The stable phases of the Cs₂CuCl_4-xBr_x mixed systems

N. van Well*†, F. Dinges†, S. Waldschmidt†,A. A. Haghighirad†, F. Ritter†,
W. Aßmus†, C. Krellner†

†Physikalisches Institut, Goethe-Universität Frankfurt am Main, Frankfurt am Main

Crystals of the Cs₂CuCl_4-xBr_x mixed system were grown using aqueous solution (typically at room temperature or at 50°C) or at far higher temperatures (around 690°C) using the Bridgman method[1]. One aim is to find out, if there is any influence of the growth method on the structural or physical properties of the samples. Especially, we want to clarify, whether a characteristic ordering of the Cl and Br atoms at the crystallographic nonequivalent halogen positions that we observed for the crystals grown from aqueous solutions takes place also in samples that are grown with the Bridgman method. In case that crystal growth is realised by way of aqueous solution the border compounds crystallises in an orthorhombic crystalline structure (space group Pnma). If the growth of the mixed systems will take place at room temperature, the existence of the orthorhombic phase will be interrupted in the intermediate composition range (x=1 – 2). Within this range a tetragonal phase is found (space group I4/mmm). However, if the growing temperature is increased to 50°C, the orthorhombic crystalline structure is obtained for the whole Cs₂CuCl_4-xBr_x mixed system.[2] Crystal growth by using the Bridgman method was only applied within the Br-rich segments of the Cs₂CuCl_4-xBr_x mixed system.

Another question concerns phase stability and stems from the marked polymorphism of the Cs₂CuCl_4-xBr_x mixed system series. The triangular frustrated orthorhombic phase is obtained within the whole concentration range from aqueous solutions, if the growth temperature does not fall below a certain composition dependent stability limit. To ensure reliable characterization results of this phase in low temperature experiments we need to know the preparative conditions that have to be fulfilled to prevent its transformation in other structural modifications. Furthermore, an additional objective of our investigation is to prove the phase stability of the Cs₂CuCl_4-xBr_x mixed system at low temperatures (up to 20K). The results show that the orthorhombic phase will remain stable for the whole Cs₂CuCl_4-xBr_x mixed systems. The same applies for the whole tetragonal phase of the investigated Cs₂CuCl_4-xBr_x mixed systems at low temperatures. Furthermore, we applied substitutions not only on the halogen sites of this compound series, but also on its cationic site of the Cs₂CuCl_4-xBr_x mixed system. First experiments have been carried out with Rb and K as substitutes for Cs. We will discuss the influence of the substitution at the alkali position of the Cu-coordination, which will be of particular importance for further investigations. This project was supported by Deutsche Forschungsgemeinschaft (SFB/TRR 49).

[1] Ono et al., Phase Transitions and Disorder Effects in Pure and Doped Frustrated Quantum Antiferromagnet Cs2CuBr4, Journal Physical Society Japan 74, 2004

[2] N. Krueger et al., Crystal Growth & Design, Vol. 10, Issue 10, pp 4456-4462 (2010)

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