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High-pressure studies of SrNi3(P2O7)2 pyrophosphate by Raman spectroscopy and X-ray diffraction
Journal of Molecular Structure 794 (2006) 334–340 www.elsevier.com/locate/molstruc
High-pressure studies of SrNi3(P2O7)2 pyrophosphate by Raman spectroscopy and X-ray diffraction Bouchaib Manoun a,b,*, Brahim El Bali c, Surendra K. Saxena a, Revansidha P. Gulve a a
Center for the Study of Matter at Extreme Conditions (CeSMEC), Florida International University, VH-140, University Park, Miami, FL 33199, USA b Laboratoire d’Analyses, d’Essais et d’Environnement (LAEE), De´partement de Chimie, Faculte´ des Sciences ‘Dhar Mehraz’, Universite´ Sidi Mohamed Ben Abdellah, 30000 Fe`s, Morocco c Laboratory of Mineral Solid and Analytical Chemistry “LMSAC”, Department of Chemistry, Faculty of Sciences, University Mohamed I, PO. Box 624, 60000 Oujda, Morocco Received 18 September 2005; received in revised form 16 February 2006; accepted 1 March 2006 Available online 19 April 2006
Abstract High-pressure investigations of SrNi3(P2O7)2 pyrophosphate, in diamond anvil cell have been performed at room temperature using in situ Raman spectroscopy and X-ray synchrotron radiation source. The endeavor was to acquire information on pressure-induced structural transformations such as phase transitions and amorphization occurring in the crystal lattice. Group theory yields to 60 Raman active modes 30AgC30Bg for SrNi3(P2O7)2, of which only 37 bands are observed at ambient conditions. The pressure-induced phase transition sequence of SrNi3(P2O7)2 pyrophosphate to pressures up to 61.5 GPa was explored. Raman spectra showed that the investigated compound, SrNi3(P2O7)2, compressed smoothly up to the highest investigated pressures, transforms to another structure at pressures of 50.5 GPa. The new high-pressure phase persists metastably down to 1 atm. upon release of pressure. We also measured the pressure dependence of the lattice parameters of the polycrystalline SrNi3(P2O7)2 sample up to a pressure of 48 GPa. The compressibility of SrNi3(P2O7)2 along the b-axis is larger than that along the a-axis. At 48 GPa, a and b cell parameters are almost equal indicating that a phase transition from the monoclinic to a tetragonal structure started. The bulk modulus of SrNi3(P2O7)2 (P21/c) is 124G2 GPa with a pressure derivative of 4.37G0.12. q 2006 Elsevier B.V. All rights reserved. Keywords: High-pressure; SrNi3(P2O7)2; Pyrophosphate; Phase transition; Raman spectroscopy; X-ray diffraction
1. Introduction Crystalline and glassy inorganic phosphate materials are of considerable industrial interest because of their utility as laser hosts [1] and their wide application in ceramic [2], dielectric [3], electric [4], magnetic [5], and catalytic [6] processes. Of special interest, orthophosphates, pyrophosphates and triphosphates are of biological importance for their various applications as catalysts, molecular sieves, or ion exchangers [7]. Systematic studies on simple and mixed pyrophosphates (M2P2O7 or (A,M)2P2O7 with A and/or M an alkaline earth or divalent 3d-metal ion) have been undertaken during the * Corresponding author. Address: Center for the Study of Matter at Extreme Conditions (CeSMEC), Florida International University, 11200 SW 8th Street, VH-140, University Park, Miami FL 33199, USA. Tel.: C1 305 348 3445; fax: C1 305 348 3070. E-mail address: [email protected] (B. Manoun).
0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.03.001
last two decades. In fact, pyrophosphates constitute the largest family of condensed phosphates with variable stoichiometries [8]. Crystallographic studies of pyrophosphates have indicated that the geometric configuration of the pyrophosphate ion is mainly dependent on the crystalline environment. The P2 O4K 7 ion consists of two PO4 tetrahedra sharing a common oxygen atom. Hezel and Ross [9] have shown that the ion is non-linear with unequal P–O bond lengths for the terminal and bridging bonds. In certain cases, the terminal bond lengths are of unequal length, and two PO3 groups are imperfectly staggered. El Bali et al. [10] determined the crystal structure of SrNi3(P2O7)2, from single crystal data, with P21/c, ZZ2, aZ ˚ , bZ112.216(9)8. 7.4092(9), bZ7.6594(8), cZ9.4474(10) A SrNi3(P2O7)2 is isostructural to AM3(P2O7)2 (AZCa, Pb and MZFe, Co, Ni). Beside the crystal structure study, authors reported on a correlation of absorption spectra and coordination geometry of the [NiOx] chromophore for the title compound. Ni2Ccations occupy two non-equivalent crystallographic positions in the framework of SrNi3(P2O7)2, both showing slightly distorted octahedral coordination. The [NiO6]
B. Manoun et al. / Journal of Molecular Structure 794 (2006) 334–340
octahedra share edges to form chains along the monoclinic axis. The structure of SrNi3(P2O7)2 might be thus described in terms of chains of edge-sharing [NiO6] octahedra, held together by [P2O7] groups. This ‘covalent’ network comprises tunnels parallel to [010], hosting Sr2Cions. The Ni-coordination can be moreover distinguished by the ‘ligating’ forms. Ni(1) is surrounded by five [P2O7] groups, one of which is a bidendate ligand. Ni(2) is coordinated by six monodendate [P2O7] groups. Despite crystallographic investigations, while the physicochemical properties of inorganic phosphates have received a great deal of attention [11], the effect of high external pressures on their structures has been comparatively little studied. To the best of our knowledge, few papers are dealing with highpressure Raman studies on pyrophosphates such as that reporting on TiP2O7 [12] where they observed a densification and a partial amorphization occurred in the crystal lattice. Also recently, the pressure dependence of the phonons of the ambient- and high-pressure phases of the vanadyl catalyst, (VO)2P2O7, has been investigated by polarized Raman spectroscopy [13]. The V–O stretching modes exhibit an anomalous mode softening at w3 GPa, which was attributed to the interlayer interaction of neighboring V and O atoms. More recently, Baril et al. [14] reported on the effect of high external pressures on the Raman spectra of four metal pyrophosphate dihydrates, KMHP2O7$2H2O[MZCo(II), Mn(II), Zn(II) and Mg(II)], for pressures up to w4.2 GPa. They suggested the occurrence of a pressure-driven structural transformation, at w2.9 GPa, or possibly even the first stages of amorphization for Co(II) compound and no serious changes for the rest of the series. The present work on Raman spectroscopy and X-ray diffraction at room and high-pressures is a part of a systematic study of some other pyrophosphates. The principal aim of our experiments is to follow and to identify the pressure-induced structural transformations, such as phase transitions and amorphization, occurring in the crystals. The investigation of SrNi3(P2O7)2 by Raman spectroscopy at high-pressures will characterise this material; and will allow us to report on the behaviour of SrNi3(P2O7)2 upon increasing (and decreasing) pressure. Another important point is to study the reversibility of the possible high-pressure induced phase transitions. The investigation of SrNi3(P2O7)2 by X-ray diffraction will allow us to measure the pressure dependence of the lattice parameters of this material and try to follow the phase transition sequences.
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2.1.2. Single crystals A powder sample was prepared according to the chemical formula SrNiP2O7, which has been melt at 1473 K in a platinium crucible. After controlled cooling (rate 108/h) to 773 K the sample was quenched to room temperature. The product was found to contain two kind of crystals distinguishable by their colors, red–orange (SrNiP2O7) and green– yellowish (SrNi3(P2O7)2). Both chemical formulae have been fixed after structure determination as reported in [10]. 2.2. Raman spectroscopy Raman spectra were measured in a back scattering arrangement, on both compression and decompression. Raman spectra were collected at room temperature by using high throughput holographic imaging spectrograph with volume transmission grating, holographic notch filter and thermoelectrically cooled CCD detector (Physics Spectra), with the resolution of 4 cmK1. The spectrometer was regularly calibrated using the neon lines. Ti3C-sapphire laser pumped by an argon ion laser was tuned at 785 nm. The laser power was operated at 100 mW in the entire measured pressure range; both, on compression and decompression. The incident laser beam was focused with the spot size of 5 mm by the objective (20!) to excite the sample. The spectrometer and the Raman spectra were calibrated using the Raman modes of diamond and sulfur, as well as, of the neon emission spectrum. The exposure time was 100 s with 10 accumulations. High-pressure Raman measurements were conducted at room temperature using a gasketed high-pressure Diamond Anvil Cell (DAC). Powder samples of SrNi3(P2O7)2 were loaded in Mao-Bell-type diamond anvil cells. Diamonds with 300 mm culets were used and a steel gasket, indented to 83 mm, with a sample chamber of 125 mm contained the sample. Ruby chips were loaded with the sample at various spots as pressure markers. Pressures were determined by using the pressuredependent spectral shift of the sharp Ruby fluorescence R1 line shift [15], excited by an argon ion laser with the wavelength of 514.5 nm. Because any pressure medium only maintains the sample at a high hydrostatic state at relatively low pressure, but at pressure O15 GPa, it mostly becomes quasi-hydrostatic due to the solidification of the pressure medium, including liquid and gas. In this study, the material is relatively soft and no pressure medium was employed. The focused laser spot on the sample was close to the Ruby chip, so the pressure difference will be small.
2. Experimental 2.3. X-ray diffraction 2.1. Sample preparation 2.1.1. Powder samples SrNi3(P2O7)2 was synthesized from stoichiometric amounts of nickel, alkaline earth carbonate and (NH4)2HPO4. After dissolution of the starting materials in nitric acid, the homogenous solutions have been evaporated to dryness on a heating stirrer. The residue was ground and heated in air at increasing temperatures up to 1373 K.
For X-ray diffraction, measurements were conducted at room temperature; powdered samples were pressurized using a gasketed Diamond Anvil Cell (DAC) with a 300 mm culet. A 100 mm initial thickness rhenium gasket was indented to about 50 mm. Since high purity aluminum does not undergo structural phase transition at high-pressure, has low shear strength, as in our previous work [16–20], it was the pressure-transmitting medium of choice. The presence of aluminum in the cell
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rendered the sample pressure nearly hydrostatic and also served as the pressure standard because of its well established pressure–volume relation [21]. Powdered samples were placed between two pieces of Al foil (z15 mm thickness) and packed in the 100 mm hole. X-ray diffraction measurements of SrNi3(P2O7)2 were conducted at room temperature using an angle dispersive ˚ of a monochromatic synchrotron radiation. Using a lZ0.496 A beam focused down to a 35 mm spot size, the X-ray spectra were collected at CHESS (Cornell University, Ithaca, NY). Diffraction rings were recorded between 2qZ1 and 358. The FIT2D program was employed to convert the image plate records into 2q’s and intensities. The cell parameters were determined using least squares refinement on individually fitted peaks. The peaks were assigned to the monoclinic structure with the space group P21/c. 3. Results and discussions SrNi3(P2O7)2 crystallizes in the monoclinic system P21 =cðC52h Þ with two formula units per unit cell [10]. Its structure can be described in terms of chains of edge-sharing [NiO6] octahedra, held together by [P2O7] groups as shown in Fig. 1. In fact, Ni2Coccupy two non-equivalent cristallographic positions in the framework of the title compound, both showing slightly distorted octahedral coordination. Two neighbouring [NiO6] octahedra share an edge to form a zigzag chains along the monoclinic axis. These chains are linked via O–P–O–P–O bridges to form the ‘covalent’ network, which comprises tunnels parallel to [010] where Sr2Cions are located. The Ni-coordination can be moreover distinguished
Fig. 1. A Projection of the crystal structure of SrNi3(P2O7)2 along [001], NiO6 octahedra (blue) form chains along b-axis, each two PO4 tetrahedra (Yellow) share one corner and link the NiO6 chains. The SrO8 polyhedra were omitted for clarity and the circles represent Sr2Ccations.
by the ‘ligating’ schemes. In fact, Ni(1) is surrounded by five [P2O7] groups, one acting as bidendate ligand whereas Ni(2) is coordinated by six monodendate [P2O7] units. The pyrophosphate P2 O4K anion consists of two distorted 7 tetrahedra sharing one apex. These anions set on two different kinds of general sites in the framework of SrNi3(P2O7)2. Conventionaly, P2 O4K 7 is characterized by its P–O–P bridging angle as by its conformation. In our case, it possesses a non eclipsed form and the P–O–P is non-linear. The Sr2Cion and one of the three Ni2Cions are on two distinct Ci sites while the remaining two Ni2Cions are on equivalent C1 sites. The irreducible representation of the title compound in the C2h factor group (excluding three acoustic modes) led to 30AgC 30BgC35AuC34Bu (30AgC30Bg are Raman active while 35AuC34Bu are infrared active). Fig. 2 shows the Raman spectrum of SrNi3(P2O7)2 recorded at ambient conditions. According to the factor group analysis [22], 60 Raman active modes, represented as MZ30AgC 30Bg, should be observed for the P21/c structure of SrNi3(P2O7)2 in the Raman experiment. However, the number of observed modes is 37, significantly smaller than expected. This may be attributed to accidental degeneracy of a number of modes. Moreover, some weak bands are most likely overlapped by much stronger bands and others bands are observed as weak shoulders. The pyrophosphate ion consists of two PO3 groups and a P–O–P bridge. So the assignments of P2O7 modes are carried out in terms of PO3 and P–O–P group vibrations [23]. The performed assignment for SrNi3(P2O7)2 compared to that of PbNi3(P2O7)2 [24] compound is presented in Table 1. Since the SrNi3(P2O7)2 specimen studied here is a polycrystalline powder, we cannot precisely assign the observed modes using a combination of the single-crystal orientation and the polarized Raman scattering. However, the observed Raman modes can be classified into three general families of lattice vibrations: external modes and Ni2Ctranslational vibrations at frequencies below 300 cmK1; the ranges assigned to the symmetric and antisymmetric components of the PO 3 deformational modes is based on known spectroscopic data
Fig. 2. Raman spectrum of SrNi3(P2O7)2 pyrophosphate recorded at ambient conditions out of DAC.
B. Manoun et al. / Journal of Molecular Structure 794 (2006) 334–340 Table 1 Vibrational frequencies (cmK1) and their assignments of SrNi3(P2O7)2 compared to that of PbNi3(P2O7)2 [24] SrNi3(P2O7)2 114 146 151 171 183 186 217 234 252 257 292 302 375 393 421 473 494 528 538 561 593 624 637 742 933 1038 1055 1098 1146 1163
PbNi3(P2O7)2 110 115 142 155 175 180 203 215
Assignments
External modes
Ni2C translational vibrations 250 255 290 375 392 420 470 495 525 535 562 590
742 927 945 1030 1050 1095 1135 1180 1215
d(P–O–P) Ni2Ctranslational vibrations? ds(PO3) and n(M–O)
das(PO3)
ns(P–O–P) nas(P–O–P) ns(PO3)
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bridging angle (P–O–P) is not linear in these compounds. In SrNi3(P2O7)2, the bridging angle (P–O–P) is also not linear and was determined to be 134.08 [10]. The asymmetric bridge P–O stretching mode gives only one Raman band at 933.4 cmK1 for SrNi3(P2O7)2 while two bands were observed in PbNi3(P2O7)2 and PbCo3(P2O7)2 where the authors attributed this splitting to the correlation field effect [24]. SrNi3(P2O7)2 and PbNi3(P2O7)2 are very similar but slightly different from that of PbCo3(P2O7)2, which could be explained by the distortion of 2C the P2 O4K ions in 7 anion and the low symmetry field of the Co the cobalt compound [24]. The symmetric P–O–P stretching vibration is observed in the Raman spectra region as strong band centered at 742 cmK1. In compounds with linear P–O–P bridge, nsP–O–P is observed at a higher frequency than those of nsPO3 and nasPO3 [25]. The appearance of nasP–O–P at a lower frequency than those of PO3 stretching frequencies again supports the non-linearity of the P–O–P bridge in SrNi3(P2O7)2 in agreement with El Bali’s et al. results [10]. Raman spectra of SrNi3(P2O7)2 were collected in situ (DAC) in compression and decompression cycles at room temperature and elevated pressures, up to 61.5 GPa; the pressure-induced changes in the Raman active modes are shown in Fig. 3. The features of the changes shown by the bands can be summarized as follows: all the bands in the whole frequency region (100–1300 cmK1) exhibit important frequency shifts (Fig. 4). All the bands change in intensity; in general, smoothing down as the pressure increases. Finally, some bands appear, suddenly; some others merge; and some
nas(PO3)
1279
of divalent metal diphosphates [24]: ds(PO3) and n(M–O) modes are observed in the region 390–500 cmK1 and the das(PO3) modes are lying in the region 520–650 cmK1. The deformation frequency of the P–O–P bridge occurs around 300 cmK1 [23]. ns(P–O–P) and nas(P–O–P) are observed in the 740–950 cmK1 frequency region. The asymmetric PO 3 terminal stretching vibrations (nasPO3) of the P2O7 group in different compounds are usually observed in the highfrequency region above 1050 cmK1. The frequencies of the symmetric PO3 terminal stretching vibrations are, in general, expected to be below the asymmetric ones in the region between 1000 and 1070 cmK1. These frequency families were also observed in other isoformular pyrophosphates such as PbNi3(P2O7)2 and PbCo3(P2O7)2 [24]. The P–O stretching modes in SrNi3(P2O7)2 are observed at higher wavenumbers than those in PbNi3(P2O7)2, indicating that the P–O bonds in Sr compound is stronger than those in the Pb compound. The P–O–P bridge stretching vibrations are observed in the usual range and could be easily identified in the Raman spectrum. In previous work by Santha et al. [24], vibrational spectra showed that ns(POP) is observed in IR as well as in Raman for PbNi3(P2O7)2 and PbCo3(P2O7)2 pointing that the
Fig. 3. Raman spectra of SrNi3(P2O7)2 recorded for the compression cycle at various pressures up to 61.5 GPa in the range 100–1300 cmK1, a phase transition is observed at 50.5 GPa.
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Fig. 4. The pressure dependence of the observed Raman shifts in SrNi3(P2O7)2 across the phase transition.
others split. In the whole frequency range, upon elevation of pressure, Raman spectra show that up to 48 GPa, all SrNi3 (P2O7)2 bands assigned to the P21/c structure maintain, with positive pressure shifts of the frequencies (Fig. 4). Note that some negative shifts are observed for some external modes at frequencies below 200 cmK1, these soft modes might be attributed to P2O7 librations. Also change in the shift slope was observed in most of the Raman modes after 20.5 GPa. The spectral changes pointed mostly to the densification of the crystal lattice. This behavior was also observed in the highpressure Raman study of TiP2O7 [12]. The bands observed at 375 and 393 cmK1, at room pressure, become more separated at 10.2 GPa. The splitting is not due to a phase transition, but results from an overlapping of two bands attributed to Ni2Ctranslational modes (375 cmK1) and ds(PO3) and n(M–O) (393 cmK1). The bands become separated at highpressures, owing to their different pressure dependence. As pressure increases (14.9 GPa), the ds(PO3) and n(M–O) band overlap with the 420.5 cmK1 band also attributed to ds(PO3) and n(M–O) and again moved faster then the 420.5 cmK1 band to separate at 38.9 GPa with a higher frequency. The terminal stretching vibrations nas(PO3) band observed at 1098 cmK1, at room pressure, shifted less than that of the symmetric one observed at 1055 cmK1 and therefore nas(PO3) became a shoulder of ns(PO3). With increasing pressure, the left weak shoulder of the 1055 cmK1 strong band, gets stronger (starting at 38.9 GPa), until it becomes an independent band at around 50.5 GPa. Upon further elevation of pressure, both bands show strong pressure dependence. Therefore, this splitting cannot be explained by an overlapping of two bands: it is a sign of a phase transition. Moreover, this new band gets stronger in intensity while the band observed, at room pressure, at 1055 cmK1 decreased significantly in intensity. The transformation pressure, for SrNi3(P2O7)2 at room temperature, is then given as (48.04C50.48)/2Z49.26G1.22 GPa. A right shoulder of the 742 cmK1 symmetric P–O–P stretching vibration is observed in the Raman spectrum at pressures over 34.2 GPa. This band will be clearly seen on decompression (Fig. 5). Also at pressure of 50.5 GPa we notice the
Fig. 5. Raman scattering spectra recorded on decompression from 61.5 GPa to ambient conditions. The new high-pressure phase is recovered at 1 atm.
appearance of other new bands for example the one centered at 1018 cmK1 (50.5 GPa) and became clear at higher pressures which provide us again direct evidence of pressure induced phase transformation occurring in SrNi3(P2O7)2 structure. More experiments are needed to complete this work; we will use synchrotron radiation source and a diamond anvil cell to measure the pressure dependence of the lattice parameters of this material and try to follow the phase transition sequences. Note that the presence of most Raman peaks over the whole pressure range indicates that the P2O7 units are present in the compressed material. The bands increase in widths without considerable changes up to 48 GPa, few modifications were observed which might be due to local distortion of the lattice forming units that affect the inter- and intra-tetrahedral angles without breaking of the bonds [12]. The vibrational modes of the SrNi3(P2O7)2 (high-pressure structure) are measured to 61.5 GPa and on decompression. Fig. 5 shows the plot of the Raman spectra on decompression. Pressure shifts of these modes are plotted in Fig. 6. As
Fig. 6. The pressure dependence of the observed Raman shifts in SrNi3(P2O7)2 after release of pressure.
B. Manoun et al. / Journal of Molecular Structure 794 (2006) 334–340
339
Fitting the same results to the Birch–Murnaghan equation [26] P Z 3=2K0 ½ðV=V0 ÞK7=3 KðV=V0 ÞK5=3 f1 C 3=4ðK00 K4Þ½ðV=V0 ÞK2=3 K1g
(2)
yields a K0 value of 124G2 GPa, with a pressure derivative, K00 of 4.37G0.12, Least squares fits of the curves shown in Fig. 7, yield a Z 7:4050K0:0195ðP=P0 Þ C 0:0002ðP=P0 Þ2
(3)
b Z 7:6741K0:0229ðP=P0 Þ C 0:0002ðP=P0 Þ2
(4)
and Fig. 7. Pressure dependencies of the a, b and c parameters for SrNi3(P2O7)2. a and b unit cell parameters join to the same value around 51 GPa. In the Raman study, we showed that the transition began around (48–50 GPa).
illustrated in Fig. 5, the frequency of the vibrations decreases continuously on decompression. The new high-pressure phase is clearly seen upon pressure release and it is recovered at ambient conditions. The XRD pattern showed that SrNi3(P2O7)2 was a single phase; it crystallizes with a monoclinic unit cell, space group P21/c with two formula units per unit cell, and with lattice ˚ , bZ parameters aZ7.407(3), bZ7.678(4), cZ9.501(8) A ˚ , these results are close to those reported by El 113.1(3) 8A Bali et al. [10] determined the crystal structure of SrNi3(P2O7)2. The pressure dependence of the lattice parameters of the polycrystalline SrNi3(P2O7)2 sample were measured up to a pressure of 48 GPa (Fig. 7). A least squares fit of the changes in the relative unit cell volume, V/V0, with pressure (Fig. 8) yields V=V0 Z 0:9982K0:0068 P=P0 C 6 !10K5 ðP=P0 Þ2 R Z 0:999 ˚ 3. where P0Z1 GPa, and V0Z496.8G0.5 A
Fig. 8. Unit cell volume of SrNi3(P2O7)2 as a function of pressure.
(1)
c Z 9:5073K0:0209P=P0 C 8 !10K5 ðP=P0 Þ2
(5)
The compressibility of SrNi3(P2O7)2 along the b-axis is relatively larger than that along the a-axis; one reason for that is that there is –Ni2C–Sr2C– Ni2C– cation chains along a-axis which make it more difficult to be compressed. Note also that, along a-axis, there is the effect of Ni2C–Sr2Crepulsion and the size of the Sr2Ccations. At 48 GPa, a and b cell parameters are almost equal indicating that a phase transition from the monoclinic to tetragonal structure starts. The extrapolation of the curves of the a and b changes versus pressure shows that a equals b at 51.5 GPa, this is consistent with the Raman study where we showed that the investigated compound, SrNi3(P2O7)2, compressed smoothly up to the highest investigated pressures, transforms to another structure at pressures of 50.5 GPa. The tetragonal unit cell parameters of the new high˚. pressure phase are aZbZ6.91 and cZ8.25 A 4. Conclusion High-pressure investigations of SrNi 3(P 2O 7) 2 pyrophosphate, in diamond anvil cell have been performed at room temperature using in situ Raman spectroscopy. The endeavor was to acquire information on pressure-induced structural transformations such as phase transitions and amorphization occurring in the crystal lattice. Group theory yields to 60 Raman active modes 30AgC30Bg for SrNi3 (P2O7)2, of which only 37 bands are observed at ambient conditions. The pressure-induced phase transition sequence of SrNi3(P2O7)2 to pressures up to 61.5 GPa was explored. Raman spectra showed that the investigated compound, compressed smoothly up to the highest investigated pressures; the bands increase in widths without considerable changes up to 48 GPa and the material transforms to another structure at pressures of 50.5 GPa. Upon release of pressure to ambient conditions, the high-pressure phase remained unchanged. We also measured the pressure dependence of the lattice parameters of a polycrystalline SrNi3(P2O7)2 sample up to a pressure of 48 GPa. The compressibility of SrNi3(P2O7)2 along the b-axis is greater than that along the a-axis. The extrapolation of the a and b curves show that, at 51.5 GPa, a and b cell parameters are almost equal indicating that a phase transition from the
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monoclinic to tetragonal structure occurred. The bulk modulus of SrNi3(P2O7)2 (P21/c) is 124G2 GPa with a pressure derivative of 4.37G0.12. Acknowledgements This work was financially supported by a grant from the National Science Foundation (DMR 0231291). Part of this work was conducted at Cornell High Energy Synchrotron Source (CHESS), supported by NSF grant and NIH/NIGMS under award DMR 0225180. Special thanks to Dr C.S. Zha for his help. References [1] A. Jouini, J.C. Gacon, M. Ferid, M. Trabelsi-Ayadi, Opt. Mater. 24 (2003) 175 (and references therein). [2] T. Kitsugi, T. Yamamuro, T. Nakamura, M. Oka, Biomaterials 16 (1995) 1101. [3] B. Jian-Jiang, K. Dong-Wan, H. Kug Sun, J. Eur. Ceram. Soc. 23 (2003) 2589 (and references therein). [4] A. Mogus-Milankovi, B. Santic, C.S. Ray, D.E. Day, J. Non-Cryst. Solids 263 (2000) 299. [5] P. Carmen, P. Josefina, S.P. Regino, R.V. Caridad, S. Natalia, Chem. Mater. 15 (2003) 3347. [6] I.C. Marcu, J.M. Millet, I. Sandulescu, Prog. Catal. 10 (2001) 71. [7] H.Y.P. Hong, Mater. Res. Bull. 11 (1976) 173. [8] A. Durif, Crystal Chemistry of Condensed Phosphates, Plenum Press, New York, 1995.
[9] A. Hezel, D. Ross, Spectrochim. Acta 23A (1967) 1583. [10] B. El Bali, A. Boukhari, J. Aride, K. Maaß, D. Wald, R. Glaum, F. Abraham, Solid State Sci. 3 (2001) 669. [11] M.J. Kohn, J. Rakovan, J.M. Hughes (Eds.), Phosphates: Geochemical, Geobiological, and Materials Importance Reviews in Mineralogy and Geochemistry Series, 48, Mineralogical Society of America, Washington, DC, 2002. [12] K.E. Lipinska-Kalita, M.B. Kruger, S. Carlson, A.M. Krogh Andersen, Physica B: Condens. Matter 337 (2003) 221. [13] U. Kuhlmann, C. Thomsen, A.V. Prokofiev, F. Bu¨llesfeld, E. Uhrig, M. Anton, C. Gross, W. Assmus, Phys. Rev. B 66 (2002) 64420. [14] M. Baril, H. Assaaoudi, I.S. Butler, J. Mol. Struct. 751 (2005) 168. [15] H.K. Mao, J. Xu, P.M. Bell, J. Geophys. Res. 91 (1986) 4673. [16] B. Manoun, H.P. Liermann, S.K. Saxena, et al., Appl. Phys. Lett. 84 (2004) 2799. [17] H.P. Liermann, A.K. Singh, B. Manoun, S.K. Saxena, V.B. Prakapenka, G. Shen, Int. J. Refract. Met. Hard Mater. 22 (2004) 129–132. [18] B. Manoun, S.K. Saxena, R. Gulve, et al., Appl. Phys. Lett. (2004) 85. [19] B. Manoun, S.K. Saxena, M.W. Barsoum, Appl. Phys. Letts. 86 (2005) 101906. [20] H.P. Liermann, A.K. Singh, B. Manoun, S.K. Saxena, C.S. Zha, Int. J. Refract. Met. Hard Mater. 23 (2005) 109–114. [21] R.G. Greene, H. Luo, A.L. Ruoff, Phys. Rev. Lett. 73 (1994) 2075–2078. [22] E. Kroumova, M.I. Aroyo, J.M. Perez-Mato, A. Kirov, C. Capillas, S. Ivantchev, H. Wondratschek, PhaseTransit. 76 (2003) 155–70http:// www.cryst.ehu.es [23] D. Philip, B.L. George, G. Aruldhas, J. Raman. Spectrosc. 21 (1990) 523. [24] N. Santha, V.U. Nayar, Spectrochim. Acta 49A (1993) 47. [25] M. Gebelica-Robert, J. Mol. Struct. 79 (1982) 255. [26] F. Birch, J. Geophys. Res. 83 (1978) 1257.
High-pressure studies of SrNi3(P2O7)2 pyrophosphate by Raman spectroscopy and X-ray diffraction Bouchaib Manoun a,b,*, Brahim El Bali c, Surendra K. Saxena a, Revansidha P. Gulve a a
Center for the Study of Matter at Extreme Conditions (CeSMEC), Florida International University, VH-140, University Park, Miami, FL 33199, USA b Laboratoire d’Analyses, d’Essais et d’Environnement (LAEE), De´partement de Chimie, Faculte´ des Sciences ‘Dhar Mehraz’, Universite´ Sidi Mohamed Ben Abdellah, 30000 Fe`s, Morocco c Laboratory of Mineral Solid and Analytical Chemistry “LMSAC”, Department of Chemistry, Faculty of Sciences, University Mohamed I, PO. Box 624, 60000 Oujda, Morocco Received 18 September 2005; received in revised form 16 February 2006; accepted 1 March 2006 Available online 19 April 2006
Abstract High-pressure investigations of SrNi3(P2O7)2 pyrophosphate, in diamond anvil cell have been performed at room temperature using in situ Raman spectroscopy and X-ray synchrotron radiation source. The endeavor was to acquire information on pressure-induced structural transformations such as phase transitions and amorphization occurring in the crystal lattice. Group theory yields to 60 Raman active modes 30AgC30Bg for SrNi3(P2O7)2, of which only 37 bands are observed at ambient conditions. The pressure-induced phase transition sequence of SrNi3(P2O7)2 pyrophosphate to pressures up to 61.5 GPa was explored. Raman spectra showed that the investigated compound, SrNi3(P2O7)2, compressed smoothly up to the highest investigated pressures, transforms to another structure at pressures of 50.5 GPa. The new high-pressure phase persists metastably down to 1 atm. upon release of pressure. We also measured the pressure dependence of the lattice parameters of the polycrystalline SrNi3(P2O7)2 sample up to a pressure of 48 GPa. The compressibility of SrNi3(P2O7)2 along the b-axis is larger than that along the a-axis. At 48 GPa, a and b cell parameters are almost equal indicating that a phase transition from the monoclinic to a tetragonal structure started. The bulk modulus of SrNi3(P2O7)2 (P21/c) is 124G2 GPa with a pressure derivative of 4.37G0.12. q 2006 Elsevier B.V. All rights reserved. Keywords: High-pressure; SrNi3(P2O7)2; Pyrophosphate; Phase transition; Raman spectroscopy; X-ray diffraction
1. Introduction Crystalline and glassy inorganic phosphate materials are of considerable industrial interest because of their utility as laser hosts [1] and their wide application in ceramic [2], dielectric [3], electric [4], magnetic [5], and catalytic [6] processes. Of special interest, orthophosphates, pyrophosphates and triphosphates are of biological importance for their various applications as catalysts, molecular sieves, or ion exchangers [7]. Systematic studies on simple and mixed pyrophosphates (M2P2O7 or (A,M)2P2O7 with A and/or M an alkaline earth or divalent 3d-metal ion) have been undertaken during the * Corresponding author. Address: Center for the Study of Matter at Extreme Conditions (CeSMEC), Florida International University, 11200 SW 8th Street, VH-140, University Park, Miami FL 33199, USA. Tel.: C1 305 348 3445; fax: C1 305 348 3070. E-mail address: [email protected] (B. Manoun).
0022-2860/$ - see front matter q 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2006.03.001
last two decades. In fact, pyrophosphates constitute the largest family of condensed phosphates with variable stoichiometries [8]. Crystallographic studies of pyrophosphates have indicated that the geometric configuration of the pyrophosphate ion is mainly dependent on the crystalline environment. The P2 O4K 7 ion consists of two PO4 tetrahedra sharing a common oxygen atom. Hezel and Ross [9] have shown that the ion is non-linear with unequal P–O bond lengths for the terminal and bridging bonds. In certain cases, the terminal bond lengths are of unequal length, and two PO3 groups are imperfectly staggered. El Bali et al. [10] determined the crystal structure of SrNi3(P2O7)2, from single crystal data, with P21/c, ZZ2, aZ ˚ , bZ112.216(9)8. 7.4092(9), bZ7.6594(8), cZ9.4474(10) A SrNi3(P2O7)2 is isostructural to AM3(P2O7)2 (AZCa, Pb and MZFe, Co, Ni). Beside the crystal structure study, authors reported on a correlation of absorption spectra and coordination geometry of the [NiOx] chromophore for the title compound. Ni2Ccations occupy two non-equivalent crystallographic positions in the framework of SrNi3(P2O7)2, both showing slightly distorted octahedral coordination. The [NiO6]
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octahedra share edges to form chains along the monoclinic axis. The structure of SrNi3(P2O7)2 might be thus described in terms of chains of edge-sharing [NiO6] octahedra, held together by [P2O7] groups. This ‘covalent’ network comprises tunnels parallel to [010], hosting Sr2Cions. The Ni-coordination can be moreover distinguished by the ‘ligating’ forms. Ni(1) is surrounded by five [P2O7] groups, one of which is a bidendate ligand. Ni(2) is coordinated by six monodendate [P2O7] groups. Despite crystallographic investigations, while the physicochemical properties of inorganic phosphates have received a great deal of attention [11], the effect of high external pressures on their structures has been comparatively little studied. To the best of our knowledge, few papers are dealing with highpressure Raman studies on pyrophosphates such as that reporting on TiP2O7 [12] where they observed a densification and a partial amorphization occurred in the crystal lattice. Also recently, the pressure dependence of the phonons of the ambient- and high-pressure phases of the vanadyl catalyst, (VO)2P2O7, has been investigated by polarized Raman spectroscopy [13]. The V–O stretching modes exhibit an anomalous mode softening at w3 GPa, which was attributed to the interlayer interaction of neighboring V and O atoms. More recently, Baril et al. [14] reported on the effect of high external pressures on the Raman spectra of four metal pyrophosphate dihydrates, KMHP2O7$2H2O[MZCo(II), Mn(II), Zn(II) and Mg(II)], for pressures up to w4.2 GPa. They suggested the occurrence of a pressure-driven structural transformation, at w2.9 GPa, or possibly even the first stages of amorphization for Co(II) compound and no serious changes for the rest of the series. The present work on Raman spectroscopy and X-ray diffraction at room and high-pressures is a part of a systematic study of some other pyrophosphates. The principal aim of our experiments is to follow and to identify the pressure-induced structural transformations, such as phase transitions and amorphization, occurring in the crystals. The investigation of SrNi3(P2O7)2 by Raman spectroscopy at high-pressures will characterise this material; and will allow us to report on the behaviour of SrNi3(P2O7)2 upon increasing (and decreasing) pressure. Another important point is to study the reversibility of the possible high-pressure induced phase transitions. The investigation of SrNi3(P2O7)2 by X-ray diffraction will allow us to measure the pressure dependence of the lattice parameters of this material and try to follow the phase transition sequences.
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2.1.2. Single crystals A powder sample was prepared according to the chemical formula SrNiP2O7, which has been melt at 1473 K in a platinium crucible. After controlled cooling (rate 108/h) to 773 K the sample was quenched to room temperature. The product was found to contain two kind of crystals distinguishable by their colors, red–orange (SrNiP2O7) and green– yellowish (SrNi3(P2O7)2). Both chemical formulae have been fixed after structure determination as reported in [10]. 2.2. Raman spectroscopy Raman spectra were measured in a back scattering arrangement, on both compression and decompression. Raman spectra were collected at room temperature by using high throughput holographic imaging spectrograph with volume transmission grating, holographic notch filter and thermoelectrically cooled CCD detector (Physics Spectra), with the resolution of 4 cmK1. The spectrometer was regularly calibrated using the neon lines. Ti3C-sapphire laser pumped by an argon ion laser was tuned at 785 nm. The laser power was operated at 100 mW in the entire measured pressure range; both, on compression and decompression. The incident laser beam was focused with the spot size of 5 mm by the objective (20!) to excite the sample. The spectrometer and the Raman spectra were calibrated using the Raman modes of diamond and sulfur, as well as, of the neon emission spectrum. The exposure time was 100 s with 10 accumulations. High-pressure Raman measurements were conducted at room temperature using a gasketed high-pressure Diamond Anvil Cell (DAC). Powder samples of SrNi3(P2O7)2 were loaded in Mao-Bell-type diamond anvil cells. Diamonds with 300 mm culets were used and a steel gasket, indented to 83 mm, with a sample chamber of 125 mm contained the sample. Ruby chips were loaded with the sample at various spots as pressure markers. Pressures were determined by using the pressuredependent spectral shift of the sharp Ruby fluorescence R1 line shift [15], excited by an argon ion laser with the wavelength of 514.5 nm. Because any pressure medium only maintains the sample at a high hydrostatic state at relatively low pressure, but at pressure O15 GPa, it mostly becomes quasi-hydrostatic due to the solidification of the pressure medium, including liquid and gas. In this study, the material is relatively soft and no pressure medium was employed. The focused laser spot on the sample was close to the Ruby chip, so the pressure difference will be small.
2. Experimental 2.3. X-ray diffraction 2.1. Sample preparation 2.1.1. Powder samples SrNi3(P2O7)2 was synthesized from stoichiometric amounts of nickel, alkaline earth carbonate and (NH4)2HPO4. After dissolution of the starting materials in nitric acid, the homogenous solutions have been evaporated to dryness on a heating stirrer. The residue was ground and heated in air at increasing temperatures up to 1373 K.
For X-ray diffraction, measurements were conducted at room temperature; powdered samples were pressurized using a gasketed Diamond Anvil Cell (DAC) with a 300 mm culet. A 100 mm initial thickness rhenium gasket was indented to about 50 mm. Since high purity aluminum does not undergo structural phase transition at high-pressure, has low shear strength, as in our previous work [16–20], it was the pressure-transmitting medium of choice. The presence of aluminum in the cell
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rendered the sample pressure nearly hydrostatic and also served as the pressure standard because of its well established pressure–volume relation [21]. Powdered samples were placed between two pieces of Al foil (z15 mm thickness) and packed in the 100 mm hole. X-ray diffraction measurements of SrNi3(P2O7)2 were conducted at room temperature using an angle dispersive ˚ of a monochromatic synchrotron radiation. Using a lZ0.496 A beam focused down to a 35 mm spot size, the X-ray spectra were collected at CHESS (Cornell University, Ithaca, NY). Diffraction rings were recorded between 2qZ1 and 358. The FIT2D program was employed to convert the image plate records into 2q’s and intensities. The cell parameters were determined using least squares refinement on individually fitted peaks. The peaks were assigned to the monoclinic structure with the space group P21/c. 3. Results and discussions SrNi3(P2O7)2 crystallizes in the monoclinic system P21 =cðC52h Þ with two formula units per unit cell [10]. Its structure can be described in terms of chains of edge-sharing [NiO6] octahedra, held together by [P2O7] groups as shown in Fig. 1. In fact, Ni2Coccupy two non-equivalent cristallographic positions in the framework of the title compound, both showing slightly distorted octahedral coordination. Two neighbouring [NiO6] octahedra share an edge to form a zigzag chains along the monoclinic axis. These chains are linked via O–P–O–P–O bridges to form the ‘covalent’ network, which comprises tunnels parallel to [010] where Sr2Cions are located. The Ni-coordination can be moreover distinguished
Fig. 1. A Projection of the crystal structure of SrNi3(P2O7)2 along [001], NiO6 octahedra (blue) form chains along b-axis, each two PO4 tetrahedra (Yellow) share one corner and link the NiO6 chains. The SrO8 polyhedra were omitted for clarity and the circles represent Sr2Ccations.
by the ‘ligating’ schemes. In fact, Ni(1) is surrounded by five [P2O7] groups, one acting as bidendate ligand whereas Ni(2) is coordinated by six monodendate [P2O7] units. The pyrophosphate P2 O4K anion consists of two distorted 7 tetrahedra sharing one apex. These anions set on two different kinds of general sites in the framework of SrNi3(P2O7)2. Conventionaly, P2 O4K 7 is characterized by its P–O–P bridging angle as by its conformation. In our case, it possesses a non eclipsed form and the P–O–P is non-linear. The Sr2Cion and one of the three Ni2Cions are on two distinct Ci sites while the remaining two Ni2Cions are on equivalent C1 sites. The irreducible representation of the title compound in the C2h factor group (excluding three acoustic modes) led to 30AgC 30BgC35AuC34Bu (30AgC30Bg are Raman active while 35AuC34Bu are infrared active). Fig. 2 shows the Raman spectrum of SrNi3(P2O7)2 recorded at ambient conditions. According to the factor group analysis [22], 60 Raman active modes, represented as MZ30AgC 30Bg, should be observed for the P21/c structure of SrNi3(P2O7)2 in the Raman experiment. However, the number of observed modes is 37, significantly smaller than expected. This may be attributed to accidental degeneracy of a number of modes. Moreover, some weak bands are most likely overlapped by much stronger bands and others bands are observed as weak shoulders. The pyrophosphate ion consists of two PO3 groups and a P–O–P bridge. So the assignments of P2O7 modes are carried out in terms of PO3 and P–O–P group vibrations [23]. The performed assignment for SrNi3(P2O7)2 compared to that of PbNi3(P2O7)2 [24] compound is presented in Table 1. Since the SrNi3(P2O7)2 specimen studied here is a polycrystalline powder, we cannot precisely assign the observed modes using a combination of the single-crystal orientation and the polarized Raman scattering. However, the observed Raman modes can be classified into three general families of lattice vibrations: external modes and Ni2Ctranslational vibrations at frequencies below 300 cmK1; the ranges assigned to the symmetric and antisymmetric components of the PO 3 deformational modes is based on known spectroscopic data
Fig. 2. Raman spectrum of SrNi3(P2O7)2 pyrophosphate recorded at ambient conditions out of DAC.
B. Manoun et al. / Journal of Molecular Structure 794 (2006) 334–340 Table 1 Vibrational frequencies (cmK1) and their assignments of SrNi3(P2O7)2 compared to that of PbNi3(P2O7)2 [24] SrNi3(P2O7)2 114 146 151 171 183 186 217 234 252 257 292 302 375 393 421 473 494 528 538 561 593 624 637 742 933 1038 1055 1098 1146 1163
PbNi3(P2O7)2 110 115 142 155 175 180 203 215
Assignments
External modes
Ni2C translational vibrations 250 255 290 375 392 420 470 495 525 535 562 590
742 927 945 1030 1050 1095 1135 1180 1215
d(P–O–P) Ni2Ctranslational vibrations? ds(PO3) and n(M–O)
das(PO3)
ns(P–O–P) nas(P–O–P) ns(PO3)
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bridging angle (P–O–P) is not linear in these compounds. In SrNi3(P2O7)2, the bridging angle (P–O–P) is also not linear and was determined to be 134.08 [10]. The asymmetric bridge P–O stretching mode gives only one Raman band at 933.4 cmK1 for SrNi3(P2O7)2 while two bands were observed in PbNi3(P2O7)2 and PbCo3(P2O7)2 where the authors attributed this splitting to the correlation field effect [24]. SrNi3(P2O7)2 and PbNi3(P2O7)2 are very similar but slightly different from that of PbCo3(P2O7)2, which could be explained by the distortion of 2C the P2 O4K ions in 7 anion and the low symmetry field of the Co the cobalt compound [24]. The symmetric P–O–P stretching vibration is observed in the Raman spectra region as strong band centered at 742 cmK1. In compounds with linear P–O–P bridge, nsP–O–P is observed at a higher frequency than those of nsPO3 and nasPO3 [25]. The appearance of nasP–O–P at a lower frequency than those of PO3 stretching frequencies again supports the non-linearity of the P–O–P bridge in SrNi3(P2O7)2 in agreement with El Bali’s et al. results [10]. Raman spectra of SrNi3(P2O7)2 were collected in situ (DAC) in compression and decompression cycles at room temperature and elevated pressures, up to 61.5 GPa; the pressure-induced changes in the Raman active modes are shown in Fig. 3. The features of the changes shown by the bands can be summarized as follows: all the bands in the whole frequency region (100–1300 cmK1) exhibit important frequency shifts (Fig. 4). All the bands change in intensity; in general, smoothing down as the pressure increases. Finally, some bands appear, suddenly; some others merge; and some
nas(PO3)
1279
of divalent metal diphosphates [24]: ds(PO3) and n(M–O) modes are observed in the region 390–500 cmK1 and the das(PO3) modes are lying in the region 520–650 cmK1. The deformation frequency of the P–O–P bridge occurs around 300 cmK1 [23]. ns(P–O–P) and nas(P–O–P) are observed in the 740–950 cmK1 frequency region. The asymmetric PO 3 terminal stretching vibrations (nasPO3) of the P2O7 group in different compounds are usually observed in the highfrequency region above 1050 cmK1. The frequencies of the symmetric PO3 terminal stretching vibrations are, in general, expected to be below the asymmetric ones in the region between 1000 and 1070 cmK1. These frequency families were also observed in other isoformular pyrophosphates such as PbNi3(P2O7)2 and PbCo3(P2O7)2 [24]. The P–O stretching modes in SrNi3(P2O7)2 are observed at higher wavenumbers than those in PbNi3(P2O7)2, indicating that the P–O bonds in Sr compound is stronger than those in the Pb compound. The P–O–P bridge stretching vibrations are observed in the usual range and could be easily identified in the Raman spectrum. In previous work by Santha et al. [24], vibrational spectra showed that ns(POP) is observed in IR as well as in Raman for PbNi3(P2O7)2 and PbCo3(P2O7)2 pointing that the
Fig. 3. Raman spectra of SrNi3(P2O7)2 recorded for the compression cycle at various pressures up to 61.5 GPa in the range 100–1300 cmK1, a phase transition is observed at 50.5 GPa.
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Fig. 4. The pressure dependence of the observed Raman shifts in SrNi3(P2O7)2 across the phase transition.
others split. In the whole frequency range, upon elevation of pressure, Raman spectra show that up to 48 GPa, all SrNi3 (P2O7)2 bands assigned to the P21/c structure maintain, with positive pressure shifts of the frequencies (Fig. 4). Note that some negative shifts are observed for some external modes at frequencies below 200 cmK1, these soft modes might be attributed to P2O7 librations. Also change in the shift slope was observed in most of the Raman modes after 20.5 GPa. The spectral changes pointed mostly to the densification of the crystal lattice. This behavior was also observed in the highpressure Raman study of TiP2O7 [12]. The bands observed at 375 and 393 cmK1, at room pressure, become more separated at 10.2 GPa. The splitting is not due to a phase transition, but results from an overlapping of two bands attributed to Ni2Ctranslational modes (375 cmK1) and ds(PO3) and n(M–O) (393 cmK1). The bands become separated at highpressures, owing to their different pressure dependence. As pressure increases (14.9 GPa), the ds(PO3) and n(M–O) band overlap with the 420.5 cmK1 band also attributed to ds(PO3) and n(M–O) and again moved faster then the 420.5 cmK1 band to separate at 38.9 GPa with a higher frequency. The terminal stretching vibrations nas(PO3) band observed at 1098 cmK1, at room pressure, shifted less than that of the symmetric one observed at 1055 cmK1 and therefore nas(PO3) became a shoulder of ns(PO3). With increasing pressure, the left weak shoulder of the 1055 cmK1 strong band, gets stronger (starting at 38.9 GPa), until it becomes an independent band at around 50.5 GPa. Upon further elevation of pressure, both bands show strong pressure dependence. Therefore, this splitting cannot be explained by an overlapping of two bands: it is a sign of a phase transition. Moreover, this new band gets stronger in intensity while the band observed, at room pressure, at 1055 cmK1 decreased significantly in intensity. The transformation pressure, for SrNi3(P2O7)2 at room temperature, is then given as (48.04C50.48)/2Z49.26G1.22 GPa. A right shoulder of the 742 cmK1 symmetric P–O–P stretching vibration is observed in the Raman spectrum at pressures over 34.2 GPa. This band will be clearly seen on decompression (Fig. 5). Also at pressure of 50.5 GPa we notice the
Fig. 5. Raman scattering spectra recorded on decompression from 61.5 GPa to ambient conditions. The new high-pressure phase is recovered at 1 atm.
appearance of other new bands for example the one centered at 1018 cmK1 (50.5 GPa) and became clear at higher pressures which provide us again direct evidence of pressure induced phase transformation occurring in SrNi3(P2O7)2 structure. More experiments are needed to complete this work; we will use synchrotron radiation source and a diamond anvil cell to measure the pressure dependence of the lattice parameters of this material and try to follow the phase transition sequences. Note that the presence of most Raman peaks over the whole pressure range indicates that the P2O7 units are present in the compressed material. The bands increase in widths without considerable changes up to 48 GPa, few modifications were observed which might be due to local distortion of the lattice forming units that affect the inter- and intra-tetrahedral angles without breaking of the bonds [12]. The vibrational modes of the SrNi3(P2O7)2 (high-pressure structure) are measured to 61.5 GPa and on decompression. Fig. 5 shows the plot of the Raman spectra on decompression. Pressure shifts of these modes are plotted in Fig. 6. As
Fig. 6. The pressure dependence of the observed Raman shifts in SrNi3(P2O7)2 after release of pressure.
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339
Fitting the same results to the Birch–Murnaghan equation [26] P Z 3=2K0 ½ðV=V0 ÞK7=3 KðV=V0 ÞK5=3 f1 C 3=4ðK00 K4Þ½ðV=V0 ÞK2=3 K1g
(2)
yields a K0 value of 124G2 GPa, with a pressure derivative, K00 of 4.37G0.12, Least squares fits of the curves shown in Fig. 7, yield a Z 7:4050K0:0195ðP=P0 Þ C 0:0002ðP=P0 Þ2
(3)
b Z 7:6741K0:0229ðP=P0 Þ C 0:0002ðP=P0 Þ2
(4)
and Fig. 7. Pressure dependencies of the a, b and c parameters for SrNi3(P2O7)2. a and b unit cell parameters join to the same value around 51 GPa. In the Raman study, we showed that the transition began around (48–50 GPa).
illustrated in Fig. 5, the frequency of the vibrations decreases continuously on decompression. The new high-pressure phase is clearly seen upon pressure release and it is recovered at ambient conditions. The XRD pattern showed that SrNi3(P2O7)2 was a single phase; it crystallizes with a monoclinic unit cell, space group P21/c with two formula units per unit cell, and with lattice ˚ , bZ parameters aZ7.407(3), bZ7.678(4), cZ9.501(8) A ˚ , these results are close to those reported by El 113.1(3) 8A Bali et al. [10] determined the crystal structure of SrNi3(P2O7)2. The pressure dependence of the lattice parameters of the polycrystalline SrNi3(P2O7)2 sample were measured up to a pressure of 48 GPa (Fig. 7). A least squares fit of the changes in the relative unit cell volume, V/V0, with pressure (Fig. 8) yields V=V0 Z 0:9982K0:0068 P=P0 C 6 !10K5 ðP=P0 Þ2 R Z 0:999 ˚ 3. where P0Z1 GPa, and V0Z496.8G0.5 A
Fig. 8. Unit cell volume of SrNi3(P2O7)2 as a function of pressure.
(1)
c Z 9:5073K0:0209P=P0 C 8 !10K5 ðP=P0 Þ2
(5)
The compressibility of SrNi3(P2O7)2 along the b-axis is relatively larger than that along the a-axis; one reason for that is that there is –Ni2C–Sr2C– Ni2C– cation chains along a-axis which make it more difficult to be compressed. Note also that, along a-axis, there is the effect of Ni2C–Sr2Crepulsion and the size of the Sr2Ccations. At 48 GPa, a and b cell parameters are almost equal indicating that a phase transition from the monoclinic to tetragonal structure starts. The extrapolation of the curves of the a and b changes versus pressure shows that a equals b at 51.5 GPa, this is consistent with the Raman study where we showed that the investigated compound, SrNi3(P2O7)2, compressed smoothly up to the highest investigated pressures, transforms to another structure at pressures of 50.5 GPa. The tetragonal unit cell parameters of the new high˚. pressure phase are aZbZ6.91 and cZ8.25 A 4. Conclusion High-pressure investigations of SrNi 3(P 2O 7) 2 pyrophosphate, in diamond anvil cell have been performed at room temperature using in situ Raman spectroscopy. The endeavor was to acquire information on pressure-induced structural transformations such as phase transitions and amorphization occurring in the crystal lattice. Group theory yields to 60 Raman active modes 30AgC30Bg for SrNi3 (P2O7)2, of which only 37 bands are observed at ambient conditions. The pressure-induced phase transition sequence of SrNi3(P2O7)2 to pressures up to 61.5 GPa was explored. Raman spectra showed that the investigated compound, compressed smoothly up to the highest investigated pressures; the bands increase in widths without considerable changes up to 48 GPa and the material transforms to another structure at pressures of 50.5 GPa. Upon release of pressure to ambient conditions, the high-pressure phase remained unchanged. We also measured the pressure dependence of the lattice parameters of a polycrystalline SrNi3(P2O7)2 sample up to a pressure of 48 GPa. The compressibility of SrNi3(P2O7)2 along the b-axis is greater than that along the a-axis. The extrapolation of the a and b curves show that, at 51.5 GPa, a and b cell parameters are almost equal indicating that a phase transition from the
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