Characterization of quasi-one-dimensional S=1/2 Heisenberg antiferromagnets Sr 2Cu(PO 4) 2 and Ba 2Cu(PO 4) 2 with magnetic susceptibility, specific heat, and thermal analysis

Abstract

Properties of Sr 2Cu(PO 4) 2 and Ba 2Cu(PO 4) 2 having [Cu(PO 4) 2] ∞ linear chains in their structures with Cu–O–P–O–Cu linkages were studied by magnetic susceptibility ( T=2–400 K, H=100 Oe) and specific heat measurements ( T=0.45–21 K). Magnetic susceptibility versus temperature curves, χ( T), showed broad maxima at T M=92 K for Sr 2Cu(PO 4) 2 and T M=82 K for Ba 2Cu(PO 4) 2 characteristic of quasi-one-dimensional systems. The χ( T) data were excellently fitted by the spin susceptibility curve for the uniform S=1/2 chain (plus temperature-independent and Curie–Weiss terms) with g=2.153(4) and J/ k B=143.6(2) K for Sr 2Cu(PO 4) 2 and g=2.073(4) and J/ k B=132.16(9) K for Ba 2Cu(PO 4) 2 (Hamiltonian H= JΣS i S i+1 ). The similar J/ k B values were obtained from the specific heat data. No anomaly was observed on the specific heat from 0.45 to 21 K for both compounds indicating that the temperatures of long-range magnetic ordering, T N, were below 0.45 K. Sr 2Cu(PO 4) 2 and Ba 2Cu(PO 4) 2 are an excellent physical realization of the S=1/2 linear chain Heisenberg antiferromagnet with k B T N/ J<0.34% together with Sr 2CuO 3 ( k B T N/ J≈0.25%) and γ-LiV 2O 5 ( k B T N/ J<0.16%). Sr 2Cu(PO 4) 2 and Ba 2Cu(PO 4) 2 were stable in air up to 1280 and 1150 K, respectively.