Magnetic Resonance Study of a Series of Phosphorus-Containing Zintl Compounds: Ca14AlP11, Ca14MnP11, and Eu14MnP11

Abstract

The compounds Ca14AlP11, Ca14MnP11, and Eu14MnP11 have been synthesized and are isostructural to the Zintl compound, Ca14AlSb11. Eu14MnP11 is a ferromagnet showing colossal magnetoresistance at TC = 52 K, Ca14MnP11 shows paramagnetic behavior down to 5 K, and Ca14AlP11 is nonmagnetic. The formula unit for these Zintl compounds is A14MPn11 and comprises 14 A cations, a MPn4 tetrahedron, four isolated Pn anions, and a Pn3 linear unit. The central Pn of the Pn3 unit is believed to be positionally disordered, and it has been speculated that the disorder is static rather than dynamic. To address this issue, 31P solid-state NMR was applied to these new compounds to provide insight into the magnetic ordering and structural changes. The NMR chemical shift anisotropy and the dipolar coupling for the 31P nuclei as well as the quadrupolar coupling for the 27Al nucleus in the diamagnetic compound Ca14AlP11 provide structural details consistent with the Zintl structure. Wide-line 31P NMR and EPR were applied to the paramagnetic compounds Ca14MnP11 and Eu14MnP11 to obtain similar structural information. The lack of any EPR spectra for the Eu-substituted compound below 50 K confirms the assigned paramagnetic-to-ferromagnetic transition temperature of 52 K. Hyperfine couplings in the Eu-substituted compound suggests localization of the electron density above 50 K.