Pressure, Temperature Reaction Phase Diagram for Ammonium

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J. Phys. Chem. 1996, 100, 3248-3251

Pressure, Temperature Reaction Phase Diagram for Ammonium Dinitramide T. P. Russell* NaVal Research Laboratory, Chemistry DiVision, Code 6110, Washington, D.C. 20375-5320

G. J. Piermarini and S. Block† Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899

P. J. Miller NaVal Surface Warfare Center, SilVer Spring, Maryland 20901 ReceiVed: July 26, 1995; In Final Form: October 24, 1995X

The pressure, temperature reaction phase diagram for ammonium dinitramide (ADN) was determined using a high-temperature-high-pressure diamond anvil cell with optical polarizing light microscopy, Fourier transform infrared spectroscopy, laser-Raman spectroscopy, and energy dispersive X-ray diffraction. The phase diagram was determined between ambient pressure and 10.0 GPa over the temperature range from -75 °C to decomposition temperatures, or 120 °C. The diagram delineates the melting curve for R-ADN, a reversible phase transition in R-ADN forming a new high-pressure monoclinic polymorph, β-ADN, and also identifies the pressure and temperature conditions at which a solid phase rearrangement occurs to form ammonium nitrate (AN) and N2O. Energy dispersive X-ray diffraction and Raman spectra were obtained for both R- and β-ADN as a function of pressure at room temperature.

Introduction The recent synthesis of ammonium dinitramide (ADN; ammonium nitronitramide) has provided an example of a new class of stabilized energetic inorganic salt compounds.1 ADN is an inorganic oxidizer with minimum signature characteristics as well as reduced hydrochloric acid production from commercial rocket propellant plumes and is considered a potential replacement for ammonium perchlorate in propellant applications. Moreover, ADN addresses today’s upper atmosphere concerns and also wetland stress near launch pads, because its use as a propellant would significantly reduce the level of harmful contaminants introduced to these environments. A molecular model for ADN in Figure 1 shows the arrangement of the atoms constituting one molecular unit. The dinitramide anion is semiplanar with the appended N-NO2 groups twisted in a propeller-like orientation.2 Recent studies have focused on the thermal decomposition and structure of ammonium dinitramide and other dinitramide salts.3-8 Two decomposition pathways under atmospheric pressure and elevated temperature have been identified.3-8 The low-temperature decomposition reaction mechanism is a molecular rearrangement which permits the formation of ammonium nitrate (AN) and N2O(g). The high-temperature rapid thermal decomposition mechanism is initiated by N-N homolysis to form NO2(g). ADN melts between 92 and 95 °C, depending upon the amount of ammonium nitrate impurity present.8 The melting point is as low as 55 °C at the ADN/AN (70/30 mol %) eutectic point.8 Until the present work, no high-pressure polymorph of ADN had been discovered, even though high-pressure studies of dinitramide compounds have been reported.9,10 In the present study, we report the discovery and identification of a highpressure polymorph, monoclinic β-ADN, delineate its pressure/ * To whom correspondence should be addressed. † Guest Scientist. X Abstract published in AdVance ACS Abstracts, January 1, 1996.

This article not subject to U.S. Copyright.

Figure 1. Model diagram of ammonium dinitramide (ADN) depicting the arrangement of the atoms constituting one molecular unit. Note that two NO2 groups are appended to a single nitrogen atom in the anion.

temperature stability field, identify and describe the pressure/ temperature dependency of a solid state unidirectional molecular rearrangement in ADN to form AN, and also define the pressure/ temperature dependence of the thermal decomposition reaction. Experimental Section11 The experimental techniques employed in the present work were reported in detail earlier.12-20 Only a brief description is given here for the sake of clarity. The diamond anvil pressure cell (DAC) used in this work is fabricated from a hightemperature, high-strength superalloy, Inconel 718, and is designed for 180° transmission and reflection measurements with a sustained static temperature range between -125 and 600 °C.12 It is based on the original NBS DAC designed for use at high temperatures.20 Temperature is measured by a chromel-alumel thermocouple with the thermocouple bead in contact with an Inconel X750 alloy gasket which confines the sample under pressure. The high-pressure-high-temperature DAC can be mounted on a micrometer positioning device for optical polarizing light microscopy, ruby fluorescence pressure measurements, Fourier transform infrared spectroscopy (FTIR), and micro-Raman spectroscopy measurements, as described briefly below. The starting material for all experiments was the R polymorph of ADN.

Published 1996 by the American Chemical Society

Reaction Phase Diagram for Ammonium Dinitramide Optical Microscopy and Ruby Fluorescence Measurements. Optical polarizing light microscopy (OPLM) studies were carried out with a system originally designed for ruby fluorescence pressure measurements described in detail earlier.12 The DAC is positioned on the optic axis of the microscope with the sample or ruby in focus. Pressures are measured by the ruby fluorescence technique with a precision of (0.05 GPa when the environment is hydrostatic and at room temperature.12,13 Birefringence changes on single crystals of ADN are readily visible under polarized white (tungsten source) light when liquid N2 (LN2), and n-pentane/isopentane (1:1 by volume) are used as pressure transmitting liquids. Both liquids permit the observation of clear, sharp images of ADN single crystals sensitive to small changes in birefringence with polarized white light. Fourier Transform Infrared Spectroscopy and Raman Microscopy. Infrared absorption spectra were recorded on a Nicolet 7199 FTIR spectrometer employing an MCT-A (narrow band) detector for improved sensitivity. Sample thicknesses of 0.01-0.05 mm were found to be necessary for these infrared transmission experiments and were prepared as thin films in a gasketed DAC containing either dry NaCl powder compacted to transparency or LN2.8,15 Micro-Raman spectra were recorded using a SPEX 1877A triple monochromator and a Photometrics LN2 cooled CCD detector for improved sensitivity. A Spectra Physics Ar ion laser operating at 514.5 nm and a power of 55 mW at the sample was employed. Raman spectra were collected on the samples prepared as a thin film with LN2 as the pressure transmitting medium. Both infrared and Raman spectra were collected for the same ADN sample and at the same pressure and temperature conditions because the thin film sample under LN2 in the DAC was suitable for both kinds of measurements. Energy Dispersive X-ray Diffraction (EDXD). For in situ X-ray diffraction measurements with the DAC, the energy dispersive X-ray diffraction technique described in detail earlier was used.16,21-23 X-ray powder patterns were obtained from samples of ADN powder compacted in the DAC using an Inconel X750 gasket, with a gasket hole 250 µm in diameter and thickness. No pressure transmitting liquid was used. Following the methods described in an earlier report, X-ray patterns were collected at 2θ + 4° using tungsten radiation.16 Results The pressure, temperature reaction phase diagram determined for ADN is shown in Figure 2. It includes the P, T parameters for the solid state molecular rearrangement reaction which forms ammonium nitrate (AN) and N2O. The general features of the diagram show the stability fields for three phases of ADN: the R and β polymorphs and the liquid phase. A region at higher pressures and temperatures defines the parameters for a solid state rearrangement forming AN (I) and N2O. The monoclinic R phase is stable from atmospheric pressure to 2.0 GPa over a large range of temperatures. The R phase melts above 92 °C with a positive slope to the melting curve extending to about 1.0 GPa. At about 2.0 GPa, R-ADN transforms to β-ADN with the phase transition pressure essentially temperature independent within the experimental uncertainty of our measurements. The new high-pressure monoclinic β phase is stable above 2.0 GPa between -75 and 140 °C. There was no evidence that the β phase melts and exists in equilibrium with liquid. Hence, no triple point (R-β-liquid) is indicated on the diagram. The solid state rearrangement reaction forming AN (I) and N2O occurs between 2.0 and 10.0 GPa between 120 and 140 °C. Accurate delineation of the R- and β-ADN stability fields was not possible, owing to the large uncertainty in the data points

J. Phys. Chem., Vol. 100, No. 8, 1996 3249

Figure 2. Pressure, temperature reaction phase diagram for ammonium dinitramide (ADN) showing the estimated thermodynamic stability fields for the R and β polymorphs and the liquidus curve. The monoclinic R phase is stable up to about 2.0 GPa between -75 and 120 °C. The high-pressure β phase, which is also monoclinic, is stable above 2.1 GPa between -75 and 120 °C. Above 140 °C between 1.0 and 10.0 GPa, ADN undergoes a molecular rearrangement to form ammonium nitrate (AN) and N2O. The R-β transition pressure is estimated to be 2.0 ( 0.2 GPa and is the result of a least squares fit of the data points.

associated with the transition. The R-β transition is reversible and can be approached from both the R and β phases, but the spread in the data points is large, on the order of 0.7 GPa, so that assigning a transition pressure was difficult. An estimate of 2.0 ( 0.2 GPa was obtained from a least squares fit of the data between -75 and 120 °C. The liquidus for R-ADN is given as a solid line to indicate firm results, while the molecular rearrangement is shown as a dash line to indicate high uncertainty associated with the line position. The β phase cannot be retrieved to ambient conditions but reverts to the R phase as the pressure is lowered below about 2.0 GPa. When R-ADN is heated at ambient conditions, melting is observed at 95 ( 3 °C. The liquid immediately begins to decompose slowly forming AN (I) and N2O(g). Thus, each data point on the liquidus curve had to be determined using a fresh undecomposed sample of R-ADN loaded to the desired pressure and then heated slowly (2 °C/min) until initial melting was detected. Uncertainty in the liquidus curve may arise from several factors. One is the fact that the transition is observed only unidirectionally, i.e., as the temperature is increased using a 2 °C/min heating rate. Superpressing the ADN could occur which would have the effect of raising the melting temperature. Another is that liquid ADN begins to decompose almost immediately, although slowly, to form AN (I) and N2O. Introduction of AN as an impurity in the original ADN sample depresses the melting temperature. For example, the eutectic melting temperature for ADN mixed with AN (70/30 mol %) has been measured to be 55 °C, considerably lower than the 95 °C reported for pure ADN.8 Finally, the onset of melting is determined in these experiments by the visual presence of a liquid phase. The temperature at which the liquid phase becomes visible may be higher than the correct melting temperature. No other phase transitions were found in the ADN system up to 2.0 GPa between -75 °C and the melting temperatures. Under microscopic examination with polarized light, a significant shrinkage in the physical size of the ADN single crystal is observed when it is pressurized to 3.0 GPa at room temperature. A change in birefringence of the crystal is observed from 1.9 to 2.8 GPa, but the change does not appear to be discontinuous. When the pressure is increased to 3.0 GPa

3250 J. Phys. Chem., Vol. 100, No. 8, 1996

Figure 3. Pressure dependency of the 1174 and 739 ∆ cm-1 Raman vibrational modes of ammonium dinitramide (ADN) at 23 °C. A discontinuity in the shift of the vibrational modes is observed near 2.0 GPa, indicating a first-order phase transition between R and β ADN polymorphs. The transformation is reversible. The uncertainty in the wavenumber measurement is 0.5 cm-1 on the basis of instrument settings and calibration. The standard uncertainty in the pressure measurement is (0.05 GPa.12-14

and then decreased to below 2.0 GPa, large cracks appear in the crystal and a subtle birefringence change was detected. After several pressure cycles, the crystal breaks apart. Similar effects were observed when single crystals of ADN were pressurized at constant temperatures between -75 and 120 °C. The hysteresis associated with these observations and the subtle birefringence changes do not permit an accurate determination of the transformation pressure. The subtle birefringence changes and the development of large cracks indicate that a first-order phase transition occurs between 1.9 and 2.6 GPa. The elusive birefringence change indicates that a similar molecular conformation probably exists in the structure of the new phase or that a subtle change in the crystal packing is associated with the transition. Raman and FTIR spectra for ADN crystals were measured as a function of pressure in a LN2 hydrostatic medium. No detectable changes were observed in spectra collected below 1.6 GPa and above 2.8 GPa. In the infrared spectra, significant pressure broadening is observed which masks any potential subtle spectral changes due to the R T β transformation. Therefore, infrared spectra are not reported for the R T β transformation. However, the pressure dependence of the individual Raman modes indicates the first-order phase transformation. The wavenumber shifts of the two Raman modes (1174 and 739 cm-1) as a function of pressure are shown in Figure 3. Assignments for the observed Raman modes were reported elsewhere.24 The 1174 and 739 cm-1 shift modes correspond to the NO2 symmetric stretch and the rocking modes of the dinitramide anion. As the pressure is increased to 2.0 GPa, a smoothly varying monotonic shift is observed for all R-ADN Raman active modes, indicating uniform compression. At 2.1 GPa, a discontinuous shift in all Raman modes is observed. Figure 3 shows the discontinuity in the 1174 and 739 ∆ cm-1 modes. The uncertainty in the wavenumber measurement is (0.5 cm-1, and that in the pressure measurement is (0.05 GPa for all reported spectra. Above 2.1 GPa, a uniform shift is again observed as a function of pressure up to 14.0 GPa. As the pressure is decreased to 1.6 GPa, a discontinuity is detected, again indicating a reversible effect. The discontinuous shift in the Raman spectrum is consistent with the OPLM observations. The ADN EDXD patterns were collected as a function of pressure from atmospheric pressure to 4.9 GPa. Up to six diffraction lines were observed for ADN under these conditions. The crystal structure of ADN is monoclinic with Pbca sym-

Russell et al.

Figure 4. Pressure dependence of the unit cell volume derived from the observed energy dispersive X-ray diffraction data. Uniform volume compression is exhibited to 2.1 GPa. A discontinuity in compression is observed at 2.1 GPa, and at the same time a new diffraction line appears, indicating the formation of the high-pressure β phase. Uniform compression resumes up to 4.9 GPa for the β phase. The vertical lines at each data point indicate the magnitude of the error associated with the volume measurement. The standard uncertainty in the pressure is (0.05 GPa.12-14

metry.2 The diffraction lines for the ambient monoclinic phase were measured by single-crystal X-ray diffraction2 and were used to index the observed diffraction lines obtained by the EDXD technique. The calculated unit cell volume was determined by a least squares method using the data obtained by the EDXD method and was found to agree with the single-crystal results. This procedure was applied to the X-ray data obtained as a function of pressure up to 4.9 GPa. The results are shown in Figure 4. The uncertainty in the unit cell dimensions is given by the indicated error bars. Uniform compression of the unit cell volume is observed up to 2.1 GPa. Between 2.1 and 2.3 GPa, however, a discontinuous shift in the unit cell volume is apparent. Above 2.3 GPa, uniform compression in the unit cell volume is observed again. Although it is not readily apparent from the EDXD data, at 2.3 GPa the pressure shift of what was originally assigned the [040] diffraction line no longer fits the monoclinic unit cell calculated by least squares. This is a new diffraction peak which comes in at 2.3 GPa. The discontinuity in the unit cell volume and the appearance of a new diffraction line indicate that a phase transformation has occurred around 2.2 GPa. The diffraction lines measured at 2.3 GPa for the new high-pressure β phase can be indexed on the basis of a unit cell also with monoclinic symmetry. The new diffraction peak can be indexed as a [411] line. Because only a limited number of diffraction lines were observed, a definitive space group assignment was not possible. No hkl ) 2n or 0k0 lines were detected which could indicate possibly a monoclinic primitive cell, with a space group corresponding to P* or P*/*. The reverse transition is observed when the pressure is decreased. The [040] diffraction line reappears when the pressure is lowered below 2.2 GPa. The total volume compression of ADN to 4.9 GPa is 25% ( 3% and includes the volume reduction for the R to β transformation which occurs between 1.9 and 2.8 GPa. The volume of R-ADN is reduced by 15% ( 2% up to the transition. The volume change at the transition is 5% ( 2%. The compression of the β phase to 4.9 GPa is 5% ( 3%. At temperatures above 120 °C between 2.0 and 10.0 GPa, a definite change in birefringence is observed in single crystals of ADN under polarized white light. The change appears as a color wave progressing through the crystal very rapidly, while at the same time the single crystal becomes polycrystalline.

Reaction Phase Diagram for Ammonium Dinitramide

J. Phys. Chem., Vol. 100, No. 8, 1996 3251 at temperatures between 120 and 140 °C, where ADN forms AN (I) and N2O. The total volume compression for ADN to 4.9 GPa, including the transition from R to β phase, is 25% ( 3%. The volume reduction due to the R to β transformation is about 5% ( 2%. Acknowledgment. The authors wish to acknowledge financial support from the Office of Naval Research and the Naval Surface Warfare Center Independent Research Program. References and Notes

Figure 5. Infrared (FTIR) absorption spectra for (A) β-ADN at 2.5 GPa and RT, (B) β-ADN at 2.5 GPa and 125 °C, (C) products from part B at 2.5 GPa and 145 °C, (D) products from part C at ambient pressure and RT, and (E) ammonium nitrate at ambient pressure and RT.

Figure 5 shows infrared (FTIR) spectra used to provide additional confirmation of the irreversible chemical reaction in ADN. Spectrum A shows β-ADN at 2.5 GPa and room temperature. Spectrum B, at 2.5 GPa and 125 °C, shows no significant change in β-ADN. However, at 145 °C and 2.5 GPa, spectrum C shows a fundamental change in the FTIR pattern which is irreversible on decreasing the temperature to 50 °C. There is the appearance of a large absorption peak at 2223 cm-1 associated with N2O. Because its peak intensity is large relative to the intensity scale in spectrum C, only the peak edge is shown here. If pressure is decreased to ambient, the relatively large absorptions in the region of 2223 cm-1 disappear (spectrum D). As the pressure is reduced to 1 atm, the N2O comes off as a gas and the absorptions in the 2223 cm-1 region go to zero. The residue has been identified as ammonium nitrate (spectrum E). Thus, an irreversible chemical reaction forming AN (I) and N2O takes place in the 2.0-10.0 GPa pressure range between 120 and 140 °C. Conclusions The pressure, temperature reaction phase diagram for ADN has been determined up to 10.0 GPa between -125 and 120 °C. A high-pressure reversible phase transition in R-ADN has been discovered which occurs at about 2.0 ( 0.2 GPa. The new high-pressure phase, β-ADN, is monoclinic, and its existence has been confirmed by X-ray diffraction, FTIR, and micro-Raman scattering measurements. R-ADN melts at about 95 °C with the liquidus having a positive slope to 1.0 GPa. Melting is accompanied by thermal decomposition above 136 °C out to 1.0 GPa. At higher pressures between 2.0 and 10.0 GPa, an irreversible solid state molecular rearrangement occurs

(1) Bottaro, J. C.; Schmidt, R. J.; Renwell, P. E.; Ross, D. S. World Intellectual Property Organization, International Application Number PCT/ US91/04268, December 26, 1991. (2) Gilardi, R. Private communication. (3) Doyle, R. J. Org. Mass Spectrosc. 1993, 28, 83-91. (4) Schmidt, R. J.; Krempp, M.; Bierbaum, V. M. Int. J. Mass Spectrosc. Ion Processes 1992, 117, 621. (5) Politzer, P.; Murrary, J. S.; Seminario, J. M.; Miller, R. J. Mol. Struct. 1992, 262, 155-170. (6) Russell, T. P.; Stern, A. G.; Koppes, W. M.; Bedford, C. D. CPIA Publication, Proceedings of the JANNAF Combustion Meeting, Hampton, VA, 19-23 October 1992. (7) Brill, T. B.; Brush, P. J.; Patil, D. G. Combust. Flame 1993, 92, 178. (8) Russell, T. P.; Tran, Y.; Gotezmer, C.; Bedford, C. D. CPIA Publication, Proceedings of the JANNAF Propulsion Meeting, Indianapolis, IN, Feb 1992. (9) Russell, T. P.; Miller, P. J.; Piermarini, G. J.; Block, S. Structure and Properties of Energetic Materials, Materials Research Society Symposium Proceedings, Boston, MA, Nov 30 to Dec 2, 1992; Liebenberg, D., Armstrong, R. W., Gilman, J. J., Eds.; Vol. 296. (10) Russell, T. P.; Miller, P. J.; Piermarini, G. J.; Block, S. To be submitted to J. Phys. Chem. (11) Certain trade names and products are mentioned in the text in order to adequately specify the experimental procedure and equipment used. In no case does such identification imply recommendation or endorsement by NIST nor does it imply that the product necessarily is the best available for the purpose. (12) Barnett, J. D.; Block, S.; Piermarini, G. J. ReV. Sci. Instrum. 1973, 44, 1. (13) Block, S.; Piermarini, G. J. Phys. Today 1976, 29, 44. (14) Piermarini, G. J.; Block, S.; Barnett, J. D.; Forman, R. S. J. Appl. Phys. 1975, 46, 2774. (15) Miller, P. J.; Piermarini, G. J.; Block, S. Appl. Spectrosc. 1984, 38, 680. (16) Mauer, F. A.; Block, S.; Piermarini, G. J. American Crystallographic Association, Program and Abstracts of Summer Meeting, Boston University, Boston, MA, Aug 12-17, 1979, Paper G3. (17) Piermarini, G. J.; Block, S.; Miller, P. J. The Chemistry and Physics of Energetic Materials, NATO Advanced Study Institute, Altavilla Milicia, Sicily, September 3-15, 1989; Bulusu, S., Ed.; 1990; Vol. 309, pp 369412. (18) Miller, P. J.; Block, S.; Piermarini, G. J. Combust. Flame 1991, 83, 174. (19) Piermarini, G. J.; Block, S.; Barnett, J. D. J. Appl. Phys. 1973, 44, 5377. (20) Weir, C. E.; Lippincott, E. R.; Van Valkenberg, A.; Bunting, E. N. J. Res. Natl. Bur. Stand. 1959, 63A, 55. (21) Giessen, B. C.; Gordon, G. E. Science 1968, 159, 973. (22) Albritton, L. M.; Margrave, J. L. High Temp.-High Pressures 1972, 4, 13. (23) Buras, B.; Staun Olsen, J.; Gerward, L.; Selmark, B.; Lindegaard Andersen, A. Acta Crystallogr. 1975, A31, 327. (24) Nadler, M. Private communications.

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