Deformed spin-$\frac12$ square lattice in antiferromagnetic NaZnVOPO$_4$(HPO$_4$)

We report the structural and magnetic properties of a new spin- 12 antiferromagnet NaZnVOPO 4 (HPO 4 ) studied via x-ray diffraction, magnetic susceptibility, high-field magnetization, specific heat, and 31 P nuclear magnetic resonance (NMR) measurements, as well as density-functional band-structure calculations. While thermodynamic properties of this compound are well described by the J 1 − J 2 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 ¯ J 1 (cid:39) − 1 . 3 K and ¯ J 2 (cid:39) 5 . 6 K, respectively. Experimental saturation field of 15.3 T is consistent with these estimates if 20% spatial anisotropy in J 1 is taken into account. Specific heat data signal the onset of a magnetic long-range order at T N (cid:39) 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 noneqivalent P sites where the broad line with the strong hyperfine coupling and short T 1 is identified as the P(1) site located within the magnetic planes, while the narrow line with the weak hyperfine coupling and long T 1 is designated as the P(2) site located between the planes.