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Responsive Rare-Earth Materials and their Applications

By: Yaroslav Mudryk, Material Matters, 2016, 11.4

Yaroslav Mudryk
Ames Laboratory of U.S. Department of Energy,
Iowa State University
Ames, Iowa 50011 USA
Email: slavkomk@ameslab.gov

Introduction

The unique properties of the rare-earth elements and their alloys have brought them from relative obscurity to high profile use in common hightech applications. The broad technological impact of these remarkable materials may have never been known by the general public if not for the supply concerns that placed the rare-earth materials on the front page of newspapers and magazines. Neodymium and dysprosium, two essential components of Nd2Fe14B-based high-performance permanent magnets, have drawn much attention and have been deemed critical materials for many energy-related applications. Ironically, the notoriety of rare-earth elements and their alloys is the result of a global movement to reduce their use in industrial applications and, thus, ease concerns about their supply and ultimately to reduce their position in high-tech supply chains.

Research into the applications of lanthanide alloys has been de-emphasized recently due to the perception that industry is moving away from the use of rare-earth elements in new products. While lanthanide supply challenges justify efforts to diversify the supply chain, a strategy to completely replace the materials overlooks the reasons rare earths became important in the first place—their unique properties are too beneficial to ignore. Rare-earth alloys and compounds possess truly exciting potential for basic science exploration and application development such as solid-state caloric cooling. In this brief review, we touch upon several promising systems containing lanthanide elements that show important and interesting magnetism-related phenomena. In particular, we will review several R5T4 (R = rare-earth element, T = 13–15th group p-element)1 and RM2 [M = either p- (e.g., Mg, Al) or d- (e.g., Fe, Ni, Co) metal]2 compounds and their unusual responsiveness to external stimuli. Both systems are known to produce strong effects in response to applied magnetic fields and are sensitive to changes in composition, temperature, and pressure.

R5T4 Compounds—Unique Multifunctional Intermetallics for Basic Research and Applications

When a compound shows an extraordinarily strong response to a relatively weak external force, such as a moderately applied magnetic field, there is often significant potential for new fundamental science and technological applications. Systems such as R5T4, for example, can produce a drastic change in electrical transport or linear dimension in response to a relatively weak stimuli. The discovery of the giant magnetocaloric effect (GMCE) in Gd5Si2Ge2 in 1997 by Pecharsky and Gschneidner3,4 immediately triggered a wave of both applied and fundamental research aimed at alloys with R5(Si1xGex)4 stoichiometry, or, in general, R5T4 compounds. Soon many other useful properties were found in these systems, including giant magnetoresistance (GMR),5,6 giant magnetostriction (GMS),7 and spontaneous generation of voltage (SGV),8 in addition to the observation of basic and very interesting phenomena, such as unconventional magnetic glass state,9 spin-flop transitions,10 magnetic deflagration,11 and short-range magnetic correlations (Griffiths-like phase).12 The fact that all these phenomena can occur simultaneously in the same material is interesting for fundamental research and shows a promising multifunctionality for use in applications. While most of these effects were originally discovered in Gd5Si2Ge2 or similar compositions (e.g., Gd5Si1.8Ge2.2 6,7), many other R5T4 systems also exhibit useful properties. For example, similar extraordinary responsive behavior is observed in compounds with a quasi-two-dimensional slab crystal structure such as orthorhombic Sm5Ge4- or monoclinic Gd5Si2Ge2-types (Figure 1).

Schematic representation of the three main structure types

Figure 1. Schematic representation of the three main structure types, O(II)-Sm5Ge4, O(I)-Gd5Si4, and M-Gd5Si2Ge2 commonly associated with magnetostructural transformations in R5T4 alloys. The slabs (shaded blue) do not change substantially during the transitions but the interslab T–T (interslab T atoms are marked red) bonds may break and re-form either fully or partially. In most cases, ferromagnetism occurs when both interslab T–T bonds are present (d = 2.6 A, middle structure).

 

During magnetic transitions these slabs may shift with respect to each other, typically resulting in a structural transformation. As a result, another slab structure, orthorhombic Gd5Si4, commonly forms when a material enters the ferromagnetic (FM) state. Consequently, the above mentioned physical effects are caused by the intimate coupling between magnetic and lattice states, and the transitions between the structures are followed by drastic changes in magnetization, lattice dimensions (DV/V up to 1.2%), lattice and magnetic entropy, electrical transport properties, and microstructure. The three main structure types involved in magnetostructural transformations in R5T4 systems are shown in Figure 1.

To describe the magnetostructural behavior in R5T4, we’ll use the Gd5Si0.5Ge3.5 compound as an example.13 At room temperature, this material has the Sm5Ge4-type structure and, when it orders antiferromagnetically at TN = 130 K, there is no structural transformation. However, at TC above 70 K, there is a sharp order–order antiferromagnetic/ ferromagnetic transformation coupled with a Sm5Ge4 to Gd5Si4 structural transition (Figure 2). Due to such an abrupt and large change in magnetization, this alloy shows a giant magnetocaloric effect, DSM = –44 J/kg K, near the temperature of nitrogen liquefaction. In addition, as seen in Figure 2B, there is an unusually large and anisotropic change of lattice parameters at the transition since the upper limit for magnetostriction is above 18,000 ppm along the a-axis of the Gd5Si0.5Ge3.5 crystal near liquid nitrogen temperature.

The transition from one structure to another can be triggered by changes in composition, temperature, applied pressure/stress, and magnetic field. A spontaneous generation of electric signal occurs during these transformations, making these systems good candidates for no-power multifunctional standby sensors. The fact that structural change can be triggered by applied pressure makes these materials interesting for stress/ strain-related applications. For example, in addition to magnetocaloric effect, the R5T4 compounds produce the barocaloric effect, a change of a material’s temperature in response to applied pressure, either hydrostatic or uniaxial. The barocaloric effect for the Gd5Si2Ge2 is shown in Figure 3. The presence of both magneto- and barocaloric effects in the same alloy has far reaching implications because it allows utilization of caloric cooling using three-dimensional p-H-T space. In practice, it means that a combination of magnetic field and stress used as the external driving force for the caloric-capable transformations may allow for large caloric effects to be achieved without the need for impractically high (and expensive) magnetic fields.14

Magnetostructural transformation

Figure 2. Magnetostructural transformation in Gd5Si0.5Ge3.5: A) contour plot of the temperature-dependent X-ray powder diffraction patterns around the transition temperature TC; B) temperature (at 0 T) and magnetic-field (at 80 K) dependence of the lattice parameters and unit-cell volume; C) temperature and magnetic-field dependence of magnetization; and D) magnetocaloric effect (magnetic entropy change) as a function of temperature determined from the M(H) data.13

 

Barocaloric effect

Figure 3. Barocaloric effect in Gd5Si2Ge2: symbols show adiabatic temperature change corresponding to the release of pressure, p = 2 kbar. Solid line indicates isothermal entropy change corresponding to the release of pressure, p = 2 kbar.15

 

RM2 Compounds – Simple Structures with Complex Physical Behavior

Intermetallic compounds with RM2 stoichiometry crystallize at room temperature in the three closely related C14 hexagonal MgZn2-type, C15 cubic MgCu2-type, and C36 hexagonal MgNi2-type structures.2 They are commonly known as Laves phases and are one of the largest families of alloys in R-M systems. Many of the cubic RM2 compounds undergo structural distortions at low temperatures, and these transitions are often coupled with magnetic ordering/reordering transformations. Depending upon the direction of the easy magnetization axis (EMA) below the transformation, RM2 compounds with the room temperature cubic crystal structure may exhibit rhombohedral (for EMA <111>), tetragonal (EMA <100>), or orthorhombic (EMA <110>) distortions.16,17 Sometimes a compound may exhibit two structural transitions, first at the magnetic ordering temperature and then at the point of spin-reorientation (change of EMA). This has been observed, for example, in HoCo2 (Figure 4).18

HoCo2, as well as some other RM2, has a first-order transition at TC; but in the majority of RM2 alloys the magnetic ordering transitions are second-order. The commonly shared understanding associates firstorder behavior with the instability of transition metal moments. This instability means that even a small change to the external stimulus may lead to a sudden occurrence of magnetically ordered state which, as mentioned above, may result in a plethora of useful phenomena, such as the giant magnetocaloric effect, giant magnetoresistance, and giant magnetostriction. At the same time, the most famous magnetostrictive material, Terfenol-D (Tb0.7Dy0.3Fe2), derives its properties from the strong anisotropic response (not first-order) of its lattice to applied magnetic field around room temperature. Unlike the abrupt response that occurs when a material undergoes a first-order transformation, the change of materials dimensions with field in Terfenol-D is gradual and can be precisely controlled. This fact illustrates the versatility of the RM2 alloys, where both first- and second-order materials can find technological usefulness. The RM2 alloys with M = Al, Co, and Ni are well-established candidates for low temperature magnetocaloric applications such as hydrogen liquefaction.

Magnetostructural transitions

Figure 4. Magnetostructural transitions in HoCo2. High temperature cubic phase distorts tetragonally below TC, then further distorts at TSR, resulting in the orthorhombic structure. A) Temperature dependence of the HoCo2 lattice parameters. B) Evolution of the normalized c/a ratio. C) Temperature dependence of the unit-cell volume at 0 and 30 kOe applied magnetic field.18

 

The basic science of RM2 compounds is also very intriguing. For example, PrAl2 shows a tetragonal distortion at TC, which is fully suppressed by an applied magnetic field as low as 10 kOe, as confirmed by both heat capacity and X-ray powder diffraction data (Figure 5).19 Interestingly, the nuclear Schottky-specific heat anomaly observed at very low temperatures is unusually high in this compound. DFT calculations point to modification of the energy levels of a nuclear spin system due to the 4f band splitting as a likely reason for the higher Schottky-specific heat close to 0 K in PrAl2 compared to the elemental Pr. The nuclear-specific heat coefficient, CN, increases with the increase in the applied magnetic field (Figure 5). The rapid increase of the material’s heat capacity below 1 K makes it a promising passive regenerator material for the ultra-low temperature cryo-cooler applications.

X-ray powder diffraction diagrams of PrAl2 collected as a function of temperature

Figure 5. X-ray powder diffraction diagrams of PrAl2 collected as a function of temperature in A) 0 kOe and B) 30 kOe applied magnetic field. The structural distortion is seen on the zero field data, but it is suppressed in 30 kOe field. C) The heat capacity of PrAl2 plotted in C/T vs T2 coordinates highlighting a sharp transition at TC that becomes smeared above 10 kOe and an upturn below 2 K. D) Schottky nuclear-specific heat anomaly in magnetic fields from 0 to 140 kOe; the inset shows the magnetic field dependence of the nuclear-specific heat coefficient CN.

 

Conclusion

Many intermetallic compounds containing lanthanides possess interesting physical behaviors that have great potential for use in energy-related applications. Two interesting and well-studied systems, R5T4 and RCo2, were highlighted in this review; however, there are undoubtedly many other rare-earth systems both known (e.g., RFe13-xSix) and waiting to be discovered. These anticipated discoveries will push the limits of our basic knowledge and enable the creation of smart multifunctional materials for the technologies of tomorrow.

Acknowledgments

The Ames Laboratory is operated for the U. S. Department of Energy by Iowa State University of Science and Technology under contract No. DE-AC02-07CH11358. The work presented in this review was supported by the Department of Energy, Office of Basic Energy Sciences, Materials Sciences Division.

Materials

     

References

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