Stability of the Local Ni${}^{2+}$ Electronic Structure to $A$-site Disorder in the Pyrochlore Antiferromagnet NaCaNi${}_2$F${}_7$

Abstract

NaCaNi${}_2$F${}_7$ is a unique example of spin-1 Heisenberg antiferromagnet on the pyrochlore lattice, but the presence of Na${}^{1+}$/Ca${}^{2+}$ $A$-site disorder complicates the local electronic and magnetic environment of the Ni${}^{2+}$ $B$-site. We utilize resonant inelastic X-ray scattering (RIXS) to study the influence of $A$-site disorder on the $B$-site electronic structure of NaCaNi${}_2$F${}_7$. Ni L-edge RIXS measurements reveal a Ni${}^{2+}$ electronic structure in nearly ideal octahedral coordination, with only a small trigonal compression ($\delta$ = -200meV) required to capture all spectral features. Within the $D_{3d}$ symmetry of the Ni local environment, we extract an anisotropic $g$-factor of $g_{\parallel }=2.26$ and $g_{\perp }=2.27$, and a corresponding paramagnetic moment of ${\mu }_{\mathrm{eff}}=3.2{\mu }_B$. To simulate disorder, RIXS spectra were calculated with realistic distributions of crystal field parameters; however, these spectra are invariant relative to a disorder-free model, demonstrating the robustness of the Ni${}^{2+}$ electronic environment to the $A$-site disorder, within the resolution of our measurement.

12 pages, 7 figures