New Transition-Metal Doped Germanium Clathrates
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New Transition-Metal Doped Germanium Clathrates Yang Li and Joseph H. Ross, Jr. Department of Physics, Texas A&M University College Station, TX 77843-4242, U.S.A. ABSTRACT We have investigated transition-metal substitutions into germanium clathrates, and describe the properties of two different Fe-substituted Ge clathrates, one with the chiral Ba6Ge25-type clathrate structure, and one based on the Ba6Ga16Ge30 clathrate with the type-I structure. In both cases Fe exhibits a high-spin local moment, with 5.5 µB and 5.6 µB per Fe. We observe ferromagnetic ordering, with Tc up to 278 K in the most heavily substituted Ba6Ga16Ge30 clathrate. X-ray powder diffraction and electron microprobe measurements confirmed the formation of these substituted phases. Ni-doped type I clathrates were found to be diamagnetic. Strong competition from Mn11Ge8 makes it difficult to produce pure Mn-doped clathrates, however a composite material with partially substituted chiral structure is described. Similar efforts to produce Co, Cr, and V-substituted clathrates have not been successful.
INTRODUCTION Si, Ge, and Sn clathrates have attracted considerable attention as potential thermoelectric materials. This is due in particular to the "rattling" behavior of atoms loosely held within the covalently-bonded cages [1]. In addition, the open structure can lead to large band masses and sharp electron density of states features [2] which may lead to the enhancement of collective electronic behavior such as superconductivity [3] and magnetism, and can also enhance the thermopower if these features are close to EF. Substitution of transition atoms on the clathrate framework may lead to magnetic or Kondo behavior which can also have a significant effect on the thermopower [4]. Here we focus on the synthesis and properties of Ge clathrates substituted with 3d-elements. A class of type-I structure Ge and Si clathrates with transition-elements and noble-metal atoms substituting on the framework was discovered by Cordier and Woll [5]. These include clathrates with the third-row substitutions Ba8Ni6Ge40 and Ba8Cu6Ge40. We examined some of these materials, and found that that the Cu clathrate forms with a definite composition close to 5 per formula unit [6], with an equilibrium vacancy concentration which follows the Zintl mechanism, for which the electron count is balanced to satisfy filled bonding orbitals. The Zintl mechanism implies possible semiconducting behavior, however NMR measurements [7] show that the material is a low-carrier density metal or semimetal. Thus the transition-metalsubstituted clathrates can exhibit low-carrier density behavior, and furthermore the magnetic response of some of these materials, in combination with the rattling behavior of cage-centered atoms, may lead to interesting behavior. Our recent work has included examination of new transition-metal substituted clathrates, and here we describe two Fe-substituted compositions, as well as others from the 3d series, with a particular focus on characterization of the magn
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