Deformation of the triangular spin-1/2 lattice in Na$_2$SrCo(PO$_4$)$_2$

Abstract

Crystal structure and thermodynamic properties of Na2SrCo(PO4)2, the chemical sibling of the triangular quantum spin-liquid candidate Na2BaCo(PO4)2, are reported. From single crystal x-ray diffraction and high-resolution synchrotron x-ray powder diffraction, the compound was found to crystallize in the monoclinic space group P 21/a at room temperature, in contrast to the trigonal Na2BaCo(PO4)2. Above 650 K, the symmetry of Na2SrCo(PO4)2 changes to C2/m, while around 1025 K a further transformation toward trigonal symmetry is observed. The monoclinic symmetry leads to a small deformation of the CoO6 octahedra beyond the trigonal distortion ubiquitous in this structure type, and results in the stronger g-tensor anisotropy (gz/gxy = 1.6) as well as the increased XXZ anisotropy (Jz/Jxy = 1.7) compared to the Ba-compound, while the average coupling strength, Jav = (2Jxy +Jz)/3 1.3 K, remains unchanged. The Néel temperature increases from 140 mK (Ba) to 600 mK (Sr), and a weak spin canting with the net in-plane moment of 0.066(4)µB/f.u. appears. We show that the ordering temperature of a triangular antiferromagnet is capably controlled by its structural distortions.