Crystal growth and magnetic properties of spin-$1/2$ distorted triangular lattice antiferromagnet CuLa$_2$Ge$_2$O$_8$

Abstract

CuLa$_2$Ge$_2$O$_8$ forms a distorted triangular lattice of quantum spin-1/2 Cu$^{2+}$ ions. A crystal growth method was developed using the traveling-solvent floating zone technique resulting in the synthesis of a large single crystal (4 mm$\times$4 mm$\times$10 mm). The crystal was characterized with regard to phase purity and crystallinity using powder X-ray diffraction, energy dispersive X-ray analysis and Laue diffraction, and found to be of excellent quality. The magnetic properties were characterized using dc-susceptibility, magnetization, and heat capacity measurements which revealed weak magnetic frustration with long-range magnetic order occurring below $T_N=1.14(1)$~K. The magnetic structure determined using neutron powder diffraction is a commensurate, noncollinear antiferromagnetic, different from the 120$^{\circ}$ order of an equilateral triangular antiferromagnet. The ordered moments lie in the {\bf bc}-plane, with components $m_b=0.50(3)$~$μ_{B}$ and $m_c= 0.73(5)$~$μ_{B}$ along the {\bf b}- and {\bf c}-axes respectively, giving a total ordered moment of $M_{total}$= 0.89(6)$μ_{B}/$Cu$^{2+}$ at 20~mK.

Figures (13)

• Figure 1: The crystal structure of CuLa$_2$Ge$_2$O$_8$ where the Oxygen atoms are removed for clarity. The Cu$^{2+}$ ions are displayed as blue spheres, Germanium atoms are represented as green spheres and Lanthanum as yellow spheres, b) The Cu$^{2+}$ ions form distorted triangular layers within the ac-plane where the distance between the neighbouring Cu$^{2+}$ ions varies from 5.14 to 5.17 Å. The smallest interlayer distance is 6.37 Å.
• Figure 2: Differential Thermal Analysis (DTA) (blue line, right axis) measured on phase pure poly-crystalline CuLa$_2$Ge$_2$O$_8$. The compound decomposes into La$_2$Ge$_2$O$_7$ and Cu$_2$O at a temperature of 1148 $^\circ$C before melting at a temperature range 1170-1180 $^\circ$C. The DDTA shown on the left axis is the derivative of DTA, where black arrows indicates the decomposition temperature.
• Figure 3: Phase diagram for the GeO$_2$-La$_2$O$_3$-CuO system. The colored dots (a,b,c,d) are the different solvent compositions tried during the crystal growth experiments.
• Figure 4: (a) Photograph of the as-grown crystal from the optimized TSFZ method, (b) Single crystalline part from the (a), (c) X-ray Laue diffraction image of the single crystal part shown in (b) with [010]-axis parallel to the incident X-ray beam. The growth direction is slightly off from the [010]-axis (by $\sim5^\circ$).
• Figure 5: Powder XRD pattern (open circles) measured on the crushed single crystal at room temperature. The red solid line represents the Rietveld refinement, with the green vertical bars showing the fitted Bragg peak positions, and the lower blue solid line represents the difference between the observed and calculated intensities.