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We have succeeded in realizing a single ferroelectric phase in CuFe 0.963 Ga 0.037 O 2 by substituting nonmagnetic Ga 3+ for Fe 3+ sites in CuFeO 2 . Ferroelectric polarization P in CuFe 0.963 Ga 0.037 O 2 is observed below 7.5 K, and has the relatively large value of ϳ250 C / m 2 , which is comparable to P = 300ϳ 400 C / m 2 in the magnetic-field-induced ferroelectric phase of CuFeO 2 . In neutron-diffraction measurements, a single magnetic diffraction peak with an incommensurate wave number was observed below 7.5 K in CuFe 0.963 Ga 0.037 O 2 , indicating that the ferroelectric-incommensurate ͑FEIC͒ phase is realized as a single phase. Therefore, CuFe 0.963 Ga 0.037 O 2 with a single FEIC phase is strongly expected to provide the best opportunity to investigate unresolved problems regarding the ferroelectric mechanism in CuFeO 2 . In this paper, we report measurements of magnetic susceptibility, specific heat, pyroelectric, dielectric constant, and neutron diffraction of a single crystal of CuFe 0.963 Ga 0.037 O 2 .

We have performed dielectric measurements and neutron diffraction experiments on the delafossite AgFeO2. A ferroelectric polarization P is approximately equal to 300 μC/m2 was observed in a powder sample, below 9 K. The neutron diffraction experiment demonstrated successive magnetostructural phase transitions at T(N1)=15 K and T(N2)=9 K. The magnetic structure for 9 K≤T≤15 K is a spin-density wave with a temperature dependent incommensurate modulation k=(-1, q, 1/2), q is approximately equal to 0.384. Below 9 K, the magnetic structure turns into elliptical cycloid with the incommensurate propagation vector k=(-1/2,q,1/2), q is approximately equal to 0.2026 Based on the deduced magnetic point-group symmetry m1' of the low-temperature polar phase, we conclude that the ferroelectric polarization in AgFeO2 is perpendicular to the monoclinic b axis and is driven by the inverse Dzyaloshinskii-Moriya effect with two orthogonal components p1 is proportional to r(ij)×(S(i)×S(j)) and p2 is proportional to S(i)×S(j).