³¹P NMR study of Na₂CuP₂O₇: an S = 1/2 two dimensional Heisenberg antiferromagnetic system

Abstract

The magnetic properties of Na 2 CuP 2 O 7 were investigated by means of 31 P nuclear magnetic resonance (NMR), magnetic susceptibility, and heat capacity measurements. We report the 31 P NMR shift, the spin-lattice (1/T 1 ), and spin-spin (1/T 2 ) relaxation-rate data as a function of temperature T . The temperature dependence of the NMR shift K(T ) is well described by the S = 1/2 square lattice Heisenberg antiferromagnetic (HAF) model with an intraplanar exchange of J/k B ≃ (18 ± 2) K and a hyperfine coupling A = (3533 ± 185) Oe/µ B . The 31 P NMR spectrum was found to broaden abruptly below T ∼ 10 K signifying some kind of transition. However, no anomaly was noticed in the bulk susceptibility data down to 1.8 K. The heat capacity appears to have a weak maximum around 10 K. With decrease in temperatures, the spin-lattice relaxation rate 1/T 1 decreases monotonically and appears to agree well with the high temperature series expansion expression for a S =1/2 2D square lattice.

Magnetic model forA2CuP2O7(

“…6, with the fitting parameters listed in Table II. For both Na 2 CuP 2 O 7 and Li 2 CuP 2 O 7 , the square-lattice model apparently fails to describe χ(T ) below 10 K. In contrast, the NN chain model provides a much better description of the data down to 2 K. This contradicts the previous conclusion by Nath et al, 19 although our susceptibility data for Na 2 CuP 2 O 7 are very similar to those previously reported. The discrepancy originates from different model expressions used in our analysis.…”

“…Compared to a planar arrangement of the plaquettes in other quasi-1D Cu +2 phosphates, such as K 2 CuP 2 O 7 exhibiting a pronounced quasi-1D magnetic behavior, 21 the tilting could enhance the interchain couplings and might be responsible for the proclaimed quasi-2D magnetism. 19,20 On the other hand, CuSe 2 O 5 with a similar tilting angle as in Na 2 CuP 2 O 7 was clearly shown to be of 1D type, 34 although the distances between the bc-planes are significantly smaller and even the chains within the bc-plane are somewhat closer together than in Na 2 CuP 2 O 7 . Accordingly, it could be expected from structural considerations that Na 2 CuP 2 O 7 is a quasi-1D system.…”

“…Although Salunke et al 20 claim that "the interchain interactions in Na , and should be considered either as a quais-1D or as a threedimensional (3D) system, in contrast to the 2D behavior proposed in the experimental study. 19 While the discrepancy between the computational and experimental results is actually resolved on the experimental side (Sec. IV B), we also check for possible shortcomings of our calculations, and supplement our model estimate of J AFM i with the full exchange couplings J i obtained from LSDA+U calculations.…”

“…However, the magnetic dimensionality is hard to decide a priori, because both intrachain and interchain couplings involve complex Cu-O-O-Cu superex-change pathways that can not be assessed in terms of simple empirical considerations, such as the Goodenough-Kanamori-Anderson rules. [16][17][18] The experimental data for Na 2 CuP 2 O 7 reported by Nath et al 19 did not fit to the 1D spin model, and showed better agreement with the 2D scenario. Later, Salunke et al 20 performed electronic structure calculations and argued that the leading couplings run along the structural chains, but the interchain couplings are stronger than in other quasi-1D compounds (e.g., in the chemically related K 2 CuP 2 O 7 presented in Ref.…”

Pyrophosphate-Bridged CuII Chain Magnet: {[Na3Cu(P2O7)(NO3)]·3H2O}n

“…These structural differences support the 2D spin dynamic reported in Na 2 Cu(P 2 O 7 ). 34,35 The difference in the magnitudes of 2J for the two compounds is significant; however, the support of the models on which the calculations are based renders a straightforward interpretation of the differences rather doubtful.…”