characterization
M. ROUBINb
and G. NIHOULa
Multiphases et Interfaces, EA2135, Universitt de Toulon, BP 132,83957 La Garde cedex, France des Mattriaux et Milieux Marins, EA1356, Universite de Toulon, BP 132,83957 La Garde cedex, France
(M.T.,
received
October
1, 1998; accepted
November
30, 1998.)
ABSTRACT. - Powders of HtQ doped with various amounts of vanadium were by pyrolysis of oxalic precursors at relatively low tempemturc (700’9 This pyrolysis was carried out under different atmospheres in order to insert vanadium under different oxidation states into hafhia. The various amounts of inserted vanadium into HtQ were determined by energy dispersive spectroscopy. The powders were structurally characterized by X-my diflixction, and electron diffraction. Nano particles of monoclinic I-ED2 were obtained when incorporating VSf. The solubility limit in that case was found to be less than 10 at. %. The insertion of vanadium with a lower oxidation state than 5+ led to the stabilization of the cubic phase, with a solubility limit higher than 30% at. prepared
INTRODUCTION Zirconia, Zr02, and hafhia, HfOz, are two compounds with similar physical and chemical properties. Like Z1-02, haGum oxide is a thermal polymorph compound, which can exist under three different crystalline forms: the monoclinic structure, stable up to 17OO”C, the tetragonal structure, and the cubic fluorite type structure, appearing at 2600°C [l]. Most applications require a stabilization of the cubic structure at room temperature in order to avoid the volume expansion appearing at the Solid State Sciences,
1293-2558/99/1/O
Elsevier,
Paris
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transformation monoclinic -+ tetragonal. This was already achieved by inserting trivalent ions like Y3+. Moreover, new physical or chemical properties can be obtained by doping HI?& with various elements. Extensive studies were already performed on the ionic conductivity of Ydoped haI%ia [l-6]. This paper presents the first attempts, to our knowledge, to insert vanadium under different oxidation states into hafma. We could not find any Hf-V-O phase diagram in the literature, although the existence of the finite compound HfVz07 is well-established [7]. Concerning V-doped ZrQ, most attempts up to now were about inserting V5+ into ZrOz. Let us mention the work of Sohn et al [S] and Rojas et al [9] about VzO5-ZrO2 catalysts and the extensive study of Ren et al. [lo] about the use of Vdoped ZrO2 as yellow pigments in glazes. V5’ corresponds to the maximum oxidation state for vanadium, but vanadium can be found under other oxidation states, namely V2’, V3’, and v4’, leading to stoichiometric oxides VO, V2O3, VO2, and to many non stoichiometric oxides [ 111. We inserted various amounts of vanadium into Hf02 by an oxalic complex precursor method, and the oxidation state of vanadium was changed by elaborating the powders under different atmospheres: reducing, neutral or oxidizing. We present first the synthesis of these V-doped HID2 powders. The amount of inserted vanadium into the powders was determined by energy dispersive spectroscopy, using HfV207 as a standard. The crystallographic structure of the different powders were determined by X-ray diffraction and electron diffraction patterns. The grain morphology was also studied by transmission electron microscopy. Finally, some highresolution electron microscopy experiments were done. ELABORATION The V-inserted hafhia powders were prepared by thermal decomposition of a mixture of hafbyl oxalic acid complex, H~(HM)(C~0&),nH~0 and ammonium oxodioxalato-vanadium IV, (NH&(VO(C~O&),mH20. A high purity hafnium tetrachloride (HfCh) was dissolved into distilled water. The addition of aqueous ammonia to the solution until pH-8 led to the formation of a white precipitate of hahrium hydroxide Hf(OH)4. This precipitate was washed with distilled water until the chloride ions (Cl-) were no more detected. The washed precipitate was then introduced into an oxalic acid solution and stirred until it becomes translucent i.e. the complete arrangement of the hafnyl oxalic acid complex, H,(HfO(C20~)2),nH~0, occurs [12-131. Ammonium vanadate (‘N&VO3), powdered oxalic acid (C2H204) and ammonium oxalate ((NH&C204) were dissolved in an aqueous environment (distilled water). The solution was continually stirred and TOME 1 -
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slightly heated (SOOC) until the specific blue color of ammonium oxodioxalato-vanadium IV, (NH.&(VO(CzO&),mH~O, appeared[ 141. In this complex, vanadium has an oxidation state 4+. The two precursor complexes were powdered by evaporation. The weight percentages of hatnium and vanadium in the complexes were deduced from gravimetric experiments and were found to be in agreement with the complexes chemical formulas. The two precursor complexes were then mixed in definite proportions in an aqueous environment to form a new complex: Hzcl-y)(NH4)2y(Hfi-yVyO(C204)2),tH20, with O
_
Y
CHARACTERIZATION X-ray Diffraction (XRD) experiments were performed on a SIEMENS D5000 powders diffractometer, using CuKa radiation filtered by a nickel window. Samples were placed on an aluminum holder and measurements were done between 20” and 70” 20, with steps of 0.01” and 2 seconds each step. Energy dispersive spectroscopy (EDS) was performed on an EDAX instrument using a Si(Li) detector with a thin beryllium window, which allows analysis of elements with z>l 1. A well-crystallized HfV207 powder was used as a standard and the K-factor for hafirium - vanadium was determined as described by P. Sheridan [ 151: we found a value of 3.68 for KHEN . For one given powder, analysis were performed over 12 grains. A SOLID
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beryllium holder and nickel grids were preferred to the usual holder and copper grids in order to have a better deconvolution near the HfL peaks. TEM observations were performed on a PHILIPS EM 400 T, with an accelerating voltage of 120 kV (wave length, A= 0.0354 A). The camera constant Lh was calibrated using a polycrystalline ahunhmm sample. The first information reached out by transmission electron microscopy is the shape, the size and the homogeneity of distributions of the grains using imaging mode. The second information reached is the crystallographic structure of the grains using the diffraction mode. Finally, HREM images were obtained on a JEOL 2010 FEG (wave length, h= 0.0251A). RESULTS Energy dispersive spectroscopy (EDS) EDS results are presented Table II. For one given monocrystalline grain, a EDS spectrum was recorded and the amount of vanadium versus hafnium was quantified. For each powder, this was carried out over 12 grains and the reported results correspond to the obtained mean value. For powders prepared under neutral (N-10, N-20, N-30) and reducing (R-10, R-20, R-30) atmospheres, the obtained compositions are in good agreement with the expected compositions due to the synthesis.
Table II: Atomic percentage of vanadium and ha&Gum for the different powders, as determined by EDS.
Moreover, these powders are very homogeneous in composition: the highest deviation from one grain to another is about 2%. For powders prepared under air (O-10, O-20, and O-30), there is a great discrepancy between the expected and measured compositions. For powders 0- lO, O-20 and O-30, the mean atomic percentage of vanadium, was found to be 7 at. % and no grains containing more than 8 at. % of vanadium were
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found. For all the observed powders, the grains contain both Hf and V, i.e. we did not find grains corresponding to pure vanadium oxides. X-ray diffraction
(XRD)
All over the different powders, three different structures were identified: the HIQ monoclinic structure, the HQ fluorite structure and the hafirium pyrovanadate (HfV20,) cubic structure. The unit cell parameters for the different structures were refined using a method of least squares with mean squares less than 0.0001mn. The crystallographic results deduced from XRD are presented in Table III. For comparison, cell parameters found in the literature for the pure HfQ monoclinic phase and for the pure HtQ cubic phase (without inserted cations) are given. Concerning the latter, different results can be found (see the review paper El]), ranging from 0.5 1 I mn to 0.53 mn. Monoclinic Hfioz [I] Cubic Hf& [I] a=O.5 117 mn, b=0.5 1754 nm, 0.51mnIaI0.53mn ~0.52915 nm, p=99.22’ o-5 o-10* Monoclinic Monoclinic a=O,5117Nn a=O.5114mn b=0,5139nm b=0.5164nm c=O.5293nm c=O.5289nm p99.3 1” p=99.22O N-5* N-IO* Monoclinic Cubic a=O.5 107 a=O,5066mn b=0.5 128 c=O.5272 p=99.26O I R-10 I R-20 1R-30 Cubic Cubic Cubic a=O,4984mn a=O.5047mn a=O.5027mn Table III: Crystalline systems and retined cell parameters. For two-phased powders(* only the cell parameters of the majority phase were refined.
The powders R-10, R-20, R-30, N-20, N-30 all correspond to a single phase, with the same cubic average symmetry (fluorite-type) (tig.la), but with different cell parameters, smaller than the cell parameters that can be found in the literature for the pure cubic HI& phase. The powders N-5 and N-10 exhibit a mixing of phases: HQ monoclinic and HfQ cubic (or tetragonal). As the tetragonal phase corresponds to a very small distortion of the cubic phase, the X-ray diagram of these twoSOLID
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phased powders N-5 and N-10 were interpreted in terms of monoclinic hafnia and a phase of average cubic symmetry. For N-5, the majority phase is clearly monoclinic, while for N-10, the cubic phase is predominant.
Fig 1: X-ray diffraction diagrams of V-doped hathia. Peaks labeled Al are due to the aluminum sample holder. a) Powder R-20. This X-ray diagmm is chamcteristic for a f.c.c phase. b) Powder O-10, c) Powder O-20. These X-ray diagrams am characteristic of the monoclinic Hfoz phase, with supplementary small peaks (labelled C2) corresponding to the cubic HfV207 phase. Ike inset shows an enhancement of diagram (b) at low angles. The amount of HfVz07 phase is more important in c).
The cell parameter of the fluorite-type cubic phase varies with the nature of the atmosphere decomposition complexes and with the amount of inserted vanadium. For powders prepared under a given atmosphere (reducing or neutral), the cell parameter, a, decreases when the percentage of vanadium increased. For powders with the same amount of inserted vanadium but prepared respectively under reducing and neutral atmosphere, the cell parameters are smaller for the powders prepared under reducing atmosphere. Therefore, the elaboration of V-doped hafnia under neutral or reducing atmospheres stabilizes the cubic phase of HfQ. Powders R-30 and N-30 TOME
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only contain the cubic phase, so the solubility limit of vanadium into cubic hafiria is not reached with 30 at. % of vanadium. All the powders prepared under oxidizing atmosphere (O-5, O-10, O-20, O-30) contain as the majority phase the monoclinic HtQ and the values found for the cell parameters were not different Corn those reported in the literature [l]. These powders contain also small amounts of HtVzOr. The X-ray diffiction diagram for powder O-10 shows small HfVz07 peaks (fig.lb). The X-ray diB+action diagram for powder O-20 shows very distinctly two phases: HfQ monoclinic and HfV,O, cubic (fig. lc). Therefore, the solubility limit of vanadium into monoclinic HtQ is less than 10 at. % and by adding more vanadium, one only increases the amount of HfvzOr in the powder. These results are consistent with the performed EDS analysis. Finally, no vanadium oxide peaks were noticed in the XRD diagrams. Transmission
electron microscopy
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In order to obtain information on the grains sixes and shapes, powders O-20, N-20 and R-20 were studied through TEM experiments. Powder O-20 shows a great homogeneity in grain sizes (fig. 2a). The grains, about 50nm in size, are facetted and strongly tend to aggregate increasing the difkulty to obtain monocrystalline diffraction patterns. Fig. 2b shows a diBaction pattern often encountered. One can see the presence of many spots in all directions due to the diffraction of disoriented grams, almost forming a ring pattern. This is characteristic of a polycrystalline powder. This observed pattern is similar to those encountered for ideal H.tQ monoclinic structure. However, we did not find grams with the HfVz07 cubic structure, as seen in the XRD diagram.
Fig. 2: TEM results on the powder O-20: a) hage of a grain aggregate, b) Corresponding polycrystalline SOLID
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Powders N-20 and R-20 are very similar: they are less agglomerated and exhibit bigger grain sizes than the powder O-20 (within 70nm up to 5OOmn). The grain shape is irregular (fig. 3a), but the grains often give rise to mono-crystalline difiaction patterns. Figs. 3b, 3c show typical diffraction patterns for these powders. If we consider the strong intensity spots, the diffraction patterns are explained by a f.c.c., fluorite type, structure with a cell parameter of about OSrmr, which corresponds to the cubic HtQ structure. Fig. 3b corresponds to a [ 1lo] zone axis and fig. 3c corresponds to a [-1121 zone axis. However, there are also weak spots on these diffraction patterns (see x on fig.3). Fig. 4 shows different profiles obtained from the fig. 3c.
Fig. 3: TEM results on powder R-20. a) Image of a grain aggregate. b) and c) diffraction patterns indexed in the cubic Hf& structure and corresponding to a [l lo] and to a [-1121 zone axis.
Comparison of profiles represented fig. 4a and 4b shows that the distance between two consecutive weak spots is similar to the distance between two consecutive strong spots. Thus, the weak spots belong to the same crystal than the strong spots. Moreover, fig. 4c shows that the weak spots are exactly at half the distance between two consecutive strong intensity spots. Weak spots were observed for various zone axis such as [OOl], [ill], [103], [233]. These spots are thus regular and have integer indices in the HfQ cubic lattice but correspond to planes that do not verify the extinction condition imposed by the faced centered cubic HfQ structure. TOME
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Fig. 4: Intensityprofilestakenalongdifferentdirectional of the diffractionpatternfig. 3.b a) alongtherow of spotsindexed220,31l,... i.e.-allowed reflectionsin the f.c.c. lattice b) alongtherow of spotsindexed110,201,...i.e forbiddenreflectionsin a f.c.c. lattice c) alongtherow of spotsindexed,311,201,l-11,... alternativelyallowedandforbidden reflectionsin a f.c.c. lattice
Fig. 5: HREMresultsonpowderR-20. a)imagetakenon onegrainorientedalonga [l lo] zoneaxis,showingtwo different regions.b) FFI of thelowerpartof image5.a,c) FFT of theupperpartof image5.a. SOLID
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No double diffraction phenomenon explains these weak spots. The allowed reflections for the HfQ cubic structure verify h,k,l all odd or all even (i.e. the conditions for a simple f.c.c. structure). Consequently, any combination of two allowed reflections always leads to an exiting third reflection but not to supplementary spots. So these weak spots must be related to an ordering phenomenon. Thus, powder R-20 was investigated by means of High Resolution Electron Microscopy (HREM). Fig. 5a shows an HREM image of one grain with a [llO] zone axis. In the lower part of the image only (111) type planes are imaged (see fig. 5b for the Fast Fourier Transform of this region). On the upper part of the image, the image is quite different: other planes, with interreticular distances greater than dill can be observed. The Fast Fourier Transform, (fig. 5c) of this region exhibits 001 and l-10 superstructures spots, which are the spots encountered in fig. 3b. Thus, the weak spots observed in electron diffraction patterns of the cubic V-doped Hf02 are related to small ordered domains into the grains. These domains are roughly 5 mn in size. CONCLUSION According to the XRD results, doping hafhia with vanadium under specific atmospheres can stabilize the hafnia high-temperature f.c.c. structure. This is well verified by MET with, however, the existence of some weak spots, which can be surprising with so few doping elements. However, the structure mainly remains f.c.c. HREM images showed the existence of small domains as the origin of the weak supplementary spots. The amount of vanadium inserted is not sufficient to have a vanadium ordering all over the grains, but is compatible with a distribution of small domains with a higher concentration of vanadium. The domain size is small compared to the regions analyzed by EDS, so that the composition appears homogeneous over one grain. Our results show that, for different decomposition atmospheres (i.e. for different vanadium oxidation states), the structure varies, and so does the solubility limit. About 30 at. % of vanadium were inserted into hafnia cubic lattice. So the solubility limit of vanadium in cubic hafnia is higher than 30 at. %. Moreover, these cubic structure powders were obtained under neutral or reducing atmos heres, so the oxidation state of vanadium in these powders is either vf +, as in the initial vanadium complex or inferior (V3+ or V2’). These powders are stable at room temperature under air, and are stable up to 850 “C under neutral atmosphere. As we never observed more than about 7 at. % of vanadium in the grains of monoclinic crystalline structure, the solubility limit of vanadium in the TOME 1 -1999-N”
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grains of powders synthesized under air is effectively 7 at. %. Moreover, an DTA-ATG experiment in air up to 700°C showed that these powders do not undergo any weight variations. This implies that the vanadium is already in is highest oxidation state, V”. Therefore, the solubility limit of V5’ into monoclinic ha&a is 7 at. %. In these powders, the remaining vanadium appears as HfVzO,. As an example, the percentage of HfVzOr in the powder O-20 corresponds to less than 8.5 mol. %. Such a low amount explains why we did not observe grains of this phase during TEM experiments. Further investigations concerning the effective oxidation states of vanadium in these compounds are under way, along with an exhaustive study of their stability versus temperature. Acknowledgements. - We would lie to thank the Provence Alpes Cote d’Azur region and Gagno Entreprise for their financia.l supports (CAR No981 l/2167).
REFERENCES [l] J. WANG, H. P. LI, R. STEVENS, J. Mat. Sci., 1992,27, p. 5397. [2] YU. K. VORON’KO, A. V. GORBACHEV and A. A. SOBOL’, Whys. Solid State, 1995,7,37, p.1055 [3] R. MACHAY, A. W. SLEIGHT and M. A. SUBRAMANIAN, J Solid State Chem., 1996,121,437. [4] A. IKESUE, K. AMATA and K. YOSHIDA, J. Am. Ceram. Sot., 1996,72, p.359 [5] A. WEYL AND D. JANKE,J. Am. Ceram. Sot., 1996,79, p.2145. [6] M. YASHIMA, H. TAKAHASHI, K. OHTAKE, T. HIROSE, M. KAKIHANA, H. ARASHI, Y. IKUMA, Y. SUZUKI and M. YOSHIMURA, J. Phys. Chem. SW., 1996, 57, p. 289. [7] E. J. BARAN, .J.Less Corn. Met.1976,46, p.343 [S] J. SOHN, S. CHO, Y. PAE, S. HAYASHI, J. Catal., 1996,159, p.170. [9] M. L. ROJAS-CERVANTES, A. J. LOPEZ-PEINADO, J. de D. LOPEZGONZALES, F. CARRASCO-MARIN, J. Mt. Sci., 1996,31, p.437. [lo] F. REN, S. ISHIDA and N. TAKEUCHI, J Am. Ceram. Sot., 1993,76, p. 1825. [ 1 l] T.B. MASSALSKI, “Binaq~ AZZoyPhase Diagrams”, ASM International, The Material Information Society, 1990,3, p.2930. [12] A. LAKHLIFI, P. SATRE, A. SEBAOUN, M. ROUBIN, Ann. Chim. Fr., 1993,18 p.565 [ 13 ] A. LAKHLIFI, CH. LEROUX, P. SATRE, B. DURANT, M. ROUBIN and G. NIHOUL, J Solid State Chem., 1995,119, p.289. [14] W. SCHRAMM, Z. Anorg. All. Chem, 1927, 161, p.250. [15] P. J. SHERIDAN, J. EZec. Micro. Tech., 1989,11, p.41.
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State Sciences
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Vanadium-doped elaboration and structural C. TURQUATa, a Materiaux b Physico-Chimie
C. LEROUXa*,
hafnia: characterization
M. ROUBINb
and G. NIHOULa
Multiphases et Interfaces, EA2135, Universitt de Toulon, BP 132,83957 La Garde cedex, France des Mattriaux et Milieux Marins, EA1356, Universite de Toulon, BP 132,83957 La Garde cedex, France
(M.T.,
received
October
1, 1998; accepted
November
30, 1998.)
ABSTRACT. - Powders of HtQ doped with various amounts of vanadium were by pyrolysis of oxalic precursors at relatively low tempemturc (700’9 This pyrolysis was carried out under different atmospheres in order to insert vanadium under different oxidation states into hafhia. The various amounts of inserted vanadium into HtQ were determined by energy dispersive spectroscopy. The powders were structurally characterized by X-my diflixction, and electron diffraction. Nano particles of monoclinic I-ED2 were obtained when incorporating VSf. The solubility limit in that case was found to be less than 10 at. %. The insertion of vanadium with a lower oxidation state than 5+ led to the stabilization of the cubic phase, with a solubility limit higher than 30% at. prepared
INTRODUCTION Zirconia, Zr02, and hafhia, HfOz, are two compounds with similar physical and chemical properties. Like Z1-02, haGum oxide is a thermal polymorph compound, which can exist under three different crystalline forms: the monoclinic structure, stable up to 17OO”C, the tetragonal structure, and the cubic fluorite type structure, appearing at 2600°C [l]. Most applications require a stabilization of the cubic structure at room temperature in order to avoid the volume expansion appearing at the Solid State Sciences,
1293-2558/99/1/O
Elsevier,
Paris
4
C.TURQUAT
et&.
transformation monoclinic -+ tetragonal. This was already achieved by inserting trivalent ions like Y3+. Moreover, new physical or chemical properties can be obtained by doping HI?& with various elements. Extensive studies were already performed on the ionic conductivity of Ydoped haI%ia [l-6]. This paper presents the first attempts, to our knowledge, to insert vanadium under different oxidation states into hafma. We could not find any Hf-V-O phase diagram in the literature, although the existence of the finite compound HfVz07 is well-established [7]. Concerning V-doped ZrQ, most attempts up to now were about inserting V5+ into ZrOz. Let us mention the work of Sohn et al [S] and Rojas et al [9] about VzO5-ZrO2 catalysts and the extensive study of Ren et al. [lo] about the use of Vdoped ZrO2 as yellow pigments in glazes. V5’ corresponds to the maximum oxidation state for vanadium, but vanadium can be found under other oxidation states, namely V2’, V3’, and v4’, leading to stoichiometric oxides VO, V2O3, VO2, and to many non stoichiometric oxides [ 111. We inserted various amounts of vanadium into Hf02 by an oxalic complex precursor method, and the oxidation state of vanadium was changed by elaborating the powders under different atmospheres: reducing, neutral or oxidizing. We present first the synthesis of these V-doped HID2 powders. The amount of inserted vanadium into the powders was determined by energy dispersive spectroscopy, using HfV207 as a standard. The crystallographic structure of the different powders were determined by X-ray diffraction and electron diffraction patterns. The grain morphology was also studied by transmission electron microscopy. Finally, some highresolution electron microscopy experiments were done. ELABORATION The V-inserted hafhia powders were prepared by thermal decomposition of a mixture of hafbyl oxalic acid complex, H~(HM)(C~0&),nH~0 and ammonium oxodioxalato-vanadium IV, (NH&(VO(C~O&),mH20. A high purity hafnium tetrachloride (HfCh) was dissolved into distilled water. The addition of aqueous ammonia to the solution until pH-8 led to the formation of a white precipitate of hahrium hydroxide Hf(OH)4. This precipitate was washed with distilled water until the chloride ions (Cl-) were no more detected. The washed precipitate was then introduced into an oxalic acid solution and stirred until it becomes translucent i.e. the complete arrangement of the hafnyl oxalic acid complex, H,(HfO(C20~)2),nH~0, occurs [12-131. Ammonium vanadate (‘N&VO3), powdered oxalic acid (C2H204) and ammonium oxalate ((NH&C204) were dissolved in an aqueous environment (distilled water). The solution was continually stirred and TOME 1 -
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slightly heated (SOOC) until the specific blue color of ammonium oxodioxalato-vanadium IV, (NH.&(VO(CzO&),mH~O, appeared[ 141. In this complex, vanadium has an oxidation state 4+. The two precursor complexes were powdered by evaporation. The weight percentages of hatnium and vanadium in the complexes were deduced from gravimetric experiments and were found to be in agreement with the complexes chemical formulas. The two precursor complexes were then mixed in definite proportions in an aqueous environment to form a new complex: Hzcl-y)(NH4)2y(Hfi-yVyO(C204)2),tH20, with O
_
Y
CHARACTERIZATION X-ray Diffraction (XRD) experiments were performed on a SIEMENS D5000 powders diffractometer, using CuKa radiation filtered by a nickel window. Samples were placed on an aluminum holder and measurements were done between 20” and 70” 20, with steps of 0.01” and 2 seconds each step. Energy dispersive spectroscopy (EDS) was performed on an EDAX instrument using a Si(Li) detector with a thin beryllium window, which allows analysis of elements with z>l 1. A well-crystallized HfV207 powder was used as a standard and the K-factor for hafirium - vanadium was determined as described by P. Sheridan [ 151: we found a value of 3.68 for KHEN . For one given powder, analysis were performed over 12 grains. A SOLID
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beryllium holder and nickel grids were preferred to the usual holder and copper grids in order to have a better deconvolution near the HfL peaks. TEM observations were performed on a PHILIPS EM 400 T, with an accelerating voltage of 120 kV (wave length, A= 0.0354 A). The camera constant Lh was calibrated using a polycrystalline ahunhmm sample. The first information reached out by transmission electron microscopy is the shape, the size and the homogeneity of distributions of the grains using imaging mode. The second information reached is the crystallographic structure of the grains using the diffraction mode. Finally, HREM images were obtained on a JEOL 2010 FEG (wave length, h= 0.0251A). RESULTS Energy dispersive spectroscopy (EDS) EDS results are presented Table II. For one given monocrystalline grain, a EDS spectrum was recorded and the amount of vanadium versus hafnium was quantified. For each powder, this was carried out over 12 grains and the reported results correspond to the obtained mean value. For powders prepared under neutral (N-10, N-20, N-30) and reducing (R-10, R-20, R-30) atmospheres, the obtained compositions are in good agreement with the expected compositions due to the synthesis.
Table II: Atomic percentage of vanadium and ha&Gum for the different powders, as determined by EDS.
Moreover, these powders are very homogeneous in composition: the highest deviation from one grain to another is about 2%. For powders prepared under air (O-10, O-20, and O-30), there is a great discrepancy between the expected and measured compositions. For powders 0- lO, O-20 and O-30, the mean atomic percentage of vanadium, was found to be 7 at. % and no grains containing more than 8 at. % of vanadium were
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found. For all the observed powders, the grains contain both Hf and V, i.e. we did not find grains corresponding to pure vanadium oxides. X-ray diffraction
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All over the different powders, three different structures were identified: the HIQ monoclinic structure, the HQ fluorite structure and the hafirium pyrovanadate (HfV20,) cubic structure. The unit cell parameters for the different structures were refined using a method of least squares with mean squares less than 0.0001mn. The crystallographic results deduced from XRD are presented in Table III. For comparison, cell parameters found in the literature for the pure HfQ monoclinic phase and for the pure HtQ cubic phase (without inserted cations) are given. Concerning the latter, different results can be found (see the review paper El]), ranging from 0.5 1 I mn to 0.53 mn. Monoclinic Hfioz [I] Cubic Hf& [I] a=O.5 117 mn, b=0.5 1754 nm, 0.51mnIaI0.53mn ~0.52915 nm, p=99.22’ o-5 o-10* Monoclinic Monoclinic a=O,5117Nn a=O.5114mn b=0,5139nm b=0.5164nm c=O.5293nm c=O.5289nm p99.3 1” p=99.22O N-5* N-IO* Monoclinic Cubic a=O.5 107 a=O,5066mn b=0.5 128 c=O.5272 p=99.26O I R-10 I R-20 1R-30 Cubic Cubic Cubic a=O,4984mn a=O.5047mn a=O.5027mn Table III: Crystalline systems and retined cell parameters. For two-phased powders(* only the cell parameters of the majority phase were refined.
The powders R-10, R-20, R-30, N-20, N-30 all correspond to a single phase, with the same cubic average symmetry (fluorite-type) (tig.la), but with different cell parameters, smaller than the cell parameters that can be found in the literature for the pure cubic HI& phase. The powders N-5 and N-10 exhibit a mixing of phases: HQ monoclinic and HfQ cubic (or tetragonal). As the tetragonal phase corresponds to a very small distortion of the cubic phase, the X-ray diagram of these twoSOLID
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phased powders N-5 and N-10 were interpreted in terms of monoclinic hafnia and a phase of average cubic symmetry. For N-5, the majority phase is clearly monoclinic, while for N-10, the cubic phase is predominant.
Fig 1: X-ray diffraction diagrams of V-doped hathia. Peaks labeled Al are due to the aluminum sample holder. a) Powder R-20. This X-ray diagmm is chamcteristic for a f.c.c phase. b) Powder O-10, c) Powder O-20. These X-ray diagrams am characteristic of the monoclinic Hfoz phase, with supplementary small peaks (labelled C2) corresponding to the cubic HfV207 phase. Ike inset shows an enhancement of diagram (b) at low angles. The amount of HfVz07 phase is more important in c).
The cell parameter of the fluorite-type cubic phase varies with the nature of the atmosphere decomposition complexes and with the amount of inserted vanadium. For powders prepared under a given atmosphere (reducing or neutral), the cell parameter, a, decreases when the percentage of vanadium increased. For powders with the same amount of inserted vanadium but prepared respectively under reducing and neutral atmosphere, the cell parameters are smaller for the powders prepared under reducing atmosphere. Therefore, the elaboration of V-doped hafnia under neutral or reducing atmospheres stabilizes the cubic phase of HfQ. Powders R-30 and N-30 TOME
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only contain the cubic phase, so the solubility limit of vanadium into cubic hafiria is not reached with 30 at. % of vanadium. All the powders prepared under oxidizing atmosphere (O-5, O-10, O-20, O-30) contain as the majority phase the monoclinic HtQ and the values found for the cell parameters were not different Corn those reported in the literature [l]. These powders contain also small amounts of HtVzOr. The X-ray diffiction diagram for powder O-10 shows small HfVz07 peaks (fig.lb). The X-ray diB+action diagram for powder O-20 shows very distinctly two phases: HfQ monoclinic and HfV,O, cubic (fig. lc). Therefore, the solubility limit of vanadium into monoclinic HtQ is less than 10 at. % and by adding more vanadium, one only increases the amount of HfvzOr in the powder. These results are consistent with the performed EDS analysis. Finally, no vanadium oxide peaks were noticed in the XRD diagrams. Transmission
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In order to obtain information on the grains sixes and shapes, powders O-20, N-20 and R-20 were studied through TEM experiments. Powder O-20 shows a great homogeneity in grain sizes (fig. 2a). The grains, about 50nm in size, are facetted and strongly tend to aggregate increasing the difkulty to obtain monocrystalline diffraction patterns. Fig. 2b shows a diBaction pattern often encountered. One can see the presence of many spots in all directions due to the diffraction of disoriented grams, almost forming a ring pattern. This is characteristic of a polycrystalline powder. This observed pattern is similar to those encountered for ideal H.tQ monoclinic structure. However, we did not find grams with the HfVz07 cubic structure, as seen in the XRD diagram.
Fig. 2: TEM results on the powder O-20: a) hage of a grain aggregate, b) Corresponding polycrystalline SOLID
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Powders N-20 and R-20 are very similar: they are less agglomerated and exhibit bigger grain sizes than the powder O-20 (within 70nm up to 5OOmn). The grain shape is irregular (fig. 3a), but the grains often give rise to mono-crystalline difiaction patterns. Figs. 3b, 3c show typical diffraction patterns for these powders. If we consider the strong intensity spots, the diffraction patterns are explained by a f.c.c., fluorite type, structure with a cell parameter of about OSrmr, which corresponds to the cubic HtQ structure. Fig. 3b corresponds to a [ 1lo] zone axis and fig. 3c corresponds to a [-1121 zone axis. However, there are also weak spots on these diffraction patterns (see x on fig.3). Fig. 4 shows different profiles obtained from the fig. 3c.
Fig. 3: TEM results on powder R-20. a) Image of a grain aggregate. b) and c) diffraction patterns indexed in the cubic Hf& structure and corresponding to a [l lo] and to a [-1121 zone axis.
Comparison of profiles represented fig. 4a and 4b shows that the distance between two consecutive weak spots is similar to the distance between two consecutive strong spots. Thus, the weak spots belong to the same crystal than the strong spots. Moreover, fig. 4c shows that the weak spots are exactly at half the distance between two consecutive strong intensity spots. Weak spots were observed for various zone axis such as [OOl], [ill], [103], [233]. These spots are thus regular and have integer indices in the HfQ cubic lattice but correspond to planes that do not verify the extinction condition imposed by the faced centered cubic HfQ structure. TOME
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Fig. 4: Intensityprofilestakenalongdifferentdirectional of the diffractionpatternfig. 3.b a) alongtherow of spotsindexed220,31l,... i.e.-allowed reflectionsin the f.c.c. lattice b) alongtherow of spotsindexed110,201,...i.e forbiddenreflectionsin a f.c.c. lattice c) alongtherow of spotsindexed,311,201,l-11,... alternativelyallowedandforbidden reflectionsin a f.c.c. lattice
Fig. 5: HREMresultsonpowderR-20. a)imagetakenon onegrainorientedalonga [l lo] zoneaxis,showingtwo different regions.b) FFI of thelowerpartof image5.a,c) FFT of theupperpartof image5.a. SOLID
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No double diffraction phenomenon explains these weak spots. The allowed reflections for the HfQ cubic structure verify h,k,l all odd or all even (i.e. the conditions for a simple f.c.c. structure). Consequently, any combination of two allowed reflections always leads to an exiting third reflection but not to supplementary spots. So these weak spots must be related to an ordering phenomenon. Thus, powder R-20 was investigated by means of High Resolution Electron Microscopy (HREM). Fig. 5a shows an HREM image of one grain with a [llO] zone axis. In the lower part of the image only (111) type planes are imaged (see fig. 5b for the Fast Fourier Transform of this region). On the upper part of the image, the image is quite different: other planes, with interreticular distances greater than dill can be observed. The Fast Fourier Transform, (fig. 5c) of this region exhibits 001 and l-10 superstructures spots, which are the spots encountered in fig. 3b. Thus, the weak spots observed in electron diffraction patterns of the cubic V-doped Hf02 are related to small ordered domains into the grains. These domains are roughly 5 mn in size. CONCLUSION According to the XRD results, doping hafhia with vanadium under specific atmospheres can stabilize the hafnia high-temperature f.c.c. structure. This is well verified by MET with, however, the existence of some weak spots, which can be surprising with so few doping elements. However, the structure mainly remains f.c.c. HREM images showed the existence of small domains as the origin of the weak supplementary spots. The amount of vanadium inserted is not sufficient to have a vanadium ordering all over the grains, but is compatible with a distribution of small domains with a higher concentration of vanadium. The domain size is small compared to the regions analyzed by EDS, so that the composition appears homogeneous over one grain. Our results show that, for different decomposition atmospheres (i.e. for different vanadium oxidation states), the structure varies, and so does the solubility limit. About 30 at. % of vanadium were inserted into hafnia cubic lattice. So the solubility limit of vanadium in cubic hafnia is higher than 30 at. %. Moreover, these cubic structure powders were obtained under neutral or reducing atmos heres, so the oxidation state of vanadium in these powders is either vf +, as in the initial vanadium complex or inferior (V3+ or V2’). These powders are stable at room temperature under air, and are stable up to 850 “C under neutral atmosphere. As we never observed more than about 7 at. % of vanadium in the grains of monoclinic crystalline structure, the solubility limit of vanadium in the TOME 1 -1999-N”
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grains of powders synthesized under air is effectively 7 at. %. Moreover, an DTA-ATG experiment in air up to 700°C showed that these powders do not undergo any weight variations. This implies that the vanadium is already in is highest oxidation state, V”. Therefore, the solubility limit of V5’ into monoclinic ha&a is 7 at. %. In these powders, the remaining vanadium appears as HfVzO,. As an example, the percentage of HfVzOr in the powder O-20 corresponds to less than 8.5 mol. %. Such a low amount explains why we did not observe grains of this phase during TEM experiments. Further investigations concerning the effective oxidation states of vanadium in these compounds are under way, along with an exhaustive study of their stability versus temperature. Acknowledgements. - We would lie to thank the Provence Alpes Cote d’Azur region and Gagno Entreprise for their financia.l supports (CAR No981 l/2167).
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