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Crystal structure, vibrational spectra and optical properties of praseodymium cyclotriphosphate PrP3O9·3H2O
Materials Research Bulletin 41 (2006) 1370–1377 www.elsevier.com/locate/matresbu
Crystal structure, vibrational spectra and optical properties of praseodymium cyclotriphosphate PrP3O93H2O Anis Jouini a,*, Mokhtar Fe´rid b, Jean-Claude Gaˆcon a, Laurent Grosvalet a, Alain Thozet c, Malika Trabelsi-Ayadi d a
Laboratoire de Physico-Chimie des Mate´riaux Luminescents, UMR CNRS No. 5620, Universite´ Claude-Bernard Lyon 1, 69622 Villeurbanne, France b Laboratoire des Proce´de´s Chimiques, Institut National de Recherche Scientifique et Technique, B.P. 95 Hammam-Lif 2050, Tunisia c Centre de Diffractome´trie, Universite´ Claude-Bernard Lyon 1, 69622 Villeurbanne, France d Laboratoire de Physico-Chimie Mine´rale, Faculte´ des Sciences de Bizerte, 7021 Zarzouna, Bizerte, Tunisia Received 9 September 2005; received in revised form 9 December 2005; accepted 16 December 2005 Available online 18 January 2006
Abstract Single crystals of the trihydrated praseodymium cyclotriphosphate PrP3O93H2O were grown for the first time, using a classical method of aqueous chemistry and characterized by single crystal X-ray diffraction. PrP3O93H2O is isostructural with 1 LnP3O93H2O (Ln = La, Ce and Nd). It crystallizes in the hexagonal system with space group P6 ðC3h Þ, with lattice parameters: ˚ , b = 6.7677(4) A ˚ , c = 6.0501(4) A ˚ , V = 239.98(3) A ˚ 3, Z = 1 and Dx = 2.988 g cm3. The crystal structure is a = 6.7677(4) A resolved for the first time, with final R(F 2) = 0.0175 and Rw(F 2) = 0.0417 for 396 independent reflections ðF02 2sðF02 ÞÞ. The P3O93 cyclic anions have a plane configuration with C3h symmetry. The nearest neighbours of the rare earth ion are six oxygen atoms belonging to the P3O93 anions. Pr3+ ions occupy sites with C3h symmetry. The energies of the crystal vibrational modes are obtained from infrared (IR) and Raman spectra. Optical absorption measurements and emission spectra under selective excitation in the Pr3+ (4f2) 3PJ (J = 0, 1, 2) levels, at room and liquid-helium temperatures, are reported. The observed fluorescence originates mainly from 3P0 with a decay time on the order of 10 ns, regardless of temperature. # 2006 Elsevier Ltd. All rights reserved. Keywords: C. X-ray diffraction; C. Infrared spectroscopy; C. Raman spectroscopy; D. Crystal structure; D. Luminescence
1. Introduction Investigations dealing with hydrated lanthanide simple cyclotriphosphates LnP3O9nH2O (where Ln = rare earth) are relatively scarce. The synthesis of trihydrate cyclotriphosphates with Ln = La, Ce and Pr was first described in 1968, by Serra and Giesbrecht [1]. Then, Bagieu-Beucher et al., identified the same series by X-ray analysis [2] and determined the structure of CeP3O93H2O crystalline powder [3]. Later NdP3O93H2O was synthesized and characterized by Gushikem et al. [4]. Simonot-Grange and Gobled studied the thermal dehydration of LnP3O93H2O (where Ln = La, Ce and Pr) [5,6]. More recently, praseodymium-activated compounds have gained interest as * Corresponding author at: Fukuda Laboratory, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Fax: +81 22 217 5102. E-mail address: [email protected] (A. Jouini). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.12.007
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potential scintillating materials for medical imaging devices [7]. The present work is a continuation of studies devoted to the synthesis and characterization of rare earth phosphate crystals, conducted within the framework of a collaboration between Tunisian and French laboratories [8–13]. These investigations aim to prepare phosphate compounds exhibiting electrical and luminescence properties of interest for applications. To the best of our knowledge, suitable PrP3O93H2O single crystals have been prepared, for the first time. As a consequence, the structural analysis and optical properties of these materials are also described here. 2. Experimental methods, results and discussion 2.1. Synthesis The synthesis of PrP3O93H2O single crystals consists in slowly mixing PrCl36H2O and Na3P3O9 0.1 M aqueous solutions in a 1:1 ratio under mechanical stirring at room temperature (RT). The chemical reaction is as follows: Na3 P3 O9 þ PrCl3 6H2 O ! PrP3 O9 3H2 O þ 3H2 O þ 3NaCl
(1)
The preparation of Na3P3O9 cyclotriphosphate is reported in Ref. [9]. After 2 days, single crystals may be isolated from the resulting pale green precipitate after filtering, washing with hot water and drying in air. They were mechanically separated, using an optical microscope, from a mass of smaller crystals of the same composition. A single crystal of 0.30 mm 0.20 mm 0.25 mm dimensions was selected for the structural analysis. 2.2. Structural analysis The X-ray diffraction intensities were collected using a Nonius Kappa CCD diffractometer. One hundred and twenty-eight frames were measured, with an exposure time of 10 s per frame and a scan angle of 28. A w scan was used for 91 frames and an V scan for the others. It was possible to determine the positions of the Pr atoms using the Patterson heavy atom method because Pr is much heavier than P and O [14]. Successive Fourier analyses were used to locate the other atoms [15]. The recording conditions and crystallographic data are reported in Table 1. The structural Table 1 Crystallographic data and experimental parameters of the X-ray intensity data collection for PrP3O93H2O Diffractometer Monochromator Empirical formula Formula weight Temperature Wavelength Crystal system Space group Unit cell dimensions Volume, Z Density (calculated) Absorption coefficient F(0 0 0) Crystal size Theta range for data collection Limiting indices Reflections collected Independent reflections Absorption correction Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2s(I)] R indices (all data)
Nonius Kappa CCD Graphite PrP3O93H2O 431.87 293(2) K ˚ 0.71073 A Hexagonal P6¯ ˚ ; b = 6.7677(4) A ˚; a = 6.7677(4) A ˚ a = 90; b = 90; g = 120 c = 6.0501(4) A ˚ 3, 1 239.98(3) A 2.988 g/cm3 5.635 mm1 206 0.30 mm 0.20 mm 0.25 mm 3.37–27.488 8 h 8, 7 k 6, 7 l 6 396 396 Integration Full-matrix least-square on F2 396/0/31 1.071 R1 = 0.0175, Rw2 = 0.0417 R1 = 0.0175, Rw2 = 0.0417
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Table 2 ˚ ) and equivalent isotropic displacement parameters (A ˚ 2 103) for PrP3O93H2O Atomic coordinates (A Atom
x/a
y/b
z/c
Ueq
Pr P(1) O(1) O(2) O(3) H(1)
0.33333 0.7215(2) 0.9471(7) 0.6028(7) 0.6848(9) 0.74066
0.66667 0.9371(2) 0.11827(7) 0.9155(6) 0.618(1) 0.58576
1 0.5 0.5 0.2883(6) 1 1.10001
11 11 25 33 39 60
Ueq is defined as the one-third of the trace of the orthogonalized Uij tensor, Ueq ¼ 13
P
i; j
Ui j jai j2 ja j j2 .
Table 3 ˚ 2 104) Anisotropic displacement parameters for PrP3O93H2O (A Atom
U11
U22
U33
U12
U13
U23
Pr P(1) O(1) O(2) O(3)
106(2) 77(7) 130(2) 380(2) 290(3)
106(2) 117(7) 130(2) 290(2) 580(4)
75(2) 103(7) 430(3) 240(2) 200(3)
53(1) 41(6) 50(2) 190(2) 300(3)
0 0 0 220(2) 0
0 0 0 120(1) 0
determination converges to R1 = 0.0175 and Rw2 = 0.0417 for 396 independent reflections having I > 2s(I). The atomic coordinates, as well as isotropic and anisotropic displacement parameters, were refined using the full-matrix least-square method. They appear in Tables 2 and 3. Main bond distances and angles calculated from the final atomic coordinates are given in Table 4. Our results confirm that the PrP3O93H2O cyclotriphosphate is isostructural with LnP3O93H2O with (Ln = La, Ce and Nd) [1–4]. These compounds present the advantage of having only one formula unit in the crystallographic cell. The P3O93 ring is planar with C3h symmetry (Figs. 1 and 2). Pr3+ ions occupy sites with C3h symmetry. The environment of the Pr3+ is shown in Fig. 3. The nearest neighbours of the rare earth ion are six oxygen atoms (O2) belonging to the P3O93 anion. These oxygen atoms are located at the corners of two parallel equilateral triangles forming a right prism, with its center occupied by the rare earth ion. Moreover, three oxygen atoms (O3) belonging to the water molecules form another equilateral triangle, which is antiparallel to the two others Table 4 ˚ ) and angles (deg.) in PrP3O93H2O Bond distances (A Bond type
˚) Distance (A
Angles (8)
Pr environment Pr–O(2)
6 2.477(4)
O(2)–Pr–O(2) O(2)–Pr–O(2) O(2)–Pr–O(2) Pr–O(2)–PR Pr–O(2)–P(1)
6 75.9(2)a 3 89.6(2)b 6 138.4(2)c 14 0.0 12 148.8(3)
O(3)–H(1)
2 0.800(4)
H(1)–O(3)–H(1)
98.3(6)
P3O9 cycle P(1)–O(1) P(1)–O(1) P(1)–O(2)
2 1.599(5) 2 1.595(5) 2 1.480(4)
O(1)–P(1)–O(1) O(1)–P(1)–O(1) O(1)–P(1)–O(2) O(1)–P(1)–O(2) O(2)–P(1)–O(2) P(1)–O(1)–P(1) P(1)–O(1)–P(1)
0.0 4 103.6(3) 4 107.0(2) 4 109.1(2) 119.9(3) 0.0 4 136.4(3)
a b c
For two oxygen atoms belonging to the same P3O9 cycle. For two oxygen atoms belonging to different P3O9 cycles but situated in the same plane. For two oxygen atoms belonging to different P3O9 cycles and situated in different planes.
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Fig. 1. Projection along the c axis of the P3O93 anion in the PrP3O93H2O structure.
Fig. 2. Projection along the b axis of the P3O93 anion in the PrP3O93H2O structure.
and located in the same plane as the rare earth ion. The lanthanide ion is at a site of C3h point symmetry. An ab projection of the PO4 tetrehedra in the PrP3O93H2O is shown in Fig. 4. 2.3. Infrared and Raman spectra The infrared (IR) spectrum of a KBr-pressed pellet of the powdered sample was studied in the 4000–400 cm1 range using a Spectrum GX FT-IR system from Perkin-Elmer. Usually, the hydrated salts absorb between 3300 and
Fig. 3. Environment of the praseodymium atom in the PrP3O93H2O structure.
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Fig. 4. An ab projection of the PO4 tetrahedra in the PrP3O93H2O structure.
1640 cm1 and absorption bands at frequencies higher than 3450 cm1 are absent in most of them. One of the most noteworthy features of the IR spectrum shown in Fig. 5 is the presence of O–H stretching bands in the 3700– 3500 cm1 region. Moreover, these bands are much sharper than in most other hydrated salts. Such a feature is also observed for the O–H bending band peaking at 1626 cm1. As a matter of fact, O–H stretching bands are known (i) to be shifted to higher frequencies in hydrated cyclophosphates and (ii) to be much sharper than in most other hydrated salts [16]. Such observations suggest that quasi-free water molecules may be associated to these bands. As a conclusion, at least some of the water molecules could be only loosely held in the PrP3O93H2O crystal lattice, probably in an interstitial manner. The strong absorption bands peaking near 1036 and 1298 cm1 usually appear in the spectra of cyclophosphates [8,9,16,17]. They are assigned to nas(OPO) and nas(POP) vibrations, respectively. The observation of these bands is a good criterion to differentiate cyclophosphates from chain structure polyphosphates [11]. Yet another characteristic feature of cyclophosphate IR spectrum is the occurrence of two strong absorption peaks at 760 and 500 cm1, usually assigned to d(POP) and d(OPO) vibrations, respectively. The room temperature PrP3O93H2O Raman spectrum shown in Fig. 6 was obtained using a Raman microprobe combined with a Dilor XY spectrometer. The 514.5 nm radiation beam from an argon ion laser was sent through a microscope, allowing the selection of a single crystal of good optical quality. The strong line at 1176 cm1 is due to OPO symmetric vibrations within the P3O9 rings. Additional lines of variable intensity are also observed in the 360– 200 cm1 region, namely the sharp ones at 357 and 306 cm1. They are due to ring bending and breathing modes. The low frequency part of the Raman spectrum (n < 170 cm1) has a characteristic far infrared profile, which can be assigned to active Pr3+ translation modes [17]. The characteristic vibration frequencies measured from the spectra displayed in Figs. 5 and 6 are presented in Table 5, with a tentative assignment of the observed main bands.
Fig. 5. Room temperature FT-IR spectrum of PrP3O93H2O.
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Fig. 6. Room temperature Raman spectrum of PrP3O93H2O.
2.4. Luminescence investigations Absorbance in the 300–1000 nm range were measured using a Lambda 20 UV–vis Perkin-Elmer spectrophotometer, with the cell filled with the obtained crystals just as it is with liquid solutions. The absorption spectrum displayed in Fig. 7 was used to locate the energies of the Pr3+ (4f2) 3PJ (J = 0, 1, 2) levels in which laser selective excitations of PrP3O93H2O were performed. Because of their small size, the samples were ground to obtain a fine powder for luminescence measurements. Fluorescence spectra and decays under pulsed laser excitation were measured using an excimer-pumped dye laser delivering pulses of 10 ns duration and 0.1 cm1 spectral width with a 10 Hz repetition rate. Coumarin 480, 460 and 440 dyes were used for selective excitation in the 3P0, 3P1 and 3P2 levels, respectively. The dye laser output beam was focused onto the powder holder under glancing incidence with its energy always kept below 250 mJ per pulse. The luminescence intensity at room temperature is very low, regardless of the absorbing 3PJ level. The observed emission bands were assigned by comparing the spectra to those reported in other rare earth phosphate compounds [11,14,15,17,18]. They originate from 3P0 ! 3HJ (J = 4, 5, 6) and 3P0 ! 3FJ (J = 2, 4) transitions occurring in the visible region. No emission originating from the 1D2 level was observed. Moreover, no emission from 3P1 and 3P2 levels was detected. These general features hold at low temperature, except that the fluorescence intensity becomes higher. The 6 K emission spectra under 3P2 excitation is shown in Fig. 8. The observed 3 P0 fluorescence decays exponentially with a very short decay time (10 ns) on the order of the time resolution of our equipment, regardless of temperature in the 6–300 K range. Such a lack of thermal dependence of the fluorescence decay was already observed in NdP3O93H2O [8]. The high-energy vibrational modes of the H2O molecules were suspected to play a predominant role in the quenching of the Nd3+ luminescence in this hydrated material. This
Table 5 Frequencies (cm1) and assignment of vibration modes in PrP3O93H2O IR
Raman
Assignment
3620, 3510 1626 1298 1108 1036 764 501 – –
– – 1248 1176–1103 900 656–622 481 357–306, 269–202 169, 131–78
n(OH) d(OH) nas(OPO) ns(OPO) nas(POP) ns POP) d(OPO) and d(POP) Ring bending and ring breathing modes Translation modes of Pr3+
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Fig. 7. Room temperature absorption spectrum of PrP3O93H2O.
Fig. 8. Emission spectra of PrP3O93H2O under excitation in the 3P2 multiplets at 444.8 nm at 6 K.
assumption was confirmed by comparing the results concerning NdP3O93H2O with those obtained for the correspondant anhydrous Nd(PO3)3 polyphosphate [8]. A similar comparison between PrP3O93H2O and Pr(PO3)3 [13] leads to the same conclusion, that the coupling of the Pr3+ excited states to the O–H stretching bands (3510– 3620 cm1) of the water molecules is responsible for the low luminescence intensity and extremely short temperatureindependent fluorescence decays in PrP3O93H2O. 3. Main conclusions Single crystals of the PrP3O93H2O cyclotriphosphate were grown for the first time, using a classical method of aqueous chemistry. The crystal structure is resolved for the first time, with final R(F 2) = 0.0175 and Rw(F 2) = 0.0417 for 396 independent reflections ðF02 2sðF02 ÞÞ. It is shown that PrP3O93H2O crystallizes in the 1 hexagonal system with P6¯ ðC3h Þ space group. Infrared and Raman spectroscopy measurements have confirmed these results. Under pulsed laser selective excitation in the Pr3+ 3PJ (J = 0, 1, 2) levels, this hydrated concentrated praseodymium phosphate exhibits a low intensity fluorescence in the visible range. This emission originates from the Pr3+ 3P0 level. It is characterized by a very short (10 ns), temperature-independent, exponential decay. The vibrational modes of the water molecules play a predominant role in the Pr3+ luminescence quenching in PrP3O93H2O.
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Acknowledgment The authors are grateful to Prof. Bernard Champagnon, Director of CECOMO (Centre Commun de Microscopie Optique, Universite´ Claude-Bernard Lyon 1, France), for his assistance with the infrared and Raman spectra. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
O.A. Serra, E. Giesbrecht, J. Inorg. Nucl. Chem. 30 (1968) 793–799. M. Bagieu-Beucher, A. Durif, Bull. Soc. Fr. Mine´r. Crystallogr. 94 (1971) 440–441. M. Bagieu-Beucher, I. Tordjman, A. Durif, Rev. Chim. Miner. 8 (1971) 753–760. Y. Gushikem, E. Giesbrecht, O.A. Serra, J. Inorg. Nucl. Chem. 34 (1972) 2179. M.-H. Simonot-Grange, D. Gobled, J. Less-Common Met. 38 (1974) 167. D. Gobled, M.-H. Simonot-Grange, A. Thrierr-Sorel, C.R. Acad. Sci. Paris 278 (1974) 179–182. C. Dujardin, C. Pedrini, J.C. Gaˆcon, A.G. Petrosyan, A.N. Belsky, A.N. Vasil’ev, J. Phys.: Condens. Matter 9 (1997) 5229. K. Horchani, M. Fe´rid, M. Trabelsi-Ayadi, Solid State Sci. 3 (2001) 347–352. A. Jouini, J.-C. Gaˆcon, A. Brenier, M. Fe´rid, M. Trabelsi-Ayadi, J. Lumin. 99 (2002) 365. A. Jouini, J.C. Gaˆcon, M. Fe´rid, M. Trabelsi-Ayadi, Opt. Mater. 24 (1/2) (2003) 175. A. Jouini, M. Fe´rid, J.C. Gaˆcon, L. Grosvalet, A. Thozet, M. Trabelsi-Ayadi, Mater. Res. Bull. 38 (2003) 1613. K. Horchani, J.C. Gaˆcon, C. Dujardin, N. Garnier, C. Garapon, M. Fe´rid, M. Trabelsi-Ayadi, J. Lumin. 94/95 (2001) 69. K. Horchani, J.C. Gaˆcon, C. Dujardin, N. Garnier, C. Garapon, M. Fe´rid, M. Trabelsi-Ayadi, Nucl. Instrum. Meth. A 486 (2002) 283. G.M. Sheldrick, SHELXS-97 Program for Crystal Structure Determination, University of Go¨ttingen, Germany, 1977. G.M. Sheldrick, SHELXL-97 Program for Crystal Structure Determination, University of Go¨ttingen, Germany, 1977. D.E.C. Corbridge, E.J. Lowe, J. Chem. Soc. (1954) 49. A. Rulomont, R. Cahay, M. Liegois-Duyckaerts, P. Tarte, Eur. J. Solid State Chem. 28 (1991) 207. M. Malinowski, R. Wolski, W. Wolinski, J. Lumin. 35 (1986) 1.
Crystal structure, vibrational spectra and optical properties of praseodymium cyclotriphosphate PrP3O93H2O Anis Jouini a,*, Mokhtar Fe´rid b, Jean-Claude Gaˆcon a, Laurent Grosvalet a, Alain Thozet c, Malika Trabelsi-Ayadi d a
Laboratoire de Physico-Chimie des Mate´riaux Luminescents, UMR CNRS No. 5620, Universite´ Claude-Bernard Lyon 1, 69622 Villeurbanne, France b Laboratoire des Proce´de´s Chimiques, Institut National de Recherche Scientifique et Technique, B.P. 95 Hammam-Lif 2050, Tunisia c Centre de Diffractome´trie, Universite´ Claude-Bernard Lyon 1, 69622 Villeurbanne, France d Laboratoire de Physico-Chimie Mine´rale, Faculte´ des Sciences de Bizerte, 7021 Zarzouna, Bizerte, Tunisia Received 9 September 2005; received in revised form 9 December 2005; accepted 16 December 2005 Available online 18 January 2006
Abstract Single crystals of the trihydrated praseodymium cyclotriphosphate PrP3O93H2O were grown for the first time, using a classical method of aqueous chemistry and characterized by single crystal X-ray diffraction. PrP3O93H2O is isostructural with 1 LnP3O93H2O (Ln = La, Ce and Nd). It crystallizes in the hexagonal system with space group P6 ðC3h Þ, with lattice parameters: ˚ , b = 6.7677(4) A ˚ , c = 6.0501(4) A ˚ , V = 239.98(3) A ˚ 3, Z = 1 and Dx = 2.988 g cm3. The crystal structure is a = 6.7677(4) A resolved for the first time, with final R(F 2) = 0.0175 and Rw(F 2) = 0.0417 for 396 independent reflections ðF02 2sðF02 ÞÞ. The P3O93 cyclic anions have a plane configuration with C3h symmetry. The nearest neighbours of the rare earth ion are six oxygen atoms belonging to the P3O93 anions. Pr3+ ions occupy sites with C3h symmetry. The energies of the crystal vibrational modes are obtained from infrared (IR) and Raman spectra. Optical absorption measurements and emission spectra under selective excitation in the Pr3+ (4f2) 3PJ (J = 0, 1, 2) levels, at room and liquid-helium temperatures, are reported. The observed fluorescence originates mainly from 3P0 with a decay time on the order of 10 ns, regardless of temperature. # 2006 Elsevier Ltd. All rights reserved. Keywords: C. X-ray diffraction; C. Infrared spectroscopy; C. Raman spectroscopy; D. Crystal structure; D. Luminescence
1. Introduction Investigations dealing with hydrated lanthanide simple cyclotriphosphates LnP3O9nH2O (where Ln = rare earth) are relatively scarce. The synthesis of trihydrate cyclotriphosphates with Ln = La, Ce and Pr was first described in 1968, by Serra and Giesbrecht [1]. Then, Bagieu-Beucher et al., identified the same series by X-ray analysis [2] and determined the structure of CeP3O93H2O crystalline powder [3]. Later NdP3O93H2O was synthesized and characterized by Gushikem et al. [4]. Simonot-Grange and Gobled studied the thermal dehydration of LnP3O93H2O (where Ln = La, Ce and Pr) [5,6]. More recently, praseodymium-activated compounds have gained interest as * Corresponding author at: Fukuda Laboratory, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Fax: +81 22 217 5102. E-mail address: [email protected] (A. Jouini). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.12.007
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potential scintillating materials for medical imaging devices [7]. The present work is a continuation of studies devoted to the synthesis and characterization of rare earth phosphate crystals, conducted within the framework of a collaboration between Tunisian and French laboratories [8–13]. These investigations aim to prepare phosphate compounds exhibiting electrical and luminescence properties of interest for applications. To the best of our knowledge, suitable PrP3O93H2O single crystals have been prepared, for the first time. As a consequence, the structural analysis and optical properties of these materials are also described here. 2. Experimental methods, results and discussion 2.1. Synthesis The synthesis of PrP3O93H2O single crystals consists in slowly mixing PrCl36H2O and Na3P3O9 0.1 M aqueous solutions in a 1:1 ratio under mechanical stirring at room temperature (RT). The chemical reaction is as follows: Na3 P3 O9 þ PrCl3 6H2 O ! PrP3 O9 3H2 O þ 3H2 O þ 3NaCl
(1)
The preparation of Na3P3O9 cyclotriphosphate is reported in Ref. [9]. After 2 days, single crystals may be isolated from the resulting pale green precipitate after filtering, washing with hot water and drying in air. They were mechanically separated, using an optical microscope, from a mass of smaller crystals of the same composition. A single crystal of 0.30 mm 0.20 mm 0.25 mm dimensions was selected for the structural analysis. 2.2. Structural analysis The X-ray diffraction intensities were collected using a Nonius Kappa CCD diffractometer. One hundred and twenty-eight frames were measured, with an exposure time of 10 s per frame and a scan angle of 28. A w scan was used for 91 frames and an V scan for the others. It was possible to determine the positions of the Pr atoms using the Patterson heavy atom method because Pr is much heavier than P and O [14]. Successive Fourier analyses were used to locate the other atoms [15]. The recording conditions and crystallographic data are reported in Table 1. The structural Table 1 Crystallographic data and experimental parameters of the X-ray intensity data collection for PrP3O93H2O Diffractometer Monochromator Empirical formula Formula weight Temperature Wavelength Crystal system Space group Unit cell dimensions Volume, Z Density (calculated) Absorption coefficient F(0 0 0) Crystal size Theta range for data collection Limiting indices Reflections collected Independent reflections Absorption correction Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2s(I)] R indices (all data)
Nonius Kappa CCD Graphite PrP3O93H2O 431.87 293(2) K ˚ 0.71073 A Hexagonal P6¯ ˚ ; b = 6.7677(4) A ˚; a = 6.7677(4) A ˚ a = 90; b = 90; g = 120 c = 6.0501(4) A ˚ 3, 1 239.98(3) A 2.988 g/cm3 5.635 mm1 206 0.30 mm 0.20 mm 0.25 mm 3.37–27.488 8 h 8, 7 k 6, 7 l 6 396 396 Integration Full-matrix least-square on F2 396/0/31 1.071 R1 = 0.0175, Rw2 = 0.0417 R1 = 0.0175, Rw2 = 0.0417
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Table 2 ˚ ) and equivalent isotropic displacement parameters (A ˚ 2 103) for PrP3O93H2O Atomic coordinates (A Atom
x/a
y/b
z/c
Ueq
Pr P(1) O(1) O(2) O(3) H(1)
0.33333 0.7215(2) 0.9471(7) 0.6028(7) 0.6848(9) 0.74066
0.66667 0.9371(2) 0.11827(7) 0.9155(6) 0.618(1) 0.58576
1 0.5 0.5 0.2883(6) 1 1.10001
11 11 25 33 39 60
Ueq is defined as the one-third of the trace of the orthogonalized Uij tensor, Ueq ¼ 13
P
i; j
Ui j jai j2 ja j j2 .
Table 3 ˚ 2 104) Anisotropic displacement parameters for PrP3O93H2O (A Atom
U11
U22
U33
U12
U13
U23
Pr P(1) O(1) O(2) O(3)
106(2) 77(7) 130(2) 380(2) 290(3)
106(2) 117(7) 130(2) 290(2) 580(4)
75(2) 103(7) 430(3) 240(2) 200(3)
53(1) 41(6) 50(2) 190(2) 300(3)
0 0 0 220(2) 0
0 0 0 120(1) 0
determination converges to R1 = 0.0175 and Rw2 = 0.0417 for 396 independent reflections having I > 2s(I). The atomic coordinates, as well as isotropic and anisotropic displacement parameters, were refined using the full-matrix least-square method. They appear in Tables 2 and 3. Main bond distances and angles calculated from the final atomic coordinates are given in Table 4. Our results confirm that the PrP3O93H2O cyclotriphosphate is isostructural with LnP3O93H2O with (Ln = La, Ce and Nd) [1–4]. These compounds present the advantage of having only one formula unit in the crystallographic cell. The P3O93 ring is planar with C3h symmetry (Figs. 1 and 2). Pr3+ ions occupy sites with C3h symmetry. The environment of the Pr3+ is shown in Fig. 3. The nearest neighbours of the rare earth ion are six oxygen atoms (O2) belonging to the P3O93 anion. These oxygen atoms are located at the corners of two parallel equilateral triangles forming a right prism, with its center occupied by the rare earth ion. Moreover, three oxygen atoms (O3) belonging to the water molecules form another equilateral triangle, which is antiparallel to the two others Table 4 ˚ ) and angles (deg.) in PrP3O93H2O Bond distances (A Bond type
˚) Distance (A
Angles (8)
Pr environment Pr–O(2)
6 2.477(4)
O(2)–Pr–O(2) O(2)–Pr–O(2) O(2)–Pr–O(2) Pr–O(2)–PR Pr–O(2)–P(1)
6 75.9(2)a 3 89.6(2)b 6 138.4(2)c 14 0.0 12 148.8(3)
O(3)–H(1)
2 0.800(4)
H(1)–O(3)–H(1)
98.3(6)
P3O9 cycle P(1)–O(1) P(1)–O(1) P(1)–O(2)
2 1.599(5) 2 1.595(5) 2 1.480(4)
O(1)–P(1)–O(1) O(1)–P(1)–O(1) O(1)–P(1)–O(2) O(1)–P(1)–O(2) O(2)–P(1)–O(2) P(1)–O(1)–P(1) P(1)–O(1)–P(1)
0.0 4 103.6(3) 4 107.0(2) 4 109.1(2) 119.9(3) 0.0 4 136.4(3)
a b c
For two oxygen atoms belonging to the same P3O9 cycle. For two oxygen atoms belonging to different P3O9 cycles but situated in the same plane. For two oxygen atoms belonging to different P3O9 cycles and situated in different planes.
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Fig. 1. Projection along the c axis of the P3O93 anion in the PrP3O93H2O structure.
Fig. 2. Projection along the b axis of the P3O93 anion in the PrP3O93H2O structure.
and located in the same plane as the rare earth ion. The lanthanide ion is at a site of C3h point symmetry. An ab projection of the PO4 tetrehedra in the PrP3O93H2O is shown in Fig. 4. 2.3. Infrared and Raman spectra The infrared (IR) spectrum of a KBr-pressed pellet of the powdered sample was studied in the 4000–400 cm1 range using a Spectrum GX FT-IR system from Perkin-Elmer. Usually, the hydrated salts absorb between 3300 and
Fig. 3. Environment of the praseodymium atom in the PrP3O93H2O structure.
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Fig. 4. An ab projection of the PO4 tetrahedra in the PrP3O93H2O structure.
1640 cm1 and absorption bands at frequencies higher than 3450 cm1 are absent in most of them. One of the most noteworthy features of the IR spectrum shown in Fig. 5 is the presence of O–H stretching bands in the 3700– 3500 cm1 region. Moreover, these bands are much sharper than in most other hydrated salts. Such a feature is also observed for the O–H bending band peaking at 1626 cm1. As a matter of fact, O–H stretching bands are known (i) to be shifted to higher frequencies in hydrated cyclophosphates and (ii) to be much sharper than in most other hydrated salts [16]. Such observations suggest that quasi-free water molecules may be associated to these bands. As a conclusion, at least some of the water molecules could be only loosely held in the PrP3O93H2O crystal lattice, probably in an interstitial manner. The strong absorption bands peaking near 1036 and 1298 cm1 usually appear in the spectra of cyclophosphates [8,9,16,17]. They are assigned to nas(OPO) and nas(POP) vibrations, respectively. The observation of these bands is a good criterion to differentiate cyclophosphates from chain structure polyphosphates [11]. Yet another characteristic feature of cyclophosphate IR spectrum is the occurrence of two strong absorption peaks at 760 and 500 cm1, usually assigned to d(POP) and d(OPO) vibrations, respectively. The room temperature PrP3O93H2O Raman spectrum shown in Fig. 6 was obtained using a Raman microprobe combined with a Dilor XY spectrometer. The 514.5 nm radiation beam from an argon ion laser was sent through a microscope, allowing the selection of a single crystal of good optical quality. The strong line at 1176 cm1 is due to OPO symmetric vibrations within the P3O9 rings. Additional lines of variable intensity are also observed in the 360– 200 cm1 region, namely the sharp ones at 357 and 306 cm1. They are due to ring bending and breathing modes. The low frequency part of the Raman spectrum (n < 170 cm1) has a characteristic far infrared profile, which can be assigned to active Pr3+ translation modes [17]. The characteristic vibration frequencies measured from the spectra displayed in Figs. 5 and 6 are presented in Table 5, with a tentative assignment of the observed main bands.
Fig. 5. Room temperature FT-IR spectrum of PrP3O93H2O.
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Fig. 6. Room temperature Raman spectrum of PrP3O93H2O.
2.4. Luminescence investigations Absorbance in the 300–1000 nm range were measured using a Lambda 20 UV–vis Perkin-Elmer spectrophotometer, with the cell filled with the obtained crystals just as it is with liquid solutions. The absorption spectrum displayed in Fig. 7 was used to locate the energies of the Pr3+ (4f2) 3PJ (J = 0, 1, 2) levels in which laser selective excitations of PrP3O93H2O were performed. Because of their small size, the samples were ground to obtain a fine powder for luminescence measurements. Fluorescence spectra and decays under pulsed laser excitation were measured using an excimer-pumped dye laser delivering pulses of 10 ns duration and 0.1 cm1 spectral width with a 10 Hz repetition rate. Coumarin 480, 460 and 440 dyes were used for selective excitation in the 3P0, 3P1 and 3P2 levels, respectively. The dye laser output beam was focused onto the powder holder under glancing incidence with its energy always kept below 250 mJ per pulse. The luminescence intensity at room temperature is very low, regardless of the absorbing 3PJ level. The observed emission bands were assigned by comparing the spectra to those reported in other rare earth phosphate compounds [11,14,15,17,18]. They originate from 3P0 ! 3HJ (J = 4, 5, 6) and 3P0 ! 3FJ (J = 2, 4) transitions occurring in the visible region. No emission originating from the 1D2 level was observed. Moreover, no emission from 3P1 and 3P2 levels was detected. These general features hold at low temperature, except that the fluorescence intensity becomes higher. The 6 K emission spectra under 3P2 excitation is shown in Fig. 8. The observed 3 P0 fluorescence decays exponentially with a very short decay time (10 ns) on the order of the time resolution of our equipment, regardless of temperature in the 6–300 K range. Such a lack of thermal dependence of the fluorescence decay was already observed in NdP3O93H2O [8]. The high-energy vibrational modes of the H2O molecules were suspected to play a predominant role in the quenching of the Nd3+ luminescence in this hydrated material. This
Table 5 Frequencies (cm1) and assignment of vibration modes in PrP3O93H2O IR
Raman
Assignment
3620, 3510 1626 1298 1108 1036 764 501 – –
– – 1248 1176–1103 900 656–622 481 357–306, 269–202 169, 131–78
n(OH) d(OH) nas(OPO) ns(OPO) nas(POP) ns POP) d(OPO) and d(POP) Ring bending and ring breathing modes Translation modes of Pr3+
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Fig. 7. Room temperature absorption spectrum of PrP3O93H2O.
Fig. 8. Emission spectra of PrP3O93H2O under excitation in the 3P2 multiplets at 444.8 nm at 6 K.
assumption was confirmed by comparing the results concerning NdP3O93H2O with those obtained for the correspondant anhydrous Nd(PO3)3 polyphosphate [8]. A similar comparison between PrP3O93H2O and Pr(PO3)3 [13] leads to the same conclusion, that the coupling of the Pr3+ excited states to the O–H stretching bands (3510– 3620 cm1) of the water molecules is responsible for the low luminescence intensity and extremely short temperatureindependent fluorescence decays in PrP3O93H2O. 3. Main conclusions Single crystals of the PrP3O93H2O cyclotriphosphate were grown for the first time, using a classical method of aqueous chemistry. The crystal structure is resolved for the first time, with final R(F 2) = 0.0175 and Rw(F 2) = 0.0417 for 396 independent reflections ðF02 2sðF02 ÞÞ. It is shown that PrP3O93H2O crystallizes in the 1 hexagonal system with P6¯ ðC3h Þ space group. Infrared and Raman spectroscopy measurements have confirmed these results. Under pulsed laser selective excitation in the Pr3+ 3PJ (J = 0, 1, 2) levels, this hydrated concentrated praseodymium phosphate exhibits a low intensity fluorescence in the visible range. This emission originates from the Pr3+ 3P0 level. It is characterized by a very short (10 ns), temperature-independent, exponential decay. The vibrational modes of the water molecules play a predominant role in the Pr3+ luminescence quenching in PrP3O93H2O.
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Acknowledgment The authors are grateful to Prof. Bernard Champagnon, Director of CECOMO (Centre Commun de Microscopie Optique, Universite´ Claude-Bernard Lyon 1, France), for his assistance with the infrared and Raman spectra. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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