Growth of single crystals of crowningshieldite (α-NiS) by chemical-vapour transport

Abstract

Single crystals of crowningshieldite (α-NiS) were synthesized in three runs via chemical-vapour transport (CVT) over a 20 cm horizontal gradient of 700 → 600 °C (No. 1) and 800 → 700 °C (No. 2 & 3) in evacuated fused silica capsules using natural millerite from Coleman mine in Sudbury, Ontario, Canada with the composition (Ni_0.976Fe_0.013Co_0.003)_Σ0.992S and I₂ as a transport reagent. Products were characterized via PXRD and SEM methods. Resultant crystals are euhedral, 10 to 400 μm in diameter, and occur as three distinct populations: 1) platy crystals exhibiting dominant pinacoid {$0001$} and minor dipyramid {$10\overline{1}1$}; 2) equant crystals with equal development of both the pinacoid {$0001$} and dipyramid {$10\overline{1}1$}; and 3) blocky crystals showing only the dipyramid {$10\overline{1}1$}. The largest crystals were produced at higher temperatures, but experiments lasting longer than 12 days did not result in significant additional crystal growth. In addition to euhedral crystals, anhedral droplets were observed to form during run No. 2 & 3 suggesting deposition as a liquid as opposed to solid crystals. Synthesis of crystals from a pure NiS compound as opposed to that of natural composition may improve the quality of the resulting crystals and better constrain the ideal growth temperature.