Deformation of the triangular spin- 12 lattice in Na2SrCo(PO4)2

Abstract

Crystal structure and thermodynamic properties of Na 2 SrCo(PO 4 ) 2 , the chemical sibling of the triangular quantum spin-liquid candidate Na 2 BaCo(PO 4 ) 2 , are reported. From single crystal x-ray diffraction and highresolution synchrotron x-ray powder diffraction, the compound was found to crystallize in the monoclinic space group P2 1 /a at room temperature, in contrast to the trigonal Na 2 BaCo(PO 4 ) 2 . Above 650 K, the symmetry of Na 2 SrCo(PO 4 ) 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 CoO 6 octahedra beyond the trigonal distortion ubiquitous in this structure type and results in the stronger g-tensor anisotropy (g z /g xy = 1.6) as well as the increased XXZ anisotropy (J z /J xy = 2.1) compared to the Ba compound (g z /g xy = 1.1, J z /J xy = 1.5), while the average coupling strength, J av /k B = (2J xy + J z )/3k B 1.3 K, remains unchanged. The Néel temperature increases from 140 mK (Ba) to 600 mK (Sr), and an uncompensated in-plane moment of 0.066(4)μ B /f.u. appears. We show that the ordering temperature of a triangular antiferromagnet is capably controlled by its structural distortions.