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

Abstract

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) Å , b=3.8967(4) Å , c=12.0777(11) Å , β=95.5220(10)°, R1 = 0.0148, wR2 = 0.0325 (EuGa 2 P 2 ) and a = 9.4953(7) Å , b = 4.0294(3) Å , c = 12.4237(9) Å , β = 95.3040(10)°, R1 = 0.0155, wR2 = 0.0315 (EuGa 2 As 2 )). 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 Ga 2 Pn 6 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%

Role of the dimensionality of the [GaX]2 network in the Zintl phases EuGa2X2

“…5,6 The replacement of magnetically inert cations with high moment metal cations in rare earth Zintl compounds can lead to interesting magneto-electronic properties. The magnetic properties and negative MR of EuGa 2 P 2 and EuGa 2 As 2 have been reported by Goforth et al 12 The larger Weiss constant and higher ordering temperatures of the phosphide indicate a higher exchange energy. Recently, Schellenberg et al 13 have studied the structure and magnetic properties of orthorhombic EuGa 2 Sb 2 and found a three dimensional (3D) network between the Ga and Sb atoms which leaves large cages for the Eu atoms.…”

“…Therefore, EuGa 2 P 2 and EuGa 2 As 2 order ferromagnetically, while EuGa 2 Sb 2 orders antiferromagnetically, in agreement with the experimental situation. 12,13 The calculated BSs of EuGa 2 X 2 for the majority and minority spin states are similar to each other except for the spin minority Eu 4f states, which lie 9 eV above the Fermi energy (E F ). moment for a Eu þ2 ion (7 l B ).…”

“…The calculations are carried out at the experimental lattice constants of EuGa 2 X 2 . 12,13 The calculated energy differences between FM and AFM coupling in EuGa 2 P 2 ; EuGa 2 As 2 , and EuGa 2 Sb 2 are 8.6 meV, 4 meV, and À4.8 meV, respectively. Therefore, EuGa 2 P 2 and EuGa 2 As 2 order ferromagnetically, while EuGa 2 Sb 2 orders antiferromagnetically, in agreement with the experimental situation.…”

“…The crystal structures of EuGa 2 P 2 12 and EuGa 2 Sb 2 13 are presented in Fig. 1 as views along the short unit cell axis.…”

Enhanced thermoelectric properties of the Zintl phase BaGa2Sb2via doping with Na or K

“…[30][31][32][33][34][35][36][37][38] Most of the A-T-Pn phases crystallize in the well-known CaAl 2 30-32 When the divalent transition metal is substituted with a triel element (e.g., Ga or In), completely different crystal structures are formed with two or three-dimensional polyanions (Fig. 1).…”