Infrared and Raman spectra of magnesium

Vibrational Spectroscopy 68(2013)122–128Contents lists available at SciVerse ScienceDirectVibrationalSpectroscopyj o u r n a l h o m e p a g e :w w w .e l s e v i e r.c o m /l o c a t e /v i b s p ecInfrared and Raman spectra of magnesium ammonium phosphate hexahydrate (struvite )and its isomorphous analogues.IX:Spectra of protiated and partially deuterated cubic magnesium caesium phosphate hexahydrateV.Stefov a ,∗,Z.Abdija a ,b ,M.Najdoski a ,V.Koleva c ,V.M.Petruˇs evski a ,T.Runˇc evski d ,R.E.Dinnebier d ,B.ˇSoptrajanova ,e aInstitute of Chemistry,Faculty of Natural Sciences and Mathematics,Ss.Cyril and Methodius University,P.O.Box 162,1001Skopje,MacedoniabFaculty of Natural Sciences and Mathematics,State University of Tetovo,Tetovo,Macedonia cInstitute of General and Inorganic Chemistry,Bulgarian Academy of Sciences,1113Sofia,Bulgaria dMax Planck Institute for Solid State Research,Stuttgart,Germany eMacedonian Academy of Sciences and Arts,Skopje,Macedoniaa r t i c l ei n f oArticle history:Received 3April 2013Received in revised form 12June 2013Accepted 14June 2013Available online 24June 2013Keywords:Caesium magnesium phosphate hexahydrate CubicInfrared spectra Raman spectraX-ray powder diffractiona b s t r a c tInfrared and Raman spectra of cubic magnesium caesium phosphate hexahydrate,MgCsPO 4·6H 2O (cF100),and its partially deuterated analogues were analyzed and compared to the previously studied spectra of the hexagonal analogue,MgCsPO 4·6H 2O (hP50).The vibrational spectra of the cubic and hexagonal dimorphic analogues are similar,especially in the regions of HOH stretching and bending vibrations.In the difference IR spectrum of the slightly deuterated analogue (<5%D),one distinctive band appears at 2260cm −1with a small shoulder at around 2170cm −1,but only one band is expected in the region of the OD stretchings of isotopically isolated HDO molecules.The small weak band could possibly result from second-order transitions (a combination of HDO bending and some libration of the same species)rather than statistical disorder of the water molecules.By comparing the IR spectra in the region of external vibrations of water molecules of the protiated compound recorded at RT (room temperature)and at LNT (liquid nitrogen temperature)and those in the series of the partially deuterated analogues,it can be stated with certainty that the bands at 924and 817cm −1result from librations of water molecules,rocking and wagging respectively.And the band at 429cm −1can be safely attributed to a stretching Mg–O w mode.In the 3(PO 4)and 4(PO 4)region in the infrared spectra,one band in each is observed,at 995and 559cm −1,respectively.In the region of the 1modes,in the Raman spectrum of the protiated compound,one very intense band was observed at 930cm −1which is only insignificantly shifted to 929cm −1in the spectrum of the perdeuterated compound.The band at 379cm −1in the Raman spectrum could be assigned to the 2(PO 4)modes.With respect to the phosphate ion vibrations,the comparison between the two polymorphic forms of MgCsPO 4·6H 2O and their deuterated compounds shows that 1(PO 4)and 3(PO 4)appear at lower wavenumbers in the cubic phase than in the hexagonal phase.These data are in full agreement with the lower repulsion potential at the cubic lattice sites compared with that for the hexagonal lattice sites.©2013Elsevier B.V.All rights reserved.1.IntroductionSpectroscopic studies of metal phosphates and arsenates have previously been carried out [1–9].The focus has been put on the struvite-type compounds [10–17]with the general formula M I M II PO 4·6H 2O (M I =NH 4,K,Rb,Cs,Tl;M II =Mg,Ni).All studied∗Corresponding author.Tel.:+38923249942;fax:+38923226865.E-mail address:viktorst@im.mk (V.Stefov).compounds from this group crystallize in the orthorhombic system except the hexagonal caesium analogue,MgCsPO 4·6H 2O.The vibrational spectra of the hexagonal form of MgCsPO 4·6H 2O have been studied earlier in our laboratory [16].Recently we have managed to synthesize the cubic form of MgCsPO 4·6H 2O and,in this work,the study of the Fourier transform infrared (FT-IR)and Raman spectra of this compound (recorded at room temperature,RT,and at the boiling temperature of liquid nitro-gen,LNT)and those of the series of its deuterated analogues is carried out and is discussed in detail.Also,the diffractograms of the title compound at room and low temperature were0924-2031/$–see front matter ©2013Elsevier B.V.All rights reserved./10.1016/j.vibspec.2013.06.003V.Stefov et al./Vibrational Spectroscopy68(2013)122–128123 recorded in order to check if phase transitions occur at lowtemperatures.MgCsPO4·6H2O is dimorphic.Depending on the preparationprocedure it can crystallize either in the cubic system(cF100,spacegroup F¯43m i.e.T2d)with Z=4[18]or in the hexagonal system(hP50,space group P63mc i.e.C46v )with Z=2[19,20].Therefore,thestudied compound is not,strictly speaking,isomorphous to stru-vite(MgNH4PO4·6H2O)as is implied by the general title of the present series of papers,although both dimorphic entities by their structural features belong to struvite-type compounds.The crystal structure of the title compound has been determined by single crystal X-ray diffraction[18].It crystallizes in the cubic system(space group F¯43m),with Z=4and thus it is isostructural with MgCsAsO4·6H2O and FeCsPO4·6H2O[21,22].The Cs+,Mg2+ and PO43−ions in the unit cell lie on positions with T d symme-try,while the water molecules occupy C2v positions.All entities in the structure are of one crystallographic type.The Mg2+ions are at the centre of regular octahedra formed by the water oxygen atoms(O1)with a Mg–O1distance of206.5pm.Every[Mg(H2O)6]2+ cation is tetrahedrally surrounded by four regular PO43−anions and vice versa,every PO43−is connected to four neighbouring [Mg(H2O)6]2+cations by three“face-to-face”hydrogen bonds to the water molecules.Each phosphate oxygen atom(O2)is accep-tor of three hydrogen bonds from three water molecules belonging to three neighbouring[Mg(H2O)6]2+cations,thus forming a three-dimensional framework.The Cs+cations are located in the voids of this arrangement and exhibit a regular cuboctahedral coordina-tion of12H2O belonging to four[Mg(H2O)6]2+cations.There are twelve hydrogen bonds per formula unit,all of a single crystallo-graphic type,O1···O2with a length of264.9pm(strong hydrogen bonds).What makes this system special and interesting for study is the fact that both water molecules and phosphate ions retain their highest molecular symmetry in the structure.Regarding the phosphate ions,the site symmetry of which is T d,it is worth men-tioning that the point group,which is isomorphous to the factor group of the space group(also known as a unit cell symmetry group),has T d symmetry,too.This structural peculiarity,i.e.the retaining of the highest symmetry of both water molecules and phosphate ions,is observed very seldom,such as in this compound and its recently studied isostructural analogues MgCsAsO4·6H2O and FeCsPO4·6H2O[21,22].It urged a more detailed spectroscopic analysis of the vibrational modes of the water molecules and phos-phate ions,which was carried out very thoroughly in this study.The RT IR spectrum of the cubic form of MgCsPO4·6H2O and its anhydrous analogue has already been published[23]but,to the best of our knowledge,there are no data or studies on the Raman spectra and on the vibrational spectra recorded at low temperatures of its deuterated analogues.2.ExperimentalThe cubic form of MgCsPO4·6H2O has been synthesized employ-ing high pressure and high temperature(P=7×106Pa,T=553K) [18].In this work,a relatively simple precipitation procedure was developed.The synthesis is basically identical with the procedure by which the hexagonal analogue was synthesized[16],the only difference being that the obtained crystals were immediately sep-arated from the mother liquor after precipitation and dried at room temperature.The cubic phase is metastable at room temperature [18]and,as we established,it very quickly transforms into the hexagonal phase at prolonged staying in the mother liquor.Partially deuterated analogues were obtained employing the same general procedure,but using H2O–D2O mixtures with appropriate compo-sitions as solvents.The highest content of deuterium in the samples was achieved using pure D2O(the corresponding samples will be Table1Crystallographic and Rietveld refinement data CsMgPO4·6H2O(cF100).At273K At100K Wavelength(Å) 1.54059 1.540592Ârange(◦)2–902–90Step(◦2Â)0.01720.0172Cell setting and space group Cubic,F¯43m Cubic,F¯43m Quantitative amount100%100%a(Å)10.031(6)10.017(8)R Bragg a(%) 2.28 3.12R wp a(%) 2.52 2.72R p a(%) 1.62 1.68R exp a(%) 1.10 1.057No.of variables3838a R Bragg,R wp,R p and R exp are as defined in TOPAS4.1(Bruker AXS)[28]. referred to as perdeuterated).X-ray diffraction patterns of all syn-thesized(protiated and deuterated)compounds were recorded to confirm that they are,indeed,all cubic(meaning,that no measur-able quantities of the hexagonal phase were present).IR spectra were recorded from pressed KBr disks and from mulls, at both RT and LNT.FT-IR spectra were recorded using a Perkin-Elmer System2000infrared interferometer.Low-temperature measurements were carried out using the variable-temperature cell P/N21525(Graseby Specac).For a good signal-to-noise ratio, 64scans were collected and averaged at LNT,whereas32scans were enough at RT.The resolution of the instrument was2cm−1 in all of the performed measurements.GRAMS ANALYST2000[24] and GRAMS32[25]packages were used for spectra acquisition and management.The RT Raman spectra of the title compound and its deuter-ated analogues were recorded at RT on a LabRam300(Horiba Jobin-Yvon)micro-Raman spectrometer equipped with two lasers: He–Ne laser operating at633nm(with6mW power on the sam-ple)and a double frequency Nd:YAG laser at532nm(with5mW power at the sample).An Olympus MPlanN microscope with50×and100×magnification was used.The spectra were recorded with spectral resolution of2cm−1.Acquisition times varied between10 and40s,with15–30scans.The LabSpec package was used for spec-tra acquisition[26]and GRAMS32[24]for spectra management.X-ray powder diffraction patterns at variable temperature were recorded on a high-resolution laboratory powder diffractometer, StoeStadi-P with Dectris-MYTHEN1K strip PSD with an opening angle of12◦in2Â,in Debye–Scherrer,geometry with the samples sealed in borosilicate glass capillaries of0.5mm diameter(Hilgen-berg glass No.50).One diffraction pattern was recorded at273K, and then the temperature was lowered to a value of100K(using a cold air-blower)and a diffraction pattern was collected at this tem-perature as well.The samples were spun during measurement for better particle statistics.Experimental details are given in Table1. Data were taken in steps of0.0172◦in a2Ârange from2◦to90◦. Diffraction data were collected at temperatures of100and273K.Unconstrained and unrestrained Rietveld refinement[27]was performed using the program TOPAS4.1[28].Thefinal plots of the diffraction pattern measured at RT and LNT are given in Fig.1 and selected crystallographic and refinement data in Table1.As a starting model for the refinements,the crystal structures of the cubic forms of MgCsPO4·6H2O were used[18].The sample was shown to crystallize exclusively in the cubic polymorphic form.The refined overall parameters were:phase scale factor,background coefficients(Chebyshev polynomials),unit cell parameters,zero-error shift,and parameters for the strain contribution to the peak profile.The anisotropy of width and asymmetry of the Bragg reflec-tions was successfully modelled by applying symmetry-adapted spherical harmonics of eighth order to Gaussian,Lorentzian and124V.Stefov et al./Vibrational Spectroscopy 68(2013)122–128Fig.1.Scattered X-ray intensities of CsMgPO 4·6H 2O (cF100)as a function of diffraction angle,measured at (A)273K and (B)100K.The observed pattern measured in Debye–Scherrer geometry,the best Rietveld fit profile (line)and the difference curve between the observed and the calculated profiles (below)are shown.exponential distributions which are then convoluted with geomet-rical and instrumental contributions to the final peak profile.3.Results and discussionThe primitive unit cell of the cubic form of MgCsPO 4·6H 2O con-tains 25atoms and,thus the total number of the fundamental vibrations is 75distributed as =4A 1+A 2+5E +7F 1+13F 2,with 1F 2being due to the acoustic modes.Under the influence of the crystal field with T d symmetry the components with F 2symmetry are IR and Raman active,these with A 1and E symmetry are only Raman active,whereas A 2and F 1are non-active modes.Therefore,a low number of bands are expected in the vibrational spectra:13IR and 19Raman bands from normal modes.The FT-IR spectra of the cubic form of MgCsPO 4·6H 2O recorded at RT and at LNT and the Raman spectrum recorded at RT are given in Figs.2and 3.These spectra are similar to those obtained for the hexagonal form of MgCsPO 4·6H 2O,especially in the region of the stretching and bending HOH vibrations [16].3.1.Internal vibrations of the water moleculesAs mentioned earlier,there is one crystallographic type of water molecules in the crystal structure.The water molecule retains the high C 2v symmetry with a trigonal planar coordination [18].According to the unit cell group analysis (UCGA)in an absence of a correlation field,three IR active and three Raman active internal modes are expected.Under the influence of a correlation field,the number of the IR bands remains the same (3F 2),while the number of the Raman bands increases to seven (2A 1+2E +3F 2).Fig.2.IR spectra of the cubic form of MgCsPO 4·6H 2O recorded at RT (lower curve)and at LNT (upper curve).Two intense bands are observed in the stretching region in the vibrational spectra at around 3200and 2900cm −1(Figs.2and 3).Similarly as for the hexagonal analogue,these result from the anti-symmetric and the symmetric stretchings of the HOH molecules,lowered in wavenumber (with respect to the gas-phase HOH val-ues)due to the existence of strong hydrogen bonds in the structure of this compound.Having in mind the fact that the water molecules occupy pos-itions with C 2v symmetry,only one band is expected in the region of the OD stretchings of isotopically isolated HDO molecules.In the difference IR spectrum of the slightly deuterated analogue (<5%D),one distinctive band appears at 2260cm −1with a small weak band at around 2170cm −1(Fig.4).Since the diffractograms recorded at RT and LNT of the protiated compound are identical (see Fig.1)and correspond to the cubic form of MgCsPO 4·6H 2O,the possibil-ity for phase transitions at LNT is eliminated.In addition,to check for any phase transformation of the studied substance during the preparation of KBr discs,a disc from the pure substance was pre-pared and it was kept under pressure of 10t/cm 2(the same pressure used for KBr discs preparation)for 1min (as for preparation of KBr discs).The diffractograms of these discs corresponded to a pure cubic form of MgCsPO 4·6H 2O,which confirms that no transforma-tion occurs during preparation of KBr discs.Therefore,the small weak band at 2170cm −1could be a result of second-order transi-tions involving motions of the HDO molecules i.e.a combination of ı(HDO)and some HDO libration.Another (less likely)alterna-tive could possibly imply a slight statistical disorder of the water molecules.However,in this case the difference in the two OD stretches (2260and 2170cm −1)of the disordered HDO molecules appears to be too large (90cm −1).Moreover,the powder neutron diffraction data of the isostructural compound FeCsPO 4·6H 2OdoFig.3.Raman spectra of the cubic form of MgCsPO 4·6H 2O recorded at RT.V.Stefov et al./Vibrational Spectroscopy68(2013)122–128125Fig.4.Difference IR spectrum recorded at LNT in the region of the (OD)vibrations, obtained by subtracting the spectrum of the protiated compound(cubic form of MgCsPO4·6H2O)from the spectrum of the analogue with low deuterium content (≈5%D).not give any evidence for statistical disorder of the water molecule [22].Furthermore,the band at2170cm−1has rather low posi-tion although bands at low enough frequencies(even lower than 2100cm−1)have already been detected in the difference infrared spectra of other struvite type compounds studied in our laboratory [10–14,16].These bands have been attributed to the OD stretch-ings of isotopically isolated HDO molecules.It should be mentioned that the observation of an additional band is more pronounced in the spectrum of the slightly deuterated isomorphous arsenate ana-logue,which is currently under study in our laboratory.Otherwise, the spectra of the partially deuterated analogues in the region ofthe OH and OD stretching vibrations(Fig.5)indeed supporttheFig.5.IR spectra of partially and nearly fully deuterated analogues of the cubic form of MgCsPO4·6H2O recorded at LNT in the region of the OH and OD stretching vibrations(the content of deuterium increases from bottom to top,offset spectra arepresented).Fig.6.IR spectra recorded at LNT of the hexagonal form of MgCsPO4·6H2O(lower curve)and cubic form of MgCsPO4·6H2O(upper curve)in the region of the HOH bending vibrations.existence of symmetric water molecules in the crystal structure. Namely,as expected[29,30],in the region of the OD stretching vibrations,upon increase of the deuterium content new bands are observed that are positioned to the left and right of the band at2260cm−1.These bands result from the symmetric and anti-symmetric stretching vibrations of the D2O molecules and,in the spectrum of the perdeuterated compound(x D≈0.95),they appear at2294and2185cm−1.In the HOH bending region in the IR spectra,very similarly to the case of the hexagonal form of MgCsPO4·6H2O,one complex band appears with more distinctive maxima at around1750,1550 and1470cm−1despite the expected single band,according to the UCGA in absence of a correlationfield(Fig.6).The spectral pat-tern in this region is practically the same for both hexagonal and cubic forms of MgCsPO4·6H2O(Fig.6),regardless of the fact that in the former compound there are two crystallographically different types of water molecules(one is trigonal planar and the other is trigonal pyramidal),compared to only one type(trigonal planar) in the cubic analogue.As the coordination of the water molecules in both polymorphs is trigonal,it could be postulated that despite the structural differences(cubic vs.hexagonal),the effective force fields for the HOH bendings in both polymorphs are essentially the same.Following previous assignments for other studied struvite-type compounds[10–17],as well as for other isomorphous series (like alums[31,32],Tutton salts[33]etc.)this complex band can be attributed to second-order transitions of HOH librations,possi-bly further enhanced by interactions of the Fermi-resonance type (overtones and combinations of lower frequency modes in reso-nance withı(HOH)vibrations).Coupling with some low frequency lattice modes may also be important(like those found by Bertie and Falk[34]).3.2.External vibrations of the water moleculesAccording to the UCGA,in this region of the IR spectrum only one band from the rocking and one from the wagging modes is expected,the twisting remaining inactive due to the high site sym-metry.On the other hand,in the Raman spectra,one band from each of the three librational modes is expected.The influence of the correlationfield should not change the number of IR and Raman bands.126V.Stefov et al./Vibrational Spectroscopy68(2013)122–128Fig.7.IR spectra of the cubic form of MgCsPO4·6H2O recorded at RT(lower curve) at LNT(upper curve)in the region of the HOH external and PO4internal vibrations.Assignments of the librational modes of the water molecules have been carried out by many authors[29,30,35,36].Nevertheless, the two most useful(albeit somewhat tentative)approaches have been shown to be:(1)that relying on the different H2O/D2O and H2O/HDO isotopic shifts of the librational bands in the vibrational spectra[37,38],and(2)the one based on measuring the intensities of the librational bands of the water molecules[39,40].Thefirst one relies on the assumption that the hindered rotations of water molecules are performed around one of the three anticipated main axes of the angular momentum.It is assumed that the librating water molecules have C2v symmetry and can be treated as a rigid rotors(or rigid hindered rotors,to be more specific).The essence of the second approach is in the expectation that the wagging mode gives rise to the most intense band in the IR spectra.By comparing the IR spectra in the region of the external vibra-tions of water molecules of the protiated compound recorded at RT and at LNT(Fig.7)and those in the series of the partially deuterated analogues(Fig.8),it can be suggested with certainty that the bands at924and817cm−1result from librations of water molecules.The high frequencies of the H2O librations are in line with the exist-ence of strong hydrogen bonds[35].The assignment of the bands originating from these modes has been performed by consider-ing their shape,intensity,negative temperature coefficient and the changes in the spectra upon increase of the deuterium content. Hence,in the spectrum of the perdeuterated analogue,the bands at729and615cm−1can be with certainty attributed to librations of D2O molecules.The factors of the H2O/D2O isotopic shifts of the libration bands are equal to1.27and1.33,respectively.A closer inspection(Figs.7and8)reveals the following:tentatively,the higher-frequency band is assigned to the HOH rocking librational mode,and the lower-frequency one to the wagging libration.This working hypothesis is motivated by the lower isotopic ratio forthe Fig.8.IR spectra of partially and nearly fully deuterated analogues of the cubic form of MgCsPO4·6H2O recorded at LNT in the region of the HOH external and PO4internal vibrations(the content of deuterium increases from bottom to top,offset spectra are presented).rocking,but higher integrated intensity of the wagging mode.It is further strengthened by the possibility that the rocking mode is at highest frequency for trigonally coordinated water molecules [37].Somewhat surprisingly,no bands could be assigned to HDO librations in the IR spectra of the partially deuterated analogues. Providing this is really the case,the rocking HDO counterpart could easily remain undetected,or hidden under the intense band at 817cm−1assigned to wagging HOH libration.It should be kept in mind that for HDO molecules the form of the normal librational modes changes(there is one H motion and another,D motion, that fall at some mid-frequencies of the wag and twist HOH and DOD modes,respectively[37]).Thus,one can easily understand the absence of intermediate bands due to the HDO motions in the spectra of the partly deuterated analogues.In short:(1)the rocking HDO remains hidden under the wagging HOH mode,as mentioned above;(2)the H motion of the HDO molecules is expected to be roughly in this same region(according to the model calcula-tions of Eriksson and Lindgren[37]),and(3)the corresponding D motion of HDO should overlap with the wagging DOD mode(the above-mentioned band at615cm−1).In the Raman spectra of the protiated and partially deuterated compounds,no band with sig-nificant intensity could be observed in the region of the librational modes(Fig.9)even though a band with considerable intensity due to the twisting mode is expected[30,40].The translations of the H2O molecules are normally referred to M–O w stretching vibrations.In an absence of a correlationfield, one band in the Raman spectra is expected from the stretching Mg–O w mode.In the presence of thisfield,the Raman mode with A1symmetry could give rise to three components with A1,E and F2symmetry respectively.In the IR spectra,even under the influ-ence of the correlationfield,only one band from the (Mg–O w) modes is expected(that of F2symmetry).In the IR spectrum of the studied compound the band at429cm−1(Fig.7)can be safely attributed to a stretching Mg–O w ly,when going from the spectrum of the protiated to the spectra with increasing deu-terium content(Fig.8),this band shifts to lower frequencies and is found at422cm−1in the spectrum of the perdeuterated analogue.V.Stefov et al./Vibrational Spectroscopy68(2013)122–128127Fig.9.Raman spectra of the cubic form of MgCsPO4·6H2O(lower curve)and its almost completely deuterated analogue(upper curve)recorded at RT in the region of the HOH external and PO4internal vibrations.The appearance of:(1)a single band in this region,and(2)the low value(1.022)of the H2O/D2O isotopic shift together with(3)the fact that in the IR spectrum no band is expected from the 2(PO4) mode,supports this conclusion.On the other hand,the low value of the frequency shift(1.022)implies that the (Mg–O w)modes are not pure but probably coupled with other low frequency motions of the same symmetry.In the Raman spectrum of the protiated com-pound,the small intensive asymmetric band at458cm−1(Fig.9) can be assigned to the stretching Mg–O w modes.The correspond-ing band in the Raman spectrum of the perdeuterated analogue is observed at446cm−1.The difference in the frequencies of the cor-responding bands in the IR and Raman spectrum of the protiated compound suggests a significant influence of the correlationfield on these (Mg–O w)modes.Similarly as in the hexagonal form of MgCsPO4·6H2O[16],in the Raman spectra of the cubic analogue,in the region around400–300cm−1(Fig.9),three more bands with considerable intensity are observed that are only slightly shifted to lower frequencies upon deuteration,implying they are also related to Mg–O w modes,probably the O–M–O bendings.3.3.Vibrations of the phosphate ionsAs previously mentioned,in the structure of the studied com-pound the phosphate ions occupy positions with T d symmetry and the primitive unit cell group has T d symmetry,too.This would mean that the influence of the correlationfield on the phosphate vibra-tions is eliminated.Accordingly,one band each for 3and 4modes of the phosphate ions is expected to appear in the IR spectra(F2 symmetry)whereas four bands are expected in the Raman spectra (one for each of the 1, 2, 3and 4modes with symmetries A1,E, F2,F2,respectively).In the IR spectrum of the protiated compound and of the perdeuterated one(Fig.8),in the region of the 3modes,one band is observed at995and987cm−1,respectively.As for the hexag-onal analogue[16],the frequency decrease of the band from the 3mode in the spectrum of the perdeuterated compound can be attributed to the coupling of this vibration with some of the H2O librations(rocking or wagging).Because of close frequencies,this coupling of the 3(PO4)vibrations is more likely to happen with the libration at924cm−1,that we attributed to the rocking mode. In the Raman spectra of the protiated and mostly deuterated com-pound,in the region from1065to975cm−1,two bands with low intensity appear at1048and985cm−1,and1035and985cm−1 (Fig.9),respectively.One of these bands can be assigned to the 3mode whereas the other is most probably a combination of 1 vibrations with a low-frequency lattice mode(possibly reinforced by Fermi resonance with the 3fundamental).A very interesting phenomenon was observed in the IR spectra of the partially deuter-ated analogues(with5,25and50%D)where a splitting of the band from the 3(PO4)mode was detected.Actually an additional band at 975cm−1grows on the low-frequency side of the original 3(PO4) band and then again disappears.It can be speculated that it origi-nates from the 3(PO4)modes that are in the vicinity of HDO or D2O molecules to which the phosphate oxygens serve as proton acceptors.In this way,in the course of the H/D substitution,the ideally symmetric phosphate anions‘see’an environment,with H statistically substituted by D.This destroys the ideal T d symmetry of that very environment.The lower symmetry of the environment is back-reflected on the symmetry of the phosphate anions,which now vibrate in a forcefield that is not of T d symmetry anymore. This,naturally,induces splitting of the 3(PO4)band.As the mole ratio of D approaches1,the two bands coalesce into a single band, in a complete agreement with the expectations.In the region of the 1modes in the RT Raman spectrum of the protiated compound one very intense band was observed at 930cm−1which is only insignificantly shifted to929cm−1in the spectrum of the perdeuterated compound(Fig.9).In contrast to the hexagonal form of MgCsPO4·6H2O[16],in the IR spectrum of the perdeuterated cubic analogue,as expected,no band was observed that could be attributed to 1modes.In the region of the 4(PO4)modes in the IR spectra of the pro-tiated compound,one band appears at559cm−1,that is shifted to lower frequencies with a simultaneous decrease in intensity(Fig.8) and is found at535cm−1in the spectrum of the perdeuterated ana-logue.In this respect it completely parallels the behaviour of the corresponding band in the IR spectra of the hexagonal analogue [16]during partial deuteration.In the Raman spectra of the proti-ated and perdeuterated compound(Fig.9)these bands are observed at559and538cm−1,respectively.All thesefindings strongly sug-gest that the 4(PO4)modes are coupled to water librational modes (more efficiently in the perdeuterated samples,in which case the 4(PO4)vibrations are simply‘pushed’downwards by the DOD librational modes).The band at379cm−1in the RT Raman spectrum(Fig.9)could be assigned to the 2(PO4)modes.This band is slightly shifted to lower frequencies in the spectra of the deuterated analogue (Fig.9)suggesting that it is also not a pure phosphate vibration, but coupled with other vibrations involving protons.The other two bands with considerable intensity in the Raman spectrum of the protiated compound at306and220cm−1appear at frequencies that are too low to be attributed to 2(PO4)modes.The two are slightly sensitive to deuteration,implying some degree of mix-ing with lattice modes(most probably bending vibrations of the Mg(H2O)6skeleton).With respect to the phosphate ion vibrations the comparison between the two polymorphic forms of MgCsPO4·6H2O shows that 1and 3appear at lower wavenumbers in the cubic phase(930。