The infrared and Raman spectra of α-zirconium phosphate

Spectrochimica Acta, ¥ol. 30A, pp. 535 to 541. Pergamon Press 1974. Printed in Northern Ireland

The infrared and Raman spectra of e~-zirconinm phosphate S. E. ~-~ORSLEY,* D. V. ~TOWELL and D. T. STEWART~ Department of Chemical Sciences, The Hatfield Polytechnic, P.O. Box 109, Hatfield, ALIO 9AB, Hertfordshire, England and British Ab~mlniumCompany Limited, Chalfont Park, Gerrards Cross, SL90QB, Buckinghamahire, England (Received 29 August 1973)

Abstract--The infrared and Raman spectra of crystMllrm ~-zlroonium phosphate have been studied. The spectra are interpreted in relation to the crystallographic structure and conclusions are presented concerning the interactions between the crystal lattice and the lattice water. INTRODUCTIO1~

~-Zn~com~M phosphate, ~-Zr(HPO4)~.H~O, has a layer lattice structure [1]. The crystals are monoclinic with a space group P21/~. Each layer of zirconium cations is sandwiched between two layers of phosphate anions. The adjacent layers of phosphate anions are so arranged as to provide cavities large enough to hold single molecules of water. The oxygen of the lattice water molecule is held at the centre of the cavity, but the position of the hydrogens is not known. The cavities have 'windows' with 'free' diameters of up to 0-24 nm through which water molecules and exchanging cations m a y pass [2]. The molecule of lattice water can be removed, reversibly, without change in the lattice dimensions and hence ~-zirconium phosphate can be compared with some zeolitic structures. However. in contrast to most zeolites, the cavities in ~-zirconium phosphate can accommodate only one water molecule. Other investigators have reported the infrared spectra of zirconium phosphates. V~SELY and PE~A~.K [3] observed the spectral changes caused by heat t r e a t m e n t and concluded t h a t orthophosphate groups were present in the original material which condensed on heating to form pyrophosphate groups. ~/[OUNIERand WrNA~aI) [4] supported these conclusions and attributed two sharp bands, at approximately 3600 em -I and 3500 cm -1, to be due to %asic' hydroxyl groups. They also assigned a broad band at 3150 cm -1 to vibration of the lattice water molecule which was considered to be strongly bound to Zr(IV) ions. I t has been found t h a t 'lattice' water, t h a t is water held in a crystal lattice b y hydrogen bonds to anions, or b y weak co-ordinate bonds to the metal, or by both, absorbs in the region 3550-3200 cm -1 and 1630-1600 cm -1, the ~3 and ~1 and the ~ * Present address." Department of Metallurgy and Materials Technology, University of Surrey, Guildford, Surrey, England. t Present address: Medical Research Council, 20 Park Crescent, London WIN 4AL, England. [1] A . Cr.~.AI~FIELD a r i d J. SMITH, Iflo#'g. ~ 7 8 . 8, 431 (1969). [2] A. C L E ~ , ~V. L. DU~, A. S. MED~A, G. I). S~ITH, and J. R. THOMAS,J. Phys. Ghern. 73, 3424 (1969). [3] V. V~.s~,LYand V. PF.lrAR~X,J . !norg. iVucl. Chem. ~ , 697 (1968). [4] F. MOUNIERand L. WIN~RD, Bull. Soe. Chim. _l~rance 1829 (1968). 535

536

S . E . HORSLEY, D. V. NO~,VELL and D. T. STEWART

modes respectively [5]. Detailed studies of lattice water, by site or group factor analysis, have been presented for several compounds, e.g., CaSO4.2H~O [6]. KIS~LEV and co-workers have studied water molecules held in zeolite structures [7]. This paper reports a study of the infrared and R a m a n spectra of crystalline ~-zirconium phosphate. The spectra are analysed and related to the crystal structure. The interactions between the water molecule and the lattice are discussed. EXPERIMENTAL Crystalline ~-zirconium phosphate was prepared by precipitation from homogeneous solution [8]. Dehydrated forms were obtained by heating in an oven at 383K until the theoretical weight loss occurred. Ion exchanged forms were prepared by suspending the acid form in a solution of the metal chloride and adding a solution of the metal hydroxide [9]. The infrared spectra were run on Perkin-Elmer 257 and Grubb Parsons Speetromaster instruments. Samples were prepared as 'Nujol' mulls and potassium bromide discs. The frequencies of the bands were corrected against a calibration spectra of polystyrene. The R a m a n spectra was run on a Cary 81 spectrophotometer, fitted with a tunable laser source, using radiation of 488 nm. RESULTS The spectra of ~-zirconium phosphate are given in Fig. 1 and Table 1. Several of the bands in the infrared spectra have been assigned by considering the spectral ~hanges caused by heat treatment. Dehydration, at 383K, removed bands A, B, D ~m 3 [

2.5 JO0 - -

3"5 I

4 I

5 I C'

6 I

7 I

8 I

I0 [

12 !

C"

C

AB

4000

D

E

2500 Wove

1500 number,

1000

cm -I

Fig. 1. The infrared spectra of a-zirconium phosphate. [5] K. N)~AMOTO Infra-red Spectra of Inorganic and Co-ordination Compounds, Vol. III-3, p. 155; II-9, p. 114 Wiley, New York (1963). [6] B. J. BER~NBLUT,P. DAWSONand G. R. WILK~SO~,Spectrochim. Acta 27A, 1849 (1971). [7] V. N. AB~AvOv, A. V. KIs~T.~.vand V. I. LYGIN, Zh. fiz. Khim. 89, 123 (1965). [8] S. E. ~IORSLEY,and D. V. ~owEIm, J. Appl. Chem. Biotechnol. ~3, 215 (1973). [9] S.E. HoRseY and D. V. I~OW~LL,Thermal Analysis, Vol. 2, P~'oc. Third I C T A , Davos, 611 (1971).

The infrared and l~aman spectra of ~-zirconium phosphate

537

Table 1. Infrared and Raman spectra frequencies of zirconium phosphate A

B

3590m 3510m 3280 s, b 3150 s, b

C

D 3596m 3513m

3280 s, b

E 3618m 3220m

3168m 3135m

2300 vw 2100 vw 1635vw 1622 w

1618m 1250w

1142w ll15w 1075 s, b 1050 s, b 980w s = strong

1070 s, b

1080 s, b

935w

970m

m = medium A B C D E

= = ~ = =

w = weak

1200 s 1170 s l140w ll00m

1055 s 985m 962m

v = very

~-Zr(HPO4)u.H20 infrared Zr(HPO4) z --infrared ZrP20 ~ --infrared ~-Zr(HPOa)~H20--Raman Zr(NaPO4)2H20 infrared

1640m

1020 s 980 s, b

b = broad

Frequencies cm-1

a n d E, Fig. 1, a n d spectra of partially d e h y d r a t e d samples showed t h a t t h e i n t e n s i t y of these four b a n d s decreased equally. As it was considered t h a t no significant c o n d e n s a t i o n of t h e o r t h o p h o s p h a t e g r o u p h y d r o x y l s w o u l d occur a t 383K, these four b a n d s were a t t r i b u t e d t o t h e lattice w a t e r molecule [9]. B a n d C was r e m o v e d d u r i n g t h e f o r m a t i o n of p y r o p h o s p h a t e groups, a t > 6 7 0 K , a n d b y ion exchange, Table 1. H e n c e C was a t t r i b u t e d t o a ( P ) - O - H s t r e t c h i n g m o d e ; t h e b r e a d t h o f this b a n d indicated t h a t the o r t h o p h o s p h a t e g r o u p was involved in h y d r o g e n bonding. Associated w i t h C were t h e w e a k b a n d s C' a n d C ~. The frequencies of these b a n d s corresponded t o characteristically weak ( P ) - O - H s t r e t c h i n g v i b r a t i o n s f o u n d in o t h e r c o m p o u n d s [5]. The a b s o r p t i o n s b e t w e e n 1075-980 cm -1 were a t t r i b u t e d t o v i b r a t i o n s o f t h e o r t h o p h o s p h a t e group. The b a n d at 1250 c m -1 was possibly due t o a P - O - H d e f o r m a t i o n mode. A f t e r t h e f o r m a t i o n of p y r o p h o s p h a t e groups, a characteristic b a n d a t 970 c m -1 was found, Table 1. DISCUSSION The interest in t h e infrared spectra o f ~-zirconium p h o s p h a t e eentres on t h e four b a n d s A, B, I) a n d E, w h i c h are a t t r i b u t e d a b o v e to t h e lattice w a t e r molecule. These b a n d s can be c o m p a r e d to those p r o d u c e d b y w a t e r molecules in zeolites. BERTSCtt a n d HABGOOD [10] h a v e r e p o r t e d t h e s p e c t r u m of p a r t l y h y d r a t e d L i n d e X molecular sieve. The s p e c t r u m of t h e s o d i u m f o r m c o n t a i n e d a sharp b a n d a t [10] L. BERTSCHand H. W . I-I-ABGOOD,J. Phys. Chem. 67, 1621 (1963).

538

S . E . HORSLEY, D. V. NOW'ELLand D. T. STEWART

3695 cm -1, a broad band centred at 3400 cm -1 with a weaker band at 3250 cm -1 and a band at ~_~1650 cm -1. These workers considered that molecular water could be adsorbed in two ways: (a) b y hydrogen bonding to the lattice oxygens; (b) b y interaction of the lone pair electrons on the water oxygen with the lattice cations. In the first case, (a), a strong broad OH stretching band was predicted to appear <3500 cm -1. In the second case, (b), if both water hydrogens were free then two sharp bands would be observed >3500 cm -1. The experimental results were attributed to a combination of both types of bonding, that is to a simultaneous bonding by the water oxygen to the exchangeable cation and b y one o5 the water hydrogens to the lattice oxygens. Such water was expected to give a parallel development in the intensity of the four bands, at 3695, 3400, 3250 and 1650 cm -1, during progressive hydration, as was experimentally observed. Further experimental support was given for cation-water interaction b y the high heat of adsorption of the lattice water, up to 125 k J mo1-1 at low coverages. Furthermore, the change in the frequency of the highest frequency band ( ~ 3 7 0 0 cm -1) for the series lithium, sodium and potassium exchanged Linde X sieves, followed the frequency changes found in the spectra of the hydroxides of those alkali metals. In the case of ~-zirconium phosphate, the application of these ideas leads to the conclusion that there are two distinct types of water molecule present. Thus water molecules bound only b y the oxygen to the lattice could give the two sharp bands A and B. Water molecules bound b y both hydrogens to the lattice could give the broad band D. The single ~ band observed would then be due to the ~9 modes of both types of water. Other arrangements are possible; however all result in the proposal that there are two types of water in the lattice. Experimentally, it was found that the four bands A, B, D and E decreased in intensity at an equal rate during progressive dehydration. Such behaviour would not be expected if two differently bonded water molecules existed in the lattice. The X-ray structure analysis of ~-zirconium phosphate gave no evidence for the presence of two differently bonded water molecules [1]. The heat of dehydration of ~zirconium phosphates, as determined b y DSC, range from 50 to 58 k J mo1-1. These results are approximately half those quoted above for zeolitic Linde X sieve and argue against substantial water-cation interaction. After ion exchange the two sharp bands A and B were replaced b y a single sharp band, Table 1, the frequency of which did not vary in the same w a y as found for zeolite Linde X sieve. From this evidence it was concluded that the spectra of ~-zirconium phosphate could not be interpreted b y the Bertsch and Habgood approach. KISELEV and co-workers have considered the infrared spectra of water molecules subjected to symmetrical and asymmetrical 'stress' [7, 11]. They conclude that for symmetrically 'stressed' molecules, for example, molecules hydrogen bonded equally through both hydrogens, the frequency of the symmetric (~1) and asymmetric (~3) [I1] A. V. I~ISELEV and V. I. LYGII¢,Infra.red Spectra of Adsorbed Species (Edited by L. H. Lx~I"LE), Chap. 14. Academic Press, New York {1966).

The infrared and Raman spectra of ~-zirconlum phosphate

539

modes would decrease equally as the strength of the bonding increased, the frequency separation being constant at _~100-110 cm -1. Under conditions of asymmetric 'stress,' for example, molecules bonded through only one hydrogen, relatively little change was predicted for the ~3 mode whilst the ~1 mode decreased towards ~-~3300 cm -1 as the strength of the bonding increased. Interpreting the spectra of ~-zirconium phosphate b y these considerations again leads to the conclusion that two differently bonded water molecules are present, in this case both under asymmetrical 'stress' or bonding. As mentioned above, this conclusion is contrary to the available experimental evidence. As an alternative approach, the spectral changes which would result when a water molecule was placed at a given symmetry site in the lattice of ~-zirconium phosphate was investigated. A site group analysis was carried out using the method described by FATELEY, ~[CDEVITT and BENTLEY [12]. The crystal structure of ~-zirconium phosphate is P21/~[1] and the crystallographic unit cell is identical with the primitive Bravais lattice. The possible site symmetries in P21/o are 4 Ci (2) and C 1 (4) that is there m a y be two equivalent atoms occupying C~ sites of which there are 4 types and there m a y be four equivalent atoms occupying C 1 sites. As there are 4 equivalent water molecules in the Bravais lattice t h e y m a y be placed on C 1 sites. To obtain the predicted spectra of the lattice water molecule in ~-zirconium phosphate it is thus necessary to correlate the molecular symmetry of free water, C~, with the site group symmetry of C 1 and the space group symmetry of C~h6. (P21/~). This operation is shown in Fig. 2. The predicted spectra contained six infrared and R a m a n active bands, and a proposed assignment of the observed bands is given in Fig. 2. It was not possible to resolve the two Au bands resulting from the • 3 and ~I modes in the very broad band at 3150 cm-L Likewise, separation of the Au and Bu bands of the ~2 mode was not possible, although the Ag and Bg modes were resolved. Factor

Molecular symmetry

Correlation

Site symmetry

Correlation

group Observed ~alf symmetry frequency ~vidth

~ A g 3657viAl

~1

1595 vzA 1

.41

-Bg Au B~

~ ~ A g

3756v a B 1

-Bg

~ - - - - - - - A ~ ~ B u ~ A g .Bx ~ Bg ~ A ~ B~

3135 3513 3150 3510 1622 1635 1618 1618 3168 3596 3150 3590

20 120 20 30 30 15 120 25

Fig. 2. Symmetry.correlation table for ~-zirconium phosphate.

[12] W. G. FA~L~Y, N. T. McD~.vrz--"and F. F. B ~ r L g r , AppL Sp6ct~y, 25, 155 (1971).

540

S . E . ~-IORSLEY, D. V. NOW'ELLand D. T. STEWART

This treatment allowed the observed infrared and R a m a n spectra to be derived from the single type of water molecule which was expected on the evidence both of the crystal structure and the behaviour of the bands A, B, D and E on dehydration. The s y m m e t r y of this water molecule is lower than C2v, and thus there are two different O---H bond lengths. This result agrees with the crystal structure which shows t h a t no two water oxygen to lattice oxygen distances are the same [1]. The range of frequencies due to O---H stretching vibrations of the water is very large, _~440 cm -1. Such a large range would be consistent with an asymmetric molecule and is discussed below. Comparing the observed frequencies with those of a free water molecule, Table 1, the average frequency shifts are, vl = --330 cm -1, va = --380 cm -1 and v2 = + 2 4 cm -x. The Lord-Merrifield correlation [13] m a y be used to calculate the water oxygen to lattice oxygen, Ow--O L, distance corresponding to these shifts. Although this correlation is not exact, it does allow a reasonable comparison to be made between the spectral data and the crystal structure. A shift of --350 cm -~ in the O-H stretching frequency corresponds to an Ow--OL distance of 0.28 rim, which agrees well with the two shortest Ow--OL distances determined by X-ray diffraction of 0.278 ~ 0.003 and 0.282 ± 0.006 nm. I t is possible to calculate the approximate energy of the hydrogen bonds holding the water molecule from the shift in the O - H stretching frequency. Assuming t h a t the energy of the hydrogen bonds is directly proportional to the shift and t h a t there are two hydrogen bonds to each water molecule, then a shift of --350 cm -~ corresponds to an energy of 44 k J tool -~ [14]. This value correlated reasonably with the value of 52 k J mo1-1, as determined by differential scanning calorimetry (DSC), for the enthalpy change involved in the removal of the lattice water from ~-zirconium phosphate [9]. The magnitude of these energy terms supports the classification of the water molecule as 'lattice' water. Assuming t h a t the lattice water molecule is involved in hydrogen bonding via the hydrogen atoms over the two shortest Ow-OL distances of 0.278 and 0.282 nm [1], then the formal difference in the two O w - H r r bond lengths is of the order of 0-004 nm. This figure can be related to the range of splitting of the OH stretching modes of 440 cm -~. These values compare reasonably with those found for CaS04" H20. I n this extensively studied system, the difference in the two O r r - H w bond lengths has been estimated as 0.002 nm and as 0,008 nm -t-0.021 where the splitting of the OH stretching modes cover 250 cm -~. [15, 16]. The study of the crystal structure of ~-zireonium phosphate showed t h a t there were two different P04 groups in the lattice, P2 and P3 [1]. These groups differed in their deviation from tetrahedral symmetry and in their P - - O H bond lengths. CRUn~SEA~K [17] has interpreted the bond order in orthophosphate groups in terms of the P - - O H bond length. For a single P - - O bond the length m a y be calculated, using the Schomaker-Stevenson equation, to be 0-171 nm (1.71/~). The average [13] [14] [15] [16] [17]

R. C. LORDand R. E. MERRIFIELD,J. Chem. Phys. 21, 166 (1953). M. FALXand G. BRINK,Cave. J. Chem. 48, 2096 (1970). V. SEIDL, O. KNOI"and M. FALK,Can,. J. Chem. 47, 1361 (1969). M. AToJI and R. E. RUNDLE, J. Chem. Phys. 29, 1306 (1958). D. J. W. CRUIKSmA.NK,J. Chem. Soc. 5486 (1961).

T h e infrared and R a m a n spectra of ~-zirconium phosphate

541

bond length for an isolated orthophosphate group is 0.154 nm (1.54 A), and Cruikshank considered that such bonds have a ~ bond order of 0.5. Assuming that the bond length is a linear function of the degree of ~r bonding, then the 7r bond order of the P - - O H bonds in ~-zirconium phosphate may be calculated as 0.33 and 0.50 for the P2 and Pa groups respectively. Thus there is a definite difference in the P - - O H bond orders and hence the hydrogens attached to these two types of orthophosphate groups will be chemically different. This conclusion is in accord with CLEA~FIELD and STr~ES interpretation of the potentiometric titration curve of ~-zirconium phosphate [18]. Cruikshank further notes that hydrogen bonding involving the hydrogen of a P - - O H group lengthens the P--O distance [17]. For example, in the low temperature form of KH2PO 4, which has strong hydrogen bonds of 0-249 nm (2.49 A) length the P - - O H and P - - O bond lengths are 0.158 nm (1.58) and 0.151 nm (1.51 A) respectively, a difference of 0.007 nm (0.07 A). In ~-zirconium phosphate the P - - O H bonds are longer than the P - - O bonds by 0.009 nm (0.09 A) and 0.002 nm (0-02 A) for the P2 and Pa groups respectively. Thus the P2 group is involved in considerably more hydrogen bonding than the P3 group, and the bonding to the P2 group is relatively strong. CONCLUSIOI~S

An analysis of the infrared and Raman spectra of ~-zirconium phosphate has been made. I t is considered that the lattice water molecule is asymmetric and that the Ow-H w bond lengths vary by up to 0-004 nm. The shift in the stretching frequencies of the water molecule were consistent with hydrogen bond lengths of 0-28 nm and such bond lengths were in agreement with the reported crystal structure. The observed enthalpy of dehydration of ~-zirconium phosphate has been related to the strength of the hydrogen bonding involving the lattice water calculated from the infrared spectra. The reported crystal structure has been interpreted in terms of two types of orthophosphate group which differ in their P - - O H ~rbond orders, and in the acidities of the hydrogen atom. The two groups also differ in the amount of hydrogen bonding they undergo with the lattice water molecule and the rest of the lattice. Aclcnowledgements--Mrs P. C A ~ , D e p a r t m e n t of Chemical Sciences, The Hatfield Polytechnic, is t h a n k e d for helpful discussion with this study. One of us (S. E. H.) acknowledges a maintenance grant from the Hertfordshire County Council. Financial assistance from British Aluminium Company Limited, Chemicals Division, Clifton Junction, Manchester, is also acknowledged.

[18] A. CLEARF][ELD a n d J. A. STYNES, J . I~q,org. ~'ucl. Ch~7~. 26, 117 (1964).

15