CRYSTAL STRUCTURE OF HIGH PRESSURE PNNH. Rosemary E. Gerald Pacalo, Kurt D. Leinenweber and Paul F. McMillan, Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604

The crystal structure of a high pressure form of PNNH (phospham) has been determined from Rietveld refinement of xray diffraction data. The new phase was synthesized at 55 kbar and 750deg.C from an amorphous starting material and is recoverable at ambient conditions. Despite numerous attempts to grow single crystals of the high pressure phase, only polycrystalline samples were produced. High pressure PNNH is monoclinic with a=9.3g84(2)Å, b=4.8188(l)Å, c=12.9713(3)Å and [[beta]]=91.855(2)deg.. In this setting the axes are approximately coincident with the axes of the low pressure tetragonal form, although the monoclinic a- and c-axes are nearly double the length af their tetragonal counterparts, resulting in a four-fold increase in unit xll volume. To preserve the similarity to the low pressure cell, the new phase was refined in space group B2.

Both forms of PNNH consist of (PN₄)^-7 tetrahedra that share all corners with neighboring tetrahedra in a cristobalite arrangement. Tetrahedra in the low pressure phase are reported to be regular, although half of the nitrogen atoms are also hydrogen bonded. The high pressure phase has more distorted tetrahedra, with the four P-N bond lengths differing by 4 - 10%. The hydrogen atoms have not yet been located. The relationship between the high pressure phase, the low pressure phase and the cubic aristotype, Fd3m, wil1 be described in terms of tilting and distortion of the tetrahedra from their ideal (cubic) arrangement. Based on the similarity of xray powder diffraction patterns, the high pressure phase of PNNH appears to be an analogue of he unquenchable SiO₂ cristobalite phase observed at 12 kbar and room temperature [1, 2].

1] Yeganeh-Haeri, A., Weidner, D.J., Parise, J., Ko, J., Vaughn, M.T.,

Liu, X., Zhao, Y., Wang, Y. and Pacalo, R. (1990) Eos Trans. Am.

Geophys. Union, 71,1671.

3] Palmer, D.C. and Finger L.W. (19g4) Amer. Mineral., 79, 1-8.