Heat Capacities of the S = ¹/₂ Two-Dimensional Heisenberg Antiferromagnet Bis(2-amino-5-chloropyridinium) Tetrabromocuprate(II) [(5CAP)₂CuBr₄] and Its Diamagnetic Analogue [(5CAP)₂ZnBr₄]

Abstract

Heat capacities of the spin quantum number S = 1/2 two-dimensional Heisenberg antiferromagnet bis(2-amino-5-chloropyridinium) tetrabromocuprate(II) [(5CAP)2CuBr4] crystal and its nonmagnetic analogue, the bis(2-amino-5-chloropyridinium) tetrabromozincate(II) [(5CAP)2ZnBr4] crystal, were measured by adiabatic calorimetry. For the (5CAP)2ZnBr4 crystal, single-crystal X-ray diffraction was also performed. The (5CAP)2ZnBr4 crystal belongs to the orthorhombic space group Pbca, with a = 16.074(2) Å, b = 7.688(2) Å, c = 30.538(6) Å, and Z = 8. In the (5CAP)2CuBr4 crystal, an antiferromagnetic phase transition occurred at T N = 5.08 K, and a thermal anomaly arising from the short-range order characteristic of two-dimensional magnetic substances was found above T N; the heat capacity of the (5CAP)2ZnBr4 crystal showed no thermal anomaly. The enthalpy and entropy gains due to the magnetic transition were estimated to be 49.3 J mol-1 and 5.65 J K-1 mol-1, respectively. The value of the entropy gain coincides well with the R ln 2 (5.76 J K-1 mol-1, R stands for the gas constant) expected for S = 1/2 spin systems. The thermal anomaly observed above T N is well accounted for in terms of the S = 1/2 two-dimensional antiferromagneic Heisenberg model of a square lattice with J/k B = −4.3 K. Spin wave analysis of the magnetic heat capacities below T N suggests that the (5CAP)2CuBr4 crystal is in a three-dimensional antiferromagnetic state, which is realized by a weak interlayer magnetic interaction between the two-dimensional layers.

Two-dimensionalS=12Heisenberg antiferromagnets: Synthesis, structure, and magnetic properties

“…In a 3D QHAF with an Ising anisotropy, the three distinct susceptibility curves are χ , dropping toward zero, χ ⊥ , staying at or close to χ max , and the powder data, χ pow , which should fall in between the two previous curves (2/3 χ max ). Note that the onset of the three dimensional behavior in the single crystal data begins at a temperature (5.1 K) which is in excellent agreement with the critical temperature determined by specific heat studies 11 . The (5MAP) 2 CuBr 4 data shown in Figure 8(b) exhibit the same dimensional crossover behavior.…”

“…The model curves based on these parameters (J and g) have been extended to zero temperature (dashed lines) and it is noted that the experimental data break away from the theoretical curves at temperatures near 5.1(2) K for (5CAP) 2 CuBr 4 and 3.8(2) K for (5MAP) 2 CuBr 4 . Since the T N temperature of (5CAP) 2 CuBr 4 is known to be 5.08 K by specific heat measurements 11 , we can establish a close agreement between T N and the temperature of divergence between the experimental data and the theoretical prediction. On the basis of this comparison, we estimate the Néel temperature of (5MAP) 2 CuBr 4 to be 3.8(2) K.…”

“…Each copper atom is related to the next by the C-centering in the lattice thus removing any possibility that any two copper sites would experience different chemical environments. It is possible that there is a structural phase change as a function of temperature which would remove this symmetry, but no anomalies were observed in the specific heat study 11 of 5CAP except at T N . To conclusively determine whether spin canting exists in the 5CAP and 5MAP systems, a determination of the magnetic lattice by neutron scattering will have to be made.…”

Two-dimensionalXYbehavior observed in quasi-two-dimensional quantum Heisenberg antiferromagnets

“…Seven years ago some of the present authors published [10,11] several studies of the molecular-based quasi-2D QHAF family which have exchange constants of 6.5 and 8.5 K, respectively, and respective saturation fields of 19 and 24 T. Although these materials did have relatively large exchange ratios, J ′ /J ≈ 0.24, they did show the characteristic susceptibilities [10,11] and heat capacities [12] of the 2D QHAF and revealed for the first time the characteristic upward curvature of the low-temperature magnetization of the 2D QHAF. Preliminary reports of this discovery led to several theoretical studies of the magnetization [13,14] and gave motivation for further investigations of the 2D QHAF in large applied fields.…”

Thermodynamic study of ibuprofen by adiabatic calorimetry and thermal analysis

“…The molar heat capacities of the solid sample were fitted to the following polynomial of heat capacities versus reduced temperature (X) by means of the least square fitting, in the light of the method described in literature [10,12]. …”