  Oxyhydroxide minerals like FeOOH have been a research focus in geology for
studying the Earth interior, and also in chemistry for studying oxygen
electrocatalysis activity. In this paper we provide first-principles evidence
of a new class of ferroelectrics or multiferroics among
them:GaOOH,InOOH,CrOOH,FeOOH, which are earth-abundant minerals and have been
experimentally verified to possess distorted rutile structures, are
ferroelectric with considerable polarizations(up to 24 muC/cm2) and
piezoelectric coefficients. Their atomic-thick layer may possess vertical
polarization robust against depolarizing field due to the formation of O-H O
bonds that can hardly be symmetrized. Moreover,CrOOH (guyanaite) is revealed to
be a combination of high-Tc in-plane type-I multiferroics and vertical type-II
multiferroics, which is strain-tunable and may render a desirable coupling
between magnetism and ferroelectricity. Supported by experimental evidence on
reversible conversion between metal oxyhydroxides and dioxides and their nice
lattice match that renders convenient epitaxial growth, heterostructure
composed of oxyhydroxides and prevalent metal dioxides (e.g. TiO2, SnO2 and
CrO2) may be constructed for various applications like ferroelectric
field-effect transistors and multiferroic tunneling junctions.
