Copyright ©JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48.
X-RAY DIFFRACTOMETRY STUDIES AND LATTICE PARAMETER
CALCULATION ON KNO3-NH4NO3 SOLID SOLUTIONS
Wen-Ming Chien1, Dhanesh Chandra1, Jennifer Franklin1, Claudia J. Rawn2 and Abdel K.
Helmy3
1
2
Metallurgical and Materials Engineering, University of Nevada, Reno, Reno, Nevada 89557
Oak Ridge National Laboratory, Metals and Ceramics Division, Oak Ridge, TN 37831-6064
3
Special Devices Inc., 14370 White Sage Road, Moorpark, CA 93021
ABSTRACT
The solid-state phase transitions of the KNO3-NH4NO3 solid solutions have been determined by
high temperature X-ray diffractometry, and lattice parameter calculation has also been
performed. Ammonium nitrate (AN) is of great use for gas generators of automobile air bag
systems. The X-ray diffraction results showed the single (AN) phase III from 5% to 20% KNO3
in NH4NO3 and up to 373 K, which is the important temperature range for the air bag gas
generator applications. The X-ray diffraction patterns of the low temperature KNO3 phase (KN
II) are from 92%-100% KNO3 composition range and up to 393 K temperature. The high
temperature KNO3 phase (KN I) showed very broad composition range from 20% up to 100%
KNO3 at various temperature ranges. The lattice parameters of the NH4NO3-rich (AN III) and
KNO3-rich (KN II and KN I) solid solutions have been calculated at different temperature range.
The volumes of AN III phase decrease from 0.3201(4) to 0.3166(1) nm3 at the room temperature
and from 0.3250(6) to 0.3215(3) nm3 at 373 K as the compositions increase from 5% to
20%KNO3. The lattice constants of the hexagonal KN I phase show that there is no significant
change in a-direction when the temperature increases. Details of X-ray results, lattice
expansions and equations during heating are presented.
INTRODUCTION
Ammonium nitrate (NH4NO3, AN) is used as an oxidizer in the propellants, explosives and gas
generators. Addition of Potassium nitrate (KNO3, KN) will avoid the solid state phase transitions
of AN near room temperature, as the solid state phase transitions will lead to change in volume
and possible disintegration of the pellets or disks made of the propellant [1-6]. Pure AN has five
different solid phases (AN V, AN IV, AN III, AN II and AN I phases), and undergoes four
different solid state phase transitions before melting. Pure KN has two different solid phases (KN
II and KN I phases), and undergoes one solid-state phase transition. The detail experimental
crystal structure information of AN and KN are given in references 7-12. The crystal structure of
AN III phase of pure NH4NO3 was determined as orthorhombic structure and stable between
305.3 K to 357 K [13]. Goodwin et al. [13] determined the lattice parameters of this
orthorhombic structure by X-ray diffraction method. Lucas et al. [9] reported the crystal
structure of AN III phase as orthorhombic, space group Pnma, and a=0.77184(1) nm,
b=0.58447(1) nm and c=0.71624(1) nm (Z=4) by the neutron diffraction study. Solid solution of
the 5%KNO3 in NH4NO3 has also been determined as AN III phase at room temperature, and the
detail structure studies are performed by X-ray diffraction [1] as well as neutron diffraction [1415] methods. Potassium nitrate (KN) undergoes one solid-state phase transition during heating
39
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Copyright ©JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48.
and contains the KN II and KN I solid-state phases. The structure of KNO3 low temperature KN
II (α) phase has been studied by Edwards [16] and redetermined by Nimmo et al. [12] with as an
orthorhombic structure with the space group of Pmcn by using single-crystal neutron diffraction
data. The unit cell of this orthorhombic structure is with a=0.54142 nm, b=0.91659 nm, and
c=0.64309 nm (Z=4) at room temperature and confirmed by Holden and Dickinson [1] with
single-crystal X-ray data. When heated the KNO3 from room temperature, the KN II (α)→KN I
(β) phase transition occurs at ~401 K at atmospheric pressure. The high temperature KN I phase
structure has been studied by Tahvonen [17], Shinnaka [18], Stromme [19], and determined by
Nimmo and Lucas [11] as hexagonal (R-3m) structure with a=0.5425(1) nm and c=0.9836(4) nm
(Z=3) at 424 K. The purpose of this study is to determine the lattice constants of the AN III, KN
II, and KN I phases of AN-KN solid solutions at various temperature ranges.
EXPERIMENTAL
Binary powder samples of different compositions were made using pure NH4NO3 and KNO3
powders. As the samples of ammonium nitrate (AN) are very sensitive to moisture, these
samples were made in special walk-in “Dry room” with extremely low humidity at TRW (air bag
propellant facility in Lockwood, Nevada). X-ray diffraction studies of the binary mixtures were
performed at University of Nevada, Reno (UNR) and Oak Ridge National Laboratories (ORNL).
Room temperature powder diffraction work was performed using a Philips Bragg-Brentano
powder diffractometer in nitrogen environment at UNR. A plastic enclosure was installed around
the room temperature diffractometer. In addition, a desiccant, Drierite (Hammond Drierite
Company, Xenia, OH) anhydrous CaSO4 (white), and 97%CaSO4-3%CoCl2 (blue), was placed
in the enclosure with constant nitrogen flow around the diffractometer, and a gas flow was
maintained within the sample holder. The humidity was monitored by change in color of
97%CaSO4-3%CoCl2 (blue) to purple (if humidity is present). A cylinder of nitrogen is placed
close to the chamber and gas released into the enclosure for approximately an hour before the
experiment. The sample preparation (mixed with the internal standard Si or LaB6) was done in
the enclosure. After placing the sample in the diffractometer, an additional dry nitrogen gas flow
was maintained directly to the sample holder housing so that there was minimal contact of water
with the sample. The X-ray data was recorded by the MDI “DataScan” program and analyzed by
MDI “Jade” program.
The high temperature X-ray diffraction studies were performed by the high-temperature X-ray
diffractometer at High Temperature Material Laboratory (HTML), Oak Ridge National
Laboratory (ORNL). The samples were mixed with the internal standard (Si or LaB6) and
prepared in a glove bag with flowing nitrogen gas. The samples were placed on the heat strip
(Rh,Pt) and heated to the desired temperatures by the heat strip. After placing the sample in the
diffractometer, we purged and vacuumed the chamber three times with UHP He before the
experiment run and kept the He gas flowing through the chamber during the experiment. The
experiment run for each sample was from room temperature to the temperature near melting
point of the solid solutions. The data were recorded by the Scintag software. The heating rate of
the heat strip is 10°C/min. There is a 5 minutes delay before the experiment starts at each
temperature run.
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Copyright ©JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48.
RESULTS AND DISCUSSIONS
The X-ray diffraction patterns show the orthorhombic AN III phase for 5% to 20%KNO3 in
NH4NO3 solid solutions at room temperature. Figure 1 shows the AN III X-ray diffraction
patterns of 5%, 14%, and 20% KNO3 solid solutions with indexing of AN III phase, internal
standards (Si or LaB6) and heat strip (Rh,Pt). The room temperature XRD patterns do not show
significant change as the compositions increase. The single AN III phase is stable up to 373 K at
this composition range. This is confirmed by the results reported by Holden et al. [1] and Cady
[2] that the AN III phase will extend stability when temperature increases from very low to
higher temperature by adding the KNO3 to form the AN-KN solid solution.
Figure 1. The X-ray diffraction patterns for (a) 5%, (b) 14%, and (c) 20% KNO3 in NH4NO3
solid solutions show the orthorhombic AN III phase. Si and LaB6 are the internal
standards. (Rh.Pt) peaks are the referred heat strip.
The lattice constants and volumes of the room temperature AN III phase for 5%-20% KNO3
compositions are calculated and listed in Table 1. The results are also compared to the data
reported by Holden et al. [1] and Choi et al. [15], which are also listed in Table 1. The lattice
constants of the orthorhombic AN III phase are a=0.7686(8) nm, b=0.5828(4) nm, and
c=0.7146(4) nm, and volume is V=0.3201(4) nm3 for 5% KNO3 solid solution at room
temperature in this study; and these results are reasonable matched the Holden et al. [1] and Choi
et al. [15] results. It should be noted that the values of lattice constants and volumes decrease as
the composition (%KNO3) increases from 5% to 20%KNO3 composition. The volumes of AN III
phase decrease from 0.3201(4) to 0.3166(1) nm3 at the room temperature and from 0.3250(6) to
0.3215(3) nm3 at 373 K as the compositions increase from 5% to 20%KNO3. The decrease in the
values of the volume in the orthorhombic structure maybe because that more NH4+ ions (larger)
are replaced by the K+ ions (smaller) as compositions (KNO3) increase. The lattice constant
results at the various temperatures (up to 373 K) for the AN III phase show that the values of
volume per unit cell increase as the temperature increases up to 373 K, but decrease as the
composition increases at the 5%-20% KNO3 composition range. The equations of the
volume/unit versus temperature for the AN III phase are obtained by using the least-square
method, and are listed in Table 2.
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Copyright ©JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48.
Table 1. Room temperature AN III phase lattice constants and volumes for 5%, 10%, 14%, 16%,
and 20% KNO3 compositions as well as the data reported by Choi and Prask [15] and
Holden and Dickinson [1].
Composition
5% KNO3
10% KNO3
14% KNO3
16% KNO3
20% KNO3
5% KNO3 [15]
5% KNO3 [1]
Lattice Parameter
Volume
a (nm)
b (nm)
c (nm)
V (nm3)
0.7686(8)
0.7660(4)
0.7662(9)
0.7679(3)
0.7668(1)
0.76772(4)
0.7694(4)
0.5828(4)
0.5824(5)
0.5822(5)
0.5829(3)
0.5805(1)
0.58208(4)
0.5827(3)
0.7146(4)
0.7131(7)
0.7121(6)
0.7121(3)
0.7114(2)
0.71396(5)
0.7158(3)
0.3201(4)
0.3182(4)
0.3177(5)
0.3187(2)
0.3166(1)
0.3191
0.3209
Table 2. Thermal expansion equations of volume/unit vs. temperature for the AN III, KN II and
KN I phases at various temperature ranges.
Phase
AN III
5%KNO3
V(nm3) = (1.446±0.208)×10-5×T(K) + (75.903±0.690)×10-3
Temperature
range
293K - 373K
10%KNO3
V(nm3) = (1.324±0.411)×10-5×T(K) + (75.907±1.441)×10-3
293K - 393K
Composition
14%KNO3
16%KNO3
20%KNO3
95%KNO3
KN II
98%KNO3
100%KNO3
95%KNO3
KN I
98%KNO3
100%KNO3
Equations for Volume/unit cell vs. Temperature
3
-5
-3
293K - 383K
3
-5
-3
293K - 373K
3
-5
-3
293K - 373K
3
-5
-3
293K - 393K
3
-5
-3
293K - 393K
3
-5
-3
293K - 393K
3
-5
-3
408K - 553K
3
-5
-3
408K - 553K
3
-5
-3
423K - 553K
V(nm ) = (1.634±0.409)×10 ×T(K) + (74.665±1.401)×10
V(nm ) = (1.165±0.197)×10 ×T(K) + (76.296±0.679)×10
V(nm ) = (1.465±0.220)×10 ×T(K) + (74.753±0.736)×10
V(nm ) = (1.178±0.336)×10 ×T(K) + (76.966±1.125)×10
V(nm ) = (1.994±0.243)×10 ×T(K) + (74.058±0.836)×10
V(nm ) = (1.585±0.220)×10 ×T(K) + (75.288±0.749)×10
V(nm ) = (2.270±0.161)×10 ×T(K) + (73.356±0.755)×10
V(nm ) = (2.071±0.146)×10 ×T(K) + (74.428±0.687)×10
V(nm ) = (2.422±0.349)×10 ×T(K) + (72.842±1.690)×10
The X-ray diffraction patterns of the KN-rich region show the low temperature KN II phase with
orthorhombic structure, and the high temperature KN I phase with hexagonal structure. Figure
2(a) shows the KN II and KN I phase X-ray diffraction patterns with indexing for the 98% KNO3
solid solution at various temperatures. As temperature increases, the orthorhombic structure KN
II phase transits to the hexagonal structure KN I phase. Figure 2(b) shows the lattice parameter
changes during the KN II→KN I phase transition for the 98% KNO3 composition. The lattice
expansion on c-axis direction of the KN I phase is much greater than the a-axis direction of KN I
phase, and a-axis, b-axis, and c-axis directions of the KN II phase. The lattice constant data of
the KN II phase show the similar results to the AN III phase. The values of the volume increase
as the temperature increases up to 393 K, but decrease as the composition increases at the 95%100% KNO3 composition range. The values of volumes decrease from 0.3208(3) nm3 to
0.3195(3) nm3 as the composition increases from 95% to 100%KNO3. Holden et al. [1] reported
the lattice constants and volume of the 95.2%KNO3 solid solution as a=0.5444 nm, b=0.9211
nm, c= 0.6458 nm and V=0.3238 nm3 which are close to the results of 95%KNO3 KN II phase in
this study as a=0.5426(2) nm, b=0.9167(5) nm, c= 0.6449(5) nm, and V=0.3208 nm3. The lattice
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Copyright ©JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48.
43
constants and volume of the hexagonal KN I phase at 523 K are a=0.5409(4) nm, c=1.0082(9),
and V=0.2554(4) nm3.
1.1
(a)
(b)
Lattice Parameter (nm)
1.0
c - KN I
0.9
b - KN II
0.8
0.7
0.6
c - KN II
0.5
a - KN II
0.4
KN II Phase
273
323
a - KN I
373
KN I Phase
423
473
523
573
Temperature (K)
Figure 2. (a) XRD patterns for 98% KNO3 solid solution show the KN II phase at 293 K and 393
K. (b) The lattice parameters of the KN II and KN I phases in the 98%KNO3
composition during heating.
The lattice constant calculation results show that the lattice expansion is decreasing in the a-axis
direction of the KN I phase during heating, but it can be considered that there is no change in the
a-axis direction because of the small amount of decrease in comparison to the large amount of
lattice expansion in c-axis direction. This means when the temperature increases, the lattice
expansion occurs only in c-axis direction, and the basal plane (a-axis) of the hexagonal structure
remains the same. Figure 3 shows the values of the volume/unit increase as the compositions
(%KNO3) increase strongly depending on the c-axis lattice expansion at 433 K, 483 K, and 523
K for the composition range of 50%-100% KNO3 solid solution. The thermal expansion
equations of volume/unit vs temperature for the KN II and KN I phases are also listed in Table 2.
0.086
(a)
1.01
523 K
1.00
483 K
0.99
(b)
523 K
0.085
3
Volume/unit (nm )
c - Lattice Parameter (nm)
1.02
433 K
483 K
0.084
433 K
0.083
0.98
0.082
0.97
40
50
60
70
%KNO3
80
90
100
40
50
60
70
80
90
100
%KNO3
Figure 3. The (a) c-axis lattice constant and (b) volume/unit cell vs composition plots for the KN
I phase at 433 K, 483 K, and 523 K.
Copyright ©JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48.
CONCLUSIONS
The phase transitions and lattice constants of the AN-KN solid solutions have been determined.
The low temperature AN III phase is stable up to 373 K at the composition range of 5%-20%
KNO3 by adding KNO3 to NH4NO3. The lattice constant calculation results show that the values
of the volume for the AN III and KN II phases decrease as the compositions increase, but the
values increase as the temperature increases. The KN II phase is stable up to 393 K. The lattice
expansion of KN I phase occurs only in c-axis direction and the basal plane (a-axis) of the
hexagonal structure remains the same when heating.
REFERENCES
[1] Holden, J.R.; Dickinson, C.W., J. Phys. Chem., 1975, 79(3), 249-256.
[2] Cady, H.H., Phase Stabilization of Ammonium Nitrate, CPIA Publication No. 377, Johns
Hopkins University, Applied Physics Laboratory, 1983, 9-14.
[3] Choi, C.S.; Prask, H.J., Phase Stabilization of Ammonium Nitrate, CPIA Publication No.
377, Johns Hopkins University, Applied Physics Laboratory, 1983, 87-96.
[4] Deimling, A.; Engel, W.; Eisenreich, N., J. Therm. Anal., 1992, 38, 843-853.
[5] Chandra, D.; Helmy, A.K., X-ray Diffraction and Differential Calorimetry Investigation of
Ammonium Nitrate Solid Solutions, Interim Report to TRW Vehicle Safety Systems, 1999.
[6] Chien, W., Solid State Phase Transition and Vapor Pressure Studies in NH4NO3-KNO3
Binary System, Ph.D. Dissertation, University of Nevada, Reno, 2003.
[7] Brown, R.N.; McLaren, A.C., Proc. Royal Soc., 1962, 266, 329-343.
[8] Lucas, B.W.; Ahtee, M.; Hewat, A.W., Acta Crystallogr., Sect. B: Struc. Cryst., 1979,
B35(5), 1038-1041.
[9] Lucas, B.W.; Ahtee, M.; Hewat, A.W., Acta Crystallogr., Sect. B: Struc. Cryst., 1980,
B36(9), 2005-2008.
[10] Ahtee, M.; Smolander, K.J.; Lucas, B.W.; Hewat, A.W., Acta Crystallogr., Sect. C: Cryst.
Struct. Commun., 1983, C39(6), 651-655.
[11] Nimmo, J.K.; Lucas, B.W., Acta Crystallogr. Sect. B: Struc. Cryst., 1976, B32(7), 19681971.
[12] Nimmo, J.K.; Lucas, B.W., J. Phys. C: Solid State Phys., 1973, 6(2), 201-211.
[13] Goodwin, T.H.; Whetstone, J., J. Chem. Soc. Abs., 1947, 1455-1461.
[14] Choi, C.S.; Prask, H.J.; Prince, E., J. Appl. Crystallogr., 1980, 13(5), 403-409.
[15] Choi, C.S.; Prask, H.J., Acta Crystallogr., Sect. B: Struc. Sci., 1982, B38(9), 2324-2328.
[16] Edwards, D.A., Z. Kristallogr., 1931, 80, 154-163.
[17] Tahvonen, P.E., Ann. Acad. Sci. Fennicae, 1947, Ser. A.(I. Math.-Phys. No. 44), 20.
[18] Shinnaka, Y., J. Phys. Soc. Jpn, 1962, 17, 820-828.
[19] Stromme, K.O., Acta Chem. Scand., 1969, 23(5), 1625-1636.
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