Spin-state equilibria in theLCoO3system fromCo59NMR

Abstract

Spin-state equilibria in the whole set of $L\mathrm{Co}{\mathrm{O}}_{3}$ (where $L$ stands for a rare-earth metal or Y) have been investigated with the use of $^{59}\mathrm{Co}$ NMR as a probe for the polycrystalline samples (except Ce) in the temperature interval 110-550 K and frequency range 3- 11.6 MHz. Besides confirming the coexistence of the high-spin---low-spin state in this temperature range, a quadrupolar interaction of \ensuremath{\sim}0.1 -0.5 MHz has been detected for the first time from $^{59}\mathrm{Co}$ NMR. The NMR line shape is found to depend strongly on the relative magnitude of the magnetic and quadrupolar interactions present. Analysis of the powder pattern reveals two basically different types of transferred hyperfine interaction between the lighter and heavier members of the rare-earth series. The first three members of the lighter rare-earth metals La, Pr (rhombohedral), and Nd (tetragonal), exhibit second-order quadrupolar interaction with a zero-asymmetry parameter at lower temperatures. Above a critical temperature ${T}_{S}$ (dependent on the size of the rare-earth ion), the quadrupolar interaction becomes temperature dependent and eventually gives rise to a first-order interaction thus indicating a possible second-order phase change. Sm and Eu (orthorhombic) exhibit also a second-order quadrupolar interaction with a nonzero asymmetry parameter (($\ensuremath{\eta}\ensuremath{\sim}0.47$)) at 300 K, while the orthorhombic second-half members (Dy,..., Lu and Y) exhibit first-order quadrupolar interaction at all temperatures. Normal paramagnetic behavior, i.e., a linear variation of ${K}_{\mathrm{iso}}$ with ${T}^{\ensuremath{-}1}$, has been observed in the heavier rare-earth cobaltites (Er,..., Lu and Y), whereas an anomalous variation has been observed in (La,..., Nd)Co${\mathrm{O}}_{3}$. Thus, ${K}_{\mathrm{iso}}$ increases with increasing temperature in PrCo${\mathrm{O}}_{3}$ and NdCo${\mathrm{O}}_{3}$. These observations corroborate the model of the spin-state equilibria in $L\mathrm{Co}{\mathrm{O}}_{3}$ originally proposed by Raccah and Goodenough. A high-spin---low-spin ratio, $r=1$, can be stabilized in the perovskite structure by a cooperative displacement of the oxygen atoms from the high-spin towards the low-spin cation. Where this ordering into high- and low-spin sublattices occurs at $r=1$, one can anticipate equivalent displacement of all near-neighbor oxygen atoms towards a low-spin cobalt ion. Thus the heavier $L\mathrm{Co}{\mathrm{O}}_{3}$ exhibits a small temperature-independent first-order quadrupolar interaction. Where $r<1$, the high- and low-spin states are disordered, giving rise to a temperature-dependent second-order quadrupolar interaction with an anomalous ${K}_{\mathrm{iso}}$ for the lighter $L\mathrm{Co}{\mathrm{O}}_{3}$.