One-Dimensional Quantum Magnetism in the S = 1/2 Mo(V) system, KMoOP2O7

We present a comprehensive experimental and ab-initio study of the \(S=1/2\) Mo\(^{5+}\) system, KMoOP\(_2\)O\(_7\), and show that it realizes the \(S = 1/2\) Heisenberg chain antiferromagnet model. Powder neutron diffraction reveals that KMoOP\(_2\)O\(_7\) forms a magnetic network comprised of pairs of Mo\(^{5+}\) chains within its monoclinic \(P2_1/n\) structure. Antiferromagnetic interactions within the Mo\(^{5+}\) chains are identified through magnetometry measurements and confirmed by analysis of the magnetic specific heat. The latter reveals a broad feature centred on \(T_\textrm{N} = 0.54\) K, which we ascribe to the onset of long-range antiferromagnetic order. No magnetic Bragg scattering is observed in powder neutron diffraction data collected at 0.05 K, however, which is consistent with a strongly suppressed ordered moment with an upper limit \(\mu_\textrm{ord} < 0.15 \mu_\textrm{B}\). The one-dimensional character of the magnetic correlations in KMoOP\(_2\)O\(_7\) is verified through analysis of inelastic neutron scattering data, resulting in a model with \(J_\textrm{1} \approx 34\) K and \(J_\textrm{2} \approx -2\) K for the intrachain and interchain exchange interactions, respectively. The origin of these experimental findings are addressed through density-functional theory calculations.