A Structural, Magnetic, and Mössbauer Spectral Study of Several Na-Mn-Fe-bearing Alluaudites


The synthesis and the chemical, structural, magnetic, and Mössbauer spectral characterization of three synthetic alluaudites, Na2Mn2Fe(PO4)3, NaMn Fe2(PO4)3 and Na2MnFeIIFeIII(PO4)3, and a natural sample with the nominal composition of NaMn Fe2(PO4)3, collected in the Buranga pegmatite, Rwanda, are reported. All four compounds have the expected alluaudite monoclinic C2/c structure with the general formula [A(2)A(2)'][A(1)A(1)'A(1)2”]M(1)M(2)2(PO4)3 in which manganese(II) is on the M(1) site and manganese(II), iron(III) and, in some cases, iron(II) on the M(2) site. The X-ray structure of Na2Mn2Fe(PO4)3 also indicates a partially disordered distribution of NaI and MnII on the M(1) and A(1) crystallographic sites. All four compounds are paramagnetic above 40 K and antiferromagnetically ordered below. Above 40 K the effective magnetic moments of NaMnFe2(PO4)3 and Na2MnFeIIFeIII(PO4)3 are those expected of high-spin manganese(II) and iron(III) with the 6 A 1 g electronic ground state and high-spin iron(II) with the 5 T 2 g electronic ground state. In contrast, the effective magnetic moment of Na2Mn2Fe(PO4)3 is lower than expected as a result of enhanced antiferromagnetic exchange coupling by the manganese(II) on the M(2) site. The Mössbauer spectra of all four compounds have been measured from 4.2 to 295 K and have been found to be magnetically ordered below 40 K for Na2Mn2Fe(PO4)3 and ∼35 K for the remaining compounds. The Mössbauer spectra of Na2Mn2Fe(PO4)3 exhibit the two expected iron(III) quadrupole doublets and/or magnetic sextets expected for a random distribution of manganese(II) and iron(III) ions on the M(2) site. Further, the Mössbauer spectra of Na2MnFeIIFeIII(PO4)3 exhibit the two iron(II) and two iron(III) quadrupole doublets and/or magnetic sextets expected for a random distribution of iron(II) and iron(III) on the M(2) site. Surprisingly, the synthetic and natural samples of NaMnFe2(PO4)3 have 19 and 10% of iron(II) on the M(2) site; apparently the presence of some iron(II) stabilizes the alluaudite structure through the reduction of iron(III)-iron(III) repulsion. The temperature dependence of the iron(II) quadrupole splitting yields a 440 to 600 cm−1 low-symmetry component to the octahedral crystal field splitting at the M(2) site. The iron(II) and iron(III) hyperfine fields observed at 4.2 K are consistent with the presence of antiferromagnetic ordering at low temperatures in all four compounds.



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