Polarized Neutron Diffraction and Mössbauer Spectral Study of Short-Range Magnetic Correlations in the Ferrimagnetic Layered Compounds (PPh₄) [Fe^IIFe^III(ox)₃] and (NBu₄) [Fe^IIFe^III(ox)₃]

Short-range antiferromagnetic correlations have been studied in the layered compounds (PPh ₄ ) [Fe ^II Fe ^III (ox) ₃ ] and (NBu ₄ ) [Fe ^II Fe ^III (ox) ₃ ] by neutron polarization analysis and Mössbauer spectroscopy. Polarized neutron diffraction profiles obtained between 2 and 50 K on (d ₂₀ -Pph ₄ ) [Fe ^II Fe ^III (ox) ₃ ] show no magnetic Bragg scattering; the lack of such scattering indicates the absence of long-range magnetic order. However, a broad asymmetric feature observed at a Q of ca. 0.8 Å ^-1 is attributed to two-dimensional short-range magnetic correlations, which are described by a Warren function. The correlation length is ca. 50 Å between 2 and 30 K and then decreases to ca. 20 Å at 50 K. The Mössbauer spectra of (PPh ₄ ) [Fe ^II Fe ^III (ox) ₃ ] and (NBu ₄ ) [Fe ^II Fe ^III (ox) ₃ ] have been measured between 1.9 and 293 K and 1.9 and 315 K, respectively, and are very similar. The paramagnetic spectra exhibit both high-spin Fe ^II and Fe ^III doublets with relative areas which indicate a 5% and 2% excess, respectively, of Fe ^III . The coexistence in (PPh ₄ ) [Fe ^II Fe ^III (ox) ₃ ] between 10 and 30 K of broad sextets and doublets in the Mössbauer spectra and the paramagnetic scattering observed in the polarized neutron measurements indicate the coexistence of spin-correlated and spin-uncorrelated regions in the layers of this compound. The polarized neutron scattering profiles and the Mössbauer spectra yield the magnetic exchange correlation length and lifetime, respectively, and the combined results are best understood in terms of layers composed of random frozen, but exchange correlated domains of ca. 50 Å diameter at the lowest temperatures, of spin-correlated domains and spin-uncorrelated regions at intermediate temperatures, and of largely spin-uncorrelated regions above the Néel temperature as determined from magnetometry. The similarity of the Mössbauer spectra of (PPh ₄ ) [Fe ^II Fe ^III (ox) ₃ ] and (NBu ₄ ) [Fe ^II Fe ^III (ox) ₃ ] leads to the conclusion that similar magnetic exchange correlations are present in the latter compound.