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Raman scattering study of the low temperature phase transitions in ammonium nitrate
1.5 May 1976
CHEMICAi PHYSICS LE-fFERS_
Volume 40, nutiber 1
XAMAN XATTERiNG
STUDY OF THE LOW TEMPERATURE
PHASE TRANSITIONS
IN AMMsNIUM NITRATE Z. IQBAL Feltman
Research
Laboratories,
Picatinny
Arsenal,
Dover.
New Jersey
07801,
USA
Received 5 February 1976
The phase changes in poiycrystalhne ammonium nitrate and its fuliy deuterated analogue have been studied by Raman scattering. The phase V and IV transition was found to be associated with the softening and intensity decrease of a NO; librational mode. The Raman line-width data suggest that the VII to V phase transition involves a change in the degree of orientational order of the ions without incurring a change in lattice structure. .:
of the NO, ions (with respect to next nearest neigh-
1. Introduction Ammonium nitrate (NH4NO$ exhibits a series of temperature dependent phases at atmospheric pressure which are characterized by successively greater motional freedom of the NH: and NOT ions. Three of these phases (commonly referred to as phases VII, V and IV) occur in the 15 to 300 K temperature range with the transition point r,,(V to VII) at ~103 K [l] and To(IV to V) at 252 K [2] _The highest tempera-
bors) about the tetragonal c axis, coupled with a 180” out-of-phase re-orientation about the a axis, which gives rise to the non-centric structure of phase V. Phase VII has been reported by various workers [I] but its crystal structure is unknown.
2. Experimental
ture phase (phase I) is analogous to the plastic phase observed in many molecular crystals since large reorientations of both NH: and NO: ions occur. A variety of spectroscopic investigations - NMR [3], X-ray diffraction [4], neutron diffraction and scattering [5,6], infrared absorption [7] and Raman scattering
The Raman scattering experiments were carried out with a Coderg T800 symmetrically mounted triple monochromator Raman system using a cooled S25 response phototube and photon counting detection. This was coupled to a stabilized Coherent Radiation argon
[S] - have been made on NH,NO,.
Low
In this communistudy of the low
cation details of a Raman scattering temperature phases of NH,N03 are reported for the first time and an attempt is made to explain the dy-
namical behaviour of this rather interesting mdecularionic solid in these phases. At 298 K and atmospheric pressure (phase IV) NH,N03 is orthorhombic with centric space group Pmmn-Dg with 2 = 2 [S]. At 252 K a distortion to 2 non-centric tetragonal structure (phase V) with space group P42-C$
with 2 = 8 [4] occurs.
The X-ray dif-
laser oscillating
at 438.0
nm with 200 mW power.
temperatures were obtained with a He closed cycle refrigerator connected to a proportional temperature controller. The absolute temperature at the base of the cold finger was measured by a chromel-gold (with 0.07 at% Fe) thermocouple with t I .O K absolilte and 0.1 K relative accuracy. The samples in the form of small pellets were mounted with a silicone heat sink compound for optimal rhermal contact. NH,NO, and ND4N03 (~99% enriched) were obtained as high purity chemicals. Appropriate single crystals for polarization measurements could net be grown.
fraction results [4] indicate that the transition from IV to V involves an approximately 90” re-orientation 41
V&ne 40,
mimtir
__ . .
1
._
_ -.-, ._
3..Results and dis+&A -_ -.
_
L_- :
-CHEMICAL
PHYSICS
:,
LETTERS
’
15 May 1976
’
‘- The &tmn spectra oi NH,NO, in the low frequency region and seIected high frequentiy regions as a function of temperature are shbwn in figs. 1 and 2
r&s@ctiveiy. The Raman lines are associated with lattice vibrations in the 10 to 350 cm-l region, NO, internal vibrations-in the 700 to 1400 cm-l region and NH: internal vibrations in the 1400 to 3400 cm-l region. .. The lattice mode frequencies associated primarily with ionic librations tind inter-ionic translational modes are listed for both NH,NO, and ND,NO, at 15 and 260 K in table 1. NOT librations are assigned on the basis of their sizable Raman intensities and lack of a deuterium shift. The NH4f librational mode would be expected near ca. 280 cm-l in accordance with incoherent neutron scattering data [6], but no Raman scattering above the background levels was observed in this frequency region. Inter-ionic translational modes would be expected to give rise to relatively weak Raman scattering. On the basis of line intensities
Fig. 2. Raman spectra of the uJ, v; and v’~ modes in polycrysbetween 15 and 260 K.
tallineNH,.+NGs at temperatures
and deuterium shifts the lines in the 10 to 90 cm-l frequency range are assigned to essentially NO, translations and the lines in the 150 to 250 cm-l region are assigned to largely NH: translational modes. These
assignments are summarized in table 1. The NO, internal frequencies (in cm-l) at 15 K and their assignments in parentheses are as follows: 709,726,727.8 (v4); 1054 (vl) and 1285, 1289 and 1308 (v3)_ At 295 K the frequencies are: 714 (zJ,): 1044 (vl) and 1288 (+)_ The NH: internal frequencies at 1.5 K are: 1387, 1400, 1412, 1422 (vi); 1657, 1682, 1697 (v;); 3030,3045 (v;); 3120,3185, 3220 (v)3). In addition, 1480 cm-1
Frequency ; cm-’
Fk. 1. Ram’an spectra of polycryst$line NH4NOs tice mode-region at temperatures between 15 and -42
in the lat295 K.
there are a group of lines in the 1440 to region which are probably associated with
the longitudinal optic (LO) frequencies of +&estrongly infrared active v3 (NOT) vibration_ At 295 K, the NH: frequencies associated with u; and v; are extremely broad. The vi and vi modes are at 1655 and 1420 cm-l respectively. The Raman spectra provide the following general information regarding the structures of phases IV, V and VII: (i) the structures of phases V and VII are similar since the lattice mode spectra for the two phases are the same; (ii) phases V and VII contain at least two crystallographiCally inequivalent NOT and NH; sites as shown by the splittings of v4,23 (NOT)
v&me-40,
number 1
15 May 1976
CHIEBIiCAL PHYSICS l&I-i-ERS
1.
_-
Table i , lattice mode frqtiencies in B&NOJ and ND,&OJ at 1.5and 260 K . -_-
I__d-_
Frequency (cm-‘)
ND4N03
NH4N03 (a) I.5 K (phase
A&&menta)
H:D ratio
t C&2.
ObS.
VII)
50.0
50.0
75.2 84.6 87.5 96.0 98.5 112.0 135.0 140.0 163.0 191.0 202.5 207.5 231.0
75.2 85.0 88.0 96.0 98.5 112.0 135.0
1.0 0.999 0.995 1.0 1.0 1.0 1.0
158.0 180.0
1.032 1.061
ca. I.105 I.105
T(NH;) T(NfZ;tf
1.0 1.0
1.0
LO 1.0
1-O 1.0 1.0 1.0
T(N0;) TWO ;I TWO;) TWO;) n
RCNO;I R(N0 ;I RiNO;)
194.0
1.042
1.105
T(NH;f)
222.0
1.040
1.105
T(NH:I
84.0 142.0 173.0
1.07 1.01 1.005
1.0 1.0 1.0
R or T(NO;j R or T(NO;) R or T(N0;)
(b) 260 K (phase IV) 90.0 143.5 174.0
._. .I
af R - rotationall or libratiorrai modes, T - translational modes.
and all the NH; internal modes; (iii) both phases V
and VII are non-centric as shown by the appearance of v3 LO modes in the spectra; (iv) phase fV has appreciable NII4f and NO3 rnotional freedom as shown by the large line-widths and asymmetric line shapes associated with its lattice made Raman spectrum fcffig. I). It has been suggested [S] that the line at 1460 cm-l in phase IV (cf. fig. 2) is associated with the NO? v3 LO mode, thus indicating that the phase IV structure is not centric. This may be partly true because large zunplitude ionic re-orientations may cause a temporal breakdown of the strict structural centricity of phase IV. The interpretation of the in-
ternal modes in the 1300 to 1500 cm-’ range may, however, be compbcated by the fact that v3 shows an upward deuterium shift in ND4NC3 indicating‘that in NH,NO, it is probably coupled to the nearby P; modes. The phase IV to V and the V to IV transition temperatures (To and p0 respectively) were determined by differential scanning calorimetry for NH4N03 to he 243 and 271 K respectively at a heatin&ooIing rate of 10°/min. For ND4NOs, To and Z$ show
an increase of 21 and 15 K respectively, indicating clearIy that the NH; ions are directly involved in the 1V to V phase change. The Raman data show that ihe growth of phase V from IV and vice versa is time dependent in the 230 to 250 K temperature range with the phases co-existing as a physical mixture. Both the lattice mode and some of the internal mode Raman lines show interesting changes with temperature which can be correlated with the dynamics of the low temperature phase transitions. The most interesting behaviour is that of the NO, librationd line at 112 cm-l which shows a somewhat non-linear softening with increasing temperature of roughly 15%, as shown in fig. 3. Atl other lattice modes show much less and essentially linear decreases in frequency with temperature. Also, as evident from the spectra shown in fig. 1, the Raman line at 112 cm-1 decreases in relative intensity quite dramatically with increasing temperature. Since a sizable re-orientation of the NOT ions occurs on going from phase IV to V 141, the softening of the 1ibratiom.G line may be due to coupling with a soft reIaxational type mction [9]. NH; ion re-orientations coupled to changes in inter-ionic hydrogen bonding
_
CHEMICAL PHYSICS LETTERER
:
_: i5 &.x$1476
.
petted pha& VII td V transition point. This suggests a singular jump in the ionic libr&ionat. amplitudes at To z= 100 K, indicating an order-disorder trtisitio+ This mechanism is consistent with NMR [3] and specific heat [I] data, but the details mustawait a diffraction studyyof phase WI.
Acknowledgement I would like to thank 63lcrnCampbell for making the differential scanning calorimetric measurements .on the samples used in this study.
Fig. 3..PIots of 1;2 cm-r :
(at 15 K) librational frequency, Wj, (shown in open circles) and the line-widths, I’, of vr and the 50 cm-i (at 15 K) translational mode (shown in open circles), as a function of temperature. The full l&es through the linekidth data represent best liiear fits and the curve through the frequency data represents only a guide to the eye.
atso play an important role as evident from the deuter. ‘ium shift of TO_All the !attice modes and the internal
2~3,ok and ~i modes in partictiar
show rapid broaden(cf. figs. 1 and 2), which can be associated with the increasing motional freedom of the ions. The deconvoluted linewidths at h&f intensity of the tranflationat NO, mode at 50 cm-l and the pi I@ mode as a function of temperature are plotted in fig. 3. Both graphs show a positive change of slope in the vicinity of 100 K, the ex-
- ina with increasing temperature
. &..--
‘I.
_:
. .
:
.
I
ill hi. Nagatani, T. Seiyama, M. Sakiyama, H. Suga and S. Seki, Bull. Chem. Sot. Japan 40 (1967) 1833.
12JR.N. Brown and AC. McLaren, Proc. Roy. Sot. A266 (1962) 329, and references therein. M-T. Rig&r, R-R. Knispel and hI.hl. Pintar, J. Chem. Phys 56 (1972) 2911. PI J.L. Amoros, F. Arrese and M. Canut, 2. Rrist. 117 (1962) 92. f51 C.S. Choi, 3-E. hiapes and E. Prince, Acta Cryst. B28 (1972) 1357. A. Baiorek, T.A. Machekhina and K. Parlinski, IAEA Proce&ngs of Conference on Neutron Scattering, Bombay (1964) p. 355. t71 A. Theoret and C. Sandorfy, Can. 1. Chem. 42 (1964) 57, and references therein. ISI D.W. James, M.T. Carrick and W-H. Leong, Chem. Phys. Letters 28 (1974) 117, and references therein. 191 Z. Iqbal and C-WV.Christoe, Solid State Commun. 17 (1975) 71.
PI
ra
CHEMICAi PHYSICS LE-fFERS_
Volume 40, nutiber 1
XAMAN XATTERiNG
STUDY OF THE LOW TEMPERATURE
PHASE TRANSITIONS
IN AMMsNIUM NITRATE Z. IQBAL Feltman
Research
Laboratories,
Picatinny
Arsenal,
Dover.
New Jersey
07801,
USA
Received 5 February 1976
The phase changes in poiycrystalhne ammonium nitrate and its fuliy deuterated analogue have been studied by Raman scattering. The phase V and IV transition was found to be associated with the softening and intensity decrease of a NO; librational mode. The Raman line-width data suggest that the VII to V phase transition involves a change in the degree of orientational order of the ions without incurring a change in lattice structure. .:
of the NO, ions (with respect to next nearest neigh-
1. Introduction Ammonium nitrate (NH4NO$ exhibits a series of temperature dependent phases at atmospheric pressure which are characterized by successively greater motional freedom of the NH: and NOT ions. Three of these phases (commonly referred to as phases VII, V and IV) occur in the 15 to 300 K temperature range with the transition point r,,(V to VII) at ~103 K [l] and To(IV to V) at 252 K [2] _The highest tempera-
bors) about the tetragonal c axis, coupled with a 180” out-of-phase re-orientation about the a axis, which gives rise to the non-centric structure of phase V. Phase VII has been reported by various workers [I] but its crystal structure is unknown.
2. Experimental
ture phase (phase I) is analogous to the plastic phase observed in many molecular crystals since large reorientations of both NH: and NO: ions occur. A variety of spectroscopic investigations - NMR [3], X-ray diffraction [4], neutron diffraction and scattering [5,6], infrared absorption [7] and Raman scattering
The Raman scattering experiments were carried out with a Coderg T800 symmetrically mounted triple monochromator Raman system using a cooled S25 response phototube and photon counting detection. This was coupled to a stabilized Coherent Radiation argon
[S] - have been made on NH,NO,.
Low
In this communistudy of the low
cation details of a Raman scattering temperature phases of NH,N03 are reported for the first time and an attempt is made to explain the dy-
namical behaviour of this rather interesting mdecularionic solid in these phases. At 298 K and atmospheric pressure (phase IV) NH,N03 is orthorhombic with centric space group Pmmn-Dg with 2 = 2 [S]. At 252 K a distortion to 2 non-centric tetragonal structure (phase V) with space group P42-C$
with 2 = 8 [4] occurs.
The X-ray dif-
laser oscillating
at 438.0
nm with 200 mW power.
temperatures were obtained with a He closed cycle refrigerator connected to a proportional temperature controller. The absolute temperature at the base of the cold finger was measured by a chromel-gold (with 0.07 at% Fe) thermocouple with t I .O K absolilte and 0.1 K relative accuracy. The samples in the form of small pellets were mounted with a silicone heat sink compound for optimal rhermal contact. NH,NO, and ND4N03 (~99% enriched) were obtained as high purity chemicals. Appropriate single crystals for polarization measurements could net be grown.
fraction results [4] indicate that the transition from IV to V involves an approximately 90” re-orientation 41
V&ne 40,
mimtir
__ . .
1
._
_ -.-, ._
3..Results and dis+&A -_ -.
_
L_- :
-CHEMICAL
PHYSICS
:,
LETTERS
’
15 May 1976
’
‘- The &tmn spectra oi NH,NO, in the low frequency region and seIected high frequentiy regions as a function of temperature are shbwn in figs. 1 and 2
r&s@ctiveiy. The Raman lines are associated with lattice vibrations in the 10 to 350 cm-l region, NO, internal vibrations-in the 700 to 1400 cm-l region and NH: internal vibrations in the 1400 to 3400 cm-l region. .. The lattice mode frequencies associated primarily with ionic librations tind inter-ionic translational modes are listed for both NH,NO, and ND,NO, at 15 and 260 K in table 1. NOT librations are assigned on the basis of their sizable Raman intensities and lack of a deuterium shift. The NH4f librational mode would be expected near ca. 280 cm-l in accordance with incoherent neutron scattering data [6], but no Raman scattering above the background levels was observed in this frequency region. Inter-ionic translational modes would be expected to give rise to relatively weak Raman scattering. On the basis of line intensities
Fig. 2. Raman spectra of the uJ, v; and v’~ modes in polycrysbetween 15 and 260 K.
tallineNH,.+NGs at temperatures
and deuterium shifts the lines in the 10 to 90 cm-l frequency range are assigned to essentially NO, translations and the lines in the 150 to 250 cm-l region are assigned to largely NH: translational modes. These
assignments are summarized in table 1. The NO, internal frequencies (in cm-l) at 15 K and their assignments in parentheses are as follows: 709,726,727.8 (v4); 1054 (vl) and 1285, 1289 and 1308 (v3)_ At 295 K the frequencies are: 714 (zJ,): 1044 (vl) and 1288 (+)_ The NH: internal frequencies at 1.5 K are: 1387, 1400, 1412, 1422 (vi); 1657, 1682, 1697 (v;); 3030,3045 (v;); 3120,3185, 3220 (v)3). In addition, 1480 cm-1
Frequency ; cm-’
Fk. 1. Ram’an spectra of polycryst$line NH4NOs tice mode-region at temperatures between 15 and -42
in the lat295 K.
there are a group of lines in the 1440 to region which are probably associated with
the longitudinal optic (LO) frequencies of +&estrongly infrared active v3 (NOT) vibration_ At 295 K, the NH: frequencies associated with u; and v; are extremely broad. The vi and vi modes are at 1655 and 1420 cm-l respectively. The Raman spectra provide the following general information regarding the structures of phases IV, V and VII: (i) the structures of phases V and VII are similar since the lattice mode spectra for the two phases are the same; (ii) phases V and VII contain at least two crystallographiCally inequivalent NOT and NH; sites as shown by the splittings of v4,23 (NOT)
v&me-40,
number 1
15 May 1976
CHIEBIiCAL PHYSICS l&I-i-ERS
1.
_-
Table i , lattice mode frqtiencies in B&NOJ and ND,&OJ at 1.5and 260 K . -_-
I__d-_
Frequency (cm-‘)
ND4N03
NH4N03 (a) I.5 K (phase
A&&menta)
H:D ratio
t C&2.
ObS.
VII)
50.0
50.0
75.2 84.6 87.5 96.0 98.5 112.0 135.0 140.0 163.0 191.0 202.5 207.5 231.0
75.2 85.0 88.0 96.0 98.5 112.0 135.0
1.0 0.999 0.995 1.0 1.0 1.0 1.0
158.0 180.0
1.032 1.061
ca. I.105 I.105
T(NH;) T(NfZ;tf
1.0 1.0
1.0
LO 1.0
1-O 1.0 1.0 1.0
T(N0;) TWO ;I TWO;) TWO;) n
RCNO;I R(N0 ;I RiNO;)
194.0
1.042
1.105
T(NH;f)
222.0
1.040
1.105
T(NH:I
84.0 142.0 173.0
1.07 1.01 1.005
1.0 1.0 1.0
R or T(NO;j R or T(NO;) R or T(N0;)
(b) 260 K (phase IV) 90.0 143.5 174.0
._. .I
af R - rotationall or libratiorrai modes, T - translational modes.
and all the NH; internal modes; (iii) both phases V
and VII are non-centric as shown by the appearance of v3 LO modes in the spectra; (iv) phase fV has appreciable NII4f and NO3 rnotional freedom as shown by the large line-widths and asymmetric line shapes associated with its lattice made Raman spectrum fcffig. I). It has been suggested [S] that the line at 1460 cm-l in phase IV (cf. fig. 2) is associated with the NO? v3 LO mode, thus indicating that the phase IV structure is not centric. This may be partly true because large zunplitude ionic re-orientations may cause a temporal breakdown of the strict structural centricity of phase IV. The interpretation of the in-
ternal modes in the 1300 to 1500 cm-’ range may, however, be compbcated by the fact that v3 shows an upward deuterium shift in ND4NC3 indicating‘that in NH,NO, it is probably coupled to the nearby P; modes. The phase IV to V and the V to IV transition temperatures (To and p0 respectively) were determined by differential scanning calorimetry for NH4N03 to he 243 and 271 K respectively at a heatin&ooIing rate of 10°/min. For ND4NOs, To and Z$ show
an increase of 21 and 15 K respectively, indicating clearIy that the NH; ions are directly involved in the 1V to V phase change. The Raman data show that ihe growth of phase V from IV and vice versa is time dependent in the 230 to 250 K temperature range with the phases co-existing as a physical mixture. Both the lattice mode and some of the internal mode Raman lines show interesting changes with temperature which can be correlated with the dynamics of the low temperature phase transitions. The most interesting behaviour is that of the NO, librationd line at 112 cm-l which shows a somewhat non-linear softening with increasing temperature of roughly 15%, as shown in fig. 3. Atl other lattice modes show much less and essentially linear decreases in frequency with temperature. Also, as evident from the spectra shown in fig. 1, the Raman line at 112 cm-1 decreases in relative intensity quite dramatically with increasing temperature. Since a sizable re-orientation of the NOT ions occurs on going from phase IV to V 141, the softening of the 1ibratiom.G line may be due to coupling with a soft reIaxational type mction [9]. NH; ion re-orientations coupled to changes in inter-ionic hydrogen bonding
_
CHEMICAL PHYSICS LETTERER
:
_: i5 &.x$1476
.
petted pha& VII td V transition point. This suggests a singular jump in the ionic libr&ionat. amplitudes at To z= 100 K, indicating an order-disorder trtisitio+ This mechanism is consistent with NMR [3] and specific heat [I] data, but the details mustawait a diffraction studyyof phase WI.
Acknowledgement I would like to thank 63lcrnCampbell for making the differential scanning calorimetric measurements .on the samples used in this study.
Fig. 3..PIots of 1;2 cm-r :
(at 15 K) librational frequency, Wj, (shown in open circles) and the line-widths, I’, of vr and the 50 cm-i (at 15 K) translational mode (shown in open circles), as a function of temperature. The full l&es through the linekidth data represent best liiear fits and the curve through the frequency data represents only a guide to the eye.
atso play an important role as evident from the deuter. ‘ium shift of TO_All the !attice modes and the internal
2~3,ok and ~i modes in partictiar
show rapid broaden(cf. figs. 1 and 2), which can be associated with the increasing motional freedom of the ions. The deconvoluted linewidths at h&f intensity of the tranflationat NO, mode at 50 cm-l and the pi I@ mode as a function of temperature are plotted in fig. 3. Both graphs show a positive change of slope in the vicinity of 100 K, the ex-
- ina with increasing temperature
. &..--
‘I.
_:
. .
:
.
I
ill hi. Nagatani, T. Seiyama, M. Sakiyama, H. Suga and S. Seki, Bull. Chem. Sot. Japan 40 (1967) 1833.
12JR.N. Brown and AC. McLaren, Proc. Roy. Sot. A266 (1962) 329, and references therein. M-T. Rig&r, R-R. Knispel and hI.hl. Pintar, J. Chem. Phys 56 (1972) 2911. PI J.L. Amoros, F. Arrese and M. Canut, 2. Rrist. 117 (1962) 92. f51 C.S. Choi, 3-E. hiapes and E. Prince, Acta Cryst. B28 (1972) 1357. A. Baiorek, T.A. Machekhina and K. Parlinski, IAEA Proce&ngs of Conference on Neutron Scattering, Bombay (1964) p. 355. t71 A. Theoret and C. Sandorfy, Can. 1. Chem. 42 (1964) 57, and references therein. ISI D.W. James, M.T. Carrick and W-H. Leong, Chem. Phys. Letters 28 (1974) 117, and references therein. 191 Z. Iqbal and C-WV.Christoe, Solid State Commun. 17 (1975) 71.
PI
ra