The crystallisation of strontium and barium molybdates and tungstates from lithium chloride melts by continuous cooling. The kinetics of crystal growth in alumina crucibles

Strontium and barium molybdate tungstate solutions in lithium chloride melts were crystallised in alumina crucibles; saturated solutions were cooled from initial temperature T 0 = 800 °C at cooling rates R T = 40°, 100° and 200°C hr −1 . The kinetics of the crystal growth were studied by chemical analysis of the solution and by optical microscopy measurements of the crystals. – Heterogeneous nuclei first grew to small crystallites (lenghts l from about 0.02 to 0.12 cm) during prolonged induction periods: the main crystal growth then started after the development of some excess solute concentration Δ C = 0.3 Δ C tot to Δ C tot (where Δ C tot was the total excess solute concentration that developed on cooling to the eutectic temperature) at temperature T . The main crystal growth, at grown times τ = 0 to τ*, was diffusion rate‐controlled. The average crystal lenghts generally varied with growth time according to the relation,where R c (= Å T ) is the rate of development of excess solute concentration.k Δ is the diffusion rate constant at temperatures near TK is ϱf N (where N is the crystal number): for the first forty percent crystallisation, this relation simplified toThe intial growth rates and l   τ 2 /τ ratios depended not on cooling rates but on the Δ C values (at the start of growth). The kinetics of crystallisation of barium molybdate (where Δ C = Δ C tot were similar to those for crystallisation from supersaturated solutions onto seeds at temperature T ; for these crystallisationswhere I D (α) is the Nielsen function for diffusion‐controlled growth. The last thirty percent growth was rate‐controlled by the rate of development of excess solute concentration only; in this range, crystal lenghts varied with growth time according to the relation,