Summary of Collaborative Investigation – Na 5 ACu 4 (AsO 4 ) 4 Cl 2 (A = Rb, Cs) Jeffrey Clayhold, Miami University, USA Shiou-Jyh Hwu, Clemson University, USA Raivo Stern & Toomas Rõõm, National Institute of Chemical Physics & Biophysics, Estonia Sergei Zvyagin, Marcelo Jaime &Neil Harrison National High Magnetic Field Laboratory, USA

Na 5 ACu 4 (AsO 4 ) 4 Cl 2 (A = Rb, Cs) 2 Na 2 O + 4 CuO + 2 As 2 O 5 + NaCl + ACl  Na 5 ACu 4 (AsO 4 ) 4 Cl 2 High-Temperature, Solid-State Synthesis 750 o C in carbon-coated fused silica 2 mm Absorbance [Arb. Unit] Energy [eV] Cu(OH) 2 Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 Na 5 CsCu 4 (AsO 4 ) 4 Cl 2 LMCT d-d transition Hwu, et al. J. Am. Chem. Soc. 2002, 124,

Na 5 ACu 4 (AsO 4 ) 4 Cl 2 (A = Rb, Cs): A Composite Structure of Alternating Magnetic and Insulating Slabs ACl / Na 6 O 8 Cu II -As-O b Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 (Fmmm, no. 69) a = (2) Å b = (2) Å c = (2) Å V = (3) Å 3 Z = 8 Cu-As-O ACl/Na 6 O 8

Tetrameric  -oxo [Cu 4 O 12 ] 16- Clusters Cu-O: ÅCu-Cl: 2.78Å  Cu-O-Cu: o o Cl a a c Cu O As

 = -86 K g = Temperature-Dependent Magnetic Susceptibility and Specific Heat Measurements A second-order phase transition Clayhold, et al. Phys. Rev. B 2002, 66,

Field Dependent Specific Heat Measurements Specific heat per temperature unit (C/T) for H along b-axis Notice the peak (marking T N ) shifting to lower temperatures as applied field grows

Field Dependence of T N from Specific Heat and NMR Measurements Field dependence of the Neel temperature (T N ) for low fields

Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 : Tunable-Frequency Electron Spin Resonance Measurements (i)Direct observation of the gap in the AFM ordered phase (Δ ~ 18 K) (ii) Potential evidence of the “gigantic” Dzyaloshinskii- Moriya interaction Zvyagin, et al., unpublished

Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 : Far-infrared spectroscopy (i)Low frequency phonon modes (ii)No structural phase transition below 60K T.Rõõm, U. Nagel, D. Hüvonen

Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 : Polarization dependence (i)Optical properties are isotropic in the ac-plane T.Rõõm, U. Nagel, D. Hüvonen Step 15deg

Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 : FIR in magnetic field T. Rõõm, et al.

Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 : magnetization up to 40T Change of slope around 15T at 0,5K Stern, et al., unpublished

Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 : magnetization up to 40T No change of slope around 15T at 20K? Stern, et al., unpublished

Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 : magnetization up to 40T Temperature dependence for B in ac plane Stern, et al., unpublished

Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 : magnetization and ESR Please ignore the black line - will check its accuracy Raivo Stern, et al., unpublished

Spin-exchange Interactions between Neighboring Cu 2+ Cations Spin-exchange coupling constants: superexchange: J a  6 meV, J c  3 meV super-superexchange: J’ a  6 meV, J’ c  5 meV a c 3.23 Å 3.24 Å 4.26 Å 4.19 Å

Partial Structure of Salt Lattice rarely seen Na 6 O 8 cluster

Connectivity at Covalent/Ionic Interface Na 6 O 8 cluster in Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 Na -O = 2.31 Å Å Na ···Na = 3.51 Å Å

Environment of Na + Cations

Resolved Na + Sites in RT MAS NMR Stern, et al., unpublished

K 72.4K 85.6K 86.3K 87.4K 104.3K 106.3K 107.4K 139.0K 203.5K 294.5K ppm Low-Temp Na MAS NMR

A single Rb + Site in RT MAS NMR Stern, et al., unpublished

Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 : single crystal 87 Rb NMR (i)Direct observation of the transition into the AFM ordered phase (T N = 14.3 K in B 0 = 14.1 T) (ii) The complicated 87 Rb NMR line shape suggests a complicated, incommensurate (IC) ordered phase (iii) A plane-wave modulated IC spin state? Stern, et al., unpublished B 0 = 14.1 T || [010]

87 Rb Quadrupolar Coupling

87 Rb NMR Linewidth

Temperature [K] PMLT1 LT2 AFM    Tentative Phase Diagram