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Mercury sublattice melting transition in the misfit intercalation compound H1.24TiS2
.I. Phys. Chm
Pergamon
MERCURY
.
I
So/ids Vol57, Nos 6-8. pp. Il29- 1132, 1996 Copyright 0 1996 Elwier Science Ltd Printed in Great Britain. All rights reserved 0022.3697/96 Si5.00 + 0.00
SUBLATTICE MELTING TRANSITION IN THE MISFIT INTERCALATION COMPOUND Hg, ,24TiS2
P. MOREAU_F, P. GANALt,
S. LEMAUX_F, G. OUVRARDf
and M. McKELVY$
tlnstitut des Materiaux de Nantes, UMR CNRS 110,2 rue de la Houssiniere, 44072 Names Cedex 03, France ICenter for Solid State Science, Arizona State University, Tempe, AZS5287-1704,U.S.A. (Received 28 May 1995; accepted 31 May 1995)
Abstract-Mercury can be intercalated into TiSs to form a compound, HgluTiSz, which exhibits novel behavior, including superstoichiometric mercury uptake, no, or at most a very small degree of, guest-host charge transfer, and the formation of incommensurate Hg chains in the guest galleries. Herein, X-ray powder diffraction (XPD) and differential scanning calorimetry (DSC) have been used to determine the effects of temperature on the Hgl,xTiSz structure from ambient temperature to 500 K. DSC studies reveal the presence of a reversible thermal transition near 473 K. The XPD patterns taken below the transition temperature are all characteristic of the ambient temperature structure together with modest sublattice thermal expansion with increasing temperature. However, above the transition temperature, all of the reflections uniquely associated with the Hg sublattice disappear, while the positions and intensities of the (001) reflections confirm the Hg remains intercalated. Thus, above the 473 K transition the in-plane Hgsublattice structure and the associated intercalant Hgchains havemelted to form guest layers with liquid-like disorder. The evolution of the host and Hg sublattice cell parameters as a function of temperature exhibits the expected discontinuous behavior associated with such a first-order transition. Keyworcis: A. chalcogenides, C. differential scanning calorimetry, C. X-ray diffraction, D. phase transitions, D. thermal expansion.
1. INTRODUCTION Layered transition-metal dichalcogenides such as lTT& and 2H-TaS2 are well known for their ability to form intercalation compounds with a wide variety of guest species. One of the most widely studied classes of transition metal dichalcogenide intercalation compounds (TMDICs) are those containing elemental metal guests, which generally exhibit guest-host charge transfer with the guest species occupying specific lattice sites in the expanded dichalcogenide lattice. However, recent studies have shown that mercury intercalated dichalcogenides such as Hg1.14 TiS2 [l] and Hgl.19TaS2 [2] exhibit novel behavior, including superstoichiometric mercury uptake, no, or at most a very small degree of, guest-host charge transfer [3], and the formation of incommensurate Hg chains in the guest galleries. Detailed structural analysis of Hg1,24TiS2 [4] has shown it has a (3 + l)dimensional structure, which is best described as interpenetrating TiS2 and Hg sublattices with common a and c axes and incommensurate b axes parallel to the Hg chains. These chains are embedded in trigonal-prismatic sulfur channels created by hostlayer restacking during the intercalation process. Similar one-dimensional Hg chains have been reported for Hg3_6MP6 (M = As, Sb, Ta, Nb) [5]
and P-Hg [6], which emphasize the important role of guest-guest interactions in this new class of MTMDICs. Previous DSC studies of the Hgl.z4TiSZ phase [l] revealed the presence of a reversible thermal transition near 468K. Herein, we demonstrate, via high temperature X-ray diffraction study, that this transition is associated with two-dimensional melting of the mercury sublattice.
2. EXPERIMENTAL Mercury intercalation into Ti!$ was performed at room temperature, without moderate temperature annealing, as mentioned in Ref. [l]. Nevertheless, TGA
and X-ray diffraction
indicate
the Hgl,24TiSZ
obtained is identical, in both composition and structure, to that prepared with moderate annealing. DSC were performed between 20 and 220°C using a Perkin-Elmer DSC4. Samples were sealed in closed containers to avoid mercury escape during analysis. In open sample containers deintercalation begins around 200°C. X-ray diffraction data have been collected with a 120” curved detector on an INEL CPS 120 diffractometer, using a monochromatized CIQ-~~ radiation (X = 1.540598 A). Samples containing crystallites between 10 and 50 pm in size were contained in experiments
1129
P. MOREAU et al.
1130
sealed 0.1 mm diameter Lindemann tubes which were rotated during analysis. This allows very accurate data collection, by rendering X-ray sample absorption and mercury deintercalation during heating insignificant. A small furnace was built for the high temperature Xray diffraction studies. It consists of a one closed end tube heater (1 cm internal diameter, 5 cm long), with Kapton windows to allow X-ray analysis. This furnace was used vertically around the Lindemann tube at sample temperatures up to 300°C. The vertical thermal gradient was less than 3°C along the sample and the temperature was very stable versus time (less than &lo(Z). A typical time exposure was 10 h at each temperature, after thermal stabilization. X-ray diffraction data have been analyzed using the PROLIX program [7] and the cell parameters have been refined following a least-squares procedure.
3. RESULTS
We have been able to reproduce in DSC experiments the reversible transition previously observed by E. Ong ef al. (Fig. 1) [l]. It shows an endothermic peak upon heating with an enthalpy of 1.60(5) calories per gramme of Hg1.24TiS2. This corresponds to an enthalpy of 0.47(2) kcal/mol of Hg which is close to that observed for the melting of elemental mercury (0.55 kcal/mol) [8]. This transition is reversible with an onset temperature of 200(2)“C for both heating and cooling. By weighing the DSC containers before and after the experiments, it was confirmed that no mercury escaped. If the experiment was performed using an open container, the DSC curve no longer represented a single event upon cooling, with two peaks observed around 200 and 190°C (Fig. 2). Weighing the DSC containers before and after analysis indicated that about 4% of the mercury was lost in these studies, corresponding to a final composition close to Hg,,,gTi&.
100
120
140 160 180 TemperaturePC)
200
220
Fig. 1. DSC curve obtained at IO”Cmn-’ on a Hg,,24TiS2 sample in a sealed container.
I
-_a’
.I!
E
2 P P)
r t~“l”‘l”‘l”‘l”‘l”~i loo
120
140 160 180 Temperature(“C)
200
220
Fig. 2. DSC curve obtained at 10”Cmn-’ on a Hg,.2.+TiS2 sample in an open container.
Figure 3 shows partial X-ray diffraction patterns taken below (175°C) and above (205%) the transition temperature observed by DSC experiments. Above the transition, the diffraction pattern is much simpler. As shown in Fig. 3, the number of diffraction peaks is reduced from 11 to 3 in the 30-44” region of 20. Between 5 and 95” in 28, the number of peaks is reduced from 34 to 14. Due to the misfit nature of the ambient temperature Hg,,24TiS2 structure along the b axis, a composite approach was used to refine its structure [4]. As a result, two different Miller indices are used for k, k, and k2 for the TiSz and Hg sublattices, respectively, which are put in the order hk, Ikz. Therefore the X-ray diffraction lines can be divided in three subsets: (1) the hk,lO (k, 2 1) reflections that are generated only by the TiS2 sublattice, (2) the hOfkz (k2 2 1) lines which correspond solely to the Hg lattice and (3) the hOI lines common to both sublattices. Above the transition temperature, only the lines corresponding to the first subset and the OOIOlines are observed. It is no longer necessary to consider a
Fig. 3. Partial view of X-ray diffraction patterns of Hg,,,, TiS2 at 175°C (thin line) and 205°C (broad line), i.e. below and above the transition temperature observed in DSC experiments. A large decrease in the number of diffraction peaks is observed at 205°C.
Intercalation compound Hg,,,,TiS, mercury network to index the diffraction pattern. Nevertheless, based on the very slight shift of the 00/O lines, it is clear the mercury remains in the van der Waals gap and has not deintercalated. The intensities of the 0010 lines are not noticeably modified by the transition. Since, theoretical intensity calculations, using the Lazy Pulverix program [9], have shown that the 0010 intensities are very sensitive to mercury concentration, due to the high diffraction power of mercury, it is apparent that the global composition of the phase has not been modified and the observed transition is not associated with mercury deintercalation. Calculations also indicate that the diffraction power of mercury is the primary contributor to the intensities of the common lines for the ambient-temperature structure (subset (3)). For example, in the 2010 series, the TiS2 network’s contribution to the intensities varies between only 0.1 and 2.4%, depending on I. All these considerations about the disappearance of a large fraction of the diffraction lines prove that mercury remains intercalated in the TiSz but its contribution to the diffraction is suppressed except for the 0010 lines which characterize the order along the c direction. The ambient temperature X-ray diffraction pattern of Hgi,24TiS2 has been previously indexed using monoclinic sublattices [4]. The TiS2 sublattice ceil exhibits a slight intralayer distortion in which its pseudohexagonal a and b parameters are inequivalent, with the a parameter being elongated by 0.34% in
1131
comparison with TiS*. The distortion is also reflected in the monoclinic angle p, which indicates an imperfect stacking of the layers along c. For the given a and c parameters, perfect stacking would correspond to a Dangle of 102.83” whereas p = 102.33” is observed [4]. The resulting double peaks, which correspond to the monoclinic distortion in the room temperature X-ray diffraction pattern, are observed to collapse above the transition temperature and a classical hexagonal cell can be used to index the resulting pattern. In order to easily follow the evolution of the cell parameters versus temperature, the original monoclinic symmetries have been used in this study to index the patterns at all temperatures, including those above the transition. Figure 4 (a-d) show the variations of the cell parameters with temperature between 20 and 250°C. The variation of the b parameter for the TiS2 subcell is not included, since the observed variations are within the accuracy of the measurements, due to the relatively few diffraction lines that contribute to bTisz_The a, c and /I parameters, which are common to both TiS2 and Hg subcells, were highly reversible with temperature, while the bus parameter was found to be slightly elongated on cooling. A shift is observed in the variation of a, c and /I between 180 and 200°C (above the transition bus no longer exists associated with the disappearance of the Hg diffraction lines). Below the transition the thermal expansions of a and b
a 5.940
9.000
0 b 0
5.930z
0 0 5.920-
-
8
0
0
0
0
8
8
8.950 M - 8.900 v
00
0 go
8.800
1” 7I ” 50
0 C 102.8
co t m
3
102.2
00
50
l
.
0
00
0
O
0
8
1”~‘1’~‘~1~“‘1”‘~,“~’
0
0 0
:
2.755-
0 O@ l
II 1’. 300 250
0
0
~ 2.760 -
d
102.4
000
”
d .
2.765 -
102.6
l
”
200 (“C)
0
0
6
, 7 7 a I”“, I50 100 Temperature
2.7701-
0
0
00
0
8.850-
0
O
100 I50 Temperature
200 (“C)
250
300
2.750
I,,, 0
I,, 1 'I. 'I '1' 1 'I I '1 "
50
100 150 Temperature
200
I ",
250
'/
300
(“C)
Fig. 4. Variations of the monoclinic cell parameters for the composite Hg , 24TiS2structure with temperature: (a) a parameter; (b) c parameter; (c) p parameter; and (d) bHg parameter. The open circles correspond to data taken during a heating cycle, while the filled circles were taken during a cooling cycle.
P. MOREAU et al.
1132
bus are very close to that observed for pristine TiSz (1.5 x 10e5 K-‘) with values of 1.7 and 1.9 x 10e5 K-‘, respectively. The thermal expansion of the c parameter (3.0 x lo-’ K-l) is intermediate between that of TiSz (1.9 x lo-’ K-‘) and solid mercury (6.0 x lo-’ K-l). The /3 parameter is not affected by heating up to the transition temperature. This indicates the monoclinic distortion is unchanged up to the transition. Above the transition, p increases to exactly the value calculated for the distortion free structure (102.75” for the corresponding a and c values at these temperatures). On heating a is observed to decrease by about 0.17% at the transition, which is probably associated with relaxation of the Hg-S interactions, since these interactions are associated with the a expansion (distortion) of about 0.34% observed for the ambient temperature structure. At the same time, c is elongated by about 0.51% at the transition in further support of weaker Hg-S guest-host interactions.
4. DISCUSSION
The changes observed in the X-ray diffraction patterns across the transition temperature indicate a dramatic change from the guest-host interactions observed at room temperature. This is mainly reflected in the changes in a and c, as discussed above, and attaining a value of p that corresponds to undistorted host-layer stacking. These changes and the disappearance of the mercury diffraction lines, which characterize the order between mercury atoms, indicate that mercury is very mobile in the galleries above the transition temperature and close to a liquid state. It has been shown at ambient temperature that the Hg chains occupy sulfur channels with trigonal prismatic coordination [4]. Above the transition, the high Hg mobility is consistent with the loss of these incommensurate Hg-S interactions, which is evidenced by the relaxation to an undistorted host-layer structure. This results in much weaker or absent modulations of the Hg chains and independent movement of the highly mobile chains. Such a transition is consistent with the absence of hysteresis in the DSC observations and the reversibility in the evolution of a, c and /3 as a function of temperature below the transition, which indicates that the structure below the transition is readily recovered on cooling. However, this is not the case for the Hg-Hg intrachain distance (bus), which remains slightly expanded (0.15%) on cooling.
This may be associated with a highly preferred mercury mobility along the chain direction, as observed by electron diffraction [7], and to the discommensuration between Hg and TiS2 networks. Due to the incommensurability, the atomic position and sulfur environment of mercury are not precisely defined along the b direction, and the pristine situation is certainly difficult to recover upon cooling, especially at the time scale of the X-ray diffraction experiments. The second DSC peak observed for open sample containers during the cooling cycle can be related to the formation of an intermediate phase during the deintercalation process. Such an intermediate phase has been previously observed by HRTEM of the deintercalation process [lo], which was performed on a similar time scale to the DSC experiments herein. This intermediate phase is easily generated from the ambient temperature Hg1,24TiSZ structure by a simple 1/2b translation of the host layers along the Hg chains. This results in distorted-trigonalantiprismatic coordination for the sulfur channels surrounding the Hg chains, which is intermediate between the trigonal prismatic host-layer stacking for Hg1,24TiS2 and the octahedral stacking found in TiSl [lo]. Acknowledgements-We thank Professor .I. Rouxel for his continuous interest and support. This work was supported by a Human Capital and Mobility EC grant (PG).
REFERENCES
1. Ong E. W., McKelvy M. J., Ouvrard G. and Glaunsinger W. S., Chem. Mater. 4, 14 (1992). 2. Moreau P. and Ouvrard G., Chemical Physics of Intercalation II (Edited by P. Bernier, J. E. Fischer, S. Roth and S. Solin), Nato AS1 Series, Vol305, p. 35 I. Plenum Press, New York (1993). 3. Moreau P., Ouvrard G., Gressier P., Ganal P. and Rouxel J., J. Phys. Chem. Solids (1996). 4. Ganal P., Moreau P., Ouvrard G., Sidorov M., McKelvy M. and Glaunsinger W., Chem. Marer. 7, 1132 (1995). 5. Brown I. D., Datars W. R., Gillespie R. J., Morgan K. R., Tun Z. and Ummat P. K., J. Solid Siare Chem. 57, 34 (1985). 6. Atoji M., Shirber J. E. and Swendson C. A., J. Chem. Phys. 31, 1628 (1959). 7. Deniard P., Evain M., Barbet J. M. and Brec R., Mar. Sci. Forum 79-82,363 (1991). 8. Weast R. C., Handbook of Chemistry and Physics (54th edn). CRC Press, Cleveland (1973). . . 9. Yvon R., Jeitschko W. and Parthe E., J. Appl. Crystallogr. 10, 73 (1977). 10. Sidorov M., McKelvy M., Sharma R., Glaunsinger W., Ganal P.. Moreau P. and Ouvrard G.. Chem. Mater. 7. 1140 (1995).
Pergamon
MERCURY
.
I
So/ids Vol57, Nos 6-8. pp. Il29- 1132, 1996 Copyright 0 1996 Elwier Science Ltd Printed in Great Britain. All rights reserved 0022.3697/96 Si5.00 + 0.00
SUBLATTICE MELTING TRANSITION IN THE MISFIT INTERCALATION COMPOUND Hg, ,24TiS2
P. MOREAU_F, P. GANALt,
S. LEMAUX_F, G. OUVRARDf
and M. McKELVY$
tlnstitut des Materiaux de Nantes, UMR CNRS 110,2 rue de la Houssiniere, 44072 Names Cedex 03, France ICenter for Solid State Science, Arizona State University, Tempe, AZS5287-1704,U.S.A. (Received 28 May 1995; accepted 31 May 1995)
Abstract-Mercury can be intercalated into TiSs to form a compound, HgluTiSz, which exhibits novel behavior, including superstoichiometric mercury uptake, no, or at most a very small degree of, guest-host charge transfer, and the formation of incommensurate Hg chains in the guest galleries. Herein, X-ray powder diffraction (XPD) and differential scanning calorimetry (DSC) have been used to determine the effects of temperature on the Hgl,xTiSz structure from ambient temperature to 500 K. DSC studies reveal the presence of a reversible thermal transition near 473 K. The XPD patterns taken below the transition temperature are all characteristic of the ambient temperature structure together with modest sublattice thermal expansion with increasing temperature. However, above the transition temperature, all of the reflections uniquely associated with the Hg sublattice disappear, while the positions and intensities of the (001) reflections confirm the Hg remains intercalated. Thus, above the 473 K transition the in-plane Hgsublattice structure and the associated intercalant Hgchains havemelted to form guest layers with liquid-like disorder. The evolution of the host and Hg sublattice cell parameters as a function of temperature exhibits the expected discontinuous behavior associated with such a first-order transition. Keyworcis: A. chalcogenides, C. differential scanning calorimetry, C. X-ray diffraction, D. phase transitions, D. thermal expansion.
1. INTRODUCTION Layered transition-metal dichalcogenides such as lTT& and 2H-TaS2 are well known for their ability to form intercalation compounds with a wide variety of guest species. One of the most widely studied classes of transition metal dichalcogenide intercalation compounds (TMDICs) are those containing elemental metal guests, which generally exhibit guest-host charge transfer with the guest species occupying specific lattice sites in the expanded dichalcogenide lattice. However, recent studies have shown that mercury intercalated dichalcogenides such as Hg1.14 TiS2 [l] and Hgl.19TaS2 [2] exhibit novel behavior, including superstoichiometric mercury uptake, no, or at most a very small degree of, guest-host charge transfer [3], and the formation of incommensurate Hg chains in the guest galleries. Detailed structural analysis of Hg1,24TiS2 [4] has shown it has a (3 + l)dimensional structure, which is best described as interpenetrating TiS2 and Hg sublattices with common a and c axes and incommensurate b axes parallel to the Hg chains. These chains are embedded in trigonal-prismatic sulfur channels created by hostlayer restacking during the intercalation process. Similar one-dimensional Hg chains have been reported for Hg3_6MP6 (M = As, Sb, Ta, Nb) [5]
and P-Hg [6], which emphasize the important role of guest-guest interactions in this new class of MTMDICs. Previous DSC studies of the Hgl.z4TiSZ phase [l] revealed the presence of a reversible thermal transition near 468K. Herein, we demonstrate, via high temperature X-ray diffraction study, that this transition is associated with two-dimensional melting of the mercury sublattice.
2. EXPERIMENTAL Mercury intercalation into Ti!$ was performed at room temperature, without moderate temperature annealing, as mentioned in Ref. [l]. Nevertheless, TGA
and X-ray diffraction
indicate
the Hgl,24TiSZ
obtained is identical, in both composition and structure, to that prepared with moderate annealing. DSC were performed between 20 and 220°C using a Perkin-Elmer DSC4. Samples were sealed in closed containers to avoid mercury escape during analysis. In open sample containers deintercalation begins around 200°C. X-ray diffraction data have been collected with a 120” curved detector on an INEL CPS 120 diffractometer, using a monochromatized CIQ-~~ radiation (X = 1.540598 A). Samples containing crystallites between 10 and 50 pm in size were contained in experiments
1129
P. MOREAU et al.
1130
sealed 0.1 mm diameter Lindemann tubes which were rotated during analysis. This allows very accurate data collection, by rendering X-ray sample absorption and mercury deintercalation during heating insignificant. A small furnace was built for the high temperature Xray diffraction studies. It consists of a one closed end tube heater (1 cm internal diameter, 5 cm long), with Kapton windows to allow X-ray analysis. This furnace was used vertically around the Lindemann tube at sample temperatures up to 300°C. The vertical thermal gradient was less than 3°C along the sample and the temperature was very stable versus time (less than &lo(Z). A typical time exposure was 10 h at each temperature, after thermal stabilization. X-ray diffraction data have been analyzed using the PROLIX program [7] and the cell parameters have been refined following a least-squares procedure.
3. RESULTS
We have been able to reproduce in DSC experiments the reversible transition previously observed by E. Ong ef al. (Fig. 1) [l]. It shows an endothermic peak upon heating with an enthalpy of 1.60(5) calories per gramme of Hg1.24TiS2. This corresponds to an enthalpy of 0.47(2) kcal/mol of Hg which is close to that observed for the melting of elemental mercury (0.55 kcal/mol) [8]. This transition is reversible with an onset temperature of 200(2)“C for both heating and cooling. By weighing the DSC containers before and after the experiments, it was confirmed that no mercury escaped. If the experiment was performed using an open container, the DSC curve no longer represented a single event upon cooling, with two peaks observed around 200 and 190°C (Fig. 2). Weighing the DSC containers before and after analysis indicated that about 4% of the mercury was lost in these studies, corresponding to a final composition close to Hg,,,gTi&.
100
120
140 160 180 TemperaturePC)
200
220
Fig. 1. DSC curve obtained at IO”Cmn-’ on a Hg,,24TiS2 sample in a sealed container.
I
-_a’
.I!
E
2 P P)
r t~“l”‘l”‘l”‘l”‘l”~i loo
120
140 160 180 Temperature(“C)
200
220
Fig. 2. DSC curve obtained at 10”Cmn-’ on a Hg,.2.+TiS2 sample in an open container.
Figure 3 shows partial X-ray diffraction patterns taken below (175°C) and above (205%) the transition temperature observed by DSC experiments. Above the transition, the diffraction pattern is much simpler. As shown in Fig. 3, the number of diffraction peaks is reduced from 11 to 3 in the 30-44” region of 20. Between 5 and 95” in 28, the number of peaks is reduced from 34 to 14. Due to the misfit nature of the ambient temperature Hg,,24TiS2 structure along the b axis, a composite approach was used to refine its structure [4]. As a result, two different Miller indices are used for k, k, and k2 for the TiSz and Hg sublattices, respectively, which are put in the order hk, Ikz. Therefore the X-ray diffraction lines can be divided in three subsets: (1) the hk,lO (k, 2 1) reflections that are generated only by the TiS2 sublattice, (2) the hOfkz (k2 2 1) lines which correspond solely to the Hg lattice and (3) the hOI lines common to both sublattices. Above the transition temperature, only the lines corresponding to the first subset and the OOIOlines are observed. It is no longer necessary to consider a
Fig. 3. Partial view of X-ray diffraction patterns of Hg,,,, TiS2 at 175°C (thin line) and 205°C (broad line), i.e. below and above the transition temperature observed in DSC experiments. A large decrease in the number of diffraction peaks is observed at 205°C.
Intercalation compound Hg,,,,TiS, mercury network to index the diffraction pattern. Nevertheless, based on the very slight shift of the 00/O lines, it is clear the mercury remains in the van der Waals gap and has not deintercalated. The intensities of the 0010 lines are not noticeably modified by the transition. Since, theoretical intensity calculations, using the Lazy Pulverix program [9], have shown that the 0010 intensities are very sensitive to mercury concentration, due to the high diffraction power of mercury, it is apparent that the global composition of the phase has not been modified and the observed transition is not associated with mercury deintercalation. Calculations also indicate that the diffraction power of mercury is the primary contributor to the intensities of the common lines for the ambient-temperature structure (subset (3)). For example, in the 2010 series, the TiS2 network’s contribution to the intensities varies between only 0.1 and 2.4%, depending on I. All these considerations about the disappearance of a large fraction of the diffraction lines prove that mercury remains intercalated in the TiSz but its contribution to the diffraction is suppressed except for the 0010 lines which characterize the order along the c direction. The ambient temperature X-ray diffraction pattern of Hgi,24TiS2 has been previously indexed using monoclinic sublattices [4]. The TiS2 sublattice ceil exhibits a slight intralayer distortion in which its pseudohexagonal a and b parameters are inequivalent, with the a parameter being elongated by 0.34% in
1131
comparison with TiS*. The distortion is also reflected in the monoclinic angle p, which indicates an imperfect stacking of the layers along c. For the given a and c parameters, perfect stacking would correspond to a Dangle of 102.83” whereas p = 102.33” is observed [4]. The resulting double peaks, which correspond to the monoclinic distortion in the room temperature X-ray diffraction pattern, are observed to collapse above the transition temperature and a classical hexagonal cell can be used to index the resulting pattern. In order to easily follow the evolution of the cell parameters versus temperature, the original monoclinic symmetries have been used in this study to index the patterns at all temperatures, including those above the transition. Figure 4 (a-d) show the variations of the cell parameters with temperature between 20 and 250°C. The variation of the b parameter for the TiS2 subcell is not included, since the observed variations are within the accuracy of the measurements, due to the relatively few diffraction lines that contribute to bTisz_The a, c and /I parameters, which are common to both TiS2 and Hg subcells, were highly reversible with temperature, while the bus parameter was found to be slightly elongated on cooling. A shift is observed in the variation of a, c and /I between 180 and 200°C (above the transition bus no longer exists associated with the disappearance of the Hg diffraction lines). Below the transition the thermal expansions of a and b
a 5.940
9.000
0 b 0
5.930z
0 0 5.920-
-
8
0
0
0
0
8
8
8.950 M - 8.900 v
00
0 go
8.800
1” 7I ” 50
0 C 102.8
co t m
3
102.2
00
50
l
.
0
00
0
O
0
8
1”~‘1’~‘~1~“‘1”‘~,“~’
0
0 0
:
2.755-
0 O@ l
II 1’. 300 250
0
0
~ 2.760 -
d
102.4
000
”
d .
2.765 -
102.6
l
”
200 (“C)
0
0
6
, 7 7 a I”“, I50 100 Temperature
2.7701-
0
0
00
0
8.850-
0
O
100 I50 Temperature
200 (“C)
250
300
2.750
I,,, 0
I,, 1 'I. 'I '1' 1 'I I '1 "
50
100 150 Temperature
200
I ",
250
'/
300
(“C)
Fig. 4. Variations of the monoclinic cell parameters for the composite Hg , 24TiS2structure with temperature: (a) a parameter; (b) c parameter; (c) p parameter; and (d) bHg parameter. The open circles correspond to data taken during a heating cycle, while the filled circles were taken during a cooling cycle.
P. MOREAU et al.
1132
bus are very close to that observed for pristine TiSz (1.5 x 10e5 K-‘) with values of 1.7 and 1.9 x 10e5 K-‘, respectively. The thermal expansion of the c parameter (3.0 x lo-’ K-l) is intermediate between that of TiSz (1.9 x lo-’ K-‘) and solid mercury (6.0 x lo-’ K-l). The /3 parameter is not affected by heating up to the transition temperature. This indicates the monoclinic distortion is unchanged up to the transition. Above the transition, p increases to exactly the value calculated for the distortion free structure (102.75” for the corresponding a and c values at these temperatures). On heating a is observed to decrease by about 0.17% at the transition, which is probably associated with relaxation of the Hg-S interactions, since these interactions are associated with the a expansion (distortion) of about 0.34% observed for the ambient temperature structure. At the same time, c is elongated by about 0.51% at the transition in further support of weaker Hg-S guest-host interactions.
4. DISCUSSION
The changes observed in the X-ray diffraction patterns across the transition temperature indicate a dramatic change from the guest-host interactions observed at room temperature. This is mainly reflected in the changes in a and c, as discussed above, and attaining a value of p that corresponds to undistorted host-layer stacking. These changes and the disappearance of the mercury diffraction lines, which characterize the order between mercury atoms, indicate that mercury is very mobile in the galleries above the transition temperature and close to a liquid state. It has been shown at ambient temperature that the Hg chains occupy sulfur channels with trigonal prismatic coordination [4]. Above the transition, the high Hg mobility is consistent with the loss of these incommensurate Hg-S interactions, which is evidenced by the relaxation to an undistorted host-layer structure. This results in much weaker or absent modulations of the Hg chains and independent movement of the highly mobile chains. Such a transition is consistent with the absence of hysteresis in the DSC observations and the reversibility in the evolution of a, c and /3 as a function of temperature below the transition, which indicates that the structure below the transition is readily recovered on cooling. However, this is not the case for the Hg-Hg intrachain distance (bus), which remains slightly expanded (0.15%) on cooling.
This may be associated with a highly preferred mercury mobility along the chain direction, as observed by electron diffraction [7], and to the discommensuration between Hg and TiS2 networks. Due to the incommensurability, the atomic position and sulfur environment of mercury are not precisely defined along the b direction, and the pristine situation is certainly difficult to recover upon cooling, especially at the time scale of the X-ray diffraction experiments. The second DSC peak observed for open sample containers during the cooling cycle can be related to the formation of an intermediate phase during the deintercalation process. Such an intermediate phase has been previously observed by HRTEM of the deintercalation process [lo], which was performed on a similar time scale to the DSC experiments herein. This intermediate phase is easily generated from the ambient temperature Hg1,24TiSZ structure by a simple 1/2b translation of the host layers along the Hg chains. This results in distorted-trigonalantiprismatic coordination for the sulfur channels surrounding the Hg chains, which is intermediate between the trigonal prismatic host-layer stacking for Hg1,24TiS2 and the octahedral stacking found in TiSl [lo]. Acknowledgements-We thank Professor .I. Rouxel for his continuous interest and support. This work was supported by a Human Capital and Mobility EC grant (PG).
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