We report on 6,7 Li nuclear magnetic resonance measurements of the spin-chain compound LiCu2O2 in the paramagnetic and magnetically ordered states. Below T ≈24 K the NMR lineshape presents a clear signature of incommensurate (IC) static modulation of the local magnetic field consistent with an IC spiral modulation of the magnetic moments.7 Li NMR reveals strong phason-like dynamical fluctuations extending well below 24 K. We hypothesize that a series of phase transitions at 24.2, 22.5, and 9 K reflects a "Devil's staircase" type behavior generic for IC systems. LDA based calculations of exchange integrals reveal a large in-chain frustration leading to a magnetical spiral.Despite extensive efforts during the last decades spin ordering in frustrated S=1/2 quantum spin chains still remains a matter of broad activities. [1,2,3,4,5, 6] Rich phase diagrams with commensurate (C) and incommensurate (IC) phases, with spin-and charge ordering, dimerization, or superconductivity have been predicted. Most studies have been focused on various cuprates with corner-or edge-shared CuO 4 plaquettes. Edge-sharing of CuO 4 plaquettes leads to CuO 2 chains with a nearly 90• Cu-O-Cu bond angle causing a reduced nearest neighbor (nn) transfer and a next-nearest neighbor (nnn) transfer of similar size allowing frustration effects. IC spiral states driven by ferromagnetic (FM) nn exchange and in-chain frustration have been predicted theoretically for CuO 2 chain compounds such as Ca 2 Y 2 Cu 5 O 10 but discarded experimentally. [5, 6] The observation of in-chain IC effects in undoped quasi-1D cuprates is so far restricted by a sharp magnetic field driven C-IC transition observed in the spin-Peierls system CuGeO 3 at high magnetic fields. [7] Here we report on 6,7 Li nuclear magnetic resonance measurements (NMR) and local density (LDA) based analysis of the electronic and magnetic structure of the chain compound LiCu 2 O 2 . We show that the observed spontaneous magnetic order can be described by a spiral modulation of the magnetic moments. Independently, LDA calculations and a subsequent Heisenberganalysis reveal strong in-chain frustration driving spiral ordering in accord with the NMR data.LiCu 2 O 2 is an insulating orthorhombic compound [1,8,9, 10] with bilayers of edge-shared Cu 2+ -O chains running parallel to the b-axis separated by Cu 1+ planes. It exhibits a high-temperature antiferromagnetic(AFM)-like Curie-Weiss susceptibility χ(T ). Low-temperature χ(T ) and specific heat stufies [1,11] point to a series of intrinsic phase transitions at T ≈24.2 K, T ≈22.5 K, and T ≈9 K pointing to a complex multi-stage rearrangement of the spin structure. Magnetization studies performed in external fields up to 5 T did not reveal any signatures of field-induced transitions. µSR data[11] point to a broad distribution of magnetic fields at the muon stopping sites. LSDA(+U) calculations point to an FM in-chain ordering.[10] However, a simple FM ordering is in conflict with the µSR data[11] and the AFM dimer liquid picture. [1,12] Thus, to...

The synthesis, chemical, structural, and magnetic properties of alkali-metal compounds with filledskutterudite structure, NaFe 4 Sb 12 and KFe 4 Sb 12 , are described. X-ray and neutron diffraction and elemental analysis established the crystal structure without defects and disorder on the cation site. The temperature and pressure dependence of the cubic unit cell of NaFe 4 Sb 12 and the displacement parameter of Na are investigated. The electronic structure is calculated by density functional methods (LMTO, FPLO). Quantum chemical calculations (electron localization function) reveal the covalent character of both Fe-Sb and Sb-Sb interactions. Electronic structure calculations within the local density approximation exhibit a band ferromagnetic ground state and predict a half-metallic behavior. In contrast to isostructural alkaline-earth compounds (CaFe 4 Sb 12 and BaFe 4 Sb 12 ), the alkali-metal skutterudites are itinerant electron ferromagnets with small magnetic moments (Ϸ0.25 B / Fe atom) and T C Ϸ 85 K. Yet the paramagnetic moments of all four compounds are between 1.5 B and 1.7 B per Fe atom, indicating similar Stoner factors. Temperature-dependent 57 Fe and 121 Sb Mössbauer spectroscopies confirm the ferromagnetic state in the sodium compound with very small hyperfine fields at the iron and antimony sites.

The hyperfine coupling constants A on p. 3 of NaFe 4 Sb 12 are too large by a factor of 10. This error comes from the conversion of units (kOe and T). The correct hyperfine coupling constants are determined to be A 1 ÿ1:449 kOe= B above T C and A 2 ÿ1:155 kOe= B below T C . We apologize for this error and any confusion that they may have caused. The results and conclusions of our Letter remain unaffected.

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