STUDY OF NA3FE2(PO4)3 AND NA3CR2(PO4)3 IN THEIR PARAMAGNETIC PHASES BY NEUTRON-SCATTERING

The paramagnetic scattering of the title compounds (noted as NaFe and NaCr, respectively) has been studied by neutron scattering on the time of flight instrument IN6 at ILL. For NaFe the strong quasielasticity observed at 60,300 and 573 K is mainly Lorentzian for small Qs and Gaussian for large Qs in conformity with de Gennes' theory. An average exchange coupling is estimated to be about 0.36 meV. For NaCr a quasi-elastic signal coexists with an inelastic feature centered between 1.5 and 1. 8 mev over the whole investigated temperature range (12 K up to 600 K). The quasi-elastic scattering, as for NaFe, can be interpreted in terms of de Gennes' theory and the corresponding average exchange coupling of 0.083 meV was found to be much smaller than that of NaFe; the interpretation of the inelastic component is discussed in a forthcoming article.