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

Abstract

The compounds Ca 14 AlP 11 , Ca 14 MnP 11 , and Eu 14 MnP 11 have been synthesized and are isostructural to the Zintl compound, Ca 14 AlSb 11 . Eu 14 MnP 11 is a ferromagnet showing colossal magnetoresistance at T C ) 52 K, Ca 14 MnP 11 shows paramagnetic behavior down to 5 K, and Ca 14 AlP 11 is nonmagnetic. The formula unit for these Zintl compounds is A 14 MPn 11 and comprises 14 A cations, a MPn 4 tetrahedron, four isolated Pn anions, and a Pn 3 linear unit. The central Pn of the Pn 3 unit is believed to be positionally disordered, and it has been speculated that the disorder is static rather than dynamic. To address this issue, 31 P solidstate 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 31 P nuclei as well as the quadrupolar coupling for the 27 Al nucleus in the diamagnetic compound Ca 14 AlP 11 provide structural details consistent with the Zintl structure. Wideline 31 P NMR and EPR were applied to the paramagnetic compounds Ca 14 MnP 11 and Eu 14 MnP 11 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.

Structure and magnetic properties of Ca14MnP11

“…X-ray Magnetic Circular Dichroism (XMCD) experimental spectra [19] for Yb 14 MnSb 11 shows that in this compound, Mn is best considered to be Mn 2+ with an antiferromagnetically coupled spin on Sb [19]. ESR measurements on Ca 14 AlP 11 show no signal, as expected from the Zintl electron counting, and Ca 14 MnP 11 show a broad signal at g ¼ 2:00, suggesting uncompensated spin density [20].…”

“…Ca 14 MnP 11 is isotypic to Ca 14 AlSb 11 and part of a family of Mncontaining transition metal Zintl phases. This new phase completes the series of Ca 14 MnPn 11 , where Pn ¼ P, As, Sb, Bi [26], and has already been studied by wide-line 31 P solid-state NMR and EPR [20]. Efforts to prepare large crystals for detailed magnetic and transport properties have been unsuccessful to date.…”

“…The Ca 14 AlSb 11 structure type has been well-studied and the phosphorus-containing analogs have been described in detail [5,20,24,25]. Ca 14 MnP 11 is the second example of a Mn-containing phosphorus analog, with the only other example being Eu 14 MnP 11 which shows ferromagnetic ordering and colossal magnetoresistance [5,20].…”

“…Ca 14 MnP 11 is the second example of a Mn-containing phosphorus analog, with the only other example being Eu 14 MnP 11 which shows ferromagnetic ordering and colossal magnetoresistance [5,20]. The structure of Ca 14 MnP 11 will be briefly described and compared to the Eu analog as well as to the main group compounds, Ca 14 AlP 11 , Ca 14 GaP 11 , and Ba 14 InP 11 [20,24,25]. The compound, Ca 14 MnP 11 crystallizes in the space group I4 1 =acd.…”

“…EPR and 31 P solid-state NMR on Ca 14 MnP 11 has been published [20]. That paper investigated the structural status of the P 3 trimer.…”