Synthesis and physical properties of new oxide AgMnO₂

A novel oxide AgMnO₂ was prepared from LiMnO₂ via Ag⁺ → Li⁺ exchange in the eutectic melt AgNO₃-KNO₃. It crystallizes in a monoclinically distorted unit cell (SG C2/m) caused by the Jahn-Teller (J-T) ion Mn³⁺ (3d ⁴). The structure was refined by isotypy with the crednerite CuMnO₂. There are two long axial Mn–O of 264.2(0) pm and four equatorial bonds of 192.7(3) pm and Mn–O–Mn adjoining (83.07°) are bent below the ideal angle. The thermal variation of the magnetic susceptibility (χ/T ⁻¹) obeys a Curie-Weiss law with manganese in a trivalent, high spin (HS) state accommodated in elongated MnO₆ octahedra (14.8%). Direct coupling between Mn³⁺ involves negative exchange interactions through long-range antiparallel moments with a temperature θ _p  = −436 K and a magnetic moment of 5.26 μ_B/Mn³⁺ slightly larger than the spin only moment. The title oxide is stable in air up to ∼680 °C before it decomposes into metal silver. It displays a semi-conducting behavior with an activation energy of ∼0.45 eV, characteristic of a conduction by low mobility polarons between Ag^+/2+ where nearly all polarons are bonded. The photoelectrochemical properties of AgMnO₂ have been investigated by photocurrent technique in 1 M KOH. The cathodic photocurrent J _ph provides unambiguous evidence of p-type character attributed to oxygen insertion (0.025 oxygen by formula unit) as required by the charge compensating mechanism. The valence band is made up of Ag−4d wave functions positioned at ∼5.14 eV below vacuum. A comparison with CuMnO₂ was also reported.