Phase Relationships, Experimental Thermodynamics, Magnetism and in situ Structural Characterization of R5(Si1-xGex)4 Systems

Personnel: K.A. Gschneidner, Jr. (PI); V.K. Pecharsky (PI); A.O. Pecharsky (Assistant Scientist); Hong Tang (Postdoc); Vitaly Ivtchenko and Paul Tomlinson (Graduate Students).

Abstract:
Research is focused on systematic experimental and theoretical studies of the unique magnetic-martensitic phase transformation in R5(SixGe1-x)4 materials, where R is Gd and other rare earth metals, to achieve an understanding of the underlying electronic structure and the microscopic interactions which bring about extremely strong coupling of the magnetic moments with the lattice. Another goal is the development and validation of a phenomenological model of the magnetic-martensitic transformation, which will allow for the design of novel material systems exhibiting extremely large responses to small changes of magnetic field, temperature, and pressure.

Recent Results:
The low temperature (~270 K) martensitic transformations in Gd5(Si1-xGex)4 with x = 0.5 and 0.52 are rapid, complete and reversible when they are coupled with the ferromagnetic ordering-disordering process on cooling-heating. A second, high temperature martensitic phase change observed to occur between ~500 to ~700 K on heating is sluggish, incomplete and irreversible in the paramagnetic state, even though the crystallographic phase change is the same as for the low temperature phase transition. Thus, the magnetic interactions and magnetic exchange energy play a crucial role in the martensitic phase transition processes in the Gd5(Si1-xGex)4 system, and the chemical bonding energy is comparable to the exchange energy. Cycling through the low temperature magnetic-martensitic phase transformation affects the properties of the monoclinic Gd5(Si1-xGex)4 alloys, which is associated with the redistribution of Si and Ge atoms between different crystallographic sites, i.e., between those located inside the subnanometer thick two-dimensional slabs and those responsible for the inter-slab bonding. Electronic transport studies revealed an unusual effect - spontaneous generation of voltage during the low temperature magnetic-martensitic phase transformation. We conclude that this voltage is thermoelectric in origin and is brought about by sporadic changes in the temperature of specimen during first order phase transformation. The density of states at the Fermi level in Gd5Ge4 remains practically unaffected by the crystallographic phase change; however, it is several times higher than that of a common metallic material. A notable variation of the Debye temperature is consistent with the change of both the magnetic order and crystal structure of the material during a magnetic field induced phase transition. Based on the magnetic, transport, heat capacity and crystallographic data, magnetic field – temperature phase diagrams were established for the Gd5(Si1-xGex)4 system (e.g., see Fig. 1). In all cases we found extended regions where the system loses homogeneity and becomes magnetically and crystallographically inhomogeneous.

Significance:
The increased complexity of novel lanthanide-based systems provides multiple degrees of freedom enabling better control over the exchange interactions, and therefore, the magnetic properties of materials. In the R5(SixGe4-x) systems, strongly interacting magnetic and non-magnetic ions are arranged in subnanometer thick two-dimensional fragments (slabs), which form a three-dimensional crystallographic framework (Fig. 1). The inter-slab interactions in these naturally occurring nano-layered magnetic materials may be controlled with a high precision by varying the stoichiometry (i.e., the value of x) at constant magnetic field, temperature and pressure, or by varying temperature, magnetic field and/or pressure at constant chemistry. Since physical properties of R5(SixGe4-x) materials are intimately related to their crystal structures, they are excellent models to achieve better understanding of the role of multiple parameters in defining the electronic structure and the microscopic interactions in these and potentially other condensed 4f-systems.

Future Work:
The use of a one-of-a-kind powder diffractometer system (a high resolution rotating anode powder diffractometer with high temperature, and low temperature - high magnetic field attachments) will allow us to examine temperature - (4 to 1000K) and magnetic field - (0 to 35 kOe between 4 and 300 K) effects on the crystal structure and bonding arrangements in different R5(Si1-xGex)4 phases. The instrument is located in the Ames Laboratory and will become fully operational in the middle of 2002. Anisotropic magnetoelasticity, magnetostriction, thermal expansion, and other structural phenomena in the R5(SixGe1-x)4 materials will be investigated in addition to direct studies of magnetic field induced magnetic-martentsitic phase transformations. Our preliminary indirect experiments indicate that in some alloys, e.g., Er5Si4, the low temperature crystallographic transition is decoupled from the ordering of magnetic moments. Unlike in Gd5(Si1-xGex)4 alloys, where similar decoupling at high temperatures makes the martensitic transition irreversible, in Er5Si4 the reversibility is preserved. Therefore, it is important to establish the necessary details of the crystal structure change in both types of materials to gain a better understanding of the mechanism of both the coupled magnetic-martensitic transformation and martensitic transformation alone.

Magnetic and transport studies have demonstrated the unusual coexistence of magnetically ordered and disordered phases with large localized magnetic moments. Therefore the kinetics of magnetic-martensitic phase transformation as a function of temperature and field will be studied in detail. Ac/dc electrical resistance, magnetization and calorimetric measurements will be carried out on isotropic and anisotropic samples over a wide range of temperatures (2 to 800 K). Combined with the atomic, magnetic and microscopic structural details, all experimental data will be used to construct and refine the magnetic phase diagrams for the various R elements and to provide the experimental data necessary for validation of modeling efforts and theoretical first principles calculations.

Interactions:
Multiple PI's in Materials and Engineering Physics Program (L.S. Chumbley, D.C. Jiles, T.A. Lograsso, J.E. Snyder), Condensed Matter Physics (V. Antropov, B.N. Harmon, C. Stassis), and Materials Chemistry (G.J. Miller).

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