Synthesis and crystal structure of the <f>α</f> polytype of HfNBr

<f>α</f>-HfNBr has been prepared at 760 °C in a sealed evacuated fused silica tube by reaction between NH4Br and Hf followed by purification through chemical vapour transport under a temperature gradient. The crystal structure of this compound at room temperature has been determined for the first time by Rietveld refinement of X-ray powder diffraction data, electron diffraction and high resolution electron microscopy. It crystallises in the orthorhombic space group Pmmn with the unit cell parameters <f>a=4.1165(2)</f>, <f>b=3.5609(2)</f>, <f>c=8.6440(3)</f> A˚. <f>α</f>-HfNBr is isotypic to FeOCl and is built from layers of composition Br&z.sbnd;Hf&z.sbnd;N&z.sbnd;N&z.sbnd;Hf&z.sbnd;Br stacked along <f>c</f> that are separated by a Van der Waals gap. The hafnium atoms are six-coordinated by four nitrogen atoms and two bromine atoms. The resulting coordination polyhedron may be described as a highly distorted octahedron. Electron diffraction and high resolution electron microscopy studies reveal that the material is almost free of defects in the inner part of the crystals and confirm the structural model refined by the Rietveld method. [ABSTRACT FROM AUTHOR]