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Ferroelectric materials exhibit an intrinsic electric polarization that can be changed or reversed by an

applied electric field. While initially ferroelectrics were little more than an academic curiosity, today they can be found at the heart of many technological devices that exploit their ferro-, piezo- and pyroelectric properties, with the most advanced application perhaps being that of ferroelectric non-volatile memories1. Although metallic behaviour and ferroelectricity have long been thought to be incompatible because conduction electrons screen the internal electric field, Anderson and Blount2 nonetheless suggested almost 50 years ago that certain structural transitions observed in metals might be ‘ferroelectric’ in nature. However, a clear example of a ferroelectric metal has so far remained elusive. Writing in Nature Materials, a collaboration led by Kazunari Yamaura and Andrew Boothroyd now reports the discovery of a structural ferroelectric-like transition in LiOsO3, which might very well be the first realization of a ‘ferroelectric metal’3.

Ferroelectric materials typically undergo a phase transition at the Curie temperature TC, from a high-temperature non-polarized paraelectric state to a low-temperature ferroelectric (polarized) state4. The spontaneous polarization is a

consequence of the structural transition that takes place at TC, and involves a (usually small) symmetry-breaking distortion. The structure of the low-temperature ferroelectric material is always non-centrosymmetric, and therefore does not display inversion symmetry, as this prevents the charge separation inherent to the electric polarization5. The compound BaTiO3 is often regarded as the prototypical ferroelectric, belonging to a family of ferroelectrics known as perovskite oxides. Below 393 K, the cubic BaTiO3 structure distorts to a ferroelectric tetragonal structure, a transition that is dominated by small displacements of Ti atoms with respect to the oxygen network. Another celebrated ferroelectric found in many technological applications is LiNbO3. Although it bears some similarities to the cubic perovskites, the high-temperature structure is rhombohedral. The low-temperature ferroelectric phase is obtained from small symmetry-breaking displacements of the Li atoms, with the loss of the inversion symmetry resulting in a spontaneous polarization at 1,483 K.

The compound discovered by Yamaura, Boothroyd and colleagues3 is closely related to LiNbO3. It displays the same rhombohedral structure at high temperatures and undergoes a structural phase transition at 140 K, involving a

+ + + + + + + +

– – – – – – – –

+ + + + + + + +

+ + + + + + + +

– – – – – – – –

+ + + + + + + +

– – – – – – – –

– – – – – – – –

Piezoelectric materiala

Pyroelectric materialb

Ferroelectric material

Applied electricfield

Temperaturechange

Appliedstress

c

Figure 1 | Piezoelectric, pyroelectric and ferroelectric materials. a, In piezoelectric materials, the coupling between mechanical and electrical energy results in an electric polarization when stress is applied. b, The polar axis of a pyroelectric material allows a net polarization when the temperature is changed. c, Ferroelectrics are a special subset of pyroelectrics, in that their polarization can be reversed on the application of an electric field. All ferroelectrics are both pyroelectric and piezoelectric.

collaborators’ work is then the successful indirect patterning of immunoglobulin G (IgG). In this experiment, Protein A, which has high affinity for the Fc domain of immunoglobulins, was functionalized with Q-peptide using a standard, nonspecific bioconjugation technique (maleimide–thiol chemistry), and then photopatterned into the hydrogel. Subsequent incubation with a fluorescently tagged IgG resulted in spatially controlled protein patterning as observed with VEGF121, FN9-10 and PDGF-BB. This demonstration exemplifies how the method could be readily translated to pattern commercially available Fc chimeric proteins, which has important implications for using dynamic protein patterning in

cell-biology studies. As this approach would circumvent the need to engineer proteins to present the Q-peptide domain, it should make the chemistry more accessible and thus facilitate its broader adoption. Ultimately, Lutolf and colleagues’ method should help shed light on fundamental questions related to stem cell biology, tissue morphogenesis and disease. ❐

Daniel L. Alge and Kristi S. Anseth are at the Department of Chemical and Biological Engineering, the BioFrontiers Institute, and the Howard Hughes Medical Institute, University of Colorado at Boulder, Boulder, Colorado 80303, USA. e-mail: Kristi.Anseth@Colorado.EDU

STRUCTURAL TRANSITIONS

‘Ferroelectricity’ in a metalThe discovery of a ferroelectric-like structural transition in metallic LiOsO3 identifies a new class of materials with unconventional properties, providing an exotic playground for theorists and experimentalists.

Veerle Keppens

References1. Mosiewicz, K. A. et al. Nature Mater. 12, 1072–1078 (2013).2. Tsien, R. Y. Angew. Chem. Int. Ed. 48, 5612–5626 (2009).3. Losonczy, A. & Magee, J. C. Neuron 50, 291–307 (2006).4. Lee, S.-H., Moon, J. J. & West, J. L. Biomaterials

29, 2962–2968 (2008).5. DeForest, C. A., Polizzotti, B. D. & Anseth, K. S. Nature Mater.

8, 659–664 (2009).6. Alge, D. L., Azagarsamy, M. A., Donohue, D. F. & Anseth, K. S.

Biomacromolecules 14, 949–953 (2013).7. Culver, J. C. et al. Adv. Mater. 24, 2344–2348 (2012).8. Kloxin, A. M., Kasko, A. M., Salinas, C. N. & Anseth, K. S. Science

324, 59–63 (2009).9. Wirkner, M. et al. Adv. Mater. 23, 3907–3910 (2011).10. Griffin, D. R. et al. Biomacromolecules 14, 1199–1207 (2013).11. DeForest, C. A. & Anseth, K. S. Angew. Chem. Int. Ed.

51, 1816–1819 (2012).12. Guvendiren, M. & Burdick, J. A. Nature Commun. 3, 792 (2012).13. Luo, Y. & Shoichet, M. S. Nature Mater. 3, 249–253 (2004).14. Wylie, R. G. et al. Nature Mater. 10, 799–806 (2011).15. Zisch, A. H., Schenk, U., Schense, J. C., Sakiyama-Elbert, S. E. &

Hubbell, J. A. J. Control. Release 72, 101–113 (2001).

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shift in the position of the Li atoms. Given the metallic nature of LiOsO3, the similarity of its structural behaviour with LiNbO3 is quite surprising: free electrons typically screen the Coulomb interactions that favour the off-centre displacements, preventing ferroelectricity.

To be considered a ferroelectric metal according to the definition of Anderson and Blount2, several criteria have to be met. First, the structural transition has to be continuous (which is to say that it cannot show discontinuous jumps in physical properties that are first derivatives of the free energy, such as the heat capacity); second, the low temperature structure has to be non-centrosymmetric; and third, the low-temperature structure needs to support a unique polar axis (which is a rotational axis of symmetry without a mirror plane perpendicular to it). Whether or not a material has such a polar axis is determined solely by its crystal structure and

establishes the subtle difference between piezo- and pyroelectrics, illustrated in Fig. 1. Almost a decade ago, the pyrochlore system Cd2Re2O7 was tentatively identified as a potential ferroelectric metal6, but it falls short: although the material displays a continuous phase transition to a structure that lacks inversion symmetry, it does not support a unique polar axis and is therefore considered ‘piezoelectric’, but not ‘ferroelectric’. The compound LiOsO3 does possess the essential polar axis, and combined with the structural transition involving Li-ion displacements, it satisfies the Anderson and Blount criteria to be considered a ferroelectric metal. Some caution, however, is warranted as the metallic nature of the compound prevents an actual polarization from being observed.

The findings reported by Yamaura and Boothroyd3 point to the very first realization of ferroelectric behaviour in

a metallic compound, and more unusual behaviour is likely to be discovered in this material. Although practical applications are at this stage hard to envisage, the ferroelectric-like transition found in LiOsO3 is quite unique, and its further study may help elucidate ferroelectric transitions in related technologically relevant materials. ❐

Veerle Keppens is at the Department of Materials Science and Engineering, the University of Tennessee, Knoxville, Tennessee 37996-2100, USA. e-mail: vkeppens@utk.edu

References1. Scott, J. F. & Paz de Araujo, C. A. Science 246, 1400–1405 (1989).2. Anderson, P. W. & Blount, E. I. Phys. Rev. Lett. 7, 217–219 (1965).3. Shi, Y. et al. Nature Mater. 12, 1024–1027 (2013).4. Rabe, K. et al. Topics in Appl. Phys. 105, 1–30 (2007).5. Halasyamani, P. S. & Poeppelmeier, K. Chem. Mater.

10, 2753–2769 (1998).6. Sergienko, I. et al. Phys. Rev. Lett. 92, 065501 (2004).

Published online: 22 September 2013

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