Труды КНЦ вып.5 (ХИМИЯ И МАТЕРИАЛОВЕДЕНИЕ вып. 5/2015(31))

E/ V Fig. Cyclic voltammograms o f the LiF-CaF 2 -SmF 3 system at different scan rates (50, 100, 200, 400, 600, 80... , 2800 mV/s) at 1175 K. WE: W; RE: Pt wire; CE: GC crucible. C°(SmF3) = 8.259 x 10 -4 mol/cm3; S = 0.231 cm 2 At temperatures of 1075 and 1125 K, the peak current of electroreduction up to at least v = 1.0 V/s is directly proportional to v -1/2, while the peak potential was independent on the potential sweep rate to the values of v = 1.0 V/s. According to the cyclic voltammetry theory [2, 3], the electrode process at these temperatures is controlled by diffusion up to the polarization rate of v = 1.0 V/s. At v > 1.0 V/s, a deviation of the experimental points was observed from the straight line passing through the origin of coordinates, which evidences a transition from a reversible process to a quasi- reversible process. At the higher temperature (1175 K) the process is controlled by diffusion at all applied scan rates. Diffusion coefficients ( D ) of Sm(III) and Sm(II) ions in LiF-CaF 2 melt were determined using cyclic voltammetry by Randels - Sevchik equation: Ip = 0.446(zF}3/2(R7y1/2SC°(Dv)1/2 (1) where Ip is the peak current in A: z is a number of electrons: S is the electrode area in cm2: C 0 is the concentration of electroactive species in mol/cm3: D is the diffusion coefficient of electroactive species in cm2/ s: v is the potential scan rate in V/s. Temperature dependencies of diffusion coefficients were following: lnD(Sm3+) = - 0.96612 - (10442/T), (2) lnD(Sm2+) = - 0.84885 - (10328/T). (3) Knowing the diffusion coefficients of Sm(III) and Sm(II) the standard rate constants of charge transfer (ks) for the redox couple Sm(III)/Sm(II) in LiF-CaF 2 melt can be determined. According the analysis described before, the studied system shows at the concentration of C°(SmF3) = 4.24 10 -4 mol/cm 3 a quasi-reversible behavior at sweep rates 1 - 2 V/s. In this range of scan rate, the constants of charge transfer could be calculated [4]. Table presents calculated values of the standard rate constants of charge transfer for the Sm(III)/Sm(II) redox couple at 1075 and 1125 K. At the higher temperature (1175 K), the constant of change transfer could not be calculated, since at this temperature the system is reversible at all applied polarization rates. The standard rate constants of charge transfer for the Sm(III)/Sm(II) redox couple at different temperatures T, K Evc, V Eva, V AEp, V v t ks (Sm(III)/Sm(II)), cm2/s 1075 -1.335 -1.028 0.307 1.803 0.0557 1125 -1.227 -0.910 0.317 1.937 0.0788 The electrochemical reduction of Sm(II) into samarium metal was not observed. It is evident that a samarium has a more negative discharge potential than the solvent cations. 280