THE VIBRATIONAL SPECTRA OF MAGNESIUM ... - Science Direct

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mode at a higher wavenumber than the symmetric bridge stretching mode is .... Mooney R. W., Toma S. V. and Brunvoll J., Spectrochim. Acta DA, 1541 (1%7).
I Phys. Chum. Solids. 1977. Vol. 38. pp. 1327-1332.

Pergamon Press.

Printed in Great l3ril.w

THE VIBRATIONAL SPECTRA OF MAGNESIUM PYROPHOSPHATE POLYMORPHSt B. C. CORNILSENSand R. A. CONDRATE,SR. Division of Engineering and Science, New York State College of Ceramics, Alfred University, Alfred, NY 14802, U.S.A. (Received 29 November 1976;accepted 25 March 1977)

Ahstraet-The Raman spectra were measured and analyzed for both cr-and BMg,PrO,. The IR and Raman spectra were interpreted for both phases using factor group analyses. The spectral features predicted with factor groups arising from the X-ray crystallographic space groups P2tlc - Cz,,and C21m- C& for a-Mg,P207 and /?-Mg2P207, respectively, fit the observed results. Bands observed in the Raman spectrum for B_Mg,PrO,are consistent with a linear bond angle while those for a-MgsP& are consistent with a bent P-O-P bond angle. No soft modes were observed in the Raman spectra indicating that the phase transition between the two phases is not a second order process. 1. mTRooucTIoN

Many investigators have reported and analyzed the IR spectra of alkaline earth pyrophosphates [ l-71. However, very few investigations have also included the Raman spectra of pyrophosphates[8,9]. The lack of this latter type of spectra can lead to the erroneous conclusion that the site group approximation is sufficient for interpreting the vibrational spectra of an investigated pyrophosphate polymorph. In this study we will discuss and interpret both the IR and Raman spectra obtained for two magnesium pyrophosphate polymorphs (a-MgZP20-r and /IMg2P207). The former polymorph is stable at room temperature, and possesses a primitive unit cell with a monoclinic structure[lO]. The latter polymorph is stable above 68”C, and possesses a thortveitite type of structure[ll]. Current investigations suggest that the PO-P bond angle in the P20rc-anion is bent for the low-temperature a-phaseHO], while the same angle is quasilinear for the higher-temperature &phase [ 111. Xray diffraction data indicates the bridging oxygen atom of the P2074--anion possesses large anisotropic thermal parameters. The analysis of IR and Raman spectra together presents related structural information for these two Mg2P207 phases. z-AL.

IR measurements IR spectra were measured on a Perkin-Elmer Model 621 double-beam grating spectrophotometer with a COZ and Hz0 purging device. Both the KBr pellet and nujol mull techniques were employed to obtain IR spectra in the 4000-200 cm-’ region. The instrument was calibrated with polystyrene ftlm and indene. Wavenumber accuracy is within &2cm-’ for narrow bands. Roman measurements Raman spectra were obtained using a Spex Industries Model 1401 double monochromator and a Coherent Radiation Laboratory Model 51 argon-ion laser excitation source. The 4880A exciting line was used to obtain the illustrated spectra. However, these spectra were checked for plasma lines, ghosts, or fluorescence by using the laser line at 5145 A. Normal 90” collection optics were used for the spectral measurements. Raman spectra were measured for both Mg,P,O, powders and sintered pellets glued with Duco cement onto a resistor. The Raman spectra of a-Mga20, were obtained at room temperature. The spectrum was obtained for /3-Mg2P207 at 115°C by passing the appropriate amount of current through the resistor. The spectrophotometer was calibrated using indene and the plasma lines of the laser. Wavenumber accuracy was within *2 cm-’ for sharp bands.

Preparation of Mg,P20, a-Mg2P207 was prepared by thermal decomposition of % m?SUL’rS ANDDiSCUsslON MgNH*P04 powder in a platinum crucible at 1270°C. MgNH.+POdwas precipitated by adding a concentrated Crystal structures and spectral predictions for a- and (NH&HP04 solution to an excess of hot concentrated BMB~P&, MgC12solution. The precipitate was digested at a boil, The space group for BMg2P207 is C2/m - C&[ll]. filtered and washed with water before decomposition. The high degree of symmetry for this polymorph sim/?-Mg,PzO, was formed in this study by heating a- plifies the normally complex analysis of the internal Mg2PzO7above 68°C on a resistor. This phase transition modes for the P2074--anion. Its primitive cell contains is reversible. All samples were characterized by wet only one formula unit. Table 1 correlates the &-free chemical, X-ray diffraction and spectrographic analyses. ion group with the &-factor group via the &,-site group. The last column in the table lists the number of tPresented at the 169th National Meeting of the American predicted fundamental modes for each symmetry species Chemical Society, Philadelphia, PA 19104,April 1975. SBased on a thesis submitted by B. C. Cornilsen for the Ph.D. in the primitive unit cell. Only accidental coincidences degree in Ceramic Science. should occur between the IR and Raman bands of this 1327

I328

B. C. CORNILSENand R. A. CONDRATE,SR. Table 1. Correlation

of &,-free

free ion group D3d symmetry species

ion species with factor group C,, for P-MgZP207 CLh site group symmetry species

3*1g

6A

C2h factor group symmetry species for nonfor primitive primitive unit cell unit cell -____ 12Ag

g

6A

g

O%g 3E

3B g

6B

5A "

lOAu

5A

7B ll

14BU

7B "

g

3Bg

!z

lALu u

3*2u 4E

u

Activities Raman

D3d 6

:

Infrared: Coincidences:

I:

centrosymmetric structure. Also, one may note that the doubly degenerate Btype modes for the &-free ion group are split into two components of A- and B-type symmetry for the CZh-factor group. Alternative possibilities for the space group of &Mg,P207 include Cm or C2[11]. However, since these space groups are noncentrosymmetric, coincidences would be expected between IR and Raman bands. Calvo has determined that the space group of aMg,P*O, is P2Jc - C:,, with four molecular units per primitive unit cell[lO]. The P20,4--anions are found on Cl-sites in this structure. Correlation of the symmetry species for the &free ion group to the &,-factor group via the C,-sites leads to the spectral predictions summarized in Table 2. The major result of this correlation is that a band cannot be active in both the IR and Raman spectra. Table 2. Factor group predictions

Raman: Infrared: Coincidences

12 0

'2h 9 12 0

Interpretation

of the vibrational spectra of /3-Mg,P*O, The Raman spectrum illustrated in Fig. 1 for p Mg,PzO, was measured at 115°C. This temperature is well above the range in which both the a- and &phases might exist simultaneously[l2-141. The spectrum is very simple in comparison to the spectrum of the a-phase (see Fig. 1) because only one formula unit exists in the primitive unit cell of /?-MgZP207. For this reason, the spectral predictions from factor group and site group analyses for the anion are the same[ IS]. Several investigators have studied the IR spectra of P20,‘- ions in crystals using different point groups which did not include Czh[l-7, 16-181. However, using IR spectra alone, they could neither distinguish unequivocally the appropriate site group nor determine the necessity of factor group analyses for these materials. Six broad bands have been reported and assigned in

for a-Mg2P207 using space group P2,/c - C&,

Cl Site Group Symmetry Species

Activities

'2h 9

Free Ion

Site Group

::

22:

17

21

C2h Factor Group Synimetry Species

Factor Group

4422 0

1329

The vibrationalspectra of magnesiumpyrophosphatepolymorphs

:

Mg2 p2Q7

j

il i

J

: i

i :

Ii

1200

LL-__,

IloG

1000

900

1

I

I

800

700

603

WAVENUMBER

(cm-’

I

500

,

400

1

BC3

;m10

)

Fig. 1. The Raman spectra of a- and /H4g2P207powders. the IR spectrum of &Mg,P207 at 100°C[l]. The DW model predicts seven fundamental IR-active modes while the Cr,,-factor group model predicts twelve fundamental IR-active modes. The predictions with the Dad-model appeared more reasonable. and Lazarev along with other investigators made their band assignments using this model[l, 16-18). Nine bands are observed in the internal mode region of the Raman spectrum for @-Mg,P207.The &-model and the &-factor group model predict six and nine Ramanactive fundamental modes, respectively, for the pyrophosphate anion. Comparing the number of observed bands with those predicted by the two models, we may note that the &,-model is insufficient, and that the factor group model generates an appropriate number of internal fundamental bands. As a matter of fact, the assignment of the broad bands in the previously ob served IR spectrum[ I] may be reasonably reinterpreted on the basis of factor-group modes (see Table 3). Also, a comparison of the band maxima in the IR and Raman spectra indicates that the mutual exclusion principle is followed. Therefore, the factor group and the space group of /?-Mg~P~o7must be centrosymmetric. This observation eliminates the possibility of the earlier mentioned alternative factor groups, C, and C2, which predict that their IR and Raman bands should be totally coincident. The applicability of &,-factor group in the interpretation of the vibrational spectra for /3-h4g2P207 is consistent with the results obtained in a study of EPR spectra[ 131. The presence of a non-linear P-O-P bond angle is not indicated in the analysis of the Raman spectrum. If this structural consideration were possible, the selection rules for the applied factor group would be violated. The spectrum would be more complex as perusal of the

spectrum of a-Mg,P& with a bent P-O-P bond angle indicates. Also, the suggestion by Lazarev that significant coupling occurs between the external modes and the low lying internal bending modes seems unqualified. Isotopic substitution for the Ca*‘-ion in the Ca2P207 polymorphs results in no large observable shifts of the internal modes. Band assignments for the fundamental modes of P20,*--anions of /?-Mg,P20r are tabulated in Table 3. Three factors were considered in order to make a reasonable set of empirical band assignments. Fist, the wavenumbers of bands resulting from factor group splitting of the same site group mode should be close together. This trend was noted for crystalline phosphate compounds such as apatites and spodiosites [ 19-213. Second, the effect of removal of degeneracy should be noted in the vibrational spectra upon placement of the P&--free ion into the unit cell of j3-Mg,P20r. Correlation of ad-free ion group with the Czh-factor group (see Table 1) produces the splitting of one band of Etype species into two bandsof A- and B-type species. Third, the relative intensities of bands should relate to the symmetry of the vibrational modes involved. For instance, a mode belonging to the A. species usually has a polarizability change that is greater than that for a related Bs species; the former mode is usually more intense in Raman spectra. The Raman bands at 576 and 561 cm-‘, as well as those at 403 and 378 cm-’ may be considered as pairs of bands split from bending modes for the En species of a PzO,‘--ion with l&.d symmetry. The lower wavenumber band at 561 cm-’ for the first pair possesses greater intensity, and therefore is a&ned to the mode with A,-symmetry. Similarly, for the. pair with lower wavenumbers, the 378cm-’ band is assigned to the A,symmetry species. The very intense higher wavenumber

1330

B. C. CORNILSEN and R. A. CONDUTE, SR. Table 3. Band assignments for the fundamental modes of the Pz074m.-anion in pMg2P207 Vibrational mode symmetry Vibration:1 modes(cm

* *

1170 1146 1136 1118 1060

Band assignment

$0

SOP v&lP

*

638

6PO2

*

586

&PO2

576

6P02

561 550

6PO2

508 440

6PO2

431

&PO2

403

APO2

378

6PO

Y

* *

?

model

grow

model

G-0

*

732

grow

species

for D3d free ion

G-0

*Cal025

978

for C2h factor

APO2

&PO2

2

6POP

1

* IR spectrum observed by Lazarev and Tenisheva at 1OO'C.' Raman spectrum run in this study at 115'C.

band at 431 cm-’ must be assigned as a mode belonging to the AZ-symmetry species of the factor group arising from the .4,,-free ion symmetry species. All of these above mentioned bands involve bending modes. The intense Raman bands at 732 and 1060cm- in the P-O stretching region are assigned to the bridge and terminal symmetric P-O stretching modes, respectively. These modes correlate with modes of the &,-free ion which possess A,,-symmetry. Therefore, these two bands should be the strongest bands in the P-O stretching region. The strong band at 1136cm-’ and a weaker band at 1146cm-’ are assigned to asymmetric P-O stretching modes with A,- and &-symmetry. These two bands originate from a band with &symmetry for the &-free ion model. Band assignments can also be made for the bending and stretching modes observed in the IR spectrum. Bands noted at 1170 and 1118cm-’ can be assigned to asymmetric P-O stretching modes with A.- and B.factor group symmetry, respectively. These bands cor.relate with a free ion band possessing &-symmetry. The splitting of the latter free ion band as well as the related band with &-symmetry occurs because of distortion of the free ion on the crystal site, and from dipole-coupling of ions in adjacent unit cells. The asymmetric bridging and terminal P-O stretching modes

possessing &-symmetry occur at 978 and 1025cm-‘, respectively. The latter band was not discussed by Lazarev but is observable in his IR spectrum[l, 221. Assignment of the asymmetric bridge P-O stretching mode at a higher wavenumber than the symmetric bridge stretching mode is consistent with analogous mode assignments for other &O,-ions (e.g. M = Si, Ge and Cr)[23-251. The pair of bands at 638 and 586 cm-‘, and the pair at 508 and 440 cm-’ are assigned to the two pairs of terminal PO2 bending modes originating from the modes with &symmetry of the &-model. A normal coordinate analysis treatment for the free ion model[l8] indicates that the two modes with E.-symmetry occur at approximately 586 and 440 cm-‘. The band at 550 cm-’ is assigned to a PO2 bending mode with B,-symmetry originating from a mode with Azu-symmetry for the &-free ion group. The band is calculated at 558 cm-’ in the normal coordinate analysis treatment. The same treatment also indicates that the P-O-P bending mode and the torsional mode are below the wavenumber region investigated. Far IR spectra will be necessary for these spectral determinations. Interpretation of the vibrational spectra of a-MgZPzO,

The Raman spectrum of a-MgzP207 is illustrated in Fig. 1. Empirical band assignments are listed for the

1331

The vibrational spectra of magnesium pyrophosphate polymorphs Table 4. Comparison of the band assignments for the stretching modes of a-MglP207 and B_Mg,P207(cm-‘) 4%2pp7 Band Assignment

Raman 1192

1211 1193

1144 1126 1113

1133 1118 1105

1064 1089

1081

1025

1005

1052

978

972

982 r'r

748

740 761

G-0

1060

G-0

vLl

&l-P

732

vLP

IR'

* present authors note splitting of rhis IR band

stretching modes in Table 4. Non-coincidences seem to exist between IR and Raman bands which are consistent with the predictions from a centrosymmetric crystal. However, the latter observation is somewhat uncertain because the IR bands are broad and their resultant peak frequencies lie between the actual transverse and longitudinal mode limits. The observation of more than twenty one bands in the internal mode region of the vibrational spectrum conclusively establishes that no site group model is su~cient for interpreting the observed spectra. The factor group generated from the space group (P21/c- C$,) determined from X-ray data can account for the number of bands observed in the IR and Raman spectra. The smaiIer number of observed bands can be due to either overlapping or weak intensities of some factor group components. A larger number of bands are observed for a-Mg2P20, than for ,@-Mg,P& because the former ~orn~und possesses a greater number of molecular formula units per primitive unit cell. Band assignments can be made for (r-Mg2P207on the basis of the assignments for &-Mg2Pz07by assuming that the differences between the wavenum~rs for related vibrational modes are not extremely large. Table 4 illustrates the relations between the band assignments for the stretching modes of a- and @-Mg2P207.These interrefations can be indicated by using the terminal symmetric P-O stretching mode as an example. This mode is only Raman active for &Mg2PZ07 because of the center of symmetry present for the PzOT’--anion on its site. The fundamental

related

band is observed at 106Ocm-

in the Raman

spectrum. This type of mode becomes both IR and Raman active for a-Mg9& because of the elimination of the center of symmetry for the anion, and the change in unit ceU symmetry. The factor group components of this mode are found at 1089 and 1064cm-’ in the Raman spectrum, and 1081cm-’ in the IR spectrum. Similar considerations allow the assignment of the lower

wavenumber bending modes. The torsional and P-O-P bending modes are not observed in the measured IR spectrum of a-Mg,P&, similarly to &Mg2P207. Low wavenumber spectra were measured for the (Iand @-phases of magnesium pyrophosphate as a function of tem~rat~e. No soft modes were observed in the Raman spectra that were sensitive to temperature change near the phase transition point. This result is consistent with an earlier conclusion in the literature that the phase ~ansformation at 68°C is not a second order process[l2,141. The volume change along with the large enthalpy change indicated a first order phase transition. A specific heat anomaly over a 20“ range led to its further classification as an anomatous first order process[ IO], REFERENCES: I. Lazarev A. N. and Tenisheva T. F., Izu. Akad. Nouk. Kaz.,

SSR, Ser. Khim. 2, 242 (1964). 2. Lazarev A. N. and TenishevaT. F., fzv. Akud. Naak. KQZ., SSR, Ser. i&m. 3,403 (1964). 3. Lazarev A. N. and TenishevaT. F., Izu.Akad. Nauk., SSSR, Neon. Mater. 5(l). 82 (1%9). 4. Stegei E. and K&ner B., i Anorg. Allg. Chem. 355, 131 (1%7). S. Hezel A. and Ross S. D., Spectrochim. Acta DA, 1583 (1967). 6. Hezel A. and Ross S. D., Spectrochim. Acto 24A,131(1968). 7. Hanuza J., Jezowska-Trzebiatowska 8. and Lukaszewicz K., J. Mol. Stnlctun 13, 391 (1972). 8. Hubin R., Rev. Roum. Cbim. i9, 947 (1974). 9. Lazarev A. N. and Aksel’rod V. S., Opt.Spectros. (U.S.S.R.) 9, 170(1960). 10. Calve C., ACMCrystaNogr.23, 289 (1967). 11. Calve C., Can. J. C&em.43, 1139(1965). 12. Roy R., Mi~Iesw~~ E. T. and Hummel F. A., Am. Minerof. x3.458 (1948. 13. Caivo C., L-e&g J. S. and Datars W. R.. 1. Gem. Phys. 46, 7% (1967). 14. Oetting Fl L. and McDonald R. A., J, Phys. &hem.67, 2737 (1%3), 15. Farmer V. C. and Lazarev A. N., In Symmetry and Crystal The Infrared Spectra of Minerals (Edited ~. - Vibrations, by V. C. Farmer), p. -. Mineralogical Society, London (1974).

I332

B. C. CORNILSEN and R. A.

16. Mooney R. W. and Goldsmith R. L., J. Inorg. Nucl. C/rem. 31, 933 (1%9). 17. Mooney R. W., Toma S. V. and Brunvoll J., Spectrochim. Acta DA, 1541(1%7). 18. Ross S. D., Inorganic-Infrared and Roman Spectra. McGraw-Hi& New York (1972). 19. Kravitz L. C., Kingsley J. D. and Elkin E. L., J. Chem. Phys. 49, 4600 (1%8). 20. Levitt S. R. and Condrate R. A., Am. Mineral. 49, 1562 (1970).

CONDRATE. SR.

21. Kowalczyk L. N. and Condrate R. A., J. Am. Ceram. Sec. 57. 102 (1974). 22. Lazarev A. N.. Vibrational Spectra and Structure of Silicates (Translated by G. D. Archard). Consultants Bureau, New York (1972). 23. Trate P., Pottier M. J. and Pro& A. M., Spectrochim. Acta 29A, 1017 (1973). 24. Hubin R. and Tarte P., Spectrochim. Acta 23A, 1815 (1%7). 25. Luu D. V. and Hillaire P., C.R. Acad. Sci. 270, 4% (1970).