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Ionic mobility in basic double salt. Part I: Hydroxycarbonates
SOLID STATE
Solid State Ionics 62 (1993) 199-204 North-Holland
IONICS Ionic mobility in basic double salt. Part I: Hydroxycarbonates R. O e s t e n a n d H. B 6 h m Institut fur Geowissenschaftender Universitdt, 6500 Mainz, Germany Received 15 January 1993; accepted for publication 11 March 1993
In this paper a possible new candidate for proton conduction is examined: basic double salts with a double layer structure. The chemical formula is [Me~_+xMe~+ ( O H - ) 2 ]x+ [A~7n.yH20]. In this part we report the results of phases with C O l - as anion and Mg-AI, Ni-AI and Zn-AI as cations. The synthesised phases (coprecipitation, hydrothermal) were characterised by XRD, SEM, IR thermogravimetry and chemical analysis, ac studies with blocking electrodes yield room-temperature conductivities in the range of I 0-4-10-6 ~r~_I c m - i , depending on the method of synthesis. The results showed no evidence of proton conduction. The mobile species is the OH--ion, which is formed inside the interlayer due to the reaction H20 + CO~z- ¢~OH- + HCO~-. With this model for the ionic conductivity the observed Arrhenius-plot data can be interpreted. At low temperatures the conductivity mechanism is formally described as Grotthus type. At higher temperatures the conductivity is controlled by OH diffusion. That model is supported by the interpretation of the tilt angle a of the semicircle.
1. Introduction
Me 2+ : Mg, Ni, Zn, Cd, ... Me 3+ :A1, Fe, Cr, Ga ....
Proton conduction in solids is of great interest because o f its potential application in fuel cells, sensors, etc. Two mechanisms o f proton conduction in solids have been discussed in the literature: (a) the Grotthus mechanisms and (b) the vehicle mechanism. In the Grotthus mechanism protons are mobile in a disordered hydrogen bond network (e.g. H U P [ l ], N 2 H 6 S O 4 [2,3 ] etc.) via reorientation o f structural units followed by jumping, whereas the vehicle mechanism is characterised by co-operative motion o f H 3 0 + and H 2 0 (e.g. HUAs [4] ); this mechanism requires a structural proton and a mobile vehicle like H 2 0 , N H 3 etc. acting as a proton acceptor. In this paper a new c o m p o u n d as a conceivable candidate for proton conduction is investigated: basic double salt with a double layer structure. The chemical formula o f the double salts can be written as follows: 2+ 3+ [Me]_~Mex (OH-)2]
with
x+ [ A x n F n - y H 2 0 ]
A ~- :CO 2-, C I - , SO~- .... The parameter x in the above formula has only two fixed values: 0.33 or 0.25, i.e. Me + :Me 3+ equals 2:1 or 3:1. The crystal structure of the basic double salts can be derived from brucite, Mg(OH)2. In this structure the O H - ions build up a hexagonal closed packing, whereby the Mg 2+ ions occupy half o f the octahedral voids in such a way, that every second sheet of voids remains empty and hence every other one is completely occupied by Mg 2+. If a fixed amount of Mgions is replaced by trivalent ions like aluminium, positively charged sheets are formed. The charge compensation occurs by anions which - together with water molecules - form the interlayer (fig. 1 ). The cations in the octahedral layer are ordered [5,6] and the two stoichiometries are the only ones in which the aluminium ion is only surrounded by magnesium ions. Due to this ordering there are only two possible stacking sequences: a hexagonal one with two ( 2 H ) and a rhombohedral one with three double layers (3R). In natural members of the double salts both stacking sequences are realised while in
0167-2738/93/$ 06.00 © 1993 Elsevier Science Publishers B.V. All fights reserved.
200
R. Oesten, 14. B6hm /Ionic mobility in basic double salt
Fig. 1. Schematicof the structure with a planar anion, water molecules are not drawn. synthetic phases only the rhombohedral polytype is formed. A recent summary of the natural members is given by Drits et al. [ 7]. It is remarkable that the carbonate ion is strongly favoured for being incorporated into the structure and that the interlayer is characterised as liquid-like [ 8 ]. Recently Z n - A I ( 2 : I ) - C 1 and Z n - C r ( 2 : I ) - C I were investigated for proton conduction [9,10 ]. A proton and chloride mobility is proposed, but no spectroscopic results (like IR or NMR) are given to support the interpretation and no interdependencies of the two mobilities are worked out.
2. Experimental The basis double salts were synthesised by the coprecipitation method [ 11-13 ]. As the carbonates of the di- and trivalent cations have a low solubility in water, a stoichiometric mixed solution of the metal chlorides was prepared, which was precipitated in a basic medium, e.g. NaOH containing NazCO3 as the carbonate source, under vigorous stirring. The precipitates were washed with distilled water until they were free of C1- and dried at 80°C. A great variety of cation-combinations was synthesised this way. The crystallinity of the samples was bad except for the Zn-AI(2 : 1 )-phase. So the samples were recrystallised hydrothermally. With our equipment (maximum temperature 200 ° C) only the Mg-A1- and Ni-Al-phases could be recrystallised successfully. For all other cation combinations the experimental conditions were not sufficient. The phases were identified and characterised by X-ray powder diffraction (Cu Kct radiation, Bragg-
Brentano focussing, graphite secondary monochromator). The scanning electron microscopy studies (SEM) were carried out with the Philips model PSEM 500 (acceleration voltage 25 kV, gold coating). Infrared data were recorded using a Perkin-Elmer Fourier-transform spectrometer, model 1760. The KBr preparation technique was employed. The ac conductivity was determined by impedance measurements with a microprocessor-controlled frequency response analyser (Solartron 1170). The samples were pressed in pellets with 8 t/cm 2 and plated with gold in vacuum. The impedance data were recorded in the frequency range of 1 Hz to 1 MHz and in the temperature range of room temperature to 230°C. To get a guess of the influence of physically adsorbed water some samples were hot pressed at 100°C and immediately measured. The amount of cations was determined by X-ray fluorescence analysis on a Philips spectrometer, model PW1404 after calcination of the samples. The water content was determined by thermogravimetric analyses using a Netzsch simultan-thermoanalyse STA, model 429. The measurement was performed in normal atmosphere and with a heating rate of 5°C/rain in platinum crucibles. The carbonate determination was carried out with the Leco microprocessor-controlled analyses system CS-125 based on the IR-absorption technique. For the chloride analyses the coulometric precipitation titration method was applied.
3. Results The synthesised samples were free of chlorine and contained the approximate maximum number of water molecules (three water molecules for the 2:1 and four for the 3:1 stoichiometry). In fig. 2 a typical thermo-analytical diagram of the basic double salts is shown. There are two steps of marked weight loss, whereby the second step consists of two reactions. The first step is caused by loss of interlayer water and the second by loss of the anion and the dehydroxylation of the octahedral layer. After these reactions Me:+O and spinel is formed, which was verified by X-ray diffractometry at elevated temperatures.
R. Oesten, H. BOhm / Ionic mobility in basic double salt
a 10o
90 80 ~ 70 61) 50
........
I .........
0
I .........
100
I .........
I .........
L .....
200 300 400 Temperature ]°CI
500
Fig. 2. D T G (a) and T G (b) diagram of a recrystallised M g AI ( 3: 1 )-phase.
100
z / '/
75 ee
50
,
400
)
II
/
Ig
i/
,
.
i
1000
.
i
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•\
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\
/
~/
.
i
,
1600
i
,
r
.
2200
)
,
i
,
2800
i
,
r
,
3400
i
,
i
4000
Wavenumber Icml" Fig. 3. IR diagram o f a recrystallised Mg-AI(2: l )-phase.
Fig. 3 shows an infrared diagram of the basic double salts. The diagram is dominated by the vibrations of the O H - , CO ] - and H20. The width of the OH-stretching mode at 3450 c m - I indicates that all OH groups form hydrogen bonds. The shoulder at 3080 cm -1 is interpreted as a hydrogen bond between interlayer water and the carbonate. The vibrations about 1400 c m - l ( P 3 ) , 870 c m - ' (v2) and 680 cm -~ (v4) belong to the carbonate anion. The splitting of the asymmetric stretch mode (1400 cm -1 ) indicates a lowering of the carbonate symmetry. This is caused by the resonance position of the anion [5] which causes the symmetric stretch mode ul (at 1050 cm -1) to become IR active. For the further discussion it is important to focus on the hydrogen bonds in the basic double salts. Therefore Pauling's second rule for ionic crystals [ 14] should be applied to the double salts: "In a sta-
201
ble ionic compound the sum of the electrostatic bonding forces between the anion and its neighbouring cations is equal to the charge of the anion (with opposite sign)." In the basic double salts the O H - of the octahedral layer has a charge of 11/6 (2: 1stoichiometry) or 11/8 (3:1 stoichiometry) respectively, which is caused by the trivalent aluminium ion. This means the hydroxide must compensate a charge bigger than one. Unlike halogenide ions the hydroxide can, in fact, compensate a bigger charge, if the O - H bond is weakened accordingly. This is realised by the formation of hydrogen bonds. The acceptors of the bonds are the water molecules and anions of the interlayer; the water molecules are necessary for the stability of the structure. Therefore the hydrogen bonds have different lengths depending on the aluminium contents of the octahedral layer. This is verified by the shift of the OH-stretching mode relative to the reference mode at 3700 cm-1 of pure Mg(OH)2. According to the theory of Lippincott and Schroeder [15 ] the bond length of the O - H is found to be 0.987 A (2:1) and 0.982 (3:1 ); the length of the whole hydrogen bond is 2.88 A (2:1) and 2.95 A (3: 1). So fairly precise predictions can be given for the cation ratio of the octahedral layer without knowing the results of the chemical analysis. The SEM recordings show that the phases crystallise as very thin hexagonal plates. The size of the crystals varies with chemistry. The recrystallised MgA 1 - C O 3 samples have particle sizes about 1-5 pm and the Zn-A1-CO3 crystals are noticeably smaller than 1 um. A lamellar habit was never found. In fig. 4 a typical impedance diagram of a recrystallised Mg-A1-CO3 phase is shown. One characteristic feature of the diagram is that the centre of the semicricle is markedly tilted below the real axis. Another characteristic is the nearly linear behaviour at low frequencies which only appears at lower temperatures. The Arrhenius plot of the conductivity is shown in fig. 5. In this diagram the conductivities have been determined by extrapolating the semicircles to the real axis. The impedance data yield the same Arrhenius diagram if the method of Jonscher [ 16 ] is applied where a log a versus log co plot is used for evaluation. At lower temperatures the conductivity decreases with increasing temperature. In a narrow temperature range the conductivity then in-
202
R. Oesten, H. B?:ihm / Ionic mobility in basic double salt
other phases indicating that the physically adsorbed water has no influence. m
2
4. D i s c u s s i o n
,<
~q e-
E
0
2
4
6
Real Axis 1106 ~1
Fig. 4. Impedance-plot (Z' versus Z") of a recrystallised MgAI(3 : I )-phase at room temperature; ol is the tilt angle of the semicircle.
-6.5
-7 E -7.5
a
-8.5 =
.
-9
,
,
L ,
i
,
j
,
.
,i
=b
. . . . .
i
. . . . . . . . . .
2.9 3.2 3.5 1000/T IK]-' Fig. 5. Arrhenius plot of recrystallised Mg-A1 phases: (a) 3:1 stoichiometry and (b) 2:1 stoichiometry. 2
2.3
2.6
creases (2: 1-stoichiometry, activation energy about 0.01 eV) or remains constant (3:1 stoichiometry). At room temperature a conductivity of about 10 - 6 f~- ~ c m - t was found. Samples, which were not recrystallised exhibit room temperature conductivities, which are higher by one or two orders of magnitude (about 10 -4 f 2 - ' c m - t ) which is caused by the lower crystallinity and a lower degree of order of the Mg-A1 distribution within the octahedral layers. If we compare these results with the ones of the Zn-AI- and Ni-Al-phases, we find that the crystallinity and ordering of the phases are the dominant parameters. The Zn-A1CO3-phase which has the best crystallinity of all measured samples shows the lowest conductivity. The hot pressed samples show no difference to the
The basic question was whether the structural features of the basic double salts promote a proton transport. The use of blocking electrodes requires the presence of so-called "acid protons". The classical example is the high-temperature phase of hydrogenuranylphosphate-tetrahydrate ( H U P ) . In this compound the U (VI) greatly reduces the basicity of the phosphate oxygens. Therefore it is possible that the proton of the OH-group in the hydrogen bond is shifted towards the H20 molecule forming H30 + units [1]. A Grotthus-type mechanism of conduction must be postulated i.e. a sequence of molecular reorientations and proton jumps in a disordered hydrogen bond network. In the low-temperature phase another conduction mechanism prevails: the vehicle mechanism [ 17 ]. It is characterised by a co-operative motion of H30 + ions and H 2 0 molecules [4]. So in both types of proton transport the hydronium ion is formed and should be detectable by spectroscopic methods. In the above examples the H30 + vibrations can clearly be identified in the I R - r o o m temperature spectra [ 18,4 ]. An important criterion to distinguish between both mechanisms is the presence or absence of a hydrogen bond network. For the Grotthus type such a network is required, whereas the diffusive motion of the "vehicle" is strongly inhibited by such a network. In the basic double salts a lowering of the OH-bond strength occurs because of the trivalent cation. The hydronium ion with a characteristic short O - H . . ' H distance (about 2.60 A), is never found in the IR spectra. From the chemical point of view it seems unlikely that the H + of the hydroxide group can behave like an "acid" proton although there is a weakening of the bond strength of the OH group. So the spectroscopic results and chemical arguments lead to the conclusion that the basic double salts cannot supply protons for conduction. In the IR recordings there is another strong hydrogen bond (at about 3080 cm-~ ), which is formed by the interlayer water and the carbonate. The length of the bridge is about 2.73 A. There is some prob-
203
R. Oesten, H. BOhm / Ionic mobility in basic double salt
ability for the formation of O H - and hydrogencarbonate according to the reaction which is well known from aqueous chemistry H20+CO
2 - ~ O H - + HCO~-.
40 a
~ ~
~~
( 1)
In the structure of the CO 2--containing double salts only one half of the anion sites are occupied. So the formed hydroxide can avoid the electrostatic forces. The reverse reaction can take place by inversion of ( 1 ) by maybe including a water molecule as an intermediate host:
.< 20 10
•
•
•
i
40
,
,
•
i
•
,
•
i
,
,
•
i
80 120 160 T e m p e r a t u r e I° CI
,
,
•
i
200
Fig. 6. Variation of the depressing angle a with temperature: (a) 3:1 stoichiometry and (b) 2:1 stoichiometry.
O H - - HOH ~ HOH + O H - , O H - + HCO3 ~ HOH + CO32- .
(2)
Therefore we conjecture that the mobile species is the hydroxide ion which is formed within the structure through reaction (1). With this model for the charge transport the observed Arrhenius plot can be interpreted. At low temperatures the conduction mechanism can formally be described as the Grotthus type, i.e. molecular reorientations and "OH jumps" in a disordered hydrogen bond network. With increasing temperature the thermal vibrations become dominant inhibiting the above described conduction mechanism. This leads to a decrease of the conductivity in the Arrhenius plot. In a narrow temperature range (between 110°C and 180°C for the 2:1 and 140°C and 170°C for the 3 : 1 stoichiometry) the conductivity increases or remains at an approximately constant value. At these elevated temperatures there already exists a noticeable loss of interlayer water, see fig. 2. The conductivity mechanism is now controlled by OH diffusion. This interpretation is supported by the diagram depicting the angle c~ (fig. 4) by which the semicricle of the impedance is tilted below the real axis. In the literature two reasons for the tilting of semicircles are discussed. The first is associated with the polycrystallinity of the samples (no single time-constant, particle size distribution, density of the pellets, contact sample-electrode etc. ). Ho et al. [ 19 ] could show that the tilt angle c~ of polycrystalline samples becomes lower with increasing temperature. So we may expect that the angle becomes lower with increasing temperature due to these effects. However, another reason is associated with non-
local process, e.g. diffusion. If the angle a is plotted versus temperature (fig. 6) a maximum value is observed at 150°C for both stoichiometries. This temperature lies inside the range of the thermally activated conductivity. So we may conclude that the process is characterised by a maximum of diffusive mobility. The interpretation of the thermally activated process is also supported by another argument: At low temperatures all water molecules are linked to the octahedral layers through hydrogen bonds. On the other hand these hydrogen bonds must be broken before the loss of the interlayer water can take place. Only afterwards the water molecules can leave the structure with further supply of energy. This means that an intermediate stage is characterised by a maximum of diffusive motion. We must assume that the thermally activated conductivity occurs in this intermediate range. Thus the conductivity data of the studied compounds is consistent with the proposed model for the charge transport; one exception in the observed impedance diagrams remains: the almost linear tail at low frequencies/low temperatures cannot be interpreted.
Acknowledgement
The project has been supported by a grant of the Deutsche Forschungsgemeinschaft, local support was given by the Materialwissenschaftliches Forschungszentrum der Universit~it Mainz.
204
R. Oesten, H. B6hm / Ionic mobility in basic double salt
References [1] A.T. Howe and M.G. Shilton, J. Solid State Chem. 28 (1979) 345. [2] S. Chandra and N. Singh, J. Phys. C16 (1983) 3083. [3] S. Chandra and N. Singh, J. Phys. C16 (1983) 3099. [4] K.D. Kreuer, A. Rabenau and A. Messer, Appl. Phys. A32 (1983) 45. [5] W, Hofmeister and H. von Platen, XVth Congress and General Assembly, IUCr (Bordeaux, France, July, 1990). [6] W. Hofmeister and H. von Platen, Cryst. Rev. 3 (1992) 3. [7]V.A. Drits, T.N. Sokolova, G.V. Sokolova and V.I. Cherkashin, Clays Clay Miner. 35 ( 1987 ) 401. [8] R. Allmann, Acta Cryst. B24 (1968) 972. [9] A. de Roy and J.P. Besse, Solid State Ionics 35 (1989) 35. [ 10] A. de Roy and J.P. Besse, Solid State Ionics 46 ( 1991 ) 95.
[ 11 ] W. Feitknecht, Helv. Chim. Acta 25 (1942) 555. [ 12 ] S. Miyata, Clays Clay Miner. 23 ( 1975 ) 369. [ 13 ] M. Gastuche, G. Brown and M.M. Mortland, Clays Clay Miner. 7 (1967) 177. [ 14] L. Pauling, Nature of the Chemical Bond, 2nd Ed. (Cornell Univ. Press, London, Ithaca, 1948 ). [ 15 ] E.R. Lippincott and R. Schroeder, J. Chem. Phys. 23 ( 1955 ) 1099. [ 16] A.K. Jonscher, Nature 267 (1977) 673. [ 17 ] K.D. Kreuer, A. Rabenau and W. Weppner, Angew. Chem. 94 (1982) 224. [ 18 ] R.W.T. Wilkens, A. Mateen and G.W. West, Am. Miner. 59 (1974) 811. [ 19 ] C. Ho, I.D. Raistrick and R.D. Huggins, J. Electrochem. Soc. 127 (1976) 343.
Solid State Ionics 62 (1993) 199-204 North-Holland
IONICS Ionic mobility in basic double salt. Part I: Hydroxycarbonates R. O e s t e n a n d H. B 6 h m Institut fur Geowissenschaftender Universitdt, 6500 Mainz, Germany Received 15 January 1993; accepted for publication 11 March 1993
In this paper a possible new candidate for proton conduction is examined: basic double salts with a double layer structure. The chemical formula is [Me~_+xMe~+ ( O H - ) 2 ]x+ [A~7n.yH20]. In this part we report the results of phases with C O l - as anion and Mg-AI, Ni-AI and Zn-AI as cations. The synthesised phases (coprecipitation, hydrothermal) were characterised by XRD, SEM, IR thermogravimetry and chemical analysis, ac studies with blocking electrodes yield room-temperature conductivities in the range of I 0-4-10-6 ~r~_I c m - i , depending on the method of synthesis. The results showed no evidence of proton conduction. The mobile species is the OH--ion, which is formed inside the interlayer due to the reaction H20 + CO~z- ¢~OH- + HCO~-. With this model for the ionic conductivity the observed Arrhenius-plot data can be interpreted. At low temperatures the conductivity mechanism is formally described as Grotthus type. At higher temperatures the conductivity is controlled by OH diffusion. That model is supported by the interpretation of the tilt angle a of the semicircle.
1. Introduction
Me 2+ : Mg, Ni, Zn, Cd, ... Me 3+ :A1, Fe, Cr, Ga ....
Proton conduction in solids is of great interest because o f its potential application in fuel cells, sensors, etc. Two mechanisms o f proton conduction in solids have been discussed in the literature: (a) the Grotthus mechanisms and (b) the vehicle mechanism. In the Grotthus mechanism protons are mobile in a disordered hydrogen bond network (e.g. H U P [ l ], N 2 H 6 S O 4 [2,3 ] etc.) via reorientation o f structural units followed by jumping, whereas the vehicle mechanism is characterised by co-operative motion o f H 3 0 + and H 2 0 (e.g. HUAs [4] ); this mechanism requires a structural proton and a mobile vehicle like H 2 0 , N H 3 etc. acting as a proton acceptor. In this paper a new c o m p o u n d as a conceivable candidate for proton conduction is investigated: basic double salt with a double layer structure. The chemical formula o f the double salts can be written as follows: 2+ 3+ [Me]_~Mex (OH-)2]
with
x+ [ A x n F n - y H 2 0 ]
A ~- :CO 2-, C I - , SO~- .... The parameter x in the above formula has only two fixed values: 0.33 or 0.25, i.e. Me + :Me 3+ equals 2:1 or 3:1. The crystal structure of the basic double salts can be derived from brucite, Mg(OH)2. In this structure the O H - ions build up a hexagonal closed packing, whereby the Mg 2+ ions occupy half o f the octahedral voids in such a way, that every second sheet of voids remains empty and hence every other one is completely occupied by Mg 2+. If a fixed amount of Mgions is replaced by trivalent ions like aluminium, positively charged sheets are formed. The charge compensation occurs by anions which - together with water molecules - form the interlayer (fig. 1 ). The cations in the octahedral layer are ordered [5,6] and the two stoichiometries are the only ones in which the aluminium ion is only surrounded by magnesium ions. Due to this ordering there are only two possible stacking sequences: a hexagonal one with two ( 2 H ) and a rhombohedral one with three double layers (3R). In natural members of the double salts both stacking sequences are realised while in
0167-2738/93/$ 06.00 © 1993 Elsevier Science Publishers B.V. All fights reserved.
200
R. Oesten, 14. B6hm /Ionic mobility in basic double salt
Fig. 1. Schematicof the structure with a planar anion, water molecules are not drawn. synthetic phases only the rhombohedral polytype is formed. A recent summary of the natural members is given by Drits et al. [ 7]. It is remarkable that the carbonate ion is strongly favoured for being incorporated into the structure and that the interlayer is characterised as liquid-like [ 8 ]. Recently Z n - A I ( 2 : I ) - C 1 and Z n - C r ( 2 : I ) - C I were investigated for proton conduction [9,10 ]. A proton and chloride mobility is proposed, but no spectroscopic results (like IR or NMR) are given to support the interpretation and no interdependencies of the two mobilities are worked out.
2. Experimental The basis double salts were synthesised by the coprecipitation method [ 11-13 ]. As the carbonates of the di- and trivalent cations have a low solubility in water, a stoichiometric mixed solution of the metal chlorides was prepared, which was precipitated in a basic medium, e.g. NaOH containing NazCO3 as the carbonate source, under vigorous stirring. The precipitates were washed with distilled water until they were free of C1- and dried at 80°C. A great variety of cation-combinations was synthesised this way. The crystallinity of the samples was bad except for the Zn-AI(2 : 1 )-phase. So the samples were recrystallised hydrothermally. With our equipment (maximum temperature 200 ° C) only the Mg-A1- and Ni-Al-phases could be recrystallised successfully. For all other cation combinations the experimental conditions were not sufficient. The phases were identified and characterised by X-ray powder diffraction (Cu Kct radiation, Bragg-
Brentano focussing, graphite secondary monochromator). The scanning electron microscopy studies (SEM) were carried out with the Philips model PSEM 500 (acceleration voltage 25 kV, gold coating). Infrared data were recorded using a Perkin-Elmer Fourier-transform spectrometer, model 1760. The KBr preparation technique was employed. The ac conductivity was determined by impedance measurements with a microprocessor-controlled frequency response analyser (Solartron 1170). The samples were pressed in pellets with 8 t/cm 2 and plated with gold in vacuum. The impedance data were recorded in the frequency range of 1 Hz to 1 MHz and in the temperature range of room temperature to 230°C. To get a guess of the influence of physically adsorbed water some samples were hot pressed at 100°C and immediately measured. The amount of cations was determined by X-ray fluorescence analysis on a Philips spectrometer, model PW1404 after calcination of the samples. The water content was determined by thermogravimetric analyses using a Netzsch simultan-thermoanalyse STA, model 429. The measurement was performed in normal atmosphere and with a heating rate of 5°C/rain in platinum crucibles. The carbonate determination was carried out with the Leco microprocessor-controlled analyses system CS-125 based on the IR-absorption technique. For the chloride analyses the coulometric precipitation titration method was applied.
3. Results The synthesised samples were free of chlorine and contained the approximate maximum number of water molecules (three water molecules for the 2:1 and four for the 3:1 stoichiometry). In fig. 2 a typical thermo-analytical diagram of the basic double salts is shown. There are two steps of marked weight loss, whereby the second step consists of two reactions. The first step is caused by loss of interlayer water and the second by loss of the anion and the dehydroxylation of the octahedral layer. After these reactions Me:+O and spinel is formed, which was verified by X-ray diffractometry at elevated temperatures.
R. Oesten, H. BOhm / Ionic mobility in basic double salt
a 10o
90 80 ~ 70 61) 50
........
I .........
0
I .........
100
I .........
I .........
L .....
200 300 400 Temperature ]°CI
500
Fig. 2. D T G (a) and T G (b) diagram of a recrystallised M g AI ( 3: 1 )-phase.
100
z / '/
75 ee
50
,
400
)
II
/
Ig
i/
,
.
i
1000
.
i
S
•\
'"
\
/
~/
.
i
,
1600
i
,
r
.
2200
)
,
i
,
2800
i
,
r
,
3400
i
,
i
4000
Wavenumber Icml" Fig. 3. IR diagram o f a recrystallised Mg-AI(2: l )-phase.
Fig. 3 shows an infrared diagram of the basic double salts. The diagram is dominated by the vibrations of the O H - , CO ] - and H20. The width of the OH-stretching mode at 3450 c m - I indicates that all OH groups form hydrogen bonds. The shoulder at 3080 cm -1 is interpreted as a hydrogen bond between interlayer water and the carbonate. The vibrations about 1400 c m - l ( P 3 ) , 870 c m - ' (v2) and 680 cm -~ (v4) belong to the carbonate anion. The splitting of the asymmetric stretch mode (1400 cm -1 ) indicates a lowering of the carbonate symmetry. This is caused by the resonance position of the anion [5] which causes the symmetric stretch mode ul (at 1050 cm -1) to become IR active. For the further discussion it is important to focus on the hydrogen bonds in the basic double salts. Therefore Pauling's second rule for ionic crystals [ 14] should be applied to the double salts: "In a sta-
201
ble ionic compound the sum of the electrostatic bonding forces between the anion and its neighbouring cations is equal to the charge of the anion (with opposite sign)." In the basic double salts the O H - of the octahedral layer has a charge of 11/6 (2: 1stoichiometry) or 11/8 (3:1 stoichiometry) respectively, which is caused by the trivalent aluminium ion. This means the hydroxide must compensate a charge bigger than one. Unlike halogenide ions the hydroxide can, in fact, compensate a bigger charge, if the O - H bond is weakened accordingly. This is realised by the formation of hydrogen bonds. The acceptors of the bonds are the water molecules and anions of the interlayer; the water molecules are necessary for the stability of the structure. Therefore the hydrogen bonds have different lengths depending on the aluminium contents of the octahedral layer. This is verified by the shift of the OH-stretching mode relative to the reference mode at 3700 cm-1 of pure Mg(OH)2. According to the theory of Lippincott and Schroeder [15 ] the bond length of the O - H is found to be 0.987 A (2:1) and 0.982 (3:1 ); the length of the whole hydrogen bond is 2.88 A (2:1) and 2.95 A (3: 1). So fairly precise predictions can be given for the cation ratio of the octahedral layer without knowing the results of the chemical analysis. The SEM recordings show that the phases crystallise as very thin hexagonal plates. The size of the crystals varies with chemistry. The recrystallised MgA 1 - C O 3 samples have particle sizes about 1-5 pm and the Zn-A1-CO3 crystals are noticeably smaller than 1 um. A lamellar habit was never found. In fig. 4 a typical impedance diagram of a recrystallised Mg-A1-CO3 phase is shown. One characteristic feature of the diagram is that the centre of the semicricle is markedly tilted below the real axis. Another characteristic is the nearly linear behaviour at low frequencies which only appears at lower temperatures. The Arrhenius plot of the conductivity is shown in fig. 5. In this diagram the conductivities have been determined by extrapolating the semicircles to the real axis. The impedance data yield the same Arrhenius diagram if the method of Jonscher [ 16 ] is applied where a log a versus log co plot is used for evaluation. At lower temperatures the conductivity decreases with increasing temperature. In a narrow temperature range the conductivity then in-
202
R. Oesten, H. B?:ihm / Ionic mobility in basic double salt
other phases indicating that the physically adsorbed water has no influence. m
2
4. D i s c u s s i o n
,<
~q e-
E
0
2
4
6
Real Axis 1106 ~1
Fig. 4. Impedance-plot (Z' versus Z") of a recrystallised MgAI(3 : I )-phase at room temperature; ol is the tilt angle of the semicircle.
-6.5
-7 E -7.5
a
-8.5 =
.
-9
,
,
L ,
i
,
j
,
.
,i
=b
. . . . .
i
. . . . . . . . . .
2.9 3.2 3.5 1000/T IK]-' Fig. 5. Arrhenius plot of recrystallised Mg-A1 phases: (a) 3:1 stoichiometry and (b) 2:1 stoichiometry. 2
2.3
2.6
creases (2: 1-stoichiometry, activation energy about 0.01 eV) or remains constant (3:1 stoichiometry). At room temperature a conductivity of about 10 - 6 f~- ~ c m - t was found. Samples, which were not recrystallised exhibit room temperature conductivities, which are higher by one or two orders of magnitude (about 10 -4 f 2 - ' c m - t ) which is caused by the lower crystallinity and a lower degree of order of the Mg-A1 distribution within the octahedral layers. If we compare these results with the ones of the Zn-AI- and Ni-Al-phases, we find that the crystallinity and ordering of the phases are the dominant parameters. The Zn-A1CO3-phase which has the best crystallinity of all measured samples shows the lowest conductivity. The hot pressed samples show no difference to the
The basic question was whether the structural features of the basic double salts promote a proton transport. The use of blocking electrodes requires the presence of so-called "acid protons". The classical example is the high-temperature phase of hydrogenuranylphosphate-tetrahydrate ( H U P ) . In this compound the U (VI) greatly reduces the basicity of the phosphate oxygens. Therefore it is possible that the proton of the OH-group in the hydrogen bond is shifted towards the H20 molecule forming H30 + units [1]. A Grotthus-type mechanism of conduction must be postulated i.e. a sequence of molecular reorientations and proton jumps in a disordered hydrogen bond network. In the low-temperature phase another conduction mechanism prevails: the vehicle mechanism [ 17 ]. It is characterised by a co-operative motion of H30 + ions and H 2 0 molecules [4]. So in both types of proton transport the hydronium ion is formed and should be detectable by spectroscopic methods. In the above examples the H30 + vibrations can clearly be identified in the I R - r o o m temperature spectra [ 18,4 ]. An important criterion to distinguish between both mechanisms is the presence or absence of a hydrogen bond network. For the Grotthus type such a network is required, whereas the diffusive motion of the "vehicle" is strongly inhibited by such a network. In the basic double salts a lowering of the OH-bond strength occurs because of the trivalent cation. The hydronium ion with a characteristic short O - H . . ' H distance (about 2.60 A), is never found in the IR spectra. From the chemical point of view it seems unlikely that the H + of the hydroxide group can behave like an "acid" proton although there is a weakening of the bond strength of the OH group. So the spectroscopic results and chemical arguments lead to the conclusion that the basic double salts cannot supply protons for conduction. In the IR recordings there is another strong hydrogen bond (at about 3080 cm-~ ), which is formed by the interlayer water and the carbonate. The length of the bridge is about 2.73 A. There is some prob-
203
R. Oesten, H. BOhm / Ionic mobility in basic double salt
ability for the formation of O H - and hydrogencarbonate according to the reaction which is well known from aqueous chemistry H20+CO
2 - ~ O H - + HCO~-.
40 a
~ ~
~~
( 1)
In the structure of the CO 2--containing double salts only one half of the anion sites are occupied. So the formed hydroxide can avoid the electrostatic forces. The reverse reaction can take place by inversion of ( 1 ) by maybe including a water molecule as an intermediate host:
.< 20 10
•
•
•
i
40
,
,
•
i
•
,
•
i
,
,
•
i
80 120 160 T e m p e r a t u r e I° CI
,
,
•
i
200
Fig. 6. Variation of the depressing angle a with temperature: (a) 3:1 stoichiometry and (b) 2:1 stoichiometry.
O H - - HOH ~ HOH + O H - , O H - + HCO3 ~ HOH + CO32- .
(2)
Therefore we conjecture that the mobile species is the hydroxide ion which is formed within the structure through reaction (1). With this model for the charge transport the observed Arrhenius plot can be interpreted. At low temperatures the conduction mechanism can formally be described as the Grotthus type, i.e. molecular reorientations and "OH jumps" in a disordered hydrogen bond network. With increasing temperature the thermal vibrations become dominant inhibiting the above described conduction mechanism. This leads to a decrease of the conductivity in the Arrhenius plot. In a narrow temperature range (between 110°C and 180°C for the 2:1 and 140°C and 170°C for the 3 : 1 stoichiometry) the conductivity increases or remains at an approximately constant value. At these elevated temperatures there already exists a noticeable loss of interlayer water, see fig. 2. The conductivity mechanism is now controlled by OH diffusion. This interpretation is supported by the diagram depicting the angle c~ (fig. 4) by which the semicricle of the impedance is tilted below the real axis. In the literature two reasons for the tilting of semicircles are discussed. The first is associated with the polycrystallinity of the samples (no single time-constant, particle size distribution, density of the pellets, contact sample-electrode etc. ). Ho et al. [ 19 ] could show that the tilt angle c~ of polycrystalline samples becomes lower with increasing temperature. So we may expect that the angle becomes lower with increasing temperature due to these effects. However, another reason is associated with non-
local process, e.g. diffusion. If the angle a is plotted versus temperature (fig. 6) a maximum value is observed at 150°C for both stoichiometries. This temperature lies inside the range of the thermally activated conductivity. So we may conclude that the process is characterised by a maximum of diffusive mobility. The interpretation of the thermally activated process is also supported by another argument: At low temperatures all water molecules are linked to the octahedral layers through hydrogen bonds. On the other hand these hydrogen bonds must be broken before the loss of the interlayer water can take place. Only afterwards the water molecules can leave the structure with further supply of energy. This means that an intermediate stage is characterised by a maximum of diffusive motion. We must assume that the thermally activated conductivity occurs in this intermediate range. Thus the conductivity data of the studied compounds is consistent with the proposed model for the charge transport; one exception in the observed impedance diagrams remains: the almost linear tail at low frequencies/low temperatures cannot be interpreted.
Acknowledgement
The project has been supported by a grant of the Deutsche Forschungsgemeinschaft, local support was given by the Materialwissenschaftliches Forschungszentrum der Universit~it Mainz.
204
R. Oesten, H. B6hm / Ionic mobility in basic double salt
References [1] A.T. Howe and M.G. Shilton, J. Solid State Chem. 28 (1979) 345. [2] S. Chandra and N. Singh, J. Phys. C16 (1983) 3083. [3] S. Chandra and N. Singh, J. Phys. C16 (1983) 3099. [4] K.D. Kreuer, A. Rabenau and A. Messer, Appl. Phys. A32 (1983) 45. [5] W, Hofmeister and H. von Platen, XVth Congress and General Assembly, IUCr (Bordeaux, France, July, 1990). [6] W. Hofmeister and H. von Platen, Cryst. Rev. 3 (1992) 3. [7]V.A. Drits, T.N. Sokolova, G.V. Sokolova and V.I. Cherkashin, Clays Clay Miner. 35 ( 1987 ) 401. [8] R. Allmann, Acta Cryst. B24 (1968) 972. [9] A. de Roy and J.P. Besse, Solid State Ionics 35 (1989) 35. [ 10] A. de Roy and J.P. Besse, Solid State Ionics 46 ( 1991 ) 95.
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