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Pressure dependence of Raman-active phonons in rare earth disulfides, α′-LnS2 (Ln: La, Pr, Nd)
Physica B 262 (1999) 426 —432
Pressure dependence of Raman-active phonons in rare earth disulfides, a -LnS (Ln: La, Pr, Nd) Andrzej Grzechnik* E! cole Normale Supe& rieure de Lyon, 46, alle& e d+Italie, 69364 Lyon Cedex 07, France Received 13 July 1998
Abstract High-pressure behavior of layered a -LnS (Ln: La, Pr, Nd) rare earth disulfides (P2 /b, C , Z"4) with the distorted F anti-Fe As structure (the PbFCl type, P4/nmm, D , Z"2) is studied with Raman spectroscopy in a diamond anvil cell F at room temperature. Upon compression, there occurs a phase transition in all three compounds to higher symmetry superstructures of the parent anti-Fe As structure. This transformation, associated with a hysteresis on decompression, takes place at about 5 GPa for a -LaS and a -PrS , and at about 8 GPa for a -NdS . 1999 Elsevier Science B.V. All rights reserved. Keywords: High pressure; Raman spectroscopy; Rare earth disulfides; Phase transitions
1. Introduction The ideal structure of rare earth dichalcogenides LnX (Ln: La—Lu; X: S, Se, Te) is of the anti-Fe As type (P4/nmm, D , Z"2, a +4 As , c +8 As ). In F this structure, layers of five chalcogen atoms in the basal face-centered square plane are separated by two slabs of alternating Ln> (the nine-fold coordination) and X\ ions. All the LnX compounds are distorted from this due to the formation of X—X pairs within the basal planes [1]. This distortion
* Corresponding author. E-mail: [email protected]. Now at: Universite´ Lyon I, Laboratoire de Physico-Chimie des Mate´riaux Luminescents, 43, Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France.
gives rise to a number of pseudo-cubic, tetragonal, orthorhombic, and monoclinic (b+90°) antiFe As super structures. Recently, charge-density waves in the planar square lattices were observed in LaTe Sb (0)x)1) [2]. The ideal anti-Fe As \V V structure is isotypical with the PbFCl type. Other compounds that crystallize with the PbFCl (ideal anti-Fe As) structure include alkali metal hydrogen halides, alkali earth metal fluorohalides, pnictides, chalconides, oxyhalides, and oxysulfides [3]. Stoichiometric light rare earth disulfides (La—Nd) are obtained at ambient pressure. Orthorhombic b polymorphs (Pnma, D, Z"8) are prepared by reaction of Ln S and S above 1023 K [4]. The same reaction below 1023 K yields monoclinic a forms (P2 /b, C , Z"4) [5]. LnS are also synthesized by reacting Ln O with CS — at 873 K
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 1 2 4 - 7
A. Grzechnik / Physica B 262 (1999) 426—432
pseudo-cubic compounds are obtained [6], while at lower temperatures, no higher than 773 K, monoclinic a polymorphs (P2 /b, C , Z"4) are prepared [7]. Additional long-time annealing of all the a, a , and b polymorphs above 1073 K leads to formation of the c variety (P4/nmm, D , Z"2) [7]. Preparation of stoichiometric heavy rare earth disulfides requires high-pressure techniques [8,9]. Depending on the conditions of the reaction of the respective elements, pseudo-cubic or tetragonal products are recovered to ambient conditions. Pseudo-cubic NdS can also be found at pressures 1.4—7.0 GPa and at temperatures above &773 K [8]. The summary of preparatory techniques shows that the higher symmetry polymorphs of the light rare earth disulfides (La—Nd) are favored at high temperatures, despite the reaction route. On the other hand, heavier rare earth disulfides are obtained at elevated pressures. In the case of NdS , the required temperature to obtain the pseudocubic form through the reaction of the elements decreases with increasing pressure [8]. The purpose of this study is to investigate the high-pressure behavior of a -LnS (Ln: La, Pr, Nd) using in situ Raman spectroscopy in a diamond anvil cell at room temperature. The obtained results could provide additional information on the structural stability of the light rare earth disulfides at different conditions.
2. Experimental Powder samples of a -LnS (Ln: La, Pr, Nd) were prepared as described previously [7]. Raman spectra, with spectral resolution of about 2 cm\, were collected using a XY Dilor Raman spectrometer (1800 groove/mm gratings) in backscattering geometry with CCD signal detection. Raman scattering was excited using an Ar> laser at a wavelength of 514.5 nm. Each sample and the pressure transmitting medium CsI were loaded into a Mao—Bell-type diamond cell with type II-a diamonds, brilliantly cut with 600 lm culets, and a sample chamber diameter of 250 lm. Pressures were determined from the R ruby fluorescence line [10].
427
3. Results and discussion Normal mode analysis of the ideal anti-Fe As and PbFCl structures (P4/nmm, D , Z"2) pre dicts that the vibrational modes are distributed as C"2A #B #3 #2A #2E , where the # A , B , and E modes are Raman active, and the A and E modes are infrared active. The distor tion of the ideal anti-Fe As structure due to the S—S bond formation in the basal plane of the a polymorphs (P2 /b, C , Z"4) leads to the lowering of symmetry and new selection rules C"9A # 9B #8A #7B , where the A and B modes are Raman active, and the A and B modes are in frared active. The symmetry types of S—S stretching oscillations of the “S—S molecular unit” (D sym metry) at C sites of C space group are correlated as #(D )PA(C )P A #B #A #B (C ). Golovin et al. [11] indicated that in all polymorphs of rare earth disulfides the most intense bands in powder Raman spectra in the range 400—500 cm\ were due to the S—S bonds in the basal planes. Accordingly, the bands at lower wave numbers were assigned to the displacements of the cationic sublattices relative to the anionic ones. Similar interpretation of the vibrational powder spectra for the LaS varieties was presented by Le Rolland et al. [7]. They also suggested that the a structures were defective, with the evidence of incomplete S\ pairs, i.e., paramagnetic delocalized S\ defects. These defects were localized within the structures at low temperature, with the result that the S—S stretching modes from different domains became visible in the powder Raman spectra, i.e., increasing resolution of the Raman band at about 420 cm\ with decrease in temperature. The results of the single crystal Raman measurement and valence force field calculation for LaSe (P2 /b, C , Z"4), isostructural with a- and a -LaSe , are in disagreement with this assignment [12]. In fact, the high-frequency modes were found to correspond to La—Se (La—X) vibrations and the only modes with the predominant Se—Se (X—X) stretching character occured in the lower frequency region. It was also demonstrated that the most intense bands in the powder Raman spectra are not necessarily the most intense ones in the Raman spectra measured on single crystals.
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It is seen in Fig. 1 that the first-order Raman spectra of a -LaS , a -PrS , and a -NdS at and near ambient conditions are almost the same, the only differences being in the absolute positions of the respective bands. The group analysis of the P2 /b (C , Z"4) anti-Fe As superstructure predicts the presence of 18 Raman bands, while only 12 are observed for a -LaS , and 11 for a -PrS and a NdS , each. This discrepancy could be associated with the fact that some of the missing modes are probably too weak in intensity to be observed in powder spectra. The strongest bands at about 400—450 cm\ are well resolved even at lowest pressures for all three compounds. Upon compression, these two bands merge and the higher frequency components have negative pressure shifts (Figs. 1 and 2). This band convergence occurs at about 5 GPa for a -LaS and a -PrS , and at about 8 GPa for a -NdS . At the same respective pres sures, the highest frequency band for each compound changes its pressure dependence. In addition to this, in every case a new band appears at about 430 cm\ at pressures above 8 GPa. It is not entirely clear from the compression data whether this band is actually a new one or it is just obscured by the two strong Raman lines at ambient conditions. It could emerge at elevated pressures due to its different pressure shift because after merging the strongest band does not shift. The pressure dependence of the bands at lower wave numbers is quite similar for all the compounds. The differences between a -LaS , on one hand, and a -PrS and a NdS , on the other, can also be attributed to the orientational effects in powder samples affecting the intensities, broadness, and resolution of weak bands at this wave number range, and not only to various responses of a -LaS , a -PrS , and a -NdS lattices to changes in pressure. Upon decompression to ambient conditions, all the spectral changes are reversible with a hysteresis of about 3—4 GPa at each case (Fig. 3). It is also noticeable that at intermediate pressures four bands are detected in the 400—500 cm\ range. This observation is contrary to normal mode analysis that predicts only two Raman active S—S stretching modes, A #B , for the P2 /b (C , Z"4) anti-Fe As superstruc F ture. If the two strong bands were assigned to the S—S oscillations [7,11], the problem would remain
with the other two weaker ones. Analysis of this superstructure shows that there are no Ln—S bonds that are much shorter than the other ones to be associated with the two weak bands in the 400—500 cm\ range, and the rest of the bonds with the bands at lower wave numbers below 300 cm\ [1,5]. The pressure dependencies of the strongest bands in the Raman spectra of a -LaS , a -PrS , and a NdS , including a hysteresis on decompression, are just opposite (Figs. 1—3) to what has been observed at low temperatures by Le Rolland et al. [7]. The low-temperature effect on the Raman spectra of a and a structural varieties could just lead to decrease in thermal broadening of the observed bands. Such a behavior does not necessarily imply the existence of incomplete S\ pairs in the basal planes that can be probed on the vibrational time scale. If the interpretation of the temperature evolution of the most intense bands in powder Raman spectra were correct, i.e., a localization of the itinerant S\ defects at low ¹, one would expect that these two bands also become better resolved at high pressures. In LnS with the ideal anti-Fc As (PbFCl) struc ture (P4/nmm, D , Z"2), all four basal Ln—S bonds are equivalent by symmetry. Another set of equivalent bonds comprises four, out of five, nonbasal Ln—S ones. Lowering of symmetry to the P2 /b (C , Z"4) superstructure causes all the Ln—S bonds to be non-equivalent with the average Ln—S bond being longer than the Ln—S U one [1,5]. On the other hand, analysis of the Raman spectra of the LnS compounds with differ ent superstructures [11,13] reveals the absence of splitting of the strongest band at about 400— 430 cm\ in the pseudo-cubic and tetragonal polymorphs at ambient conditions. The experimental observations presented above can be interpreted as an evidence for pressure-induced phase transitions in layered a -LnS (Ln: La, Pr, Nd) rare earth dis ulfides (P2 /b, C , Z"4) with the distorted anti Fe As structure (the PbFCl type, P4/nmm, D , Z"2). Upon compression, the most intense Raman bands converge indicating the absence of symmetry for the anti-Fe As superstructure. The mechanism for this structural change can be envisioned as a tendency of the nine-fold polyhedra
A. Grzechnik / Physica B 262 (1999) 426—432
Fig. 1. Raman spectra of a -LaS (a), a -PrS2 (b), and a -NdS (c) upon compression.
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A. Grzechnik / Physica B 262 (1999) 426—432
Fig. 2. Pressure dependence of the observed Raman frequencies in a -LaS (a), a -PrS (b), and a -NdS (c). Lines are guides for the eyes.
around the Ln> ions to become more symmetric within the anti-Fe As type, i.e., a tendency towards the equivalency of the four Ln—S and four, out of five, Ln—S bonds. As seen in Fig. 2, the U phase transition depends on the Ln> ion and occurs at about 5 GPa for a -LaS and a -PrS , and at about 8 GPa for a -NdS . From the lack of drastic changes in the number of the observed
Raman bands at the respective pressures, it could be argued that the number of the molecular units does not change (Z"4) and the resulting high-pressure structure is not the ideal anti-Fe As (PbFCl) one but rather a pseudo-cubic or tetragonal anti-Fe As superstructure. In all the cases of a -LnS studied here, the back transformations are sluggish and proceed with a hysteresis of about 3 GPa.
A. Grzechnik / Physica B 262 (1999) 426—432
431
Fig. 3. Raman spectra of a -LaS (a), a -PrS (b), and a -NdS (c) upon decompression.
The above interpretation is in agreement with the study by Webb and Hall [8] who found that the pressure necessary to synthesize stoichiometric heavy rare earth disulfides depends on the respective cation with the tendency to increase along the lanthanide series. As known from the summary of preparatory techniques, the higher symmetry polymorphs of the light rare earth disulfides (La—Nd) are favored at high temperatures and the required temperature to obtain the pseudo-cubic form of NdS through the reaction of the elements decreases with increasing pressure [8]. This strongly suggests that the observed pressures for structural changes in a -LnS (Ln: La, Pr, Nd) would decrease with increasing
temperature. The results of this study could be helpful to predict and understand the high-pressure behavior of other compounds with the PbFCl (anti-Fe As) structure [3]. It could be anticipated that the phase transitions in alkali metal hydrogen halides, alkali earth metal fluorohalides, pnictides, chalconides, oxyhalides, and oxysulfides would tend to retain the layered character of the PbFCl type.
Acknowledgements I thank P. McMillan for providing the samples and for several discussions.
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A. Grzechnik / Physica B 262 (1999) 426—432
References [1] J. Flahaut, in: K.A. Gschneider Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 2, North-Holland, Amsterdam, 1979. [2] E. DiMasi, B. Foran, M.C. Aronson, S. Lee, Phys. Rev. B 54 (1996) 13587. [3] A.F. Wells, Structural Inorganic Chemistry, 5th ed., Clarendon Press, Oxford, 1984, pp. 486—488. [4] J. Dugue´, D. Carre´, M. Guittard, Acta Crystallogr. B 34 (1978) 403. [5] S. Be´nazeth, M. Guittard, J. Flahaut, J. Solid State Chem. 37 (1981) 44. [6] T. Toide, T. Utsunomiya, M. Sato, Y. Hoshino, T. Hatano, Y. Akimoto, Bull. Tokyo Inst. Technol. 117 (1973) 41.
[7] B. Le Rolland, P. Molinie´, P. Colombet, P.F. McMillan, J. Solid State Chem. 113 (1994) 312. [8] A.W. Webb, H.T. Hall, Inorg. Chem. 9 (1970) 1084. [9] A.A. Eliseev, V.A. Tolstova, G.M. Kuz’micheva, Russian J. Inorg. Chem. 23 (1978) 1759. [10] H.K. Mao, P.M. Bell, J.W. Shaner, D.J. Steinberg, J. Appl. Phys. 49 (1978) 3276. [11] Yu.M. Golovin, K.I. Petrov, E.M. Loginova, A.A. Grizik, N.M. Ponomarev, Russian J. Inorg. Chem. 20 (1975) 155. [12] A. Grzechnik, J.Z. Zheng, D. Wright, W. Petuskey, P.F. McMillan, J. Phys. Chem. Solids 57 (1996) 1625. [13] B. Le Rolland, P. McMillan, P. Colombet, C.R. Acad. Sci. Paris, t. 312 (II) (1991) 217.
Pressure dependence of Raman-active phonons in rare earth disulfides, a -LnS (Ln: La, Pr, Nd) Andrzej Grzechnik* E! cole Normale Supe& rieure de Lyon, 46, alle& e d+Italie, 69364 Lyon Cedex 07, France Received 13 July 1998
Abstract High-pressure behavior of layered a -LnS (Ln: La, Pr, Nd) rare earth disulfides (P2 /b, C , Z"4) with the distorted F anti-Fe As structure (the PbFCl type, P4/nmm, D , Z"2) is studied with Raman spectroscopy in a diamond anvil cell F at room temperature. Upon compression, there occurs a phase transition in all three compounds to higher symmetry superstructures of the parent anti-Fe As structure. This transformation, associated with a hysteresis on decompression, takes place at about 5 GPa for a -LaS and a -PrS , and at about 8 GPa for a -NdS . 1999 Elsevier Science B.V. All rights reserved. Keywords: High pressure; Raman spectroscopy; Rare earth disulfides; Phase transitions
1. Introduction The ideal structure of rare earth dichalcogenides LnX (Ln: La—Lu; X: S, Se, Te) is of the anti-Fe As type (P4/nmm, D , Z"2, a +4 As , c +8 As ). In F this structure, layers of five chalcogen atoms in the basal face-centered square plane are separated by two slabs of alternating Ln> (the nine-fold coordination) and X\ ions. All the LnX compounds are distorted from this due to the formation of X—X pairs within the basal planes [1]. This distortion
* Corresponding author. E-mail: [email protected]. Now at: Universite´ Lyon I, Laboratoire de Physico-Chimie des Mate´riaux Luminescents, 43, Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France.
gives rise to a number of pseudo-cubic, tetragonal, orthorhombic, and monoclinic (b+90°) antiFe As super structures. Recently, charge-density waves in the planar square lattices were observed in LaTe Sb (0)x)1) [2]. The ideal anti-Fe As \V V structure is isotypical with the PbFCl type. Other compounds that crystallize with the PbFCl (ideal anti-Fe As) structure include alkali metal hydrogen halides, alkali earth metal fluorohalides, pnictides, chalconides, oxyhalides, and oxysulfides [3]. Stoichiometric light rare earth disulfides (La—Nd) are obtained at ambient pressure. Orthorhombic b polymorphs (Pnma, D, Z"8) are prepared by reaction of Ln S and S above 1023 K [4]. The same reaction below 1023 K yields monoclinic a forms (P2 /b, C , Z"4) [5]. LnS are also synthesized by reacting Ln O with CS — at 873 K
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 1 2 4 - 7
A. Grzechnik / Physica B 262 (1999) 426—432
pseudo-cubic compounds are obtained [6], while at lower temperatures, no higher than 773 K, monoclinic a polymorphs (P2 /b, C , Z"4) are prepared [7]. Additional long-time annealing of all the a, a , and b polymorphs above 1073 K leads to formation of the c variety (P4/nmm, D , Z"2) [7]. Preparation of stoichiometric heavy rare earth disulfides requires high-pressure techniques [8,9]. Depending on the conditions of the reaction of the respective elements, pseudo-cubic or tetragonal products are recovered to ambient conditions. Pseudo-cubic NdS can also be found at pressures 1.4—7.0 GPa and at temperatures above &773 K [8]. The summary of preparatory techniques shows that the higher symmetry polymorphs of the light rare earth disulfides (La—Nd) are favored at high temperatures, despite the reaction route. On the other hand, heavier rare earth disulfides are obtained at elevated pressures. In the case of NdS , the required temperature to obtain the pseudocubic form through the reaction of the elements decreases with increasing pressure [8]. The purpose of this study is to investigate the high-pressure behavior of a -LnS (Ln: La, Pr, Nd) using in situ Raman spectroscopy in a diamond anvil cell at room temperature. The obtained results could provide additional information on the structural stability of the light rare earth disulfides at different conditions.
2. Experimental Powder samples of a -LnS (Ln: La, Pr, Nd) were prepared as described previously [7]. Raman spectra, with spectral resolution of about 2 cm\, were collected using a XY Dilor Raman spectrometer (1800 groove/mm gratings) in backscattering geometry with CCD signal detection. Raman scattering was excited using an Ar> laser at a wavelength of 514.5 nm. Each sample and the pressure transmitting medium CsI were loaded into a Mao—Bell-type diamond cell with type II-a diamonds, brilliantly cut with 600 lm culets, and a sample chamber diameter of 250 lm. Pressures were determined from the R ruby fluorescence line [10].
427
3. Results and discussion Normal mode analysis of the ideal anti-Fe As and PbFCl structures (P4/nmm, D , Z"2) pre dicts that the vibrational modes are distributed as C"2A #B #3 #2A #2E , where the # A , B , and E modes are Raman active, and the A and E modes are infrared active. The distor tion of the ideal anti-Fe As structure due to the S—S bond formation in the basal plane of the a polymorphs (P2 /b, C , Z"4) leads to the lowering of symmetry and new selection rules C"9A # 9B #8A #7B , where the A and B modes are Raman active, and the A and B modes are in frared active. The symmetry types of S—S stretching oscillations of the “S—S molecular unit” (D sym metry) at C sites of C space group are correlated as #(D )PA(C )P A #B #A #B (C ). Golovin et al. [11] indicated that in all polymorphs of rare earth disulfides the most intense bands in powder Raman spectra in the range 400—500 cm\ were due to the S—S bonds in the basal planes. Accordingly, the bands at lower wave numbers were assigned to the displacements of the cationic sublattices relative to the anionic ones. Similar interpretation of the vibrational powder spectra for the LaS varieties was presented by Le Rolland et al. [7]. They also suggested that the a structures were defective, with the evidence of incomplete S\ pairs, i.e., paramagnetic delocalized S\ defects. These defects were localized within the structures at low temperature, with the result that the S—S stretching modes from different domains became visible in the powder Raman spectra, i.e., increasing resolution of the Raman band at about 420 cm\ with decrease in temperature. The results of the single crystal Raman measurement and valence force field calculation for LaSe (P2 /b, C , Z"4), isostructural with a- and a -LaSe , are in disagreement with this assignment [12]. In fact, the high-frequency modes were found to correspond to La—Se (La—X) vibrations and the only modes with the predominant Se—Se (X—X) stretching character occured in the lower frequency region. It was also demonstrated that the most intense bands in the powder Raman spectra are not necessarily the most intense ones in the Raman spectra measured on single crystals.
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It is seen in Fig. 1 that the first-order Raman spectra of a -LaS , a -PrS , and a -NdS at and near ambient conditions are almost the same, the only differences being in the absolute positions of the respective bands. The group analysis of the P2 /b (C , Z"4) anti-Fe As superstructure predicts the presence of 18 Raman bands, while only 12 are observed for a -LaS , and 11 for a -PrS and a NdS , each. This discrepancy could be associated with the fact that some of the missing modes are probably too weak in intensity to be observed in powder spectra. The strongest bands at about 400—450 cm\ are well resolved even at lowest pressures for all three compounds. Upon compression, these two bands merge and the higher frequency components have negative pressure shifts (Figs. 1 and 2). This band convergence occurs at about 5 GPa for a -LaS and a -PrS , and at about 8 GPa for a -NdS . At the same respective pres sures, the highest frequency band for each compound changes its pressure dependence. In addition to this, in every case a new band appears at about 430 cm\ at pressures above 8 GPa. It is not entirely clear from the compression data whether this band is actually a new one or it is just obscured by the two strong Raman lines at ambient conditions. It could emerge at elevated pressures due to its different pressure shift because after merging the strongest band does not shift. The pressure dependence of the bands at lower wave numbers is quite similar for all the compounds. The differences between a -LaS , on one hand, and a -PrS and a NdS , on the other, can also be attributed to the orientational effects in powder samples affecting the intensities, broadness, and resolution of weak bands at this wave number range, and not only to various responses of a -LaS , a -PrS , and a -NdS lattices to changes in pressure. Upon decompression to ambient conditions, all the spectral changes are reversible with a hysteresis of about 3—4 GPa at each case (Fig. 3). It is also noticeable that at intermediate pressures four bands are detected in the 400—500 cm\ range. This observation is contrary to normal mode analysis that predicts only two Raman active S—S stretching modes, A #B , for the P2 /b (C , Z"4) anti-Fe As superstruc F ture. If the two strong bands were assigned to the S—S oscillations [7,11], the problem would remain
with the other two weaker ones. Analysis of this superstructure shows that there are no Ln—S bonds that are much shorter than the other ones to be associated with the two weak bands in the 400—500 cm\ range, and the rest of the bonds with the bands at lower wave numbers below 300 cm\ [1,5]. The pressure dependencies of the strongest bands in the Raman spectra of a -LaS , a -PrS , and a NdS , including a hysteresis on decompression, are just opposite (Figs. 1—3) to what has been observed at low temperatures by Le Rolland et al. [7]. The low-temperature effect on the Raman spectra of a and a structural varieties could just lead to decrease in thermal broadening of the observed bands. Such a behavior does not necessarily imply the existence of incomplete S\ pairs in the basal planes that can be probed on the vibrational time scale. If the interpretation of the temperature evolution of the most intense bands in powder Raman spectra were correct, i.e., a localization of the itinerant S\ defects at low ¹, one would expect that these two bands also become better resolved at high pressures. In LnS with the ideal anti-Fc As (PbFCl) struc ture (P4/nmm, D , Z"2), all four basal Ln—S bonds are equivalent by symmetry. Another set of equivalent bonds comprises four, out of five, nonbasal Ln—S ones. Lowering of symmetry to the P2 /b (C , Z"4) superstructure causes all the Ln—S bonds to be non-equivalent with the average Ln—S bond being longer than the Ln—S U one [1,5]. On the other hand, analysis of the Raman spectra of the LnS compounds with differ ent superstructures [11,13] reveals the absence of splitting of the strongest band at about 400— 430 cm\ in the pseudo-cubic and tetragonal polymorphs at ambient conditions. The experimental observations presented above can be interpreted as an evidence for pressure-induced phase transitions in layered a -LnS (Ln: La, Pr, Nd) rare earth dis ulfides (P2 /b, C , Z"4) with the distorted anti Fe As structure (the PbFCl type, P4/nmm, D , Z"2). Upon compression, the most intense Raman bands converge indicating the absence of symmetry for the anti-Fe As superstructure. The mechanism for this structural change can be envisioned as a tendency of the nine-fold polyhedra
A. Grzechnik / Physica B 262 (1999) 426—432
Fig. 1. Raman spectra of a -LaS (a), a -PrS2 (b), and a -NdS (c) upon compression.
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Fig. 2. Pressure dependence of the observed Raman frequencies in a -LaS (a), a -PrS (b), and a -NdS (c). Lines are guides for the eyes.
around the Ln> ions to become more symmetric within the anti-Fe As type, i.e., a tendency towards the equivalency of the four Ln—S and four, out of five, Ln—S bonds. As seen in Fig. 2, the U phase transition depends on the Ln> ion and occurs at about 5 GPa for a -LaS and a -PrS , and at about 8 GPa for a -NdS . From the lack of drastic changes in the number of the observed
Raman bands at the respective pressures, it could be argued that the number of the molecular units does not change (Z"4) and the resulting high-pressure structure is not the ideal anti-Fe As (PbFCl) one but rather a pseudo-cubic or tetragonal anti-Fe As superstructure. In all the cases of a -LnS studied here, the back transformations are sluggish and proceed with a hysteresis of about 3 GPa.
A. Grzechnik / Physica B 262 (1999) 426—432
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Fig. 3. Raman spectra of a -LaS (a), a -PrS (b), and a -NdS (c) upon decompression.
The above interpretation is in agreement with the study by Webb and Hall [8] who found that the pressure necessary to synthesize stoichiometric heavy rare earth disulfides depends on the respective cation with the tendency to increase along the lanthanide series. As known from the summary of preparatory techniques, the higher symmetry polymorphs of the light rare earth disulfides (La—Nd) are favored at high temperatures and the required temperature to obtain the pseudo-cubic form of NdS through the reaction of the elements decreases with increasing pressure [8]. This strongly suggests that the observed pressures for structural changes in a -LnS (Ln: La, Pr, Nd) would decrease with increasing
temperature. The results of this study could be helpful to predict and understand the high-pressure behavior of other compounds with the PbFCl (anti-Fe As) structure [3]. It could be anticipated that the phase transitions in alkali metal hydrogen halides, alkali earth metal fluorohalides, pnictides, chalconides, oxyhalides, and oxysulfides would tend to retain the layered character of the PbFCl type.
Acknowledgements I thank P. McMillan for providing the samples and for several discussions.
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A. Grzechnik / Physica B 262 (1999) 426—432
References [1] J. Flahaut, in: K.A. Gschneider Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 2, North-Holland, Amsterdam, 1979. [2] E. DiMasi, B. Foran, M.C. Aronson, S. Lee, Phys. Rev. B 54 (1996) 13587. [3] A.F. Wells, Structural Inorganic Chemistry, 5th ed., Clarendon Press, Oxford, 1984, pp. 486—488. [4] J. Dugue´, D. Carre´, M. Guittard, Acta Crystallogr. B 34 (1978) 403. [5] S. Be´nazeth, M. Guittard, J. Flahaut, J. Solid State Chem. 37 (1981) 44. [6] T. Toide, T. Utsunomiya, M. Sato, Y. Hoshino, T. Hatano, Y. Akimoto, Bull. Tokyo Inst. Technol. 117 (1973) 41.
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