Deformed spin-1/2 square lattice in antiferromagnetic NaZnVOPO₄(HPO₄)

We report the structural and magnetic properties of a new spin-1/2 antiferromagnet NaZnVOPO₄(HPO₄) studied via x-ray diffraction, magnetic susceptibility, high-field magnetization, specific heat, and ³¹P nuclear magnetic resonance (NMR) measurements, as well as density-functional band-structure calculations. While thermodynamic properties of this compound are well described by the J₁-J₂ square-lattice model, ab initio calculations suggest a significant deformation of the spin lattice. From fits to the magnetic susceptibility we determine the averaged nearest-neighbor and second-neighbor exchange couplings of J₁ ≃ −1.3 K and J₂ ≃ 5.6 K, respectively, resulting in the effective frustration ratio α = J₂/J₁ ≃ −4.3 that implies columnar antiferromagnetic order as the ground state. Experimental saturation field of 15.3 T is consistent with these estimates if 20 % spatial anisotropy in J₁ is taken into account. Specific heat data signal the onset of a magnetic long-range order at T_N ≃ 2.1 K, which is further supported by a sharp peak in the NMR spin-lattice relaxation rate. The NMR spectra mark the superposition of two P lines due to two nonequivalent P sites where the broad line with the strong hyperfine coupling and short T₁ is identified as the P(1) site located within the magnetic planes, while the narrow line with the weak hyperfine coupling and long T₁ is designated as the P(2) site located between the planes.