31P MAS and 2D exchange NMR of crystalline silicon phosphates

9 August1996

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 258 (1996) 107-112

31p MAS and 2D exchange NMR of crystalline silicon phosphates P. Hartmann a, C. Jana h, j. Vogel b, C. J~tger

a

a InstitutJSr Optik und Quantenelektronik, Friedrich-Schiller-Universit~t Jena, Max-Wien-Platz 1, D-07743 Jena, Germany b Otto-Schott-lnstitut, Friedrich-Schiller-Universit~t Jena, Fraunhoferstr. 6, D-07743 Jena, Germany

Received 7 May 1996

Abstract

31p MAS NMR has been used to determine 31p chemical shift tensors of crystalline silicon phosphates. The chemical shift data of five polymorphs of SiP20 7 (hexagonal, tetragonal, cubic and two monoclinic forms) are presented besides those of Si3(PO4)4. Results of 31p 2D exchange NMR on a phase mixture of different silicon diphosphates are shown. This novel approach allows an unambiguous identification of the various phases even in multicomponent mixtures as occurring in ceramics.

1. Introduction Phosphate invert glasses and glass ceramics are very interesting in view of their biological properties [1,2]. The structures of these glasses have been investigated intensively [3]. However, only little information about the structural principles is available if silicon is introduced in phosphate invert glass structures. Silicon tetrahedrally coordinated by four oxygen atoms has been accepted as the building unit for most of the silicates and silicate based glasses in the same way as [PO 4] units in phosphate glasses. However, the existence of six-coordinated silicon in a few crystalline materials such as stishovite [4] and SiP207 [5,6] had been noted. For silica-containing phosphate invert glasses both tetrahedrally and octahedrally coordinated silicon are conceivable. High-resolution solid-state NMR is a powerful tool for investigating the structure of glasses as well as crystalline materials. Detailed knowledge of the chemical shift tensors of silicon phosphates such as

Si3(PO4) 4 and SiP20 7 is the prerequisite for the investigation of silica-containing phosphate invert glasses by 31p NMR. Crystalline SiP20 7 exists in various polymorphs. X-ray diffraction data reveal that the silicon in all these polymorphs is octahedrally coordinated by oxygen and the phosphorus is tetrahedrally coordinated [5,6]. Since in the present literature there is a deficit of 31p chemical shift data of silicon phosphates, the determination of the 31p parameters is needed. Mixtures of the various polymorphs may occur both in the glasses and in the crystalline phases of the ceramics. Hence, spectroscopic informations are needed to identify the various NMR peaks unambiguously. To this end the possibilities of 2D Exchange NMR are demonstrated.

2. Experimental The investigated Si3(PO4) 4 and polymorphic forms of SiP207 were synthesized by the procedure

0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All fights reserved. PII S 0 0 0 9 - 2 6 1 4 ( 9 6 ) 0 0 6 1 6 - 1

108

P. Hartmann et al. / Chemical Physics Letters 258 (1996) 107-112

developed by Makart [5] and Liebau et al. [6]. The samples were prepared from mixtures of crystalline ortho-phosphoric acid and silica gel 60H supplied by Merck (Germany). In some experiments, quartz powder, silica glass, analytical grade phosphoric acid, diammonium hydrogen phosphate and ammonium dihydrogen phosphate were also included. The samples were investigated using an X-ray diffractometer D5000 (Siemens, Germany). The measured X-ray diffraction powder patterns were compared with the data reported in the literature [7]. The NMR spectra were acquired using a Bruker AMX 400 NMR spectrometer. All 31p isotropic chemical shift parameters 6~so and relative line intensities were obtained from (1D) high speed MAS spectra using spinning speeds of about 13 kHz. The anisotropy (AS) and asymmetry (-q) parameters were obtained from low spinning speed (of about 2 kHz) MAS spectra using the method of Herzfeld and Berger [8,9]. The 31p chemical shifts refer to an 85% solution of phosphoric acid. Additionally, the samples were investigated by 2D Exchange NMR. In these experiments [10-14] the neighbourhood of nonequivalent phosphorous atoms are identified by magnetization exchange mediated by the dipole-dipole interaction. High-speed MAS was applied to obtain high resolved NMR spectra. The dipole interaction, averaged by MAS, was efficiently recoupled by irradiation of rotor-synchronized 27r pulses (mixing time) [10]. The typical pulse lengths are 2.2 /zs for zr/2 pulses and 4/xs for 7r pulses. Spectra were acquired at a resonance frequency of ~'0 = 161.92 MHz using spinning speeds of 12.5 kHz, repetition times of 10 s and time domain data size (TDI and TD2) of 1024 points. The mixing times (0 < tmix < 30 ms) were optimized analysing the cross peak intensities and the signal to noise ratio. The typical mixing time was about 6.5 ms.

3. Results

The silicon phosphates prepared were identified by X-ray diffraction. Beside Si3(PO4) 4 (JCPDS 221380), five polymorphs of SiP20 7 have been synthesized (hexagonal form: JCPDS 22-1318, tetragonal form: JCPDS 22-1320, cubic form: JCPDS 22-1321,

........... i................... i................... i................... i................... ~................... 1................... i................... i....... 20 0 -20 -40 -60 -80 -100 ppm Fig.

1. 3 ~ p

MAS

NMR

spectrum

of

the

monoclinic

polymorph

(JCPDS 25-755) of SiP207 (resonance frequency fo = 161.92 MHz; spinning speed vr = 5.0 kHz; receiverdelay time tel e = 15 ,US;pulse width tw = 1.5 ,us and repetitiontime / r e = 200 s).

monoclinic form: JCPDS 25-755, second monoclinic form: JCPDS 39-189). The 31p chemical shift tensor parameters were obtained with high accuracy analysing the 1D MAS NMR spectra. Fig. 1 shows the 1D 31p MAS NMR spectrum of the pure monoclinic polymorph of SiP20 7 (JCPDS 25-755). Like this, the spectra of the other monoclinic and tetragonal SiP20 7 polymorphs consist of two spectral components with an intensity ratio of 1:1. That means these diphosphate polymorphs possess two nonequivalent [PO4]-tetrahedra. This result agrees with crystallographic investigations [15,16]. However, the spectrum of the hexagonal silicon diphosphate has only a single line position. Thus it could be expected that the P207 unit of the hexagonal polymorph has equivalent phosphorus atoms. The spectrum of the cubic SiP207 polymorph is very complex and contains ten sets of lines, suggesting ten different types of PO 4 tetrahedra. However, the superstructure of the cubic SiP20 7 polymorph described by Tillmanns et al. [17] possesses eleven nonequivalent [PO4]-tetrahedra. The monophosphate Si3(PO4) 4 gives rise to a single line at 8iso = - 4 4 . 5 ppm. The results are summarized in Table 1. However, silicon diphosphate tends to form polymorphic mixtures. In these cases 2D Exchange NMR Spectroscopy was used to achieve reliable NMR data. Fig. 2 shows the 1D 31p MAS spectrum of a polycrystalline silicon phosphate sample. A line assignment is rather tricky in such a case and requires assumptions. At first glance 15 lines seem to be

109

P. Hartmann et al. / Chemical Physics Letters 258 (1996) 107-112

present, but this is not correct as shown below. Referring to Table 1 five of the lines ( - 4 4 . 5 ppm, - 4 8 . 1 p p m / - 55.9 ppm and - 4 5 . 9 p p m / - 53.3 ppm) could be identified as Si3(PO4)4, SiP20 7 monoclinic ( # ) and tetragonal ( * ) (JCPDS 3 9 - 1 8 9 and 22-1320), respectively. The remaining 10 lines indicate the presence of the cubic polymorph (JCPDS 22-1321). This assignment was confirmed by the 2D exchange pattern shown in Fig. 3. There is no cross peak between the line at - 4 4 . 5 ppm with other lines. Therefore, this line can be attributed to the silicon monophosphate (JCPDS 2 2 - 1 3 8 0 ) having only one crystallographically nonequivalent phosphorus position. However, cross peaks between the shifts - 45.9 ppm and - 53.3 ppm (monoclinic phase JCPDS 22-1320) as well as between - 48. I ppm and - 5 5 . 9 ppm (tetragonal phase) are found. These

cross peaks identify diphosphates containing P207 units with nonidentical [PO4] tetrahedra. The remaining cross peaks indicate a fourth and fifth compound in the sample, the cubic polymorph of SiP207 (JCPDS 2 2 - 1 3 2 1 ) and an unknown phase. The occurrence of an unknown silicon phosphate phase is not surprising since already Liebau et al. [6] described the preparation of additional SiO 2 . P205 phases, but the authors were not able to characterize these phases. For the cubic polymorph as well as for the unknown phase a certain number of cross peaks were found. This can easily be seen in the corresponding cross sections of Fig. 3. Indeed, there are 10 lines corresponding to the cubic phase in agreement with the 1D N M R data listed in Table 1. The unknown phase shows three lines at - 4 8 . 8 ppm _+ 0.5 ppm, - 5 4 . 8 ppm + 1 ppm and - 6 8 . 0 ppm _+ 1 ppm. They are very close to the resonances of the

!ppm

.......

.il ....

++

.... i: ........... ; ......

i

...........

........

Y

~ ~onoc,~ni+ ,

/,,+ +~

....................

-40

~

, .......................................

-50

+o

:.

+0

, ...................

-60

-70

,

ppm

-40

tetragonal

, . . . . ,

. . . . . . . . . . . . .

-40 -45-50

. , . . . ~

-55-660-

. . . .

.

~"~-~. . . . .

. . . .

5 -70 ppm

Fig. 3. 3~p 2D exchange NMR spectrum of a polycrystaUinesilicon phosphate sample (see l D spectrum in Fig. 2). The sample consists of silicon monophosphate 22-1380 (QO), silicon diphosphate cubic 22-132, tetragona122-1320, monoclinic 39-189 and an unknown phase (resonance frequencyfo = 161.92 MHz; spinning speed vr = 12.5 kHz; dephasing delay time taepb = 50 ms; zr/2 pulse width t w = 2.2 ~ts, mixing time tmix = 6.5 ms, data size 1024 points in both dimensions and repetition time tre = 100 s). The five cross sections of the 2D spectrum are scaled.

110

P. Hartmann et a l . / Chemical Physics Letters 258 (1996) 107-112

Table 1 alp chemical shift tensor parameters of Si3(PO4)4 and various polymorphs of SiP207 obtained by fit analysis of slow spinning MAS NMR Phase t~iso At5 r/ (assignment according [ppm] [ppm] to JCPDS) 22-1318 SiP207 (hexagonal)

-51.9+0.2 78+5

0.08+0.05

22-1320 SiP207 (tetragonal)

line a -45.95:0.2 72+5 lineb -53.35:0.2 85+5

0+0.05 0.01+0.05

22-1321 SiP207 (cubic)

line a line b linec lined linee linef lineg lineh linei linej

0.045:0.1 0.575:0.1 0.335:0.1 0.225:0.1 0.455:0.1 0.02+0.1 0.24+0.1 0.655:0.1 0.545:0.1 0,245:0.1

25-755 SiP207 (monoclinic)

linea -47.65:0.2 795:5 line b - 49.9 5:0.2 68 5:5

0,045:0.05 0.055:0.05

39-189 SiP207 (monoclinic)

linea -48.15:0.2 755:5 lineb -55.95:0.2 845:5

0.395:0.05 0.335:0.05

-44.55:0.2 585:5

0.075:0.05

-49.7+0.2 -53.65:0.2 -54.85:0.2 -55.65:0.2 -58.95:0.2 -59.35:0.2 -59.95:0.2 -68.85:0.2 -71.85:0.2 -73.35:0.2

22-1380 Si3(PO4)4

855:10 825:10 895:10 845:10 865:10 865:10 855:10 915:10 875:10 865:10

other polymorphs. For that reason and because of their small intensity they cannot be resolved in the 1D NMR spectrum. However, taking the line posiI#

QO

\ / ' . . . . . .-40 ....

-45' . . . .-50 . . . . . . .-55

~''

' - ~ . . . .7{)-. . . . . . . .-75 .....

ppna

Fig. 2. 3~p MAS NMR spectrum of a polycrystalline silicon phosphate sample. The sample consists of silicon monophosphate 22-1380 (QO), silicon diphosphate cubic 22-1321 (not marked), tetmgonal 22-1320 (*) and monoclinic 39-189 (#) (resonance frequency fo = 161.92 MHz; spinning speed ~'r= 14.0 kHz; receiver delay time /de = 15 /zs; pulse width t w = 1.5 p,s and repetition time tre = 200 s).

tions determined from the 2D spectrum into account, all the 31p chemical shift parameters can be determined by the MAS lineshape fit. The unambiguous line assignment and the indication of a new silicon phosphate phase in this polycrystalline sample illustrate the advantages of the 2D Exchange NMR.

4. Discussion Usually, the phosphate structures are characterized using the Q~ group classification. The Q~ groups are basic structural units ([PO 4] tetrahedra), where [n] is the number of bridging oxygen atoms per [POa]-tetrahedron. The anisotropic chemical shift is well known to be suitable for analysing the short range phosphate structure. The following rules were found for alkali and alkaline earth phosphate structures: (i) Different Q= groups result in separate values of the 31p chemical shift anisotropy [ 18-31 ]. (ii) The isotropic chemical shift correlates to the field strength of the cations surrounding the phosphate anions [18,19,27,30]. An easy relation between the isotropic shift and the Dietzel cation field strength F [32] has been proposed by Haubenreil3er [19] where the isotropic shift is given by: 3iso = A - ( B . F ) with A = 1 6 . 1 ppm, B---36.1 ppm • pm 2 for Q0 groups and A = 4.1 ppm, B = 41.3 ppm- pm 2 for Ql groups. The 31p isotropic chemical shifts of the SiP20 7 polymorphs range from - 4 5 . 9 ppm to - 7 3 . 3 ppm (average isotropic shift 3 i s o ( a V . ) = - 5 6 . 4 ppm). These values are clearly outside the range typical for Ql units in alkaline phosphates and alkaline earth phosphates (see Fig. 4). However, respecting the silicon cation field strength of F = 1.57 • 104 pm -2, the predicted shift value for Ql[Si] groups is cSi~o(calc.) = - 6 1 ppm. within the margins of error the calculated shift agrees with the obtained average shift. The shift anisotropies measured span a narrow range from 68 to 91 ppm (Fig. 4). The average shift anisotropy of A 3 = 82 ppm is outside the range typical for Q~ groups but it is shifted towards the typical values for isolated phosphate groups Q0. The obtained average shift asymmetry parameter

P. Hartmann et al./ Chemical Physics Letters 258 (1996) 107-112

/ - -

Q1 Q ltSil '

i

'

i

5 iso [ppm]

Q1

~

_

'

_

'

Ab [ p p m l

ql

~

_

_

Q l[si] '0:2'

111

sented the 31p MAS NMR spectrum of an unspecified monoclinic SiP20 7 possessing four lines with isotropic chemical shifts of - 46.4, - 47.7, - 50.2 and - 5 3 . 7 ppm. Because of the intensity ratio (1:2:2.5:1) the authors assigned these lines to four nonequivalent phosphorus positions in the structure of a monoclinic SiP20 7 phase. Corresponding to crystallographic investigations, it should be noted that both monoclinic SiP20 7 polymorphs consist of two nonequivalent [PO+] groups. Regarding the present investigations (Table 1), it could be expected that the sample investigated by Mudrakovskii et al. was a mixture of the monoclinic SiP20 7 (JCPDS 2 5 - 7 5 5 ) and the tetragonal SiP20 7 (JCPDS 2 2 1320) polymorph with the ratio 2" 1.

' '0:8'

Fig. 4. Comparison of the typical ranges of the 31p chemical shift tensor parameters (isotropic shift 6iso, shift anisotropy A6, and shift asymmetry ~) for silicon diphosphates (Qt[Si]) and Qi groups in alkali and earth alkali phosphates (Qt accordingto Refs. [8,20]).

of r/av -- 0.25 proves the assumption of an approximate C3v symmetry of the P O 4 tetrahedra in the silicon diphosphates. The 31p chemical shift anisotropy and the isotropic shift measured for the Q°[Si]-groups in the silicon monophosphate (JCPDS 2 2 - 1 3 8 0 ) are not typical for unprotonated Q°-groups in alkaline phosphates and alkaline earth phosphates ([31], Table 1). Moreover, the shift anisotropies of the Q°[Si]-groups are not significantly different from those of the Ql[Si]-groups in silicon diphosphates. However, the calculated isotropic shift of ~iso(calc.)= - 4 1 ppm agrees with the measured isotropic shift also in the case of silicon monophosphate. It should be noted that rule (i) is not generally valid for silicon phosphate structures. In contrast, the well-known correlation between the isotropic chemical shift and the cation field strength is very useful also in silicon phosphates. Therefore, the 3Xp N M R spectra of unknown silicon containing phosphate structures (e.g. glasses, glass ceramics) should be interpreted very carefully. The results presented in Table 1 are in contrast to previous investigations. Mudrakovskii et al. [33] pre-

Acknowledgements The support of the present work by the Deutsche Forschungsgemeinschaft (DFG VO 5 9 1 / 1 - 2 ) is gratefully acknowledged.

References [1] W. Vogel and W. HSland, Angew. Chem. Int. Ed. Engl. 26 (1987) 527. [2] J. Vogel, P. Hartmann and K.-J. Schulze: Resorbable, porous glasses and glass ceramics, Advances in science and technology 12: Materials in clinical applications, Faenza (1995) 59-66. [3] P. Hartmann, J. Vogel and B. Schnabel, J. Non-Cryst. Solids 176 (1994) 157. [4] S.M. Stishov and S.V. Popova, Geochimia 10 (1961) 837. [5] H. Makart, Helv. Claim. Acta 50 (1967) 399. [6] F. Liebau, G. Bissert and N. K5ppen, Z. Anorg. Allg. Chemie 359 (1968) 113. [7] Joint Committee on Powder Diffraction Standards. [8] J. Herzfeld and A.E. Berger, J. Chem. Phys. 73 (1980) 6021. [9] D. Fenzke, B. MaeB and H. Pfeifer, J. Mag. Res. 88 (1990) 172. [10] A.E. Benett, J.H. Ok, R.G. Griffin and S. Vega, J. Chem. Phys. 96 (1992) 8624. [11] R.R. Ernst, G. Bodenhausen and A. Wokaun, Principles of Magnetic Resonance in One and Two Dimensions (Clarendon Press, London, 1990). [12] K. Schmidt-Rohr and H.W. Spiess, Multidimensional Solid State NMR and Polymers (Academic Press, London, 1994). [13] C. Jager, M. Feige, R. Born and H.W. Spiess, J. Non-Cryst. Solids 180 (1994) 91.

112

P. Hartmann et a L / Chemical Physics Letters 258 (1996) 107-112

[14] R. Born, M. Feige, C. J~iger and H.W. Spiess, Z. Naturforsch. 50a (1995) 169. [15] G. Bissert and F. Liebau, Acta Cryst. B26 (1970) 233. [16] F. Liebau and K.-H. Hesse, Z. Kristall. 133 (1971) 213. [17] E. Tillmanns, W. Gebert and W.H. Baur, J. Solid State Chem. 7 (1973) 69. [18] G.L. Turner, K.A. Smith, R.J. Kirkpatrick and E. Oldfield, J. Magn. Reson. 70 (1986) 408. [19] U. Haubenreil3er, Habilitationsschrift, Jena, (1986). [20] A.R. Grimmer, Proc. XXth Congress Ampere, Tallinn, 1978. [21] A.R. Grimmer, Spectrochim. Acta 34 A (1978) 941. [22] T.M. Duncan and D.C. Douglass, Chem. Phys. 87 (1984) 339. [23] F. Ermark, B. Topic, U. Haeberlen and R. Blinc, J. Phys. Condens. Matter 1 (1989) 5489.

[24] A. Olivieri, J. of Magn. Reson. 88 (1990) 1. [25] S. Un and M.P. Klein, J. Am. Chem. Soc. 111 (1989) 5119. [26] A.R. Grimmer and U. Haubenreil3er, Chem. Phys. Lett. 99 (1983) 487. [27] A.R. Grimmer, J. Chim. Phys. 89 (1992) 413. [28] U. Haubenreil3er, H. Biirger, U. Sternberg, G. Scheler and W. Vogel, Glastechn. Berichte 59 (1986) 174. [29] R.K. Brow, R.J. Kirkpatrik and G.L. Turner, J. of Non-Cryst. Solids 116 (1990) 39. [30] U. Steinberg, Mol. Phys. 63 (1988) 249. [31 ] P. Hartmann, J. Vogel and B. Schnabel, J. Mag. Res. A 111 (1994) 110. [32] A. Dietzel, Z. Elektrochem. 48 (1942) 9. [33] I.L. Mudrakovskii, V.M. Mastikhin, V.P. Shmachkova and N.S. Kotsarenko, Chem. Phys. Lett. 120 (1985) 424.