Phase separation and frustrated square lattice magnetism of Na1.5VOPO4F0.5<

Abstract

Crystal structure, electronic structure, and magnetic behavior of the spin-1 2 quantum magnet Na1.5VOPO4F0.5 are reported. The disorder of Na atoms leads to a sequence of structural phase transitions revealed by synchrotron x-ray powder diffraction and electron diffraction. The hightemperature second-order α ↔ β transition at 500 K is of the order-disorder type, whereas the low-temperature β ↔ γ + γ transition around 250 K is of the first order and leads to a phase separation toward the polymorphs with long-range (γ) and short-range (γ ) order of Na. Despite the complex structural changes, the magnetic behavior of Na1.5VOPO4F0.5 probed by magnetic susceptibility, heat capacity, and electron spin resonance measurements is well described by the regular frustrated square lattice model of the high-temperature α-polymorph. The averaged nearestneighbor and next-nearest-neighbor couplings are J1 −3.7 K and J2 6.6 K, respectively. Nuclear magnetic resonance further reveals the long-range ordering below TN = 2.6 K in low magnetic fields. Although the experimental data are consistent with the simplified square-lattice description, band structure calculations suggest that the ordering of Na atoms introduces a large number of inequivalent exchange couplings that split the square lattice into plaquettes. Additionally, the direct connection between the vanadium polyhedra induces an unusually strong interlayer coupling having effect on the transition entropy and the transition anomaly in the specific heat. Peculiar features of the low-temperature crystal structure and the relation to isostructural materials suggest Na1.5VOPO4F0.5 as a parent compound for the experimental study of tetramerized square lattices as well as frustrated square lattices with different values of spin.

Tailoring a fluorophosphate as a novel 4 V cathode for lithium-ion batteries

“…The EPR spectrum of Na 1.5 V 4+ PO 5 F 0.5 was very intense, and the shape was in good agreement with a previous report33. However, the EPR signal of Na 1.5 V 3+ PO 4 F 1.5 was negligible, which can be explained by the fact that V 3+ , a non-Kramer's ion with integral J in LS coupling34, is silent under conventional EPR experimental conditions35.…”

“…The Hubbard parameters (GGA+U)54 were added to correct the incomplete cancellation of the self-interaction of GGA. A U value of 5.0 eV (the on-site coulomb term) and J value of 1.0 eV (the exchange term) were used for the vanadium ions5055 in Na 1.5 VPO 5 F 0.5 , LiNa 0.5 VPO 5 F 0.5 , and Na 0.5 VPO 5 F 0.5 . Activation barriers for lithium and sodium diffusion in the two model systems of Na 1.5 VPO 5 F 0.5 and LiNa 0.5 VPO 5 F 0.5 were calculated using the nudged-elastic-band (NEB) method5657.…”

Frustrated three-dimensional antiferromagnetLi2CuW2O8:Li7Ranjith1, Majumder2, Baenitz3 et al. 2015Phys. Rev. BSelf Cite19691View full textAdd to dashboardCite

“…Similar kind of T -dependence has been observed in several antiferromagnets 34,[42][43][44] and the decrease in 1/T 1 above T N is explained by the partial filtering of antiferromagnetic fluctuations from the neighboring spins. In those compounds, the probed nucleus is coupled to nearest neighbor spins having opposite directions.…”

A Family of High‐Performance Cathode Materials for Na‐ion Batteries, Na3(VO1−xPO4)2 F1+2x (0 ≤ x ≤ 1): Combined First‐Principles and Experimental Study

“…To date, only a few sodium–vanadium fluorophosphates, including Na 1.5 VOPO 4 F 0.5 , NaVPO 4 F, Na 3 V 2 (PO 4 ) 2 F 3 , and Na 3 V 2 O 2 (PO 4 ) 2 F, have been independently reported as minerals or cathode materials for electrochemical systems. Around the same time, our group and the Rojo group first recognized that some of these compounds belong to the same crystallographic system of Na 3 (VO 1− x PO 4 ) 2 F 1+2 x , where x = 0 corresponds to Na 1.5 VOPO 4 F 0.5 and x = 1 corresponds to Na 3 V 2 (PO 4 ) 2 F 3 .…”