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High temperature powder neutron diffraction studies of structural transformations in transition metal hemicarbides M2C1−x
Physica B 156 & 157 (1989) North-Holland, Amsterdam
44-46
HIGH TEMPERATURE POWDER NEUTRON DIFFRACTION STUDIES OF STRUCTURAL TRANSFORMATIONS IN TRANSITION METAL HEMICARBIDES M,C 1_-x II. COMPOUNDS
OF THE VIth GROUP
T. EPICIER’, IG.EM.P. P. M.,
J. DUBOIS’,
(Mo,C,_,
AND W,C,_J
C. ESNOUF’,
G. FANTOZZI’
~.a. CNRS 341, INSA, BBt. 502, 69621 Villeurbanne 21.L.L., BP 156X, 38042 Grenoble Cedex, France
This second transformation
and P. CONVERT’
Cedex, France
part is devoted to the hemicarbides of the VIth transition metal group (Mo,C and W?C); a new structural <-+ (E-type) is identified in Mo,C,_, compounds, while this transition is confirmed in W,C.
1. Introduction
a f-
__
_
‘(3~102)-f-
vm-C-mw--r -.---__
As for the hemicarbides of the Vth group (see part I, in these Proceedings), the crystallography of compounds of the VIth transition metal group (Mo,C, W,C) has been studied and reviewed by previous workers (e.g. refs. [l, 21). However, controversies subsist concerning the various ordered structures involved in these materials: this has motivated a complete study of the polymorphism of Mo,C and W,C by high temperature powder neutron diffraction [3]. The main results of this work are summarized here*.
*br-?a
---.,*
*
t
------_--I
b .
D
-----d
E
--*
--.-r
,**-
2. Study of MO& 2.1. Recall of literature data The following thermal evolution may be deduced from previous works on Mo,C [l, 21: <-, y (critical temperature T, = 1430°C) Several neutron refinements confirm the 5 phase at room temperature (see review in ref.
PI)* 2.2. Results The structural
evolution of Mo,C is illustrated by fig. 1. It is seen that a new transition <+ E is ob-
*Basic
information is briefly reported in part I (particularly, the different ordered structures in M,C compounds are illustrated by fig. 1).
Fig. 1. Thermal evolution of characteristic Bragg peaks intensities in MO&, (a) and Mo,C, 94 (b) (the intensity of the highest matrix reflection has been reduced by 3; encircled symbols concern the 5 superstructure reflection). Note that the <-+E transition occurs in a sharp temperature range in Mo,C,. The fitting of the tail of the E curve is discussed in section 4.
served, which ends at T, = (1350 & 15)‘C in and near 1250°C in MO,&,. The Mo,C,, transition is quite abrupt in the stoichiometric
0921-4526/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
T. Epicier et al. I Diffraction in transition metal hemicarbides
powder, which suggests a first-order type transformation. The E+ y transition_ is illustrated by the gradual vanishing of the ( 1101)~E peak; this behaviour indicates a continuous second order transition. 3. Study of W,C 3.1. Recall of literature data Controversies exist in the literature as regards the identification of ordered phases in W,C, since the OL,5 and E forms have simultaneously been invoked (see review in ref. [3]); the cxphase is frequently proposed as the stable low temperature phase, while 5 and E are competiting middle temperature structures: however, the E phase seems to be more easily retained at room temperature and previous profile refinements indicate a modification with respect to the exact e-Fe,N-type structure (this point is reviewed in ref. [4] and will not be developed here because of lack of space; in the following, the actual E-type phase will be labelled E’ to point out this difference). 3.2. Results At room temperature, the major constituent observed in normally prepared samples is the E’ phase; it may be emphasised that no evidence was obtained for the a structure, whatever the various thermal treatments performed in order
TEWERATLRE
M,C,_,
II
45
to stabilize this phase: we may then consider that the hypothesis of the (Yphase as the stable low temperature form is wrong for W,C. During heating, the E’ phase gets fully disordered near T, = 1800°C in W,C, (see fig. 2(a)), and 1625°C in W,C,,,, [3]; the transformation is very similar to that occurring in Mo,C. The evidence for the 5 modification has been occasionally obtained at room temperature [3]; it appears that this orthorhombic distortion may be stabilised by a long-term annealing [5]. In the present work, the 5+ E transition has been directly observed on a W,C, powder annealed 23 days at 900°C; the transformation is of the first order and exhibits an important hysteresis (see fig. 2(b)). 4. Discussion Both Mo,C and W,C can be consistently discussed as a whole, since it is seen that they show the same behaviour in temperature: 5 + E’ (a-type) + y. This evolution is similar to that identified in V,C (see part I). The transition between the low temperature orthorhombic 5 phase and the middle temperature hexagonal E’ phase is a first order transition. A strong hysteresis is observed in W,C, while such an effect is not detected in Mo,C, the carbon diffusion being significant at the tempera-
-luIvE
<.o
Fig. 2. Study of W,C. (a): left: direct disordering of the E’ phase at T, = (1800 2 5)“C (the intensity of the highest matrix reflection has been divided by 10); (b): right: [ + E’ transition after stabilization of the c phase: full symbols are measurements at heating, while open ones are at cooling (the hysteresis of the transition is clearly revealed).
46
T. Epicier et al. I Diffraction in transition metal hemicarbides
ture of transformation [3]. It appears that the E-Mo,C phase was not identified in the previous works [l, 21, which claim the evidence for the disordered y phase in the same temperature range: this confusion between the hypothetic y form and the actual E’ one may be explained by a careful1 re-examination of the results of these previous authors (see ref. [4]). The disordering of the E’ phase proceeds as a second order transformation. The fit of experimental data shown in figs. 1 and 2a) is consistent with a second order transition, since p ranges between 0.25 and 0.33 for all compounds: as for V,C and Nb,C (see part I), these values approach the theoretical one predicted by the 3-dimensional Ising model (i.e. 0.31). This transition is fully reversible, thus the high temperature disordered y phase cannot be retained at room temperature; nevertheless, short-range ordering can be observed in W,C after quench from T = 2200°C (TJ T 2 0.84) [4]: the results may be compared with previous ones obtained on V,C and Nb,C [6], and discussed in terms of the pair-interaction model developed by the authors of ref. [6]; this approach allows also the relative stability of various ordered forms to be discussed.
M,C,_,
II
5. General conclusion
The present in situ neutron study has allowed the crystal chemistry of metallic hemicarbides M,C to be re-analysed and corrected in many cases. It would be worth comparing the results of this macroscopic approach with a microstructural investigation, especially by Transmission Electron Microscopy; this has indeed been made .in the case of Mo,C, W,C (where high resolution T.E.M. has been performed) [4], and should be extended to other compounds such as V,C, which exhibits structural modifications at rather low temperature. References [l] E. Rudy, St. Windisch, A.J. Stosick and J.R. Hoffman. Trans. Met. Sot. AIME 239 (1967) 1247. [2] A.L. Bowman and G.P. Arnold, Adv. High Temp. Chem. 4 (1971) 243. [3] T. Epicier, J. Dubois, C. Esnouf, G. Fantozzi and P. Convert, Acta Met. 36 (1988) 1903. (41 T. Epicier These, INSA - Universite Lyon (1988). [S] V.S. Telegus, E.I. Gladyshevskii and P.I. Kripyakevich, Soviet. Phys. Cryst. 12 (1968) 813. [6] K. Hiraga and M. Hirabayashi, J. de Phys., Coll. C7, 38 (1977) 224.
44-46
HIGH TEMPERATURE POWDER NEUTRON DIFFRACTION STUDIES OF STRUCTURAL TRANSFORMATIONS IN TRANSITION METAL HEMICARBIDES M,C 1_-x II. COMPOUNDS
OF THE VIth GROUP
T. EPICIER’, IG.EM.P. P. M.,
J. DUBOIS’,
(Mo,C,_,
AND W,C,_J
C. ESNOUF’,
G. FANTOZZI’
~.a. CNRS 341, INSA, BBt. 502, 69621 Villeurbanne 21.L.L., BP 156X, 38042 Grenoble Cedex, France
This second transformation
and P. CONVERT’
Cedex, France
part is devoted to the hemicarbides of the VIth transition metal group (Mo,C and W?C); a new structural <-+ (E-type) is identified in Mo,C,_, compounds, while this transition is confirmed in W,C.
1. Introduction
a f-
__
_
‘(3~102)-f-
vm-C-mw--r -.---__
As for the hemicarbides of the Vth group (see part I, in these Proceedings), the crystallography of compounds of the VIth transition metal group (Mo,C, W,C) has been studied and reviewed by previous workers (e.g. refs. [l, 21). However, controversies subsist concerning the various ordered structures involved in these materials: this has motivated a complete study of the polymorphism of Mo,C and W,C by high temperature powder neutron diffraction [3]. The main results of this work are summarized here*.
*br-?a
---.,*
*
t
------_--I
b .
D
-----d
E
--*
--.-r
,**-
2. Study of MO& 2.1. Recall of literature data The following thermal evolution may be deduced from previous works on Mo,C [l, 21: <-, y (critical temperature T, = 1430°C) Several neutron refinements confirm the 5 phase at room temperature (see review in ref.
PI)* 2.2. Results The structural
evolution of Mo,C is illustrated by fig. 1. It is seen that a new transition <+ E is ob-
*Basic
information is briefly reported in part I (particularly, the different ordered structures in M,C compounds are illustrated by fig. 1).
Fig. 1. Thermal evolution of characteristic Bragg peaks intensities in MO&, (a) and Mo,C, 94 (b) (the intensity of the highest matrix reflection has been reduced by 3; encircled symbols concern the 5 superstructure reflection). Note that the <-+E transition occurs in a sharp temperature range in Mo,C,. The fitting of the tail of the E curve is discussed in section 4.
served, which ends at T, = (1350 & 15)‘C in and near 1250°C in MO,&,. The Mo,C,, transition is quite abrupt in the stoichiometric
0921-4526/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
T. Epicier et al. I Diffraction in transition metal hemicarbides
powder, which suggests a first-order type transformation. The E+ y transition_ is illustrated by the gradual vanishing of the ( 1101)~E peak; this behaviour indicates a continuous second order transition. 3. Study of W,C 3.1. Recall of literature data Controversies exist in the literature as regards the identification of ordered phases in W,C, since the OL,5 and E forms have simultaneously been invoked (see review in ref. [3]); the cxphase is frequently proposed as the stable low temperature phase, while 5 and E are competiting middle temperature structures: however, the E phase seems to be more easily retained at room temperature and previous profile refinements indicate a modification with respect to the exact e-Fe,N-type structure (this point is reviewed in ref. [4] and will not be developed here because of lack of space; in the following, the actual E-type phase will be labelled E’ to point out this difference). 3.2. Results At room temperature, the major constituent observed in normally prepared samples is the E’ phase; it may be emphasised that no evidence was obtained for the a structure, whatever the various thermal treatments performed in order
TEWERATLRE
M,C,_,
II
45
to stabilize this phase: we may then consider that the hypothesis of the (Yphase as the stable low temperature form is wrong for W,C. During heating, the E’ phase gets fully disordered near T, = 1800°C in W,C, (see fig. 2(a)), and 1625°C in W,C,,,, [3]; the transformation is very similar to that occurring in Mo,C. The evidence for the 5 modification has been occasionally obtained at room temperature [3]; it appears that this orthorhombic distortion may be stabilised by a long-term annealing [5]. In the present work, the 5+ E transition has been directly observed on a W,C, powder annealed 23 days at 900°C; the transformation is of the first order and exhibits an important hysteresis (see fig. 2(b)). 4. Discussion Both Mo,C and W,C can be consistently discussed as a whole, since it is seen that they show the same behaviour in temperature: 5 + E’ (a-type) + y. This evolution is similar to that identified in V,C (see part I). The transition between the low temperature orthorhombic 5 phase and the middle temperature hexagonal E’ phase is a first order transition. A strong hysteresis is observed in W,C, while such an effect is not detected in Mo,C, the carbon diffusion being significant at the tempera-
-luIvE
<.o
Fig. 2. Study of W,C. (a): left: direct disordering of the E’ phase at T, = (1800 2 5)“C (the intensity of the highest matrix reflection has been divided by 10); (b): right: [ + E’ transition after stabilization of the c phase: full symbols are measurements at heating, while open ones are at cooling (the hysteresis of the transition is clearly revealed).
46
T. Epicier et al. I Diffraction in transition metal hemicarbides
ture of transformation [3]. It appears that the E-Mo,C phase was not identified in the previous works [l, 21, which claim the evidence for the disordered y phase in the same temperature range: this confusion between the hypothetic y form and the actual E’ one may be explained by a careful1 re-examination of the results of these previous authors (see ref. [4]). The disordering of the E’ phase proceeds as a second order transformation. The fit of experimental data shown in figs. 1 and 2a) is consistent with a second order transition, since p ranges between 0.25 and 0.33 for all compounds: as for V,C and Nb,C (see part I), these values approach the theoretical one predicted by the 3-dimensional Ising model (i.e. 0.31). This transition is fully reversible, thus the high temperature disordered y phase cannot be retained at room temperature; nevertheless, short-range ordering can be observed in W,C after quench from T = 2200°C (TJ T 2 0.84) [4]: the results may be compared with previous ones obtained on V,C and Nb,C [6], and discussed in terms of the pair-interaction model developed by the authors of ref. [6]; this approach allows also the relative stability of various ordered forms to be discussed.
M,C,_,
II
5. General conclusion
The present in situ neutron study has allowed the crystal chemistry of metallic hemicarbides M,C to be re-analysed and corrected in many cases. It would be worth comparing the results of this macroscopic approach with a microstructural investigation, especially by Transmission Electron Microscopy; this has indeed been made .in the case of Mo,C, W,C (where high resolution T.E.M. has been performed) [4], and should be extended to other compounds such as V,C, which exhibits structural modifications at rather low temperature. References [l] E. Rudy, St. Windisch, A.J. Stosick and J.R. Hoffman. Trans. Met. Sot. AIME 239 (1967) 1247. [2] A.L. Bowman and G.P. Arnold, Adv. High Temp. Chem. 4 (1971) 243. [3] T. Epicier, J. Dubois, C. Esnouf, G. Fantozzi and P. Convert, Acta Met. 36 (1988) 1903. (41 T. Epicier These, INSA - Universite Lyon (1988). [S] V.S. Telegus, E.I. Gladyshevskii and P.I. Kripyakevich, Soviet. Phys. Cryst. 12 (1968) 813. [6] K. Hiraga and M. Hirabayashi, J. de Phys., Coll. C7, 38 (1977) 224.