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Experience in use of synchrotron radiation in solid state chemistry studies
192
Nuclear Instruments and Methods in Physics Research A261 (1987) 192-199 North-Holland, Amsterdam
Section V. X-ray diffractometry EXPERIENCE IN USE OF SYNCHROTRON IN SOLID STATE CHEMISTRY STUDIES
RADIATION
V l a d i m i r V. B O L D Y R E V , Y u r i i A. G A P O N O V , N i k o l a i Z. L Y A K H O V , A n a t o l i i A. P O L I T O V , Boris P. T O L O C H K O , T a t j a n a P. S H A K H T S H N E I D E R a n d M i k h a i l A. S H E R O M O V Institute of Solid State Chemistry, Novosibirsk, 630091 USSR, and Institute of Nuclear Physics, Novosibirsk, 630090, USSR The characteristics of chemical reactions in solids - (1) local process, (2) high velocity, (3) nuclei formation in the crystal volume makes special methods of investigation necessary. In this work it is shown that investigations of structural transformation which accompany chemical reactions proceeding at a high rate in local regions either in the volume, or on the crystal surface, are possible due to the unique peculiarities of synchrotron radiation.
1. Introduction The rate of chemical reactions is basically sensitive to temperature, pressure and concentration of reagents. All these parameters are easily measured in the course of a chemical reaction in gas or liquid, therefore and investigator can check the reaction, predict its progress and, consequently, control it. A quite different situation is observed when the chemical reaction occurs in a solid. Due to the nonuniformity of the distribution of the reaction zone the reaction parameters in different parts of the crystal can be essentially different: the temperature - by some thousands of degrees, the pressure - by dozens of kilobars, the concentration - by dozens of percent. The absence of information concerning the parameters of the chemical reaction in a solid results in the fact that it is difficult to check its progress and, consequently, to control it. One more characteristic of solid phase chemical processes is their high sensitivity to the defect structure of the initial reagents and it is rather important to have information just before the chemical reaction on where and what defects are present, because these defects are potential centres of the chemical reaction in the solid (fig. 1). Synchrotron radiation provides investigators with new possibilities with which they can obtain correct information concerning both the initial structure of the matrix (i.e., the initial parameters of the chemical reaction in the solid), the dynamics of its distortion in the course of the reaction and the kinetics of the product formation.
2. Diffractometry of synchrotron radiation X-ray diffractometry with the use of synchrotron radiation becomes the most informative method for the 0168-9002/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
investigation of chemical reactions in the solid [1,2]. The unique characteristics of synchrotron radiation provide perfect parameters in diffraction experiments: 1) spatial resolution 5 × 5 btm, 2) time resolution 10 3 s, 3) phase sensitivity 0.01%, 4) penetration depth 0.01-10 mm. A chemical reaction in a solid results in a change of the substance structure, therefore, one may consider the beginning of the process to be either the appearance of the reflections from the products of the reaction, or the development of stresses in the initial matrix (since the mole volumes of the reagent and product differ, as a rule). These are the traditional method, but synchrotron radiation provides a higher qualitative level of investigation [1,2]. Consider, for example, the process of formation of nickel hydride. In the course of hydrogenation, hydrogen penetrates into the crystalline lattice forming a solid solution that leads to the shifting of the metal atoms from equilibrium positions. The dislocation effect in the hydrogenated metal results in the development of a dislocation structure and intensive interaction of hydrogen with dislocations and point defects. Besides, the formation of the hydride phase in certain crystallographic phases may occur. Shifting of the reflections from their equilibrium positions is observed as hydrogen penetrates into the crystalline lattice of nickel. Simultaneously the thin structure of the reflections changes (fig. 2). In the course of the general shifting of the reflections to smaller angles, an increase of the angle interval between the (111) and (200) reflections is observed. Thus, the reflection from the (111) plane is shifted in the direction of larger angles, and the (200) reflection 3 shifted in the opposite direction [3].
193
V. V. Boldyrev et al. / Using synchrotron radiation in solid state chemistry
-'-9 a j
[0401
b
Fig. 1. Topogram of an initial crystal of Ni-en(NCS)2 (a), and phase transition in it (b) ( × 10).
Such a behaviour of the reflections shows that the formation of stacking fault defects of the crystalline lattice occurs simultaneously with the formation of a solid solution of hydrogen penetration into the nickel matrix. The dynamics of the process is shown in fig. 3. Simultaneously with the beginning of the change of the nickel structure the formation of a metastable phase of nickel hydride is observed and the reflections from its crystalline lattice are visible on the X-ray pattern. In the course of hydrogenation the nuclei of nickel hydride are increased in size and become about 200 (we have supposed that the size of the nuclei coincides with the domain size calculated by the Warren-Averbach method). Simultaneously we observe that at the initial stage of formation of a new phase the parameter of the nickel hydride lattice differs from that of the
initial metal by 3% only, and not by 6% as was measured after hydrogenation. But as the hydride phase grows the parameter of the nickel hydride lattice increases and becomes about the tabular value of 3.717 A. The growth of the nickel hydride phase correlates in time with the development of the defect structure in the nickel matrix (fig. 4). At the initial stage of hydrogenation the crystalline lattice of nickel has the greatest rate of distortion. At this very moment the stacking fault defects of the crystalline lattice of deformation and twin types appear, and the domain size decreases. At the moment when the concentration of nickel hydride reaches its maximum value (fig. 3), relaxation processes in the nickel matrix are observed: a decrease of the defect concentration of the crystalline lattice and an increase of the domain size (fig. 4). Usually recrystalliV. X-RAY DIFFRACTOMETRY
V. IC Boldyrev et al. / Using synchrotron radiation in solid state chemist O'
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angfe (28) Fig. 2. Change of nickel X-ray pattern in the course of hydrogenation.
zation of the defect structure of nickel at such a high rate as in our experiment occurs at temperatures higher than 400 ° C. Rapid recrystallization of the defect structure of nickel during cathodic h y d r o g e n a t i o n might be explained by the influence of the penetrated hydrogen on the value of the effective activation energy for the formation a n d growth of the recrystallization centres. Due to the distortion of the crystalline lattice a n d relaxation processes in the course of cathodic hydrogenation one can assume that hydrogen forming a solid solution is not the main source of a n o n e q u i l i b r i u m distortion of the nickel crystalline lattice, as was supposed earlier. More likely the formation of a new phase, i.e. nuclei formation and growth of nickel hydride particles inside the nickel matrix, results in its greatest distortion. As soon as this process is over, the developm e n t of distortions of the nickel crystalline lattice stops. In general, such a situation is typical for solid phase reactions, where, as a rule, the molar volumes of the reagent and product and their structures essentially differ. In the microcollimation experiment, when the size of the b e a m was decreased to 5 × 5 /~m, we obtained i n f o r m a t i o n o n the development of distortions of the crystalline lattice in the course of h y d r o g e n a t i o n of a crystalline grain. In this case an increase of the lattice parameter, a b r o a d e n i n g of the reflection a n d a decrease of its intensity were observed, but no formation of the hydride phase inside the crystallite was established. A t the same time, a hydride phase in the grain b o u n d a r y was observed. In our experiments reflections from nickel hydride were observed when their intensity was only 0.01% of
Fig. 3. Change of the lattice parameter a 1 and of the intensity I of nickel hydride reflections in the course of cathodic hydrogenation. 1' and a ' belong to the first process of hydrogenation, I " and a " to the second, after the decomposition of NiH.
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Fig. 4. (a) is the domain size of the NiH phase; 1 (111), 2 (100) directions. (h) is the concentration change of the stacking fault defects in nickel in the course of hydrogenation.
195
V.V. Boldyrev et aL / Using synchrotron radiation in solid state chemistry
the intensity of the nickel reflections, i.e. the start of the process was established at a p r o d u c t c o n c e n t r a t i o n 200 times less t h a n that observed w h e n working with X-ray tubes. Thus s y n c h r o t r o n radiation allows investigation of solid phase processes at an early stage of the f o r m a t i o n of chemical reaction products. Due to the shifting of the reflections from the initial position we o b t a i n i n f o r m a t i o n o n the f o r m a t i o n of stresses in the reagent during the reaction at a n early stage of the interaction, since the use of s y n c h r o t r o n r a d i a t i o n provides a higher accuracy of d e t e r m i n a t i o n of the position a n d shifting of the reflections. T h e correlation of stresses in the matrix a n d the kinetics of the chemical reaction in the solid provides additional i n f o r m a t i o n a b o u t the solid phase reaction. Unlike reactions in gases a n d solutions, the formation of the p r o d u c t of a solid state reaction is localized. Specifically, reactions of thermal decomposition of solids proceed, as a rule, topochemically, i.e., with the formation of an interface separating the solid reagent from the reaction product. The " t h i c k n e s s " of the interface is small, besides, its state m a y be changed in time. This fact limits the possibility of investigating the substance state at the interface: this m e t h o d must be local a n d fast-acting. As we have seen, diffractometry of synchrotron radiation corresponds to all these characteristics [2,4]. A more widely spread model of topochemical reactions of thermal decomposition is a sublimation model, i.e., the interface is a surface on which a j u m p of the c o n c e n t r a t i o n of water molecules is observed (in the case of d e h y d r a t i o n of crystallohydrides) (fig. 5a). A more complex model is one in which the interface is n o t a surface b u t a layer of the interstitial p r o d u c t in which the c o n c e n t r a t i o n of water molecules changes constantly (fig. 5b). Experiments of interface scanning (fig. 6) in the course of d e h y d r a t i o n of CuSO 4 • 5 H 2 0 a n d L i z S O 4 H20 crystals by a collimated b e a m of s y n c h r o t r o n r a d i a t i o n (5 ttm) c o n f i r m e d the correctness of this
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model: the width of the interface is 100-200 # (fig. 7). Structurally the interface is similar to a m u c h distorted reagent: in the d e h y d r a t i o n of CuSO 4- H 2 0 a metastable state was observed from the structure of the reagent (fig. 8). As was s h o w n above, the topochemical d e v e l o p m e n t of the reaction results in elastic stresses of the initial matrix at the p r o d u c t - r e a g e n t b o u n d a r y with a length of some microns. To u n d e r s t a n d the b e h a v i o u r of the substances near the interface we carried out experim e n t s using a c h a m b e r with d i a m o n d anvils to o b t a i n superhigh pressures up to 220 kbar. In these experim e n t s one c a n investigate the behaviour of a substance either u n d e r a pressure increase or decrease. I n fig. 9 the X-ray p a t t e r n s for AgI are given: A is the high pressure p h a s e (a structure of the NaC1 type), C is the low pressure phase (hexagonal structure), B is the interstitial structure (tetragonal structure) [5]. F o r a pressure increase a phase transition from the hexagonal to the tetragonal p h a s e a n d then cubic phases was observed. After having o b t a i n e d the high pressure p h a s e by compression, we tried to quickly decrease the load a n d observed the dynamics of the reverse transition via the interstitial and tetragonal phase. But we did not m a n a g e to establish the interstitial phase u n d e r these conditions. Taking into account that the exposure V. X-RAY DIFFRACTOMETRY
196
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Fig. 7. Dependence of intensity, interplane distance and halfwidth of the (002) reflection of initial Li2SO4.HzO on the coordinate near the interface visible by microscope as a result of mechanical scanning by the beam. The arrow points to a visible interface. It is seen that the "thickness" of the reaction zone is about 150 ~m.
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b of a single diffractogram was 60 s, this value can be considered as the m a x i m u m duration of a polymorphic transition. A n o t h e r characteristic of solid state chemistry is that the reactions proceed actually within the crystal volume and not on the surface. Therefore, the information from the crystal volume is very important. Owing to the penetrating range of the synchrotron radiation spectrum one can " l o o k " inside the substance which provides information concerning the stress distribution in the crystal, profiles of the substance concentration during the chemical reaction, and the kinetics of reactions (fig. 10). Diffractometry of synchrotron radiation also provides quite new information on mechanochemistry: we have obtained data on shift deformation which results in an acceleration of the chemical reaction (i.e., a decrease of the potential barrier). This is observed in experiments on deformation by cutting Sn and CaCO 3. In the first case one observes the formation of the Sn III metastable phase and in the second case that of aragonite (fig. 11).
Fig. 8. In the course of dehydration of Li2SO4.H20 some interplane distances, as well as do02, are increased. The rest are not changed: (a) is a dash-diagram of the initial, interstitial and final structure, (b) is the time dependence of the change of some interplane distances of a powder-like LisSO 4. H20 sample in the course of dehydration in vacuum.
Interesting results were obtained during the investigation of relaxation processes after deformation by cutting silver, copper and nickel. The investigation of the relaxation processes is promising since it provides an optimization of the mechanical activation and energy accumulation regime before its dissipation [6]. To make the mechanism of structural relaxation clear, low-temperature annealing of the samples by cutting deformed silver in the temperature interval from 25 up to 95 o C was carried out (fig. 12). Initially after the deformation (in the interval from 0.4 up to 4 s) the relaxation rate is high and practically does not depend on the annealing temperature. Since the processes at this very stage of relaxation are not
197
V. V. Boldyrev et al. / Using synchrotron radiation in solid state chemistry
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thermally activated, one can suppose that in this time interval a dislocation slip occurs due to the internal residual microstresses, resulting in a more u n i f o r m dist r i b u t i o n (or in a n a r r a n g e m e n t in the form of dislocation walls). A further slip of the dislocations becomes impossible since they are likely to b e stopped o n the stoppers; to traverse t h e m (or to avoid them) m u c h a d d i t i o n a l energy is needed. Relaxation of the silver structure at 1 0 - 1 0 0 s dep e n d s strongly o n the t e m p e r a t u r e (fig. 12) a n d is described b y a ratio which corresponds to the kinetic e q u a t i o n of the second order a n d the c o n c e n t r a t i o n of the dislocations (in this case we have supposed that the change of the half-width of the reflection is p r o p o r tional to the c o n c e n t r a t i o n s of the dislocations). Taking this into account one can consider the annihilation of dislocations of different sign to be just the process d e t e r m i n i n g relaxation in this time interval. By plotting
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Fig. 9. Dynamics of the change of AgI X-ray patterns during a phase transition from AgI II (2.9 kbar) to AgI I (2.5 bar) via the interstitial phase.
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Fig, 11. X-ray patterns of calcite and aragonite obtained in the region of the strongest distortions from the diamond indentor.
In k - l / T , where T is the absolute temperature, we defined the energy of the activation of the structural relaxation of silver as E = (1 +_ 0.1) eV. In the course of annealing at 8 0 - 9 0 o C, where relaxation processes are especially rapid, it is likely that for times longer t h a n 100 s n o annihilation of dislocations of different sign prevails, b u t their o u t p u t a n d
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Fig. 12. Change of the half-width of the (111) reflection of silver in the course of annealing at temperatures of (1) 50, (2) 75, (3) 95 o C. V. X-RAY DIFFRACTOMETRY
198
V. V. Boldyrev et aL / Using synchrotron radiation in solid state chemistry
disappearance at sinks of dislocations, since at this very stage the distance between the dislocations might be compared with that between the sinks. Consequently, the kinetics of this process should be described by an equation of first order which we observed in the experiment. At lower temperatures the transition from one kinetics to another was observed at larger times but the value of the dislocation concentration was the same. A relatively low (for the process of dislocation propagation) value of activation energy obtained in the course of the experiment can be explained as follows: though the change of the half-width of the reflections is connected with a decrease of the dislocation density, the activation of this process depends on the decrease of the concentration of point defects, i.e., dislocation stoppers. A further decrease of the concentration of point defects is confirmed by a decrease of the diffusion background near the reflections. It should be noted that the o ( t ) dependence corresponding to the system of {100} planes acts, at first, in some other way: for the first 100 s o ( t ) even increases (fig. 13b). This can be explained by the fact that dislocations distributed in different crystallographic planes
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do not act in the same way during the annealing. This is true since (111) is a preferred plane slip in the facecentered cubic lattice, therefore, the dislocations in this plane would easily move to the sinks. Dislocations in other planes move more slowly due to the mechanism of cross slip and diffuse movements. Therefore, when a sharp decrease of concentration of dislocations occurs in the system of (111) planes, the microstresses are increased in the {100} system, which results in an increase of o ( t ) reflections of this system. Further, the microstresses over all directions become equilibrated which leads to a smooth decrease of o ( t ) reflections from {100} planes.
3. X-ray topography
X-ray topography allows investigations on a micron scale. Due to the high intensity of synchrotron radiation it becomes possible to reduce the exposure times of topograms and to investigate the dynamics of the change of crystal defect structures during their transformation. Since X-ray topography offers a unique possibility to obtain the local distribution pattern of the deformed regions in the crystal, it is interesting to use this method for the investigation of solid phase processes of topochemical character accompanied by localization and autolocalization phenomena (i.e., proceeding via the formation and growth of nuclei of a new phase). Owing to the difference between the molar volumes of the phases, the formation of the regions of the new phase under the transformation should result in elastic stresses at the interface and, hence, in a deformation of the initial lattice. Relaxation of elastic stresses, in turn, affects morphology of the product and the kinetics of the transformation. We tried to investigate the defect structure of Nienz(NCS)2 single crystals (-en is the ethylenediamine) and the development of stresses under a phase transformation. Investigations during the phase transition provide a relation of the real crystal structure with the morphology of formation of the transformation product, and the degree of deformation of the initial crystal lattice during the growth of nuclei of a new phase was measured. It was shown that initiation of the phase transformations in Ni-en2(NCS)2 occurs on the point defects (fig. 1).
Acknowledgements 0,5
0,6
0,7
0,8
C, %
Fig. 13. Correlation dependence between the value of microdistortions and real deformation E for the (111) (1) and (200) (b) reflections.
We wish to thank the academician A.N. Skrinsky and Prof. G.N. Kulipanov for their support of our experimental program during all its stages.
V.V. Boldyrev et a L / Using synchrotron radiation in solid state chemistry
References [1] N.A. Mezentsev et al., Nucl. Instr. and Meth. 204 (1986) 604. [2] B.P. Tolochko, M.A. Sheromov, N.Z. Lyakhov and V.V. Boldyrev, Dokl. Akad. Nauk SSSR. 259 (1981) 1125. [3] B.P. Tolochko, N.Z. Lyakhov and A.I. Maslii, Izv. Sib. Otdel. Akad. N a u k SSSR, Ser. Khim. 1 (1985) 54.
199
[4] Y.A. Gaponov, N.Z. Lyakhov, V.V. Boldyrev and B.P. Tolochko, Izv. Sib. Otdel. Akad. Nauk SSSR, Ser. Khim. 3 (1985) 22. [5] N.Z. Lyakhov et al., Proc. 4th All U n i o n Meeting on Use of Synchrotron Radiation, Novosibirsk (1984) p. 118. [6] B.P. Tolochko and A.I. Maslii, Izv. Sib. Otdel. Akad. Nauk SSSR, Set. Khim. 1 (1985) 48.
V. X-RAY D I F F R A C T O M E T R Y
Nuclear Instruments and Methods in Physics Research A261 (1987) 192-199 North-Holland, Amsterdam
Section V. X-ray diffractometry EXPERIENCE IN USE OF SYNCHROTRON IN SOLID STATE CHEMISTRY STUDIES
RADIATION
V l a d i m i r V. B O L D Y R E V , Y u r i i A. G A P O N O V , N i k o l a i Z. L Y A K H O V , A n a t o l i i A. P O L I T O V , Boris P. T O L O C H K O , T a t j a n a P. S H A K H T S H N E I D E R a n d M i k h a i l A. S H E R O M O V Institute of Solid State Chemistry, Novosibirsk, 630091 USSR, and Institute of Nuclear Physics, Novosibirsk, 630090, USSR The characteristics of chemical reactions in solids - (1) local process, (2) high velocity, (3) nuclei formation in the crystal volume makes special methods of investigation necessary. In this work it is shown that investigations of structural transformation which accompany chemical reactions proceeding at a high rate in local regions either in the volume, or on the crystal surface, are possible due to the unique peculiarities of synchrotron radiation.
1. Introduction The rate of chemical reactions is basically sensitive to temperature, pressure and concentration of reagents. All these parameters are easily measured in the course of a chemical reaction in gas or liquid, therefore and investigator can check the reaction, predict its progress and, consequently, control it. A quite different situation is observed when the chemical reaction occurs in a solid. Due to the nonuniformity of the distribution of the reaction zone the reaction parameters in different parts of the crystal can be essentially different: the temperature - by some thousands of degrees, the pressure - by dozens of kilobars, the concentration - by dozens of percent. The absence of information concerning the parameters of the chemical reaction in a solid results in the fact that it is difficult to check its progress and, consequently, to control it. One more characteristic of solid phase chemical processes is their high sensitivity to the defect structure of the initial reagents and it is rather important to have information just before the chemical reaction on where and what defects are present, because these defects are potential centres of the chemical reaction in the solid (fig. 1). Synchrotron radiation provides investigators with new possibilities with which they can obtain correct information concerning both the initial structure of the matrix (i.e., the initial parameters of the chemical reaction in the solid), the dynamics of its distortion in the course of the reaction and the kinetics of the product formation.
2. Diffractometry of synchrotron radiation X-ray diffractometry with the use of synchrotron radiation becomes the most informative method for the 0168-9002/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
investigation of chemical reactions in the solid [1,2]. The unique characteristics of synchrotron radiation provide perfect parameters in diffraction experiments: 1) spatial resolution 5 × 5 btm, 2) time resolution 10 3 s, 3) phase sensitivity 0.01%, 4) penetration depth 0.01-10 mm. A chemical reaction in a solid results in a change of the substance structure, therefore, one may consider the beginning of the process to be either the appearance of the reflections from the products of the reaction, or the development of stresses in the initial matrix (since the mole volumes of the reagent and product differ, as a rule). These are the traditional method, but synchrotron radiation provides a higher qualitative level of investigation [1,2]. Consider, for example, the process of formation of nickel hydride. In the course of hydrogenation, hydrogen penetrates into the crystalline lattice forming a solid solution that leads to the shifting of the metal atoms from equilibrium positions. The dislocation effect in the hydrogenated metal results in the development of a dislocation structure and intensive interaction of hydrogen with dislocations and point defects. Besides, the formation of the hydride phase in certain crystallographic phases may occur. Shifting of the reflections from their equilibrium positions is observed as hydrogen penetrates into the crystalline lattice of nickel. Simultaneously the thin structure of the reflections changes (fig. 2). In the course of the general shifting of the reflections to smaller angles, an increase of the angle interval between the (111) and (200) reflections is observed. Thus, the reflection from the (111) plane is shifted in the direction of larger angles, and the (200) reflection 3 shifted in the opposite direction [3].
193
V. V. Boldyrev et al. / Using synchrotron radiation in solid state chemistry
-'-9 a j
[0401
b
Fig. 1. Topogram of an initial crystal of Ni-en(NCS)2 (a), and phase transition in it (b) ( × 10).
Such a behaviour of the reflections shows that the formation of stacking fault defects of the crystalline lattice occurs simultaneously with the formation of a solid solution of hydrogen penetration into the nickel matrix. The dynamics of the process is shown in fig. 3. Simultaneously with the beginning of the change of the nickel structure the formation of a metastable phase of nickel hydride is observed and the reflections from its crystalline lattice are visible on the X-ray pattern. In the course of hydrogenation the nuclei of nickel hydride are increased in size and become about 200 (we have supposed that the size of the nuclei coincides with the domain size calculated by the Warren-Averbach method). Simultaneously we observe that at the initial stage of formation of a new phase the parameter of the nickel hydride lattice differs from that of the
initial metal by 3% only, and not by 6% as was measured after hydrogenation. But as the hydride phase grows the parameter of the nickel hydride lattice increases and becomes about the tabular value of 3.717 A. The growth of the nickel hydride phase correlates in time with the development of the defect structure in the nickel matrix (fig. 4). At the initial stage of hydrogenation the crystalline lattice of nickel has the greatest rate of distortion. At this very moment the stacking fault defects of the crystalline lattice of deformation and twin types appear, and the domain size decreases. At the moment when the concentration of nickel hydride reaches its maximum value (fig. 3), relaxation processes in the nickel matrix are observed: a decrease of the defect concentration of the crystalline lattice and an increase of the domain size (fig. 4). Usually recrystalliV. X-RAY DIFFRACTOMETRY
V. IC Boldyrev et al. / Using synchrotron radiation in solid state chemist O'
194
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25
angfe (28) Fig. 2. Change of nickel X-ray pattern in the course of hydrogenation.
zation of the defect structure of nickel at such a high rate as in our experiment occurs at temperatures higher than 400 ° C. Rapid recrystallization of the defect structure of nickel during cathodic h y d r o g e n a t i o n might be explained by the influence of the penetrated hydrogen on the value of the effective activation energy for the formation a n d growth of the recrystallization centres. Due to the distortion of the crystalline lattice a n d relaxation processes in the course of cathodic hydrogenation one can assume that hydrogen forming a solid solution is not the main source of a n o n e q u i l i b r i u m distortion of the nickel crystalline lattice, as was supposed earlier. More likely the formation of a new phase, i.e. nuclei formation and growth of nickel hydride particles inside the nickel matrix, results in its greatest distortion. As soon as this process is over, the developm e n t of distortions of the nickel crystalline lattice stops. In general, such a situation is typical for solid phase reactions, where, as a rule, the molar volumes of the reagent and product and their structures essentially differ. In the microcollimation experiment, when the size of the b e a m was decreased to 5 × 5 /~m, we obtained i n f o r m a t i o n o n the development of distortions of the crystalline lattice in the course of h y d r o g e n a t i o n of a crystalline grain. In this case an increase of the lattice parameter, a b r o a d e n i n g of the reflection a n d a decrease of its intensity were observed, but no formation of the hydride phase inside the crystallite was established. A t the same time, a hydride phase in the grain b o u n d a r y was observed. In our experiments reflections from nickel hydride were observed when their intensity was only 0.01% of
Fig. 3. Change of the lattice parameter a 1 and of the intensity I of nickel hydride reflections in the course of cathodic hydrogenation. 1' and a ' belong to the first process of hydrogenation, I " and a " to the second, after the decomposition of NiH.
a
(L,), ,~
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Fig. 4. (a) is the domain size of the NiH phase; 1 (111), 2 (100) directions. (h) is the concentration change of the stacking fault defects in nickel in the course of hydrogenation.
195
V.V. Boldyrev et aL / Using synchrotron radiation in solid state chemistry
the intensity of the nickel reflections, i.e. the start of the process was established at a p r o d u c t c o n c e n t r a t i o n 200 times less t h a n that observed w h e n working with X-ray tubes. Thus s y n c h r o t r o n radiation allows investigation of solid phase processes at an early stage of the f o r m a t i o n of chemical reaction products. Due to the shifting of the reflections from the initial position we o b t a i n i n f o r m a t i o n o n the f o r m a t i o n of stresses in the reagent during the reaction at a n early stage of the interaction, since the use of s y n c h r o t r o n r a d i a t i o n provides a higher accuracy of d e t e r m i n a t i o n of the position a n d shifting of the reflections. T h e correlation of stresses in the matrix a n d the kinetics of the chemical reaction in the solid provides additional i n f o r m a t i o n a b o u t the solid phase reaction. Unlike reactions in gases a n d solutions, the formation of the p r o d u c t of a solid state reaction is localized. Specifically, reactions of thermal decomposition of solids proceed, as a rule, topochemically, i.e., with the formation of an interface separating the solid reagent from the reaction product. The " t h i c k n e s s " of the interface is small, besides, its state m a y be changed in time. This fact limits the possibility of investigating the substance state at the interface: this m e t h o d must be local a n d fast-acting. As we have seen, diffractometry of synchrotron radiation corresponds to all these characteristics [2,4]. A more widely spread model of topochemical reactions of thermal decomposition is a sublimation model, i.e., the interface is a surface on which a j u m p of the c o n c e n t r a t i o n of water molecules is observed (in the case of d e h y d r a t i o n of crystallohydrides) (fig. 5a). A more complex model is one in which the interface is n o t a surface b u t a layer of the interstitial p r o d u c t in which the c o n c e n t r a t i o n of water molecules changes constantly (fig. 5b). Experiments of interface scanning (fig. 6) in the course of d e h y d r a t i o n of CuSO 4 • 5 H 2 0 a n d L i z S O 4 H20 crystals by a collimated b e a m of s y n c h r o t r o n r a d i a t i o n (5 ttm) c o n f i r m e d the correctness of this
~ b A A
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Fig. 5. Schematic of the interface: (a) the first model; (b) the second model.
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model: the width of the interface is 100-200 # (fig. 7). Structurally the interface is similar to a m u c h distorted reagent: in the d e h y d r a t i o n of CuSO 4- H 2 0 a metastable state was observed from the structure of the reagent (fig. 8). As was s h o w n above, the topochemical d e v e l o p m e n t of the reaction results in elastic stresses of the initial matrix at the p r o d u c t - r e a g e n t b o u n d a r y with a length of some microns. To u n d e r s t a n d the b e h a v i o u r of the substances near the interface we carried out experim e n t s using a c h a m b e r with d i a m o n d anvils to o b t a i n superhigh pressures up to 220 kbar. In these experim e n t s one c a n investigate the behaviour of a substance either u n d e r a pressure increase or decrease. I n fig. 9 the X-ray p a t t e r n s for AgI are given: A is the high pressure p h a s e (a structure of the NaC1 type), C is the low pressure phase (hexagonal structure), B is the interstitial structure (tetragonal structure) [5]. F o r a pressure increase a phase transition from the hexagonal to the tetragonal p h a s e a n d then cubic phases was observed. After having o b t a i n e d the high pressure p h a s e by compression, we tried to quickly decrease the load a n d observed the dynamics of the reverse transition via the interstitial and tetragonal phase. But we did not m a n a g e to establish the interstitial phase u n d e r these conditions. Taking into account that the exposure V. X-RAY DIFFRACTOMETRY
196
V. [C Boldyreu et aL / Using synchrotron radiation in solid state chemistry
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Fig. 7. Dependence of intensity, interplane distance and halfwidth of the (002) reflection of initial Li2SO4.HzO on the coordinate near the interface visible by microscope as a result of mechanical scanning by the beam. The arrow points to a visible interface. It is seen that the "thickness" of the reaction zone is about 150 ~m.
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b of a single diffractogram was 60 s, this value can be considered as the m a x i m u m duration of a polymorphic transition. A n o t h e r characteristic of solid state chemistry is that the reactions proceed actually within the crystal volume and not on the surface. Therefore, the information from the crystal volume is very important. Owing to the penetrating range of the synchrotron radiation spectrum one can " l o o k " inside the substance which provides information concerning the stress distribution in the crystal, profiles of the substance concentration during the chemical reaction, and the kinetics of reactions (fig. 10). Diffractometry of synchrotron radiation also provides quite new information on mechanochemistry: we have obtained data on shift deformation which results in an acceleration of the chemical reaction (i.e., a decrease of the potential barrier). This is observed in experiments on deformation by cutting Sn and CaCO 3. In the first case one observes the formation of the Sn III metastable phase and in the second case that of aragonite (fig. 11).
Fig. 8. In the course of dehydration of Li2SO4.H20 some interplane distances, as well as do02, are increased. The rest are not changed: (a) is a dash-diagram of the initial, interstitial and final structure, (b) is the time dependence of the change of some interplane distances of a powder-like LisSO 4. H20 sample in the course of dehydration in vacuum.
Interesting results were obtained during the investigation of relaxation processes after deformation by cutting silver, copper and nickel. The investigation of the relaxation processes is promising since it provides an optimization of the mechanical activation and energy accumulation regime before its dissipation [6]. To make the mechanism of structural relaxation clear, low-temperature annealing of the samples by cutting deformed silver in the temperature interval from 25 up to 95 o C was carried out (fig. 12). Initially after the deformation (in the interval from 0.4 up to 4 s) the relaxation rate is high and practically does not depend on the annealing temperature. Since the processes at this very stage of relaxation are not
197
V. V. Boldyrev et al. / Using synchrotron radiation in solid state chemistry
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thermally activated, one can suppose that in this time interval a dislocation slip occurs due to the internal residual microstresses, resulting in a more u n i f o r m dist r i b u t i o n (or in a n a r r a n g e m e n t in the form of dislocation walls). A further slip of the dislocations becomes impossible since they are likely to b e stopped o n the stoppers; to traverse t h e m (or to avoid them) m u c h a d d i t i o n a l energy is needed. Relaxation of the silver structure at 1 0 - 1 0 0 s dep e n d s strongly o n the t e m p e r a t u r e (fig. 12) a n d is described b y a ratio which corresponds to the kinetic e q u a t i o n of the second order a n d the c o n c e n t r a t i o n of the dislocations (in this case we have supposed that the change of the half-width of the reflection is p r o p o r tional to the c o n c e n t r a t i o n s of the dislocations). Taking this into account one can consider the annihilation of dislocations of different sign to be just the process d e t e r m i n i n g relaxation in this time interval. By plotting
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Fig. 9. Dynamics of the change of AgI X-ray patterns during a phase transition from AgI II (2.9 kbar) to AgI I (2.5 bar) via the interstitial phase.
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Fig, 11. X-ray patterns of calcite and aragonite obtained in the region of the strongest distortions from the diamond indentor.
In k - l / T , where T is the absolute temperature, we defined the energy of the activation of the structural relaxation of silver as E = (1 +_ 0.1) eV. In the course of annealing at 8 0 - 9 0 o C, where relaxation processes are especially rapid, it is likely that for times longer t h a n 100 s n o annihilation of dislocations of different sign prevails, b u t their o u t p u t a n d
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Fig. 12. Change of the half-width of the (111) reflection of silver in the course of annealing at temperatures of (1) 50, (2) 75, (3) 95 o C. V. X-RAY DIFFRACTOMETRY
198
V. V. Boldyrev et aL / Using synchrotron radiation in solid state chemistry
disappearance at sinks of dislocations, since at this very stage the distance between the dislocations might be compared with that between the sinks. Consequently, the kinetics of this process should be described by an equation of first order which we observed in the experiment. At lower temperatures the transition from one kinetics to another was observed at larger times but the value of the dislocation concentration was the same. A relatively low (for the process of dislocation propagation) value of activation energy obtained in the course of the experiment can be explained as follows: though the change of the half-width of the reflections is connected with a decrease of the dislocation density, the activation of this process depends on the decrease of the concentration of point defects, i.e., dislocation stoppers. A further decrease of the concentration of point defects is confirmed by a decrease of the diffusion background near the reflections. It should be noted that the o ( t ) dependence corresponding to the system of {100} planes acts, at first, in some other way: for the first 100 s o ( t ) even increases (fig. 13b). This can be explained by the fact that dislocations distributed in different crystallographic planes
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do not act in the same way during the annealing. This is true since (111) is a preferred plane slip in the facecentered cubic lattice, therefore, the dislocations in this plane would easily move to the sinks. Dislocations in other planes move more slowly due to the mechanism of cross slip and diffuse movements. Therefore, when a sharp decrease of concentration of dislocations occurs in the system of (111) planes, the microstresses are increased in the {100} system, which results in an increase of o ( t ) reflections of this system. Further, the microstresses over all directions become equilibrated which leads to a smooth decrease of o ( t ) reflections from {100} planes.
3. X-ray topography
X-ray topography allows investigations on a micron scale. Due to the high intensity of synchrotron radiation it becomes possible to reduce the exposure times of topograms and to investigate the dynamics of the change of crystal defect structures during their transformation. Since X-ray topography offers a unique possibility to obtain the local distribution pattern of the deformed regions in the crystal, it is interesting to use this method for the investigation of solid phase processes of topochemical character accompanied by localization and autolocalization phenomena (i.e., proceeding via the formation and growth of nuclei of a new phase). Owing to the difference between the molar volumes of the phases, the formation of the regions of the new phase under the transformation should result in elastic stresses at the interface and, hence, in a deformation of the initial lattice. Relaxation of elastic stresses, in turn, affects morphology of the product and the kinetics of the transformation. We tried to investigate the defect structure of Nienz(NCS)2 single crystals (-en is the ethylenediamine) and the development of stresses under a phase transformation. Investigations during the phase transition provide a relation of the real crystal structure with the morphology of formation of the transformation product, and the degree of deformation of the initial crystal lattice during the growth of nuclei of a new phase was measured. It was shown that initiation of the phase transformations in Ni-en2(NCS)2 occurs on the point defects (fig. 1).
Acknowledgements 0,5
0,6
0,7
0,8
C, %
Fig. 13. Correlation dependence between the value of microdistortions and real deformation E for the (111) (1) and (200) (b) reflections.
We wish to thank the academician A.N. Skrinsky and Prof. G.N. Kulipanov for their support of our experimental program during all its stages.
V.V. Boldyrev et a L / Using synchrotron radiation in solid state chemistry
References [1] N.A. Mezentsev et al., Nucl. Instr. and Meth. 204 (1986) 604. [2] B.P. Tolochko, M.A. Sheromov, N.Z. Lyakhov and V.V. Boldyrev, Dokl. Akad. Nauk SSSR. 259 (1981) 1125. [3] B.P. Tolochko, N.Z. Lyakhov and A.I. Maslii, Izv. Sib. Otdel. Akad. N a u k SSSR, Ser. Khim. 1 (1985) 54.
199
[4] Y.A. Gaponov, N.Z. Lyakhov, V.V. Boldyrev and B.P. Tolochko, Izv. Sib. Otdel. Akad. Nauk SSSR, Ser. Khim. 3 (1985) 22. [5] N.Z. Lyakhov et al., Proc. 4th All U n i o n Meeting on Use of Synchrotron Radiation, Novosibirsk (1984) p. 118. [6] B.P. Tolochko and A.I. Maslii, Izv. Sib. Otdel. Akad. Nauk SSSR, Set. Khim. 1 (1985) 48.
V. X-RAY D I F F R A C T O M E T R Y