Pressure-induced phase transition in ferrielectric CuInP2S6

Solid State Communications, Vol. 108, No. 1, pp. 43–47, 1998 q 1998 Elsevier Science Ltd. All rights reserved 0038–1098/98 $ - see front matter

Pergamon

PII: S0038–1098(98)00297-X

PRESSURE-INDUCED PHASE TRANSITION IN FERRIELECTRIC CuInP 2S 6 A. Grzechnik, a V.B. Cajipe, b ,* C. Payen b and P.F. McMillan c a´

Ecole Normale Supe´rieure de Lyon, 46, alle´e d’Italie, F-69364 Lyon Cedex 07, France b Institut des Mate´riaux, UMR 6502, BP 32229, 44322 Nantes Cedex 3, France c Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287, U.S.A. (Received 4 May 1998; accepted 9 June 1998 by G. Bastard) We report the first observation of a pressure-induced phase transition in a lamellar chalcogenophosphate. Raman scattering measurements on the layered ferrielectric CuInP 2S 6 were performed at pressures of up to 17.5 GPa and room temperature. The recorded spectra indicate the onset of a transition at about 4.0 GPa which is inferred to be first-order. The significantly reduced number of bands observed at high pressure and reversibility of the transition are consistent with the conservation of the lamellar morphology and a possible ABC to AB chalcogen stacking change. A discussion is given of how compression might modify the potential for the off-center cations and the polar state of this material. q 1998 Elsevier Science Ltd. All rights reserved Keywords: A. ferroelectrics, D. phase transitions, E. inelastic light scattering.

ferrielectric arrangement [3] (space group Cc). The occurence of such polar sublattices is consistent with a second-order Jahn–Teller effect for these d 10 cations. In the paraelectric phase (space group C2/c), the Cu I undergoes thermal hopping motions within a double-well potential defined by the S 6 cage and the In III moves to a central, octahedral position. Partial filling of a tetrahedral site in the interlayer space has also been found at T . Tc . The transition has been deduced to be of the first-order, order-disorder type [3, 4]. Applying pressure on CuInP 2S 6 would be interesting for two reasons. First, squeezing could modify the potential for the off-center cations and produce a new phase that may remain polar or not. The case of a nonpolar high-pressure phase would raise the question of whether or not pressure can force monovalent copper into an octahedral environment in which it is normally unstable. Second, structural types adopted by other chalcogenophosphates could become accessible at high pressures. Loss of the lamellar morphology may not be precluded; indeed, there are several chalcogenophosphates with three-dimensional (3D) structures [5]. However, applying pressure would most probably lead to a contraction of the interlayer space before inducing a transition, so that conservation of the layered character is

It is well-known that certain crystals exhibit electrical polarity when subject to stress. For ferroelectrics in particular, this property implies a close relationship between lattice strain and a reversible spontaneous polarization. The application of hydrostatic pressure may then be expected to alter ferroelectric behavior [1]. Specifically, high pressure techniques provide a means to probe the sensitivity of electric dipole ordering to volume changes under conditions of constant temperature and chemical composition. Structural transformations related or not to the symmetry breaking responsible for the para-ferroelectric transition at T c may also be induced by applying pressure. Lamellar CuInP 2S 6 has recently been shown to be ferroelectric at T , Tc ¼ 315(5) K [2, 3]. This compound has a monoclinic structure based on an ABC close-packed stacking of the chalcogens (CdCl 2 type). The metal cations and P–P pairs occupy half of the octahedral sites such that an alternation of layers and empty galleries is formed. Each layer contains triangular arrays of Cu I, In III and (P 2S 6) ¹4 ions. Below T c, the Cu I and In III off-center along the layer normal to yield a

* Corresponding author. E-mail: [email protected] 43

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PRESSURE-INDUCED PHASE TRANSITION IN CuInP 2S 6

likely at moderate pressures. Known variations on the monoclinic CdCl 2-type structure involve a change in the alignment of cation and P–P sublattices along the c axis or, an AB chalcogen stacking which entails trigonal symmetry (CdI 2 type) [6–11]. A transition from a monoclinic (C2/m) to trigonal (R3) structure accompanied by Cd II ordering in off-center sites has in fact been observed in Cd 2P 2S 6 at 228 K and ambient pressure [11]. In this communication we present the results of Raman scattering measurements on ferrielectric CuInP 2S 6 conducted at high pressures (up to 17.5 GPa) and room temperature. The occurence of a structural transition to a more symmetric lamellar phase at about 4.0 GPa is deduced from the observed changes in the Raman spectrum. To our knowledge, this is the first pressureinduced phase transition to be reported for a layered chalcogenophosphate. Crystalline samples of CuInP 2S 6 were obtained as described previously [2]. Raman spectra, with spectral resolution of about 2 cm ¹1, 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 þ laser at a wavelength of 514.5 nm. A crystal of CuInP 2S 6 and the pressure transmitting medium CsI were loaded into a Mao–Bell-type diamond cell with type II-a diamonds, brilliant cut with 600 mm culets and a sample chamber diameter of 250 mm. Pressures were determined from the R 1 ruby fluorescence line [12]. Figure 1 displays the Raman spectrum obtained for CuInP 2S 6 under ambient conditions. The primitive cell

Fig. 1. Raman spectrum of CuInP 2S 6 at ambient conditions.

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of ferrielectric CuInP 2S 6 contains two formula units with all the atoms at general C 1 sites. Group analysis of the normal modes predicts the selection rules Gvib ¼ 28A9 þ 29A0, where non-degenerate A9 and A0 modes are both Raman and infrared active. The internal modes [13] of the (P 2S 6) 4¹ molecule have 18A9 þ 18A0 symmetry types. The remaining 10A9 and 11A0 modes are due to translational and rotational motions of the P 2S 6 unit as well as Cu I and In III oscillations. The spectrum in Fig. 1 shows a smaller number of bands than predicted by group analysis. Coupling of same-symmetry modes for the two equivalent P 2S 6 anions in the primitive cell and quasi degeneracy of the corresponding bands probably contribute to this observation. More importantly, the reduced count is consistent with the relatively small spontaneous polarization P s found in this compound and the expectation of weak intensities for bands originating from the infrared-active (13Au þ 14Bu ) modes of the paraelectric phase [4]. We assign the bands in the 400–600 cm ¹1 region to the symmetric and antisymmetric P–S oscillations. The strongly polar intense band at 376 cm ¹1 is tentatively attributed to the P–P stretching mode. Such an intense band has been identified in the 300–400 cm ¹1 region of vibrational spectra in other M9M0P 2S 6 compounds [14–17]. The strong and broad bands in the 200–350 cm ¹1 range could be assigned to the P 2S 6 deformation vibrations (S–P–S and S–P–P modes) which couple to Cu I vibrations. The weak band occuring at 28 cm ¹1 may be ascribed to local Cu I vibrations within an off-center site [4]. This agrees very well with the optical phonon observed in CuInP 2S 6 using inelastic neutron scattering [18] and with band assignments for M 1.74Cu 0.52P 2S 6 (M: Mn II, Cd II) [15]. The remaining lines at low wavenumbers are due to mixed modes with different contributions of the external P 2S 6 oscillations and Cu I and In III vibrations. The peak positions and intensities found in Fig. 1 are consistent with the combined features of spectra recorded in the Y(ZZ)X and Y(ZY)X geometries in the previous Raman spectroscopy study [4] of the ferri-paraelectric transition in CuInP 2S 6. Changes in the Raman spectrum of CuInP 2S 6 upon compression can be followed in Figs 2 and 3. Up to 4.0 GPa, all the bands shift monotonously towards higher wavenumbers, without any mode softening. The incipient splitting of the band at about 320 cm ¹1 at high pressure to about 4.0 GPa (Fig. 2), is related to the different pressure dependence of the bands that are not resolved in the Raman spectrum at ambient conditions. There is a continuous decrease in intensities of the bands in the 200–320 cm ¹1 range up to this pressure. The Raman spectrum recorded at 4.0 GPa, still showing the spectral features of the ambient pressure phase, contains a new band at about 280 cm ¹1. At 5.1 GPa, the P–S

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PRESSURE-INDUCED PHASE TRANSITION IN CuInP 2S 6

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Fig. 3. Pressure shift of the Raman bands in CuInP 2S 6 upon compressing. The solid lines are a guide to the eye.

Fig. 2. Raman spectrum of CuInP 2S 6 upon compressing. The asterisks indicate the Ar þ plasma line. stretching bands at about 450 cm ¹1 and those around 320 cm ¹1 and in the 60–120 cm ¹1 range vanish entirely. This is associated with an abrupt change in the pressure dependence of the observed Raman bands (Fig. 3). There is also a reversal in intensity magnitudes of the two bands in the 200–350 cm ¹1 region, in which S–P–S and S–P–P modes mixed with Cu I oscillations occur. These observations, along with the occurence of other new Raman bands at 5.1 GPa, clearly indicate the onset of a pressure-induced phase transition at about 4.0 GPa. Upon further compression, all the modes shift towards higher wavenumbers and the two bands in the 160– 220 cm ¹1 range merge, suggesting that the high-pressure phase accommodates further structural changes within a wide pressure range (from 4 to about 10 GPa). The intensities of all the observed bands, relative to the P–P band, continuously diminish in passage to the highest recorded pressure (Fig. 2). It is also worth noticing that the lowest wavenumber band, entirely due to the Cu I oscillation, does not soften in the entire pressure range studied here. Figure 4 demonstrates that all the pressure-induced structural changes in CuInP 2S 6

are fully reversible upon decompression to ambient conditions. This suggests that the material remains lamellar up to the highest attained pressure. The significantly smaller number of Raman bands detected at pressures above 4.0 GPa indicates that the high pressure phase of CuInP 2S 6 has a symmetry higher than found under ambient conditions. The thermally driven ferri-paraelectric transition (Cc to C2/c) at atmospheric pressure likewise leads to a reduced number of expected bands (14Ag þ 16Bg ). However, the spectral changes obtained with compression, particularly as noted for the spectra at 4.0 and 5.1 GPa where a new band appears and others vanish, are quite distinct from those observed upon heating above T c [4]. It would thus be reasonable to say that the CuInP 2S 6 structure does not remain monoclinic at pressures above 4.0 GPa. Also, a shift of the chalcogen stacking from ABC to the more compact AB type would be consistent with volume reduction at high pressure. A monoclinic to trigonal symmetry transformation analogous to that observed in Cd 2P 2S 6 at 228 K is thus conceivable [11, 19]. Indeed, describing the high-pressure structure of CuInP 2S 6 within the space group R3 would imply 9A þ 9E Raman modes, i.e. fewer than expected for non-compressed CuInP 2S 6. The degree of polarity of the high-pressure, roomtemperature phase however remains an object of

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PRESSURE-INDUCED PHASE TRANSITION IN CuInP 2S 6

Fig. 4. Raman spectrum of CuInP 2S 6 upon decompressing. The asterisks indicate the Ar þ plasma line. speculation. According to the crystallographic study of CuInP 2S 6, cooling at ambient pressure from above T c causes an anomalous contraction of both the layer thickness and the interlayer gap [3]. These effects are however associated with a thermal inhibition of copper hopping motions, i.e. it cannot be assumed that a negative volume change per se implies polar order. More useful would be a consideration of how pressure might alter the effective potential and motional behavior of Cu I. For defect Cu I ions in shallow off-center potentials of alkali halides, it has been shown that pressure enhances the defect mobility by reducing the barriers of the multiwell; further compression leads to a local off- to on-center transformation [20]. Similar scenarios might be imagined for Cu I in CuInP 2S 6 under pressure, a difference being that the initial potential here has odd rather than even parity. Also, the P 2S 6 groups play a key role in the present case. While the ferrielectric state may be primarily ascribed to a second order Jahn–Teller effect for the d 10 cations, its occurrence is facilitated by rearrangements in the PS 3 umbrellas [2, 3]. Within this picture, application of pressure would affect the energy

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lowering associated with the cation off-centering and P 2S 6 distortions via adjustments in the latter. Note that P 2S 6 twists would provide the shearing mechanism for the ABC to AB stacking change. Compression of the interlamellar space would moreover augument the S–S interactions, initially of the van der Waals type, across the gap. In our Raman experiments, the evolution with pressure of the intensities within the 200–350 cm ¹1 region of the P 2S 6 deformation modes is accompanied by an energy shift for the Cu I oscillations (Fig. 3). This is consistent with a pressure-modified shape for the copper potential. It is also reminiscent of the observed evolution of these bands at ambient pressure when heating from below the order-disorder transition in this compound [4]. A crystal symmetry change following a continuous, pressure-induced reduction of the central copper potential barrier may be envisioned. However, the transition is inferred to be first-order because: (i) the supposed monoclinic to trigonal symmetry change is discrete (no group-subgroup relationship); (ii) evidence for the coexistence of two phases is found in the spectrum at 4.0 GPa; and (iii) the frequency decrease that occurs between 4.0 and 5.1 GPa for the lowest energy Cu I vibration band is abrupt. The last observation also argues against a displacive character for the Cu I behavior under pressure, i.e. copper mode softening does not occur and Cu I dipoles, static or reorienting, still exist in the compressed state. On the other hand, the bands in the 60–120 cm ¹1 range associated with combined cation and anion external modes vanish above 4.0 GPa. In this regard, mode condensation accompanying an off- to on-center displacement for In III may not be precluded. The transition and high-pressure phase here observed for CuInP 2S 6 merit further investigation using other techniques. Initial results of our X-ray diffraction experiments agree with the hypothesis of a monoclinic to trigonal structural transformation around 4.0 GPa. Dielectric measurements on this compound are also planned, possibly with simultaneous temperature variation to explore the P–T phase diagram. Investigations of other layered chalcogenophosphates under pressure would likewise be of interest. For example, both CuCrP 2S 6 and CuVP 2S 6 are paraelectric at ambient conditions: it would be important to determine if they undergo phase transitions at high pressure. The different cation and P–P sublattice alignments within the CuMP 2S 6 (M ¼ In, Cr, V) series may also influence high-pressure behavior. CuInP 2Se 6 contains AB stacked selenium atoms and exhibits a broad transition at 233 K that is still under study; comparing its response to compression with that of CuInP 2S 6 would shed light on how the occurrence of a pressure-induced transition is influenced by the chalcogen type and arrangement.

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Acknowledgement—V.B.C. would like to thank Yu.M. Vysochanskii for helpful comments.

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