Formation of Laves Phases in Repulsive and Attractive Hard Sphere Suspensions

TL;DR

This study demonstrates the formation of colloidal Laves phases in binary hard sphere mixtures, showing that their crystallization can be accelerated and improved by adding non-adsorbing polymer, with implications for photonic materials.

Contribution

First experimental observation of Laves phases in hard sphere binary mixtures, confirming theoretical predictions and showing effects of attractive interactions on crystallization.

Findings

Laves phases of MgZn2 structure form over weeks to months.

Addition of non-adsorbing polymer accelerates crystallization.

Attractive interactions improve crystal quality.

Abstract

Colloidal Laves phases (LPs) of $MgCu_2$ type are promising precursors for diamond structure photonic materials. They have been predicted for hard sphere binary mixtures, but not yet observed. We here report a time resolved static light scattering study on their formation in a binary mixture of buoyant experimental hard sphere approximants (size ratio $\Gamma=0.77$, molar fraction of small spheres $x_S = 0.76$) for volume fractions between melting and the glass transition. In line with theoretical expectation, all samples form LPs of $MgZn_2$ structure on the time scale of weeks to months. $MgNi_2$ structures are absent, $MgCu_2$ structures and randomly stacked LPs prevail at elevated volume fraction. The addition of small amounts of non-adsorbing polymer switches the interaction to depletion attractive and results in significantly accelerated crystallization kinetics and improved crystal quality.