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. 2016 Oct 17:6:35362.
doi: 10.1038/srep35362.

Single crystal growth from separated educts and its application to lithium transition-metal oxides

F Freund  1 , S C Williams  2 , R D Johnson  2 , R Coldea  2 , P Gegenwart  1 , A Jesche  1
Affiliations

Single crystal growth from separated educts and its application to lithium transition-metal oxides

F Freund et al. Sci Rep. .

Abstract

Thorough mixing of the starting materials is the first step of a crystal growth procedure. This holds true for almost any standard technique, whereas the intentional separation of educts is considered to be restricted to a very limited number of cases. Here we show that single crystals of α-Li2IrO3 can be grown from separated educts in an open crucible in air. Elemental lithium and iridium are oxidized and transported over a distance of typically one centimeter. In contrast to classical vapor transport, the process is essentially isothermal and a temperature gradient of minor importance. Single crystals grow from an exposed condensation point placed in between the educts. The method has also been applied to the growth of Li2RuO3, Li2PtO3 and β-Li2IrO3. A successful use of this simple and low cost technique for various other materials is anticipated.

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Figures

Figure 1
Figure 1. Schematic description of the synthesis method (inset) and temperature profile.
Elemental Li and Ir are spatially separated in an open crucible. Upon heating in air Li initially forms solid LiOH that transforms to Li2O at T > 500 °C. Ir partially oxidizes to IrO2. The formation of α-Li2IrO3 takes place at T = 750–1050 °C. Single crystals grow from an exposed condensation point placed in between the educts.
Figure 2
Figure 2. Crystal growth equipment (crucible diameter 16 mm).
Arrangement of the materials before and after the growth process is depicted in (a,b), respectively. The rings with spikes are oriented like a spiral staircase in order to allow for nucleation at different positions with less intergrowth of the crystals. Formation of the largest α-Li2IrO3 single crystals is observed on spikes placed roughly 4 mm above the Ir starting material. (c) individual setup parts made from Al2O3 and (d) typical appearance of one of the lower spikes covered with α-Li2IrO3 crystals at the bottom side, scale bar 1 mm.
Figure 3
Figure 3. Sample quality and magnetic anisotropy of α-Li2IrO3.
(a) comparatively large single crystal (1.2 mm × 0.4 mm × 0.5 mm and m = 1.7 mg) grown from separated educts (scale bar 0.3 mm). The corresponding Laue-back-reflection pattern, depicted in (b), shows the (nearly) three-fold rotation symmetry perpendicular to the honeycomb layers. (c) temperature dependent specific heat of the single crystal shown in (a) in comparison with a typical polycrystalline sample. (d) an easy-plane anisotropy is apparent from the temperature dependent magnetic susceptibility (μ0H = 1 T, χab : H⊥c*, formula image).
Figure 4
Figure 4. X-ray diffraction pattern from three different α-Li2IrO3 crystals:
one un-twinned and without stacking faults (sample 1, panels (a,b)), one predominantly a single grain, but with significant stacking faults manifested in extended diffuse scattering along l (sample 2, panel c), and one multi-twin crystal (sample 3, panel d). Color is intensity on a log scale. Vertical arrows near k = 2 in panels b,c show direction along which the intensity is plotted in panel g, note the strong contrast between sample 1 with sharp peaks at integer l and sample 2 where diffuse scattering dominates. Bottom graphs (e,f,h) show the calculated X-ray diffraction pattern in the same axes as the above panels a,b,d, for the nominal monoclinic crystal structure of α-Li2IrO3. Panel h includes contribution from C± twins (grains rotated by ±120° around c* leading to the peaks at fractional coordinates (0, k, n ± 1/3), with n integer and k = ±2, ±4) and an A-type twin responsible for the peaks at (0, k, ±1) with k odd (see Supplementary Note for details).
Figure 5
Figure 5. Further materials synthesized from separated educts.
(a) β-Li2IrO3 single crystal (scale bar 0.2 mm) and (b) a single crystal of Li2RuO3 (scale bar 0.1 mm). For Li2PtO3 fine yellow powder could be obtained shown in (c) (scale bar 1 mm).
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