Synthesis and Crystal Structure of a New Potassium−Gadolinium

CRYSTAL GROWTH & DESIGN

Synthesis and Crystal Structure of a New Potassium-Gadolinium Cyclotetraphosphate, KGdP4O12

2003 VOL. 3, NO. 4 599-602

H. Ettis, H. Naı¨li,* and T. Mhiri Laboratoire de l’Etat Solide, Universite´ de Sfax 3018, Sfax, Tunisie Received November 19, 2002

ABSTRACT: The chemical preparation, crystal structure, and infrared absorption spectrum are given for a new cyclotetraphosphate, KGdP4O12. X-ray investigations showed that the newly synthesized compound crystallizes in a monoclinic structure, space group C2/c with the following unit-cell dimensions: a ) 7.875(1), b ) 12.431(2), c ) 10.537(2) Å, β ) 110.94(1)°, V ) 963.4(3) Å3, Z ) 4, and F ) 3.532 g cm-3. The structure was solved from 1390 independent reflections with R1 ) 2.99 and WR2 ) 7.37%, refined with 84 parameters. As in all atomic arrangements, we observe the formation of an infinite network of P4O124- cyclotetraphosphate anions connected with GdO8 polyhedra to form a three-dimensional framework which delimits interesting tunnels where the K+ cations are located. In this structure, the P4O12 ring develops around an inversion center. The reported IR study, recorded at room temperature in the frequency range 400-4000 cm-1, shows some characteristics bands of cyclotetraphosphates. 1. Introduction Condensed phosphates which have been the object of several research studies have considerable interest as they show different applications in electricity and catalysis. Specifically, condensed phosphates of rare earths have optical properties in the laser technology domain.1-5 However, with gadolinium, there are few numbers of crystals produced. The reason for this situation is probably the difficulty in obtaining crystals of high quality. An examination of the literature shows that only the cyclotetraphosphates with general formula MILnP4O124-13 (where MI is a monovalent cation and Ln is a rare earth: Ce, Pr, Nd, Sm, Eu, Ho, and Y) were synthesized and their structures were determined. At room temperature, they are found mainly in two different space groups: C2/c and I4h 3d.12,14 The common feature of these structures is typical of layers formed by P4O124- rings. However, at least to our knowledge, no structural studies of the corresponding gadolinium cyclotetraphosphates of monovalent cations have been conducted. Thus, KGdP4O12 reported here is the first material that is synthesized to enrich this family of compounds. In this paper, we describe the chemical preparation and report detailed structural investigation of the new cyclotetraphosphate KGdP4O12. In addition, the nominal compound has been characterized by IR spectroscopy. 2. Experimental Procedures 2.1. Crystal Growth and Characterization. Crystals of KGdP4O12 were prepared by using the flux method. At room temperature, 2.8 g of H3PO4, 3.2 g of KH2PO4, and 0.4 g of Gd2O3 were mixed respectively in a Pt crucible, preheated at 2 °C/min to 200 °C, and kept for 4 h at this temperature. Then, the temperature was increased progressively to 550 °C. Two days later, the temperature was reduced by 40 °C/day to reach 50 °C. The crystals obtained by this procedure are of about 0.22 × 0.06 × 0.09 mm3 size. They have a needle shape after double washing in boiling water and with nitric acid to eliminate the remaining oxide Gd2O3. The formula of this * Corresponding author: E-mail: [email protected].

Table 1. Results of Chemical Analysis for KGdP4O12 calculated experimental

P (%)

K (%)

Gd (%)

24.20 22.50a

7.63 7.02b

30.69 29.05c

a Determined by spectrophotometry. b Determined by atomic absorption. c Determined by ICP method.

compound is determined by chemical analysis (Table 1) and confirmed by refinement of the crystal structure. In fact, the single crystals already formed correspond to the composition KGdP4O12. It is noted that many preparations with respect to this stoichiometry always led to the same density of crystals (dmes ) 3.311 g cm-3). Infrared absorption spectra of suspensions of crystalline powders in KBr were examined by a 733 Perkin-Elmer spectrophotometer in the 400-4000 cm-1 region. Data collection was performed with an Enraf-Nonius CAD-4 diffractometer using MoKR graphite monochromated radiation (λ ) 0.71069 Å). The cell parameters obtained from singlecrystal diffractometer measurements are a ) 7.875(1), b ) 12.431(2), c ) 10.537(2) Å, and β ) 110.94(1)° with space group C2/c. The ω-2θ scan mode was used with the scan width (0.9 + 0.8 tgθ)°. The heavy-atom method was used to solve the structure. A three-dimensional Patterson function showed that the Gd atoms should occupy the special position (4e) of the space group C2/c. Atomic scattering factors were taken from the International Tables for X-ray Crystallography15; 1479 reflections were collected in the whole Ewald sphere for 3 e θ e 30°, of which 1358 reflections had an intensity of I > 2σ(I). The structure was successfully developed in the centrosymmetric space group C2/c. Gadolinium and potassium atom positions were located using SHELXS-97,16 whereas P and O atom positions were deduced from difference Fourier maps during the refinement of the structure with an adapted version of SHELXL-97 program.17 The least-squares refinement leads to the final reliability factors R1 ) 2.99 and WR2 ) 7.37%, obtained by fitting 84 parameters. The final fractional atomic coordinates and the equivalent anisotropic thermal parameters are given in Tables 3 and 4.

3. Results and Discussion 3.1. Description of the Structure. The crystal structure of KGdP4O12 projected onto the (001) and (100) planes is shown in Figures 1 and 2, respectively. The atomic arrangement is typical of a layer structure. It consists of the centrosymmetrical cyclotetraphosphate

10.1021/cg0200659 CCC: $25.00 © 2003 American Chemical Society Published on Web 05/09/2003

600

Crystal Growth & Design, Vol. 3, No. 4, 2003

Ettis et al.

Table 2. Summary of Crystal Data, Intensity Measurements, and Refinement Parameters for KGdP4O12 I. Crystal Data formula KGdP4O12 formula weight (g/mol) 512.35 crystal-system: monoclinic space group: C2/c a ) 7.875 (1) Å β ) 110.94 (1)° b ) 12.431 (2) Å Z)4 c ) 10.537 (2) Å V ) 963.4 (3) Å3 dcal ) 3.532 g.cm-3 µ(MoKR) ) 8.047 mm-1 crystal shape needle habit colorless crystal size (mm) (0.22 × 0.06 × 0.09) II. Intensity Measurements temperature: 298 K radiation: Mo KR (0.71069 Å) diffractometer: CAD4 scan mode: ω - 2θ Enraf-Nonius scan width (0.9 + 0.8 tgθ)° theta range 3-30° reference reflections 3 5 2 and 1 3 7 measurement area: (h, k, (l) h ) 11; k ) 17; l ) 13 total reflections 1479

Figure 1. Projection of KGdP4O12 crystals structure on the ab-plane.

III. Structure Determination structure solution Shelxs and Shelxl 16,17 corrections Lorentz and polarization empirical absorption correction Ψ scan 18 unique reflections included 1358 with I > 2σ(I) refined parameters 84 R1 (%) ) 2.99 WR2 (%) ) 7.37 Table 3. Fractional Atomic Coordinates and Temperature Factors for KGdP4O12a atoms

x

y

z

Ue´q

Gd K P(1) P(2) O(E11) O(E12) O(L12) O(L21) O(E21) O(E22)

0.5000 0.5000 0.2174(1) 0.0417(1) 0.2804(3) 0.3508(3) 0.0949(4) 0.0689(3) -0.0597(3) 0.2108(3)

0.6196(1) 0.6837(1) 0.4770(1) 0.6707(1) 0.4728(2) 0.4875(2) 0.3727(2) 0.5713(2) 0.7556(2) 0.7001(2)

0.7500 1.2500 0.4403(1) 0.4982(1) 0.3237(3) 0.5818(3) 0.4302(3) 0.4119(2) 0.4008(3) 0.6137(2)

0.0071(1) 0.0400(4) 0.0080(2) 0.0078(2) 0.0123(4) 0.0139(5) 0.0128(5) 0.0115(4) 0.0141(5) 0.0123(4)

a

Estimated standard deviations are given in parentheses. Ue´q)1/3∑i∑jUija*i a*j aiaj.

ring anion P4O124- consisting of PO4 tetrahedra, each of which shares two corners with the others. Inside such a layer, the phosphoric ring has 1h internal symmetry. It develops around an inversion center located at (0, 1/2, 0), so it is built up by only two independent PO4 tetrahedra. This symmetry is the frequent one in cyclotetraphosphates compared to 2, 2/m, m, mm, 4 h and 4 h 2m symmetries observed in P4O124- rings characterized until the present.19 The main interatomic distances and bond angles for the two independent tetrahedra are given in Table 5. As shown in projection into the bc

Figure 2. Projection of the structure of KGdP4O12 along [100] direction.

plane, the P4O12 rings form layers perpendicular to the [001] direction at z ) 0 and z ) 1/2. The GdO8 polyhedra interconnect the P4O12 rings to form a three-dimensional framework. This disposition creates interesting tunnels in which potassium atoms reside. All the K+ and Gd3+ ions are located on the 2-fold axis in z ) 1/4 and z ) 3/4

Table 4. Anisotropic Displacement Parameters (in 10-3Å2)a

a

atoms

U11

U22

U33

U23

U13

U12

Gd K P(1) P(2) O(E11) O(E12) O(L12) O(L21) O(E21) O(E22)

0.0072(2) 0.0864(3) 0.0069(3) 0.0073(3) 0.0134(11) 0.0125(11) 0.0152(13) 0.0117(10) 0.0120(10) 0.0110(10)

0.0054(2) 0.0131(5) 0.0074(3) 0.0060(3) 0.0129(11) 0.0126(10) 0.0083(10) 0.0095(10) 0.0114(11) 0.0095(10)

0.0072(2) 0.0209(6) 0.0088(4) 0.0091(3) 0.0130(11) 0.0132(11) 0.0169(13) 0.0115(10) 0.0176(12) 0.0127(11)

0.0000 0.0000 -0.0009(2) 0.0011(2) 0.0005(8) -0.0019(8) -0.0026(8) -0.0004(8) 0.0068(9) -0.0004(8)

0.0006(1) 0.0195(7) 0.0018(3) 0.0013(3) 0.0075(9) 0.0005(9) 0.0080(11) 0.0020(8) 0.0036(9) -0.0003(9)

0.0000 0.0000 -0.0005(2) 0.0001(2) 0.0018(8) -0.0003(8) -0.0044(8) 0.0025(8) 0.0027(8) -0.0008(8)

The anisotropic displacement exponent takes the form exp[-2Π2∑i∑jUijhihja*i a*j ].

Synthesis of Potassium-Gadolinium Cyclotetraphosphate

Crystal Growth & Design, Vol. 3, No. 4, 2003 601

Figure 3. IR spectrum at room temperature of KGdP4O12 in the frequency range 400-4000 cm-1. Table 5. Atomic Distances (Å) and Angles (°) (with standard deviations in parentheses)

Table 6. Bond Distances (Å) in GdO8 and KO10 Polyhedra

P(1) O(E11) O(E12) O(L12) O(L21)

Tetrahedron around P(1) < P-O > ) 1.544 Å O(E11) O(E12) O(L12) O(L21) 1.4819(2) 2.5807(3) 2.4719(3) 2.4989(3) 120.49(1) 1.4907(2) 2.5198(3) 2.5260(3) 106.73(1) 109.32(1) 1.5974(2) 2.4793(3) 107.96(1) 109.24(1) 101.41(1) 1.6063(2)

Gd-O(E21)h Gd-O(E12) Gd-O(E11)a Gd-O(E22)g

Polyhedra around Gd: 2.3837(2) Gd-O(E21)c 2.3937(2) Gd-O(E12)g 2.4237(2) Gd-O(E11)e 2.4319(2) Gd-O(E22)

2.3837(2) 2.3937(2) 2.4237(2) 2.4319(2)

P(2) O(E21) O(E22) O(L21) O(L12)a

Tetrahedron around P(2) < P-O > ) 1.547 Å O(E21) O(E22) O(L21) O(L12) 1.4903(2) 2.5753(3) 2.4911(3) 2.4800(3) 119.37(1) 1.4928(2) 2.5763(3) 2.4569(3) 107.69(1) 113.11(1) 1.5940(2) 2.5401(3) 106.07(1) 104.54(1) 104.79(1) 1.6122(2)

K-O(E12)e K-O(E22)h K-O(L12)k K-O(E21)i K-O(E11)f

Polyhedra around K: 2.750(2) K-O(E12)b 2.933(2) K-O(E22)d 2.943(2) K-O(L12)l 3.377(2) K-O(E21)j 3.380(2) K-O(E11)g

2.750(2) 2.933(2) 2.943(2) 3.377(2) 3.380(2)

P(1)-P(2) P(1)a-P(2)-P(1) P(1)-O(L21)-P(2) a

2.9472(1) 87.12(3) 134.12(7)

P(2)a-P(1)

P(2)a-P(1)-P(2) P(2)a-O(L12)-P(1)

2.9841(1) 92.88(3) 136.79(8)

Symmetry code: -x, -y + 1, -z + 1.

planes. Within the P4O124- ring, the P-O bonds of the ring oxygen atoms O(L12) and O(L21) (mean value 1.60 Å) are much longer than those of the terminal oxygens O(E11), O(E12), O(E21), and O(E22) (mean value 1.49 Å). Furthermore, the tetrahedral angles between terminal P-O bonds (mean value 120.49°) are much larger than the others (mean value 101.41°). Slight distortions of P4O12 rings are evidenced by P-P-P angles (Table 5). The average of these angles is 90°, and show no deviation from the ideal value of 90 ( 4° for cyclotetraphosphates.19 The P-O-P angles range from 134.12(7)° to 136.79(8)° with an average of 135.45°. An examination of the main geometrical features of the two independent PO4 tetrahedra (Table 5) shows clearly that despite the P-P-P and P-O-P angle deformations they are in accordance with all that have been previously observed for PO4 tetrahedra involved in condensed phosphate anions.19 Indeed, three different types of O-P-O angles coexist in PO4 tetrahedra. The O(L)P-O(L) angles [O(L): the oxygen of the ring] corresponding to the largest P-O bonds are always close to 100°. The O(L)-P-O(E) angles have values expected for a regular tetrahedron, while the O(E)-P-O(E) angles [O(E): external oxygen] corresponding to the shortest P-O distances have always values close to 120°, probably induced by the mutual repulsion of the

a Symmetry codes: -x + 1, -y + 1, -z + 1. b -x + 1, -y + 1, -z + 2. c -x + 1/2, -y + 3/2, -z + 1. d -x + 1/2, -y + 3/2, -z + 2. e x, -y + 1, z + 1. f x, y, z + 1. g -x + 1, y, -z + 3/2. h x + 1/2, -y + 3/2, z + 1/2. i x + 1, y, z + 1. j -x, y, -z + 3/2. k x + 1/2, y + 1/2, z + 1. l -x + 1/2, y + 1/2, -z + 3/2.

nonbridging oxygen atoms. Nevertheless, the calculated average of the distortion indices20 corresponding to the different angles and distances in the independent PO4 tetrahedra, DI(P-O) ) 0.0367, DI(O-P-O) ) 0.0387, and DI(O-O) ) 0.0145, show an above distortion of the P-O distances compared to the O-O distances. So the PO4 tetrahedra have a local symmetry C1 different from the ideal one of 4 h 3m.20 In the present structure, the gadolinium and potassium atoms are located between P4O124- layers to ensure the cohesion and the neutrality of the structure. The Gd-O and K-O distances are shown in Table 6. The shape of the gadolinium coordination in this crystal is slightly irregular. The Gd-O distance varies from 2.3837(2) to 2.4319(2) Å. It is noted that the gadolinium coordination is similar to others in rare earth cyclotetraphosphates.11,21 The eight oxygens of the GdO8 polyhedra are external and belong to six different rings P4O124-. The GdO8 polyhedra are isolated from one another since they do not share any oxygen atoms. The coordination of potassium is quite irregular, as it can be seen in other cyclotetraphosphates. In fact, the K-O distance ranges from 2.7504(7) to 3.3809(7) Å. The KYP4O12 21 and RbNdP4O12 11 are isostructural to the title compound. Yttrium and neodymium atoms have 8-fold coordination, while K and Rb atoms have 10 oxygen atom neighbors. The shortest Gd-Gd distance (5.269 Å) in KGdP4O12 is small compared to the

602

Crystal Growth & Design, Vol. 3, No. 4, 2003

others in tetraphosphates observed in the literature: RbNdP4O12 (6.129 Å)11, KYP4O12 (5.978 Å)21, and LiNdP4O12 (5.620 Å)5. The GdO8 share with the KO10 polyhedra four oxygens. 3.2. IR Spectroscopy. At room temperature, the compound KGdP4O12 exhibits a monoclinic symmetry with space group (C2/c, Z ) 4). The IR spectrum has been investigated in the frequency range 400-4000 cm-1 as shown in Figure 3. It is noted that this spectrum was recorded in a KBr pellet. To assign the IR peaks to vibrational modes, we examine the modes and frequencies observed in similar compounds.10 The broad and intense band appearing at 1260 cm-1 is assigned to the antisymmetrical vibration νas(O-P-O). The band around 1115 cm-1 can be attributed to the symmetrical vibration νs(O-P-O). Also, we attribute the band at 1033 cm-1 to the antisymmetrical vibration νas(P-O-P). The symmetrical vibration νs of (P-O-P) is represented by the band around 715 cm-1. The band at 509 cm-1 is assigned to the deformation vibration δ(P-O-P). By comparison to other cyclotetraphosphate compounds, we notice the absence of bands in the 700-1000 cm-1 interval. This result is in agreement with a cyclic structure. 4. Conclusion The results of X-ray diffraction and IR spectroscopy show that the cyclotetraphosphate KGdP4O12 is isostructural to RbNdP4O12 and KYP4O12 compounds. The analogy between these structures is remarkable concerning the cell dimensions and the environment of the different atoms. KGdP4O12 at room temperature is monoclinic with space group C2/c. The atomic arrangement is typical of a layer structure built up by P4O12 ring anions, which develop approximately parallel to the [100] and [010] directions. The cationic entities (K+ and Gd3+) are inserted between these layers, to ensure the cohesion of the structure. As with most cyclotetraphosphates of rare earths, the gadolinium atoms are in 8-fold coordination, whereas the coordination sphere of potassium cations is defined by 10 oxygen atom neighbors. The P4O12 layers are joined to each other by GdO8 polyhedra, driving to a three-dimensional framework structure and delimiting tunnels where K+ cations are lodged. The infrared spectrum at room temperature of the compound KGdP4O12 is characterized by the absence of

Ettis et al.

bands in the 700-1000 cm-1 range. Referring to the literature, this result is in agreement with X-ray data regarding the cyclic aspect of the phosphate groups. Further studies concerning new compounds belonging to this family such as CsGdP4O12, RbGdP4O12, NH4GdP4O12, and TlGdP4O12 are currently being investigated and are under discussion. The corresponding papers will be published later. Acknowledgment. We express our most grateful thanks to Prof. A. Driss for the X-ray data collection. References (1) Chinn, S. R.; Hong, H. Y. P. Appl. Phys. Lett. 1975, 26, 649651. (2) Otsuka, K.; Miyazawa, S.; Yamada, T.; Iwasaki, H. Nakano, J. J. Appl. Phys. 1977, 48, 2099-2101. (3) Tsujimoto, Y.; Fukuda, Y.; Fukai, M. J. Electrchem. Soc. 1977, 124, 553-556. (4) Hong, H. Y. P. Mater. Res. Bull. 1975, 10, 1105-1110. (5) Koizumi, H. Acta Crystallogr. 1976, B32, 266-268. (6) Hong, H. Y. P. Mater. Res. Bull. 1975, 10, 635-640. (7) Koizumi, H.; Nakano, J. Acta Crystallogr. 1978, B34, 33203323. (8) Averbuch-Pouchot, M. T.; Durif, A. Acta Crystallogr. 1983, C39, 811-812. (9) Durif, A.; Averbuch-Pouchot, M. T.; Guitel, J. C. Acta Crystallogr. 1983, C39, 812-813. (10) Ferid, M.; Ariguib, N. K.; Trabelsi, M. J. Sol. Stat. Chem. 1987, 69, 1-9. (11) Koizumi, H.; Nakano, J. Acta Crystallogr. 1977, B33, 26802684. (12) Masse, R.; Guitel, J. C.; Durif, A. Acta Crystallogr. 1977, B33, 630-632. (13) Durif, A. Crystal Chemistry of Condensed Phosphates, 1995; pp 253-255. (14) Rzaigui, M.; Ariguib, N. K. J. Sol. Stat. Chem. 1983, 49, 391. (15) International Tables for X-ray Crystallography, Vol. C, Kluwer: Dordrecht, 1992. (16) Sheldrich, G. M. Program for the Solution of Crystal Structures; University of Go¨tting, Germany, 1990. (17) Sheldrich, G. M. Program for the Crystal Structure Determination; University of Go¨tting, Germany, 1997. (18) North, A. C. T.; Philips, D. C.; Mattews, F. S. Acta Crystallogr. 1968, A39, 351. (19) Averbuch-Pouchot, M. T. Durif, A. Crystal Chemistry of Cyclotetraphosphates, Stereochemistry of Organometallic and Inorganic Compound; Elsevier Science Publisher: Amsterdam, 5, 1994. (20) Baur, W. H. Acta Crystallogr. 1974, B30, 1195. (21) Hamady, A.; Jouini, T.; Driss, A. Acta Crystallogr. 1995, C51, 1970-1972.

CG0200659