Polar metal

A polar metal, metallic ferroeletric,^[1] or ferroelectric metal^[2] is a metal that contains an electric dipole moment. Its components have an ordered electric dipole. Such metals should be unexpected, because the charge should conduct by way of the free electrons in the metal and neutralize the polarized charge. However they do exist. One substance family that can produce a polar metal is the nickelate perovskites. One example is lanthanum nickelate, LaNiO₃.^[3][4] A thin film of LaNiO₃ grown on the (111) crystal face of lanthanum aluminate, (LaAlO₃) was interpreted to be both conductor and a polar material at room temperature.^[3] Also, when grown 3 or 4 unit cells thick (1-2 nm) on the (100) crystal face of LaAlO₃, the LaNiO₃ can be a polar insulator or polar metal depending on the atomic termination of the surface.^[4] Lithium osmate,^[5] LiOsO₃ also undergoes a ferrorelectric transition when it is cooled below 140K. The point group changes from R3c to R3c losing its centrosymmetry.^[6] At room temperature and below lithium osmate is an electric conductor, in single crystal, polycrystalline or powder forms, and the ferroelectric form only appears below 140K. Above 140K the material behaves like a normal metal.^[7]

P. W. Anderson and E. I. Blount predicted that a ferroelectric metal could exist in 1965.^[6] They were inspired to make this prediction based on superconducting transitions, and the ferrorelectric transition in barium titanate. The prediction was that atoms do not move far and only a slight crystal non-symmetrical deformation occurs, say from cubic to tetragonal. This transition they called martensitic. They suggested looking at sodium tungsten bronze and InTl alloy. They realised that the free electrons in the metal would neutralise the effect of the polarization at a global level, but that the conduction electrons do not strongly affect transverse optical phonons, or the local electric field inherent in ferroelectricity.^[8]

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