q=0 long-range magnetic order in centennialite CaCu₃(OD)₆Cl₂?0.6D₂O: A spin-1/2 perfect kagome antiferromagnet with J₁-J₂-J_d

Crystal and magnetic structures of the mineral centennialite CaCu3(OH)(6)Cl-2 center dot 0.6H(2)O are investigated by means of synchrotron x-ray diffraction and neutron diffraction measurements complemented by density functional theory (DFT) and pseudofermion functional renormalization group (PFFRG) calculations. CaCu3(OH)(6)Cl-2 center dot 0.6H(2)O crystallizes in the P (3) over bar m1 space group and Cu2+ ions form a geometrically perfect kagome network with antiferromagnetic J(1). No intersite disorder between Cu2+ and Ca2+ ions is detected. CaCu3(OH)(6)Cl-2 center dot 0.6H(2)O enters a magnetic long-range ordered state below T-N = 7.2 K, and the q = 0 magnetic structure with negative vector spin chirality is obtained. The ordered moment at 0.3 K is suppressed to 0.58(2)mu(B). Our DFT calculations indicate the presence of antiferromagnetic J(2) and ferromagnetic J(d) superexchange couplings of a strength which places the system at the crossroads of three magnetic orders (at the classical level) and a spin-1/2 PFFRG analysis shows a dominance of q = 0 type magnetic correlations, consistent with and indicating proximity to the observed q = 0 spin structure. The results suggest that this material is located close to a quantum critical point and is a good realization of a J(1)-J(2)-J(d) kagome antiferromagnet.