Frustrated square lattice with spatial anisotropy: Crystal structure and magnetic properties of PbZnVO(PO4)2

Crystal structure and magnetic properties of the layered vanadium phosphate PbZnVO(PO4)(2) are studied using x-ray powder diffraction, magnetization and specific-heat measurements, as well as band-structure calculations. The compound resembles AA'VO(PO4)(2) vanadium phosphates and fits to the extended frustrated square-lattice model with the couplings J(1), J(1)' between nearest neighbors and J(2), J(2)' between next-nearest neighbors. The temperature dependence of the magnetization yields estimates of averaged nearest-neighbor and next-nearest-neighbor couplings, (J) over bar (1) similar or equal to -5.2 K and (J) over bar (2) similar or equal to 10.0 K, respectively. The effective frustration ratio alpha = (J) over bar (2)/(J) over bar (1) amounts to -1.9 and suggests columnar antiferromagnetic ordering in PbZnVO(PO4)(2). Specific-heat data support the estimates of (J) over bar (1) and (J) over bar (2) and indicate a likely magnetic ordering transition at 3.9 K. However, the averaged couplings underestimate the saturation field, thus pointing to the spatial anisotropy of the nearest-neighbor interactions. Band-structure calculations confirm the identification of ferromagnetic J(1), J(1)' and anti-ferromagnetic J(2), J(2)' in PbZnVO(PO4)(2) and yield (J(1)'-J(1)) similar or equal to 1.1 K in excellent agreement with the experimental value of 1.1 K, deduced from the difference between the expected and experimentally measured saturation fields. Based on the comparison of layered vanadium phosphates with different metal cations, we show that a moderate spatial anisotropy of the frustrated square lattice has minor influence on the thermodynamic properties of the model. We discuss relevant geometrical parameters, controlling the exchange interactions in these compounds and propose a strategy for further design of strongly frustrated square-lattice materials.