Physicists from Augsburg University together with colleagues from Oxford report on a novel method for the growth of lithium-based transition metal oxides. Augsburg/PhG/KPP -The synthesis of ceramic crystals often requires very complicated methods. Starting materials in form of powders have to be mixed, pressed and pre-reacted in order to allow for single crystal growth from the melt at elevated temperatures. Or samples are grown from solution or chemical vapor transport in complex processes. However, so far none of the established methods yields single crystals of lithium iridate - despite the great interest in this material that was initiated by the prediction of highly unusual magnetic properties.

Now physicists from Augsburg succeeded in synthesizing α-Li2IrO3 single crystals by using a rather simple and unconventional method. The novel technique is based on isothermal vapor transport and doesn’t need any pre-reaction or special requirements. It works in hot air.


Lithium iridate: a high potential material

In the search for exotic magnetic ground states lithium iridate (α-Li2IrO3) recently attracted a lot of attention. It belongs to the so-called honeycomb iridates that are considered as hot candidates for the physical realization of a novel magnetic interaction. This Kitaev interaction - named after the mathematical physicist Alexei Yurevich Kitaev - couples neighboring magnetic moments in a very special way and could be highly relevant for quantum computing. So far, α-Li2IrO3 could be grown only in form of a fine powder. The lack of single crystals compromised a better understanding of this material. The method described by Prof. Dr. Philipp Gegenwart und Dr. Anton Jesche in Nature Scientific Reports allowed to grow such single crystals for the first time. Fur-thermore, it turned out to be versatile and has been applied to the growth of several other mate-rials.

Schematic of the growth setup. The desired single crystals grow from separated educts at 1020°C via vapor transport. The condensation takes place at spikes placed in between the starting materials.  © University of Augsburg/EP VISchematic of the growth setup. The desired single crystals grow from separated educts at 1020°C via vapor transport. The condensation takes place at spikes placed in between the starting materials. © University of Augsburg/EP VI


Single crystals: a prerequisite to a better understanding of materials

The availability of single crystals is of fundamental importance for solid state research: Single crystals allow for direct access to the anisotropy of physical properties. The crystal structure analysis by X-ray diffraction is significantly improved. And solving the magnetic structure is of-ten only possible by using single crystals of sufficient size and quality.


Beyond common crystal growth techniques

Novel materials are usually grown by standard techniques such as solid state reactions, growth from melt or solution or chemical vapor transport. However, nature is not always kind and sometimes classical methods do not lead to the desired result. “When all standard methods fail we have to come up with creativity and unconventional new ideas” says Philipp Gegenwart, the chair of Experimental Physics VI at the ‘Center for Electronic Correlations and Magnetism’ (EKM). And a novel solution was found in the Emmy Noether research group of Anton Jesche. Single crystals of α-Li2IrO3 were grown for the first time. The outstanding quality of these crys-tals was confirmed by collaborators from Oxford University who were able to determine the full magnetic structure.


The novel method: isothermal vapor transport from separated educts

The educts (starting materials) for the growth of α-Li2IrO3 are elemental lithium and iridium that are vertically separated. Upon heating to temperatures of 1020°C oxides and hydroxides form, move by vapor transport and react with each other. Single crystals grow from an exposed condensation point placed in between the educts (see Figure). The formation of α-Li2IrO3 itself drives the transport by maintaining a concentration gradient. Most remarkably, a temperature gradient is of minor importance and the whole process takes place in an open crucible in air. “This is a major difference to the established chemical vapor transport where the material trav-els in a special atmosphere in a closed container from the hot to the cold end” says Jesche.


Already solved: the magnetic structure of α-Li2IrO3

Resonant X-ray diffraction measurements performed on the single crystals allowed to resolve the magnetic structure of α-Li2IrO3. These results are presented in another joint publication by the physicists from Augsburg and their colleagues from Oxford University that appeared in Physical Review B. “Furthermore, our results show very interesting magnetic anisotropies” says Gegenwart.


Application to the single crystal growth of related compounds

It should be emphasized that the novel method is not restricted to the synthesis of α-Li2IrO3.
Gegenwart says: „Since the novel method was already used to grow related compounds like Li2RuO3 and the high-temperature phase β-Li2IrO3 we are convinced to find further applica-tions”.

Publications:

F. Freund, S.C. Williams, R.D. Johnson, R. Coldea, P. Gegenwart, A. Jesche, Single crystal growth from separated educts and its application to lithium transition-metal oxides. Sci. Rep. 6, 35362 (2016), http://www.nature.com/articles/srep35362

S.C. Williams, R.D. Johnson, F. Freund, S. Choi, A. Jesche, I. Kimchi, S. Manni, A. Bombardi, P. Ma-nuel, P. Gegenwart, R. Coldea. Incommensurate counterrotating magnetic order stabilized by Ki-taev interactions in the layered honeycomb α-Li2IrO3. Phys. Rev. B 93, 195158 (2016), http://journals.aps.org/prb/abstract/10.1103/PhysRevB.93.195158
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