Vibrational spectra and structure of AsPS glasses

86

Journal of Non-Crystalline Solids 134 (1991) 86-93 North-Holland

Vibrational spectra and structure of A s - P - S glasses L. K o u d e l k a Institute of Chemical Technology, 532 10 Pardubice, Czechoslovakia

M. Pisfir6ik Institute of Inorganic Chemistry, Slovak Academy of Sciences, Bratislava, Czechoslovakia

L.N. Blinov a n d M.S. G u t e n e v Leningrad Technical University, Leningrad, USSR Received 10 January 1991 Revised manuscript received 7 May 1991

A structural model for A s - P - S glasses is proposed from an analysis of Raman and infrared spectra of two series of glasses with compositions (As2S3) 1_x(P2Ss)x with x = 0 - 0 . 7 and AsxPxSl_Ex with x = 0.10-0.16. The structural network in these glasses is composed of ASS3/2 and S=PS3/2 structural units forming 8-membered rings of As2P2S4. The Raman spectra of these glasses are characterized by a sharp strong Raman band of 418 cm-1 ascribed to the breathing vibration of the rings. In the S-rich compositional region lying under the ASES3-P2S5 line, towards the S comer of the A s - P - S phase diagram, sulfur atoms form S8 molecules inside the structural network. In the 31p M A S - N M R spectrum by a single line with the chemical shift 8(31p) = 113.4 ppm is due to the S=PS3/2 coordination of the phosphorus atom.

I. Introduction Only a few papers have been published in recent years dealing with glasses formed in the ternary A s - P - S system. Goryunova et al. [1] reported glass formation in the (As2S3)I_x(P2S3) x system, where glasses were obtained in the compositional region of 0 _< x _< 0.5 with quenching, but only in the region of x = 0-0.12 with a slow cooling in air. Blachnik and Hoppe [2] studied the glass formation and thermal properties of A s - P - S glasses in another compositional series of ASxPxSl_2x, where glasses were obtained up to x = 0.18. They report that the glass transition temperature of these glasses, 273-464 K, increases with decreasing sulfur content. The dielectric properties of another compositional series of ( A S z S 3 ) I _ x ( P 2 S s ) x glasses were measured by Gutenev [3], who prepared glasses in this series up to x = 0.7. Photo-induced para-

magnetic centers in the same glass series were observed by the electron spin resonance spectroscopy by Likholit et al. [4]. Moreover, in the A s - P - S system the formation of two crystalline compounds of AsPS4 and As2P2S 7 was also reported and their vibrational properties were measured by Wibbelmann and Brockner [5]. From their data on the vibrational spectra, they proposed for the AsPS 4 compound the chemical formula of (As2P2Ss)n, because they concluded that this compound has a macromolecular-type structure with the monomer having the structural formula of S S--P--S

--

--As /

L

~S

0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All fights reserved

~S As PJS /

--#-S

S--. |

J°

L. Koudelka et al. / Vibrational spectra and structure of A s - P - S glasses As

J/3o/

S

iO

x

20

~S 5

40

P

at.*/, P Fig. 1. Phase diagram of a part of the A s - P - S system with an assumed glass-forming boundary ( - - - ) based on data [1-3]. o, studied samples in two series ( . . . . . . ).

Our work is devoted mainly to the study of structure of A s - P - S glasses by vibrational spectroscopy. For this study, we have chosen two compositional lines, of ( m s 2 S 3 ) l _ x ( P 2 S s ) x and Asxp~s1-2~, which make it possible to investigate changes in vibrational spectra with compositional changes near the supposed glass-forming boundary (fig. 1) anc[ along the compositional line with the 1 : 1 P : S ratio, respectively. For the explanation of the observed spectra a structural model of A s - P - S glasses is proposed. Structural similarities of the crystalline and glassy phases of the same composition are discussed as well as the vibrational properties of both phases.

2. Experimental The glasses of both investigated series were prepared by direct synthesis from pure elements in batches of - 1 0 g in evacuated silica ampoules heated at 600 ° C for 3 h and then quenched in air to room temperature. The composition of the prepared samples and their location in the S-rich corner of the A s - P - S phase diagram is denoted in fig. 1 together with an assumed glass-forming boundary constructed mainly on the basis of published data on the glass formation in A s - P - S system [1-3]. X-ray diffraction patterns of all samples confirmed their amorphous state.

87

The crystalline compound of AsPS 4 was prepared by sublimation of amorphous AsPS 4 composition heated at 3 7 5 ° C in evacuated silica ampoule using temperature gradient of 55 o C. The X-ray diffraction pattern of the crystal was in agreement with that given by Wibbelman and Brockner [5] for the AsPS 4 compound. Raman spectra of A s - P - S glasses were measured at room temperature with a JEOL Raman spectrometer, JRS-S1, with excitation by H e - N e laser radiation using a slit width of 8 cm-1. Unanalyzed polarization of the scattered radiation was used in all measurements. Infrared spectra of A s - P - S glasses were measured with a PerkinElmer IR spectrometer, model IR 684, in the frequency region of 200-1000 cm 1. The glass samples were mixed with TIBr in the weight ratio 2 mg/500 mg T1Br and pressed into tablets with a thickness of 1 mm an a diameter of 10 mm. Magic-angle spinning (MAS)-NMR spectra of the 31p nuclei were measured with a Briiker MFL 200 spectrometer at a frequency of 81.01 MHz and spinning speed of 4 kHz. The spectra were obtained with 90 ° pulse length of 4.7 ms and relaxation delay of 107 s; chemical shifts were externally referenced to 85% H3PO 4.

3. Results The Raman spectra of ( A s 2 5 3 ) ! x ( P 2 S s ) x glasses are shown in fig. 2. With an increasing content of P2S5, the shape of a broad band with a maximum of 345 cm -1 at the A s 2 S 3 glass changes and on its long-wavelength side a new narrow band at 418 cm-1 appears, the strength of which increases rapidly with increasing x. In the samples with x = 0.4-0.7, it becomes dominant in the observed Raman spectra. In the high-frequency region above 500 cm-1 there appears also a broad band positioned between 690 and 650 cm-1. With increasing x. its maximum shifts slightly from 654 cm-1 at x = 0.1 to 650 cm -1 at x = 0.5 and then back towards the higher frequencies up to 690 cm-~ in the glass with x = 0.7, where this band becomes even broader. The original dominant band of 345 cm -1 changes its shape and, instead of it, two strong

88

L. Koudelka et al. / Vibrational spectra and structure of A s - P - S glasses

Table 1 Vibrational bands observed in the Raman and infrared spectra of As-P-S system glasses Frequency of vibrational bands (cm- l) Assignment infrared

Raman

-

150 200 220 230 268

-

306

P4S9

328-340 368 418 475 510-530 650-690

v(AsS3) v(S=PS3) As2 P2S4 rings S8 v(PS3) v(P=-S)

-

317-334 -

478 508-518 641-671

Ss P4S10 S8

8(S=PS3), P4S7 P4Sao

bands appear in the R a m a n spectra of the glasses with x = 0.3-0.5, having m a x i m a of 328 and 368 c m - 1 . With a further increase in x, the shape of the 328 cm -a b a n d changes and on its lowfrequency side the b a n d of 306 cm -1 appears.

800

600

400

200

W e a k b a n d s at 200, 230 and 268 cm -1 can be observed, especially in the glasses with a higher content of phosphorus. A n o t h e r weak and b r o a d b a n d can be observed between 510 and 530 cm - l in the glasses with 2 0 - 6 0 mol% of phosphorus pentasulfide. R a m a n spectra of ASxPxS1-2x glasses are shown in fig. 3. for x = 0.10-0.16. The composition with x = 0.15 c o r r e s p o n d s to the c o m p o s i t i o n (As2S3)05(P2Ss)0. 5 and also to the composition of the crystalline c o m p o u n d AsPS 4 [5]. The R a m a n spectra of this series of change from the composition with x = 0.15 with an increase in sulfur content in such a way that new b a n d s at 150, 220 and 475 c m - 1 appear in the R a m a n spectrum of the glass with x = 0.10. O n the other h a n d these bands are missing in the glass with x = 0.16, where weak bands at 206, 230, 268 and 530 cm -~ are observed. Infrared spectra of (As2S3)I_x(P2Ss) x glasses are shown in fig. 4. The starting glass, As2S 3, has a b r o a d b a n d at 312 c m -1 and a small b a n d at 460 c m - 1 . With additions of P2S5, another two strong

800

600

Roman shift (cmt) Fig. 2. Raman spectra of (As2S3) 1_x(P2S5)x glasses.

400 Roman

200

shift (cn5 I)

L. Koudelka et al. / Vibrational spectra and structure of A s - P - S glasses

89

ASxPx$1_2× 414 475

9

328

xffi0.15 sss S50

800

600

400 26o Roman shift (crfi1)

Fig. 3. Raman spectra of AsxPxS1-2x glasses.

L

4 (P:S)

___

800 bands appear in the infrared spectra between 633 and 649 cm - t and between 510 and 518 cm -1. The position of the first band at the samples with x increasing from 0 to 0.5 shifts slightly towards longer wavelengths from 648 cm -1 at x = 0.1 to 633 cm-1 at x = 0.5. With increasing x from 0.5 to 0.7 its m a x i m u m shifts back to 645 cm -1, similarly to the shift observed in the R a m a n spectra for the band between 640 and 690 c m - t . The second strong band observed between 510 and 518 cm -1, with increasing x, changes its m a x i m u m only slightly from 510 cm -~ at x = 0.1 up to 518 cm-~ at x = 0.7. In the glasses with x = 0.5-0.7, another medium strength band appears between 455 and 460 c m - l . It also increases in intensity with an increasing content of P2S5. Infrared spectra of A S x P x S l _ z x glasses are shown in fig. 5 for x =0.10 and 0.16 (ASPS4 composition) together with the infrared spectrum of crystalline AsPS 4. With an increasing content of sulfur in these glasses, the band of 633 c m - ~ shifts towards higher frequencies (671 cm-1 at x = 0.10)

I

~ (P-S-P) I

l

600

'~ (As-S) I

400

I _ _

[cE 1] 200

Fig. 4. Infrared spectra of (As2S3) 1_~(P2S5)~ glasses.

I ASo.loPo,loS0,80(ASPS8)

i Aso.mPo.16So.66 (ASPS4)

AsPS4-crystot[ine

420

)

800

760

s6o

4ha

200 # [cr61]

Fig. 5. Infrared spectra of ASxPxS] 2x glasses and the crystalline compound of ASPS4.

90

L. Koudelka et aL / Vibrational spectra and structure of A s - P - S glasses

l 2'00

s'o

6

40

- 6o -1 oppm

Fig. 6. 31p MAS-NMR spectrum of the AsPS 4 glass.

and a new band of 478 cm -~ appears in the infrared spectra. We have obtained also the 31p MAS-NMR spectrum of the AsPS 4 glass which is shown in fig. 6. The center resonance band lies at 8 = 113.4 ppm and on its both sides spinning sidebands are observed. In addition to the dominant signal, a small shoulder appears on the downfield side of the center band with 8 = 124 ppm.

4. Discussion

4.1. Band assignment of vibrational spectra For the assignment of vibrational bands in the Raman and infrared spectra of A s - P - S glasses we assume, a priori, that their structural network will be composed of AsS~ and PS~ structural units with the most common coordination 3 a n d / o r 4. In the concentration region of our samples, the sulfur content is sufficient to minimize the probability of forming P-P, As-As, or A s - P bonds. ms2S 3 glass structure is composed of ASS3/2 trigonal pyramids [6], whereas in crystalline P2S5 phosphorus forms S---PS3 coordination with the phosphorus atom bonded to three sulfur bridging atoms and one P=S isolated bond [7]. The isolated bond is shorter (1.90-1.95 ,~) than the bridging P - S bonds (2.08-2.09 A) and its bond-stretching vibration is active both in Raman and infrared

spectra [8]. The stretching frequency of the isolated P=S bond in thiophosphate compounds varies in the range of 450-750 cm-1 [8] because it depends both on the mass of the other atoms bonded to the phosphorus atom [9] and on the ~r-bond order of the isolated P=S bond [8]. Therefore, we assigned the strong bands, observed in the Raman spectra of A s - P - S glasses at 650-690 cm -1 and in the infrared spectra at 633-648 cm -1, to the stretching vibrations of the isolated P=S bond in the S=PS 3 structural group. The observed frequencies are close to those in the vibrational spectra of P4S10 molecules [7] and in the PxSl_x glasses [10]. The observed changes in the frequencies of the bands in IR and Raman spectra with decreasing content of arsenic sulfide in the (As2S3)I_x(P2Ss) x glasses can be ascribed to inductional effects of As atoms on the bond order of this isolated P=S bond. Another strong band in IR spectra of A s - P - S glasses appears between 508 and 516 cm -a, i.e. inside the frequency region of stretching vibrations of P - S bridging bonds [11] observed in compounds with P - S - P linkages in the range of 400-550 cm -1. In the Raman spectra in this region there is a weak band between 510 and 530 cm -~. Within the region lie also the frequencies of asymmetric stretching vibrations of thiophosphates [12,13] which have a strong response in IR spectra and only a weak response in the Raman spectra [12]. Therefore, we attribute the above mentioned bands to the asymmetric stretching vibration of S=PS 3 structural units. Such a vibrational mode should give a strong response in IR spectra (the band peaking between 508 and 516 cm -1) and a weak response in Raman spectra (the band observed between 510 and 530 c m - l ) , which is in agreement with our experimental data. In the Raman spectra of A s - P - S glasses there is a dominant sharp band at 418 cm -1, which in the (As2S3)I_x(P2S5) x glasses increases in intensity with increasing x up to x = 0.5. Such a sharp band in the Raman spectra of glasses, according to Galeener [14], is characteristic of an intermediate range order in glasses, usually of the breathing vibration of ring structures [14]. Such a ring structure was proposed for the crystalline compound of ASPS4 by Wibbelmann and Brockner

91

L. Koudelka et al. / Vibrational spectra and structure of A s - P - S glasses

[5] and the proposed ring consists of four sulfur atoms ad two arsenic and two phosphorus atoms as As2P2S4 (see above). Therefore, we assume that in A s - P - S glasses ASS3/2 pyramids and S=PS3/2 tetrahedra join via sulfur bridges to form similar rings. Thus, we ascribe the observed strong sharp band at 418 cm -~ in the Raman spectra to the breathing vibration of As2P2S4 rings with bridging sulfur atoms. The other strong bands observed in the Raman spectra of ( A s 2 S 3 ) I _ x ( P 2 S s ) x glasses at 368 and between 328 and 340 cm -~ we assign to the stretching vibrations of S=PS 3 and AsS 3 groups, respectively. The frequency of the first band corresponds to that observed for the stretching vibration of the PS 3 group in some thiophosphates [13] and its strength increases with increasing phosphorus content. The band between 328 and 340 cm -~ is assigned to the stretching vibration of A s S 3 groups, because its strength decreases and its frequency shifts from 340 to 328 cm -~ with a decreasing arsenic content. The changes in the shape and position of this band can be due either to some changes in arsenic coordination or to a deformation of A s S 3 pyramids. Similar changes were observed also in the Raman spectra of AsxS 1 glasses by Ward [15] at high sulfur contents (90-95 at.%), but explanation of those changes has not been given yet, The weak bands between 455 and 459 and at 371 cm 1 in the IR spectra of (As2S3) l_x(P2s5)x glasses with x = 0.6-0.7 are attributed to PaSta molecules inside the structural network of A s - P - S glasses which approach the glass-forming boundary. These bands together with the strong bands at 690 and 530 cm -~ are dominant features of infrared spectra of P4Sm compound [7]. Also the weak bands at 200 and 268 cm-1 in the Raman spectra of this series of glasses (fig. 2) are attributed to P4Sm molecules, whereas the weak band at 306 cm-1 is due to P4S9 molecules 'dissolved' in the glass matrix [16]. Based on our data, an unambiguous assignment of the weak band at 230 cm -~ in the Raman spectra of A s - P - S glasses (fig. 2) is not possible. Since the band at 230 cm-~ appears in the Raman spectra of (As2S3) 1 x(P2Ss)x glasses with x as low

as 0.2, we might assign it to bending vibrations of S=PS 3 structural groups or to small amounts of P4S7 molecules in the network [17]. Raman spectra of our samples of ASxPxSl_2x glasses (fig. 3) at higher sulfur content (x = 0.10) contain bands at 150, 220 and 475 cm -1 and in the infrared spectra at 478 cm -~ (fig. 5). The presence of these bands in vibrational spectra of A S x P x S l _ 2 x reveals the presence of $8 molecules in the glass network [15]. The assignment of vibrational bands is summarized in table 1. 4.2. A s P S 4

An interesting comparison of vibrational spectra can be made between crystalline and amorphous forms of the AsPS 4 composition. As can be seen in fig. 5, the infrared spectrum of glassy AsPS 4 can be taken as a broadened spectrum of the crystalline compound AsPS 4. The doublet at 641 cm -1 was ascribed by Wibbelmann and Brockner [5] to the stretching vibration of the isolated P=S bond. No assignment of other bands observed in IR and Raman spectra of crystalline AsPS 4 was made [5]. The supposed structural symmetry of the monomer unit (C2v) should result in 30 vibrational bands while the number of observed bands was smaller and thus an unambiguous assignment was not possible. Larger differences are observed between the Raman spectra of glassy and crystalline AsPS 4 (fig. 7). The strong narrow band at 418 cm -1 appears in the Raman spectra of both forms, but

7oo

£o

Roman shift [crfi 1) Fig. 7. Raman spectra of glassy AsPS4 and crystalline ASPS..

92

L. Koudelka et al. / Vibrationalspectra and structure of As-P-S glasses

not in the infrared spectra. This fact supports also the suggested assignment of this band to the breathing vibration of AszP2S4 rings. Therefore, we propose that the network of glassy and crystalline forms of ASPS4 have the same basic structural units, i.e., ASS3/2 and S=PS3/2 linked together in rings composed of two ASS3/2 and two S=PS3/2 groups connected via sulfur bridges. These rings have a long range order in crystalline ASPS4, but only a medium range order in glassy AsPS 4. The presence of one dominant type of coordination around phosphorus atom in glassy AsPS 4 is consistent with its P MAS-NMR spectrum (fig. 6) having one dominant resonance with a chemical shift of 113.4 ppm. A similar value of the chemical shift 3(31p) was obtained by Tullius et al. [18] for P~S l _~ glasses (~(31p) = 112 _+ 2 ppm). This agreement is consistent with the assumption of similar coordination around phosphorus atoms in P - S and A s - P - S glasses, i.e., S=PS3/2 structural units derived from the molecular structure of P4S10 [7]. A small shoulder at 124.3 ppm in 3~p MASN M R spectrum (fig. 6) could be attributed to a small fraction of molecular units in the network. Such an assignment, while also supported by the analysis of the Raman spectra, nevertheless does not allow an unambiguous assignment of this band. •

•

•

With a further increase in the phosphorus pentasulfide content, the tendency to form molecular units (P4S10, P4S9) in the network increases. This increase results in loss of glass-forming ability at x > 0.7. In the AsxPxSl_ ~ glasses, an increase in the sulfur content results in the formation of S8 molecules. The network of glassy AsPS 4 with an increasing sulfur content does not change substantially, only the relative number of S=PS3/2 and ASS3/2 structural units decrease and double sulfur bridges ( - S - S - ) are incorporated between the rings of As2P2S4.

31

4.3. Structure and bonding in A s - P - S

5. Conclusion Study of vibrational spectra of A s - P - S glasses has shown that their structure is composed of ASS3/2 and S=PS3/2 structural units. The assignment of a sharp Raman band at 418 cm -1 to the breathing vibration of AszP2S4 rings is not unambiguous. We cannot exclude also a possible formation of ring structures formed by interconnections of S=PS3/2 structural units only, because similar band at 418 cm -~ was found also in the Raman spectra of PxS1-x glasses [10]. A study of possible formation of cyclothiophosphates similar to known cyclooxophosphates could solve this problem.

glasses

From the above analysis of the experimental data, we propose a model of structural changes in A s - P - S glasses with compositional changes in both glass series. In the (AszS3)1_~(P2S~) ~ series of glasses, the addition of PzS5 to AszS3 results in the formation of S=PS3/2 structural units in the network composed of ASS3/2 trigonal pyramids. Both structural units have a tendency to form rings composed of two S=PS3/2 and two ASS3/2 structural units interconnected via sulfur bridging atoms. A maximum number of these structural rings occurs at the composition with the ratio of AszS 3 :PzS 5 = 0.5 : 0.5 (ASPS4) where the vibrational band of 418 cm -1 (breathing vibration of As2P2S4 rings) has the highest intensity.

The authors would like to acknowledge Dr Jaroslav Straka from the Macromolecular Institute of Czechoslovak Academy of Sciences, Prague for the measurement of 31p MAS-NMR spectrum of AsPS 4 glass•

References [1] N.A. Goryunova, B.T. Kolomiets and V.P. Shilo, Zh. Tekh. Fiz. 28 (1958) 981. [2] R.H. Blachnik and A. Hoppe, J. Non-Cryst. Solids 34 (1979) 191. [3] M.S. Gutenev, Fiz. Khim. Stekla 13 (1987) 308. [4] I.L. Likholit, V.F. Masterov, L.A. Baidakov and L.N. Blinov, Fiz. Tverd. Tela 29 (1987) 881. [5] C. Wibbelmann and W. Brockner, Z. Naturforsch• 36a (1981) 836.

L. Koudelka et al. / Vibrational spectra and structure of A s - P - S glasses

[6] G. Lucovsky and R.M. Martin, J. Non-Cryst. Solids 8-10 (1972) 185. [7] M. Somer, W. Bues and W. Brockner, Z. Naturforsch. 38a (1983) 163. [8] D.E. Rogers and G. Nickless, in: Inorganic Sulphur Chemistry, ed. G. Nickless (Elsevier, London, 1968) p. 282. [9] F.N. Hooge and P.J. Christen, Rec. Tray. Chim. 77 (1958) 911. [10] L. Koudelka, M. Pishr~ik, M.S. Gutenev and L.N.Blinov, J. Mater. Sci. Lett. 8 (1989) 933. [11] D.E.C. Corbridge, in: Topics in Phophorus Chemistry, Vol. 6 (Wiley, London, 1969) p. 235. [12] U. Patzmann and W. Brockner, Z. Naturforsch. 38a (1983) 27.

93

[13] R. Mercier, J.P. Malugani, B. Fahys, J. Douglade and G. Robert, J. Solid State Chem. 43 (1982) 151. [14] F.I__ Galeener, in: Raman Spectroscopy Linear and Nonlinear, eds. J. Lascombe and P.V. Huong (Wiley, New York, 1982) p. 529. [15] A.T. Ward, J. Phys. Chem. 72 (1968) 4133. [16] Z. Wang, X. Wang and W. Lu, in: Proc. 7th Int. Conf. on Raman Spectroscopy, ed. W.F. Murphy (NRCC, Ottawa, 1980) p. 132. [17] M. Gardner, J. Chem. Soc. Dalton Trans. (1973) 691. [18] M. Tullius, D. Lathrop and H. Eckert, J. Phys. Chem. 94 (1990) 2145.