Piezoelectricity and crystal structure of α-nitrogen

Abstract

Nitrogen is known to exhibit three solid modifications dcpending on temperature and pressure, and whereas the structures of the fl and 7, forms are well established, there has been some doubt in the recent literature as to whether the low temperature e-phase belongs to space group P213 or Pa3. Since almost two dozen papers have been published on nitrogen during the past two years alone there is obviously much interest in the subject, and it is important to remove any uncertainty about the correct crystal structure. The most recent evidence for the P213 assignment comes from the careful single-crystal X-ray work of Jordan, Smith, Streib & Lipscomb (1964), who observed two faint reflections in violation of Pa3 symmetry. Recent X-ray work by Schuch & Mills (1970) was unable to confirm this. An electron diffraction study supports the Pa3 structure (Venables, 1970), and numerous infrared and Raman spectroscopy experiments (St. Louis & Schnepp, 1969; Brith, Ron & Schnepp, 1969; Cahill & Leroi, 1969; Anderson, Sun & Donkersloot, 1970), have found no evidence of the P213 structure, which because of its noncentrosymmetry would tend to produce coincidences between infrared and Raman frequencies. Theorists thus far have been unable to explain why the translational distortions of the P2~3 structure should occur for nitrogen as all the assumed crystalline potential models have led to a prediction of the equilibrium molecular locations in the Pa3 structure. All the recent calculations of lattice dynamics (Schnepp & Ron, 1969; Anderson, Sun & Donkersloot, 1970) have assumed a Pa3 structure and doubts have been raised that the true structure can be P213. One simple way to distinguish between the two space groups is to test for piezoelectricity in e-nitrogen, because