Magnetism and Negative Magnetoresistance of Two Magnetically Ordering, Rare-Earth-Containing Zintl phases with a New Structure Type: EuGa2Pn2 (Pn = P, As)

Single crystals of EuGa(2)Pn(2) (Pn = P, As) were grown from a molten Ga flux and characterized by single-crystal X-ray diffraction at 100(1) K. They are isostructural and crystallize in a new structure type (monoclinic, P2/m, a = 9.2822(9) angstrom, b = 3.8967(4) angstrom, c = 12.0777(11) angstrom, beta = 95.5220(10)degrees, R1 = 0.0148, wR2 = 0.0325 (EuGa2P2) and a = 9.4953(7) angstrom, b = 4.0294(3) A, c = 12.4237(9) angstrom, beta = 95.3040(10)degrees, R1 = 0.0155, wR2 = 0.0315 (EuGa2As2)). The Structures consist of alternating layers of two-dimensional Ga(2)Pn(2) anions and Eu cations. The anion layers are composed of Ga(2)Pn(6) staggered, ethane-like moieties having a rare Ga-Ga bonding motif, these moieties are connected in a complex fashion by means of shared Pn atoms. Both structures show small residual electron densities that can be modeled by adding a Eu atom and removing two bonded Ga atoms, resulting in structures (< 2%) where most of the atoms are the same, but there is a difference in bonding that leads to one-dimensional ribbons of parallel Ga2Pn6 staggered, ethane-like moieties. The compounds can be understood within the Zintl formalism, but show metallic resistivity. Magnetization measurements performed on single crystals show low-temperature magnetic anisotropy as well as multiple magnetic ordering events that occur at and below 24 and 20 K for the phosphorus and arsenic analogs, respectively. The magnetic coupling between Eu ions is attributed to indirect exchange via an RKKY interaction, which is consistent with the metallic behavior. The compounds display large negative magnetoresistance of up to -80 and -30% (MR = [(rho(H)-rho(0))/rho(H)] x 100%) for Pn = P, As, respectively, which is maximal at the magnetic ordering temperatures in the highest measured field (5T).