The effects of Sr doping on the structural properties of Na$_{ 2}$BaCo(PO$_{ 4}$)$_{ 2}$, 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 Na$_{ 2}$Ba$_{ 1-x}$Sr$_{ x}$Co(PO$_{ 4}$)$_{ 2}$ (NBSCPO), the trigonal phase (space group $\mathit{ P}$$\bar{ 3}$$\mathit{ m}$1) with a perfect triangular lattice of Co$^{ 2+}$ ions is structurally stable when the doping level of Sr is below 30% ($\mathit{ x}$ $\le$ 0.3), while a pure monoclinic phase (space group $\mathit{ P}$2$_{ 1}$/$\mathit{ a}$) with slight rotations of CoO$_{ 6}$ octahedra and displacements of Ba$^{ 2+}$/Sr$^{ 2+}$ ions will be established when the Sr doping level is above 60% ($\mathit{ x}$ $\ge$ 0.6). Such a doping-induced structural transformation in NBSCPO is supported by first-principles calculations and Raman spectroscopy. Na$_{ 2}$SrCo(PO$_{ 4}$)$_{ 2}$, a novel spin-1/2 triangular-lattice 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 Na$_{ 2}$BaCo(PO$_{ 4}$)$_{ 2}$ with a perfect triangular lattice, suggesting the robustness of magnetic exchange interaction against the Ba/Sr substitutions.
Categories: cond-mat.str-el cond-mat.mtrl-sci