The High-pressure Studies Of Ammonium Azide And Alkaline Earth Metal Azides

In this paper, the in-situ high-pressure synchrotron X-ray diffraction studies werecarried out using angle dispersive synchrotron radiation technique to investigate thehigh-pressure behaviors of ammonium azide and alkaline earth metal azides, Ramanscattering, and infrared absorption are performed on by using diamond anvil cells.The structural elastic properties and the phase transitions of these azides under highpressure are discussed as below:1. high-pressure in situ X-ray powder diffraction study on NH4N3have beenconducted up to50.5GPa. The compressibility of orthorhombic phase is isotropic dueto the orientation of azide anions. The hydrogen bond weaken with increasingpressure due to the bending of N H…N bond, leading to the increase of N H stretchfrequency and rotation of azide anions at2b and4h Wyckoff positions up to2.9GPa.The rotation of azide anions obviously influences the intermolecular interactionsalong c axis in orthorhombic phase. The pressure induced phase transition involves aproximity of a and c, temporally assigned as a reversible second-orderorthorhombic-to-tetragonal transition. The bulk modulus of the orthorhombic phasesare determined to be B0=24.5±3.5GPa, B0’=3.4±3.2. NH4N3has also beenstudied by high pressure IR absorption and Raman scattering up to20GPa and22GPa at room temperature. In order to further analyze the experimental spectra,particularly for the far-and mid-IR modes, we calculated the vibrational spectra atambient conditions by using the CASTEP code. From the high pressure IR andRaman studies, a pressure-induced phase transition occurred at2.9GPa. All IR-activevibrational modes maintained their identity at the high pressure phase, indicating thatNH4N3still existed in the form of ammonium cations and azide anions which were linked by the hydrogen bond. From the intensity variation of the torsional mode at420cm-1and the frequency shift of the N H symmetric stretch, we determined thatthe hydrogen bonding energy weakened with the changing of pressure from zero to2.9GPa, then strengthened as the pressure further increased to12GPa, and finallyweakened again as pressure increased up to22GPa. The change at2.9GPa is inducedby a structure phase transition, whereas the change at12GPa is caused by therotational or bending behavior of the azide ions.2. The high pressure structural and elastic properties of calcium azide (Ca(N3)2)has been investigated using in situ high-pressure X-ray diffraction, Raman scattering,far-IR absorption and mid-IR absorption in a diamond anvil cell up to54,19,22and31GPa, respectively. No phase transition occurred in the pressure ranges we achievedin this study. The orientation, rotation and bending of azide anions obviouslyinfluences the compressibility properties of Ca(N3)2. The measured zero-pressure bulkmodulus is higher than the alkali metal azides, which is ascribed to their differentionic character in the metal-azide bond. The vibrational spectra of Ca(N3)2at ambientpressure have also been calculated using CASTEP to accomplish the assignments ofall vibration modes. In the high-pressure vibration studies, several external modes andinternal bending modes (ν2) of N3ions soften from ambient pressure to~7GPa or gointo reverse from~7GPa to higher pressure. These evidences coincide with thechange in the FE fEdata of the XRD result, in which the curve presents a turningpoint at7.1GPa. The compression, rotation and bending of azide ions alternate as themain behavior under pressure, which may responsible for these changes. Because thevariation of the bond energy and the distance between the atoms would lead to themigration of the electron cloud, this change may induced by an electronic phasetransition.3. The high pressure structural and elastic properties of calcium azide (Sr(N3)2)has been investigated using in situ high-pressure X-ray diffraction up to33.5GPa. Nophase transition occurred in the pressure ranges we achieved in this study. The orientation, rotation and bending of azide anions obviously influences thecompressibility properties of Ca(N3)2. The measured zero-pressure bulk modulus ishigher than the alkali metal azides, which is ascribed to their different ionic characterin the metal-azide bond.4. The high pressure synchrotron X-ray diffraction study of Ba(N3)2has beenconducted up to28GPa at room temperature. The cell parameters a, b, and c of theambient pressure phase shrink with different rates of99.47%,99.27%, and99.95%respectively, which shows a considerable anisotropic compressibility of b> a> cunder pressure. The b is most compressible because all azide ions crystallize in the (010) planes and the interlayer direction is more compressive. The a is leastcompressible due to the large repulsions between the neighboring azide (i) anions andthe slip behaviors of the (001) planes. The repulsions between azide anions and theslip behaviors of the planes dominate the compressibility of Ba(N3)2. Above2.6GPa,several changes have been observed in the X-ray diffraction patterns, indicating theonset of a pressure-induced phase transition. Phase II is identified to be a monoclinicstructure and probably has the same space group as phase I. That is, this phasetransition is likely a monoclinic to monoclinic isostructural phase transition. AsBa(N3)2was compressed to11.8GPa, several new peaks started to emerge, whichmay indicate the beginning of another transition. With further compression, no othertransformation has been observed up to28.0GPa.