?-d system Berezinskii-Kosterlitz-Thouless phase transition Heisenberg Heisenberg exchange Hamiltonian Prussian blue analogues S = 1/2 XXZ model antiferromagnetic coupling antiferromagnetism chain coordination polymers copper(II) critical behaviour crystal structure cyano bridge cyclam dioxothiadiazole effect of high pressure exact diagonalization inelastic neutron scattering magnetic conductor magnetic properties magnetism magnetocaloric effect manganese(III) molecular magnetism molecular magnets octacyanometallates octacyanotungstate(V) phase diagram quantum magnet