- 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 P21/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=2.1) compared to the Ba compound (gz/gxy=1.1, Jz/Jxy=1.5), while the average coupling strength, Jav/kB=(2Jxy+Jz)/3kB≃1.3K, remains unchanged. The Néel temperature increases from 140 mK (Ba) to 600 mK (Sr), andCrystal 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 P21/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=2.1) compared to the Ba compound (gz/gxy=1.1, Jz/Jxy=1.5), while the average coupling strength, Jav/kB=(2Jxy+Jz)/3kB≃1.3K, remains unchanged. The Néel temperature increases from 140 mK (Ba) to 600 mK (Sr), and an uncompensated 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.…
