Doping-induced structural transformation in the spin-1/2 triangular-lattice antiferromagnet Na2Ba1-xSrxCo(PO4)2

Abstract

The effects of Sr doping on the structural properties of Na2BaCo(PO4)2, a spin-1/2 triangular-lattice antiferromagnet as a quantum spin liquid candidate, are investigated by complementary x-ray and neutron powder diffraction measurements. It is found that in Na2Ba1−xSrxCo(PO4)2 (NBSCPO), the trigonal phase (space group P3̅m1) with a perfect triangular lattice of Co2+ ions is structurally stable when the doping level of Sr is below 30% (x≤ 0.3), while a pure monoclinic phase (space group P21/a) with slight rotations of CoO6 octahedra and displacements of Ba2+/Sr2+ ions will be established when the Sr doping level is above 60% (x≥ 0.6). Such a doping-induced structural transformation in NBSCPO is supported by first-principles calculations and Raman spectroscopy. Na2SrCo(PO4)2, a novel spin-1/2 antiferromagnet with glaserite-type structure, although monoclinically distorted, exhibits no long-range magnetic order down to 2 K and a similar negative Curie-Weiss temperature as Na2BaCo(PO4)2 with a perfect triangular lattice, suggesting the robustness of magnetic exchange interaction against the Ba/Sr substitutions.