We show that materials based on the yavapaiite layered structure are of potential interest as realizations of a model quasi-two-dimensional triangular-lattice antiferromagnet. The structure type is such that magnetic ions occupy a regular or very slightly distorted triangular lattice in well separated layers. We report the magnetic susceptibility versus temperature behaviour of three compounds: , which has an equilateral triangular lattice, and and which both have isosceles triangular lattices. A comparison of the behaviour of these three compounds identifies the effect of distortion and the spin value on the properties of the triangular-lattice antiferromagnet. , which we have made for the first time, has S = 1/2, and may prove to be the best example of the S = 1/2 triangular-lattice antiferromagnet yet discovered.

In the crystal structures of CsFe(SO4)2, RbFe(SO4)2, and KFe(SO4)2, the magnetic Fe3+ ions form a triangular array in well separated layers. CsFe(SO4)2 and RbFe(SO4)2 may be regarded as realizations of the highly frustrated triangular lattice antiferromagnet, whereas KFe(SO4)2 is a suspected realization of the row model. The latter model is characterized by two couplings J′ and J, and for J′/J>0.5 forms a helical spin structure with an incommensurate repeat distance. The regular triangular lattice magnet may be described by the row model with J′=J, and its “120°” spin structure may be regarded as a special case of this helical structure. We have determined the low temperature (1.3 K) magnetic structures adopted by CsFe(SO4)2, RbFe(SO4)2, and KFe(SO4)2 by powder neutron diffraction. CsFe(SO4)2 and RbFe(SO4)2 adopt the expected 120° helical spin structure of the triangular lattice magnet, but KFe(SO4)2 does not adopt the expected incommensurate helical structure of the row model. Rather, it adopts a sine wave modulated structure. Possible reasons for this behavior are discussed.

No abstract

The total energy and electronic structure of lanthanum have been calculated in the bcc, fcc, hcp, and double hcp structures for pressures up to 35 GPa. The full-potential linearized-augmented-plane-wave method was used with both the local-density approximation ͑LDA͒ and generalized-gradient approximation ͑GGA͒. The correct phase ordering has been found by the GGA results, with lattice parameters and bulk moduli in good agreement with experimental data. The GGA method shows excellent agreement with experiment while the LDA results show some discrepancies. The calculated strain energies for the fcc and bcc structures demonstrate the respective stable and unstable configurations at ambient conditions. The calculated superconductivity properties under pressure for the fcc structure are also found to agree well with measurements. Both LDA and GGA, with minor differences, reproduce well the experimental results for the superconducting critical temperature T c .

No abstract