Crystal structure and properties of phosphate PbCu2(PO4)2 with spin-singlet ground state -: art. no. 024429

Phosphate PbCu2(PO4)(2) was found in the Pb-Cu-P-O system. Its structure parameters were refined by the Rietveld method using synchrotron x-ray diffraction data [space group Pccn (No. 56); Z=8; a=8.03810(10) A, b=15.46574(14) A, and c=10.36820(10) A]. PbCu2(PO4)(2) is isotypic with SrCu2(PO4)(2). Magnetic properties of PbCu2(PO4)(2) were investigated by magnetic susceptibility, magnetization up to 65 T, electron-spin resonance, and specific-heat measurements. Magnetic susceptibilities were fit by a linear four-spin cluster model, Cu1-Cu2-Cu2-Cu1 [Hamiltonian: -2J(1)(S1S2+S3S4)-2J(2)S(2)S(3)], with -2J(1)/k(B)=97.0 K for Cu1-Cu2 and -2J(2)/k(B)=86.5 K for Cu2-Cu2. The spin gap (Delta/k(B)) estimated from the high-field magnetization was 43 K. The g values of PbCu2(PO4)(2), g(//)=2.167 and g(perpendicular to)=2.222, suggest the 3z(2)-r(2) type of the orbital ground state for Cu2+ ions. Specific heat showed that PbCu2(PO4)(2) does not undergo a long-range magnetic order transition down to 0.45 K in agreement with its spin-singlet ground state.