AAg 2 M[VO 4 ] 2 with A = Sr 2+ or Ba 2+ present a series of layered compounds featuring a triangular lattice of transition metal cations, M = Co 2+ or Ni 2+ , connected via nonmagnetic ortho-vanadates, which provide the magnetic superexchange within the layers. For this series of insulating compounds, ferromagnetic long-range order below 10 K is suggested by magnetization and specific heat measurements and confirmed by neutron diffraction experiments. We have investigated the impact of the spacer size of A 2+ separating the layers leading to a tilting of the vanadates and consequently inducing a change in the effective magnetic correlations. Magnetization and specific heat measurements corroborate the important dependence of the magnetic superexchange on the orientation of the vanadates and the respective spin system. Furthermore, the ground state properties of the spin systems, S = 1 (Ni 2+ ) and S = 3/2 (Co 2+ ) in their respective octahedral coordination of oxygen, are evaluated. Calculated magnetic moments of the single ion complexes agree well with the magnetic structure. We, furthermore, report the dependence of T c on applied isotropic pressure suggestive of a pressure effect on the effective ferromagnetic exchange coupling constants. In addition spectroscopic investigations probing the electronic structure of the [MO 6 ] complexes and the vibrational structure of the [VO 4 ] units are given.

By application of flux growth methods in combination with redox reactions, single crystals of BaAg(2)Cu[VO(4)](2) can be synthesized. A new structure type (triclinic, P1, Z = 2, a = 5.448(2) Å, b = 5.632(3) Å, c = 14.393(6) Å, α = 94.038(9)°, β = 90.347(6)°, and γ = 118.195(5)°) has been found and will be described here. Structure-properties relationships have been investigated by spectroscopic methods (IR, UV-vis-NIR, ESR) and the electronic structure will be discussed within the angular-overlap model (AOM) for Cu(2+). Furthermore, we present the magnetization and specific heat data for BaAg(2)Cu[VO(4)](2) representing a Heisenberg spin system with exclusive super-super exchange (SSE) on a frustrated magnetic triangular lattice. Considerable antiferromagnetic (AFM) low-dimensional interaction is evident, and ferromagnetic-like long-range order sets in at ≈0.7 K.

AAg 2 Fe [VO 4 ] 2 with A = K or Rb was synthesized by solid state methods and characterized by thermodynamic properties measurements (Neél order at T N ≈ 3.0 K). The respective nuclear (a K,Rb = 5.48 Å, c K = 7.212 Å, c Rb = 7.357 Å, P3̅ , Z = 1) and magnetic structures were refined using neutron diffraction in applied magnetic fields. The results indicate enhanced structural stability of the P3̅ structure and the realization of a frustrated triangular lattice with antiferromagnetic XY-anisotropies. Two magnetic structures were identified: a helical and chiral Y-phase with a propagation vector of (1/3, 1/3, ≈ 0.39) and a commensurate up−up−down phase. These unique compounds offer convenient experimental access for optimizing the features and properties of ferroaxial multiferroic materials.

The series of compounds, AAg 2 Cr[VO 4 ] 2 with A = Ag, K, or Rb, are layered S = 3/2 triangularlattice (TL) systems where the magnetic exchange interactions between Cr 3+ (3d 3 ) ions are mediated by non-magnetic [VO 4 ] 3− entities. Here, the relative orientation of the vanadate is altered with respect to the TL as a function of the A-site which corresponds to an induced symmetry change of the [CrO 6 ]-complex. All members of this series of compounds belong to the class of frustrated TL antiferromagnets. We find that the distorted TL (A = Ag) exhibits collinear antiferromagnetic long-range order (LRO) at T N ≈ 10 K, whereas the high-symmetry cases (A = K, Rb) evade LRO in zero field down to 0.03 K, the lowest temperature of our experiments. The latter members of the series belong to the undistorted TL and are candidates for spin-liquid ground states presumably not related to Ising anisotropy or dimerization.

Experimental Realization of a Unique Class of Compounds: XY-Antiferromagnetic Triangular Lattices, KAg 2Fe[VO4]2 and RbAg2Fe[VO4]2, with Ferroelectric Ground States. -The title compounds are synthesized by solid state reaction of M2CO3 (M: K, Rb), AgVO3, and Fe2O3 (773 K, 3 d) and characterized by XRD, powder neutron diffraction, and magnetic measurements. The compounds crystallize in the space group P3 with Z = 1 and exhibit a frustrated triangular lattice with antiferromagnetic XY-anisotropies. These unique compounds offer convenient experimental access for optimizing ferroaxial multiferroic materials. -(AMUNEKE, N. E.; TAPP, J.; DE LA CRUZ, C. R.; MOELLER*, A.; Chem. Mater. 26 (2014) 20, 5930-5935, http://dx.