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Raman spectrum of molten MgCl2
tolumc 43, number
f
CHEMICAL
I October 1976
PHYSICS LETTERS
RAMAN SPECTRUM OF MOLl’EN M&I, Chung-Hsi HUANG and M.H. BROOKER Deprrment Received
of Chemistry,
Memorial University.
St, John ‘r. Newfotandkvtd,
cirnadu
8 June 1976
Raman spectra obtained for molten MgCll at 1010 K indicate the prcrncc of a discrete tetrahedral MgC~- specks in cqtilibrium with polynuctear complexes of unknown structure. The ionic nature of molten MgC12 is confirmed in mntrast to extended polymeric structure of BeCl~ in agreement with the retativciy high equivalent conductivity of MgCl2.
As part
of our program to investigate the Raman
spectra of AC solid and molten phases of the mixtures \MgCl,+t (A = alkali metal ion,n = 1 to 4) we have round it necessary to rcinvestigatc the Raman spectrum
3f pure M&l,. In a previous study, Capwell [ 1] measured the Ra’man spectrum of solid and molten MgCl, ind concluded that the melt was stru~ura~y similar to the layered solid. Two peaks at 195 and 102 cm-t n the melt were assigned to residual lattice modes by analogy to the spectrum of the crystal which exhibited only two peaks at 243 and t 57 cm-t. The existence 3f discrete complex ions was not supported. This is
surprising since the equivalent conductance [2] * of molten M&l, is 33.2 ohm-*, or 60% of the value of %cl,, 55.0 ohm-t , and ions of some type must be present in the melt. Raman investigations of the MgX2-KX, s terns had suggested that the tetrahedral complex MgX’4 was the predo~nant species in the l$MgX, melts, although polylnuclear complexes became important at high Mg:K ratios [3,4]. Raman studies of K2MgC14 and Cs,MgCl,
as the solid single
crystals and the melts stronyty supported the complex-ion model for the melts [5]. Additional studies in our laboratory of the A,MgCIZ+, system indicate that an equilibrium exists between MgCl:_ and at
* Equivalent
conductance
values for molten CaCI2, MgCl2
and KC1 at 1060 K are 55.0.33.2 and 108 ohm-‘. molten B&l2 at 720 K the value is 0.0203 ohm-’ 180
For -
Ieast one polynuclear complex with the polynuclear compkx favoured at higher Mg : K ratios. A careful study of molten MgCl2 has revealed several Raman components not previously detected which indicate the presence of M&in eq~~bri~ with polynuclear complexes of unknown structure.
2. Experimental The anhydrous magnesium chloride was obtained from Alfa products, and used without further puriflcation. Due to its h@h hygroscopicity, all manipula-
tions with the powder were performed in a glove bag under dry nitrogen. Samples of MgC12 were contained in 6 mm i-d. quartz tubing and further dried under vacuum at room temperature for one day, the temperature was gradually raised to 180°C and further dehydrated at 180°C for another day. The tubes were sealed and used as Raman cells. Polarized Raman spectra were recorded with a Coderg PHO Raman spectrophotometer after sample excitation with the 488.0 nm line (= 600 nfW at the sample) of a control laser Model 550 argon laser. Plasma lines from the laser were removed with narrow bandpass interference filter. A post-sample filter solution of 0.5 M Pr (EDTA)- complex (61 in a 1.0 cm cell was placed between the sample and the spectrophotometer to attenuate the scattered laser light to decrease the background and remove tke grating
Volume
43. number
CHEMICAL PHYSICS LETTERS
1
1 Octoba 1976
1.0
ghosts which occasionally appear in the PHO monochromator. A half-wave plate before tile sample was used to control the polarization of the incident beam and a polaroid film was placed before the entrance slit of the monochromator to analyze the scattered light. Spectra of the melt were obtained by coiling nichrome wire around the Raman ~~11.Temperatures were measured with a chrome1alumel thermocouple outside the Raman cell. The spectra were scanned at least 3 times for each of three preparations to establish the consistency of the band shapes and frequendes. The Raman cnvelopcs were manually digitalliztd, base-line corrected and curve resolved to gaussian profiles which were fitted by a least squares technique.
0
\?
J
J x(rrIr
3. Results and discussion Raman spectra for solid and molten MgCl, arc shown in fig. 1. The use of the post-sample filter re, sulted in a significantly improved background and spectra could be recoreded to within 30 cm-* of the exciting line. However, even without the filter it was possible to obtain a better spectrum of molten MgCl, than has been published. The spbctra did not show any features attributable to ghosts or impurities and spectra recorded without the post-sample filter were simila: to those recorded with it although greater background scattering accompanied the latter. Peak positions, relative intensities and assignments are presented in table 1. For solid MgCl, our results are essentially identical to those of Capwell [l ] and except for a small frequency increase the results at 77 K are Table 1 Peak positions (cm-‘)
-
and intensity data a) for.mol:cn
298 K
assignment _____.
158
244
155
240
300
400
D:d spoa: group Es
*tz
x(zs)Y;II
200 Cl --I
100
1
0
Fjg. 1. Raman spectra of anhydrous hfgCl* recorded with 2.0 cm-’ slits. (a) Solid at 298 K,(b) melt at 1010 K,I@; (c) melt at 1010 K.11; insert shows the ~3 mode of hlg($obscrvcd with 5
X stale
expansion.
@Cl2 and KIMgCb ---K2hlgCb b)
MgCI. melt. 1010 K
h!gCI. solid 77 K
“‘- .... ,_., _,...T-&.;,- ‘L.,.. _.............--.,-.^.. . ,.,.,
x(zx)Y;ll
. .._._-_-_-.----
assignment
melt, 740 K
“2 (El
115 (0.13)
. . . -100 (0.14)
130 (0.62)
h&q-”
194 (1.50)
140 (0.06) 194 (0.04)
244 (1 .O)
244 (0.04)
~1(*I)
227 (0.32) 252 (1.0)
335 (0.07)
335 (0.04)
Q(l.‘Z)
350 (0.045)
y4 (I:21 MR2qy
14s (0.19)
a) Rchtive intensity data normalized to 1.0 for Y, (Al) mode. b, Data from ref. [S J.
181
Volume 43, number i
CIKMICAL
PHYSICS LETTERS
similar ts the ruum temperature data. Tugether with Capwell’s high temperature resuIts for the solid this suggests that MgCl, dues nut undergu any. phase transition between 77 K and the mdtinng paint. The most important feature in the I, spectrum Of
of the basic lattice structure as has been suggested by Capwell but melts to give a dynamic equilibrium mixture wNch cuntains ~gC@-,
the mt=Xtis the distinct shoulder a 244 cm-t on a broad peak with a maximum at q I94 cmmf. Addi-
244 and 195 cm-t in pure MgCI, compared to those at 250 and 227 cm-f in KzMgCI, is consistent with a decrease in the canccntration of M&l:- as the Clcomxntration dccrcases and a corresponding incrcasc in polynuclrar complex. Our Raman data and ttie enthalp of fusion data of Helm ct al, [S] indicate that s is aTso the dominant species in molten MgC&KMgCCt. Helm et a1. [S] also suggested a decrease: in complex stability as the temperature was intreascd. Alt bough they suggested the react ion
tianal Raman intensity can be detected * 130 cm-l the peaks at and = 335 cm- 1. In the IL p!arimtion 244,194 and 130 cm-* are essentially absent and depolarization ratios, p < U,U5 have been determined, whereas, the braad peak at 335 em- ‘i remains and weak waks at * 100 and 140 cm-l can be detected. Ckxc inspection of Capwell’s published spectrum sugt gusts t hc presence of the 244 cm-1 siioulder and the weak 335 cn; -I feature, We were unable to detect the sharp weak peak at 102 cm-l reported by CapiveU in the I, spectrum of the melt, On the basis of relative intensity and polarization &ta (table 1) the peaksat 244,335,140 and IO0 cm-l have been assigned to the q , v3, p4, and v2 modes of the discrete MgCf:- tetfahedraf campkx timitar to thztt fcwnd in molten K+gC’I, [5]. Peaks at 195 and i 30 cm-l arc probably due to symmetric stretching vibrations of polynucla~ ccmplex(es)+ It is unlikely that either peak is due to MgCiS since the symmetric stretching mode of this species would bi; cxpccted at higher frequencies than that uf MgC$-. The position
melt with retention
plynuclca~
Mg2+ + Mga:-
+ Mg,Ct, +
That ions are present in molten MgCl, appears certain
from the relatively high conductivity af Mg$ cumpared to BeC’l, which is considered to have an extended polymeric unit [23+ The Raman spectra suggest that Mgclz does nut
182
and CT
our results for K,MgCl, are mure r=onsistent with the breakdown of a polynuckar complex. Attempts to study the effect of temperat urc on the Raman spectrum of molten Mgc12 were inconcl_tivc.
This wurk was supprted by a grant from the National Research Council of Ganada. *
uf the peakat 195 cm-l is tower than the peak sbserved at 227 cm-1 in molten KzMgCl, and assigned to Mg2C& and quite possibly represents another p>lynuctear species perhaps a positive isn Mg&+ 9r Mg&, dictated by electric;il neutrality, or neutral Mg$Z14. ft is difficuIt to envisage puiymeric units containing more than tahree magnesium ions that would hve a lfetimr: long enough to be detected spectroscupicatig. Perhaps one way picturing the interaction is to consider it as an ion-pair formation
cQmpfexes, and perhaps M$
ions. The decrease in the intensity r;itia d the peaks at
References
f
CHEMICAL
I October 1976
PHYSICS LETTERS
RAMAN SPECTRUM OF MOLl’EN M&I, Chung-Hsi HUANG and M.H. BROOKER Deprrment Received
of Chemistry,
Memorial University.
St, John ‘r. Newfotandkvtd,
cirnadu
8 June 1976
Raman spectra obtained for molten MgCll at 1010 K indicate the prcrncc of a discrete tetrahedral MgC~- specks in cqtilibrium with polynuctear complexes of unknown structure. The ionic nature of molten MgC12 is confirmed in mntrast to extended polymeric structure of BeCl~ in agreement with the retativciy high equivalent conductivity of MgCl2.
As part
of our program to investigate the Raman
spectra of AC solid and molten phases of the mixtures \MgCl,+t (A = alkali metal ion,n = 1 to 4) we have round it necessary to rcinvestigatc the Raman spectrum
3f pure M&l,. In a previous study, Capwell [ 1] measured the Ra’man spectrum of solid and molten MgCl, ind concluded that the melt was stru~ura~y similar to the layered solid. Two peaks at 195 and 102 cm-t n the melt were assigned to residual lattice modes by analogy to the spectrum of the crystal which exhibited only two peaks at 243 and t 57 cm-t. The existence 3f discrete complex ions was not supported. This is
surprising since the equivalent conductance [2] * of molten M&l, is 33.2 ohm-*, or 60% of the value of %cl,, 55.0 ohm-t , and ions of some type must be present in the melt. Raman investigations of the MgX2-KX, s terns had suggested that the tetrahedral complex MgX’4 was the predo~nant species in the l$MgX, melts, although polylnuclear complexes became important at high Mg:K ratios [3,4]. Raman studies of K2MgC14 and Cs,MgCl,
as the solid single
crystals and the melts stronyty supported the complex-ion model for the melts [5]. Additional studies in our laboratory of the A,MgCIZ+, system indicate that an equilibrium exists between MgCl:_ and at
* Equivalent
conductance
values for molten CaCI2, MgCl2
and KC1 at 1060 K are 55.0.33.2 and 108 ohm-‘. molten B&l2 at 720 K the value is 0.0203 ohm-’ 180
For -
Ieast one polynuclear complex with the polynuclear compkx favoured at higher Mg : K ratios. A careful study of molten MgCl2 has revealed several Raman components not previously detected which indicate the presence of M&in eq~~bri~ with polynuclear complexes of unknown structure.
2. Experimental The anhydrous magnesium chloride was obtained from Alfa products, and used without further puriflcation. Due to its h@h hygroscopicity, all manipula-
tions with the powder were performed in a glove bag under dry nitrogen. Samples of MgC12 were contained in 6 mm i-d. quartz tubing and further dried under vacuum at room temperature for one day, the temperature was gradually raised to 180°C and further dehydrated at 180°C for another day. The tubes were sealed and used as Raman cells. Polarized Raman spectra were recorded with a Coderg PHO Raman spectrophotometer after sample excitation with the 488.0 nm line (= 600 nfW at the sample) of a control laser Model 550 argon laser. Plasma lines from the laser were removed with narrow bandpass interference filter. A post-sample filter solution of 0.5 M Pr (EDTA)- complex (61 in a 1.0 cm cell was placed between the sample and the spectrophotometer to attenuate the scattered laser light to decrease the background and remove tke grating
Volume
43. number
CHEMICAL PHYSICS LETTERS
1
1 Octoba 1976
1.0
ghosts which occasionally appear in the PHO monochromator. A half-wave plate before tile sample was used to control the polarization of the incident beam and a polaroid film was placed before the entrance slit of the monochromator to analyze the scattered light. Spectra of the melt were obtained by coiling nichrome wire around the Raman ~~11.Temperatures were measured with a chrome1alumel thermocouple outside the Raman cell. The spectra were scanned at least 3 times for each of three preparations to establish the consistency of the band shapes and frequendes. The Raman cnvelopcs were manually digitalliztd, base-line corrected and curve resolved to gaussian profiles which were fitted by a least squares technique.
0
\?
J
J x(rrIr
3. Results and discussion Raman spectra for solid and molten MgCl, arc shown in fig. 1. The use of the post-sample filter re, sulted in a significantly improved background and spectra could be recoreded to within 30 cm-* of the exciting line. However, even without the filter it was possible to obtain a better spectrum of molten MgCl, than has been published. The spbctra did not show any features attributable to ghosts or impurities and spectra recorded without the post-sample filter were simila: to those recorded with it although greater background scattering accompanied the latter. Peak positions, relative intensities and assignments are presented in table 1. For solid MgCl, our results are essentially identical to those of Capwell [l ] and except for a small frequency increase the results at 77 K are Table 1 Peak positions (cm-‘)
-
and intensity data a) for.mol:cn
298 K
assignment _____.
158
244
155
240
300
400
D:d spoa: group Es
*tz
x(zs)Y;II
200 Cl --I
100
1
0
Fjg. 1. Raman spectra of anhydrous hfgCl* recorded with 2.0 cm-’ slits. (a) Solid at 298 K,(b) melt at 1010 K,I@; (c) melt at 1010 K.11; insert shows the ~3 mode of hlg($obscrvcd with 5
X stale
expansion.
@Cl2 and KIMgCb ---K2hlgCb b)
MgCI. melt. 1010 K
h!gCI. solid 77 K
“‘- .... ,_., _,...T-&.;,- ‘L.,.. _.............--.,-.^.. . ,.,.,
x(zx)Y;ll
. .._._-_-_-.----
assignment
melt, 740 K
“2 (El
115 (0.13)
. . . -100 (0.14)
130 (0.62)
h&q-”
194 (1.50)
140 (0.06) 194 (0.04)
244 (1 .O)
244 (0.04)
~1(*I)
227 (0.32) 252 (1.0)
335 (0.07)
335 (0.04)
Q(l.‘Z)
350 (0.045)
y4 (I:21 MR2qy
14s (0.19)
a) Rchtive intensity data normalized to 1.0 for Y, (Al) mode. b, Data from ref. [S J.
181
Volume 43, number i
CIKMICAL
PHYSICS LETTERS
similar ts the ruum temperature data. Tugether with Capwell’s high temperature resuIts for the solid this suggests that MgCl, dues nut undergu any. phase transition between 77 K and the mdtinng paint. The most important feature in the I, spectrum Of
of the basic lattice structure as has been suggested by Capwell but melts to give a dynamic equilibrium mixture wNch cuntains ~gC@-,
the mt=Xtis the distinct shoulder a 244 cm-t on a broad peak with a maximum at q I94 cmmf. Addi-
244 and 195 cm-t in pure MgCI, compared to those at 250 and 227 cm-f in KzMgCI, is consistent with a decrease in the canccntration of M&l:- as the Clcomxntration dccrcases and a corresponding incrcasc in polynuclrar complex. Our Raman data and ttie enthalp of fusion data of Helm ct al, [S] indicate that s is aTso the dominant species in molten MgC&KMgCCt. Helm et a1. [S] also suggested a decrease: in complex stability as the temperature was intreascd. Alt bough they suggested the react ion
tianal Raman intensity can be detected * 130 cm-l the peaks at and = 335 cm- 1. In the IL p!arimtion 244,194 and 130 cm-* are essentially absent and depolarization ratios, p < U,U5 have been determined, whereas, the braad peak at 335 em- ‘i remains and weak waks at * 100 and 140 cm-l can be detected. Ckxc inspection of Capwell’s published spectrum sugt gusts t hc presence of the 244 cm-1 siioulder and the weak 335 cn; -I feature, We were unable to detect the sharp weak peak at 102 cm-l reported by CapiveU in the I, spectrum of the melt, On the basis of relative intensity and polarization &ta (table 1) the peaksat 244,335,140 and IO0 cm-l have been assigned to the q , v3, p4, and v2 modes of the discrete MgCf:- tetfahedraf campkx timitar to thztt fcwnd in molten K+gC’I, [5]. Peaks at 195 and i 30 cm-l arc probably due to symmetric stretching vibrations of polynucla~ ccmplex(es)+ It is unlikely that either peak is due to MgCiS since the symmetric stretching mode of this species would bi; cxpccted at higher frequencies than that uf MgC$-. The position
melt with retention
plynuclca~
Mg2+ + Mga:-
+ Mg,Ct, +
That ions are present in molten MgCl, appears certain
from the relatively high conductivity af Mg$ cumpared to BeC’l, which is considered to have an extended polymeric unit [23+ The Raman spectra suggest that Mgclz does nut
182
and CT
our results for K,MgCl, are mure r=onsistent with the breakdown of a polynuckar complex. Attempts to study the effect of temperat urc on the Raman spectrum of molten Mgc12 were inconcl_tivc.
This wurk was supprted by a grant from the National Research Council of Ganada. *
uf the peakat 195 cm-l is tower than the peak sbserved at 227 cm-1 in molten KzMgCl, and assigned to Mg2C& and quite possibly represents another p>lynuctear species perhaps a positive isn Mg&+ 9r Mg&, dictated by electric;il neutrality, or neutral Mg$Z14. ft is difficuIt to envisage puiymeric units containing more than tahree magnesium ions that would hve a lfetimr: long enough to be detected spectroscupicatig. Perhaps one way picturing the interaction is to consider it as an ion-pair formation
cQmpfexes, and perhaps M$
ions. The decrease in the intensity r;itia d the peaks at
References