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We report the structural, magnetic, transport and electronic properties of Nbsubstituted LaCo$_{1-x}$Nb$_x$O$_3$ ($x =$ 0--0.2). The Rietveld analysis ofx-ray diffraction data demonstrate structural phase transitions fromrhombohedral to orthorhombic and further to monoclinic with increasing the Nbconcentration up to $x \ge$ 0.2. Interestingly, we observed dramatic changes inthe magnetization (M) with increasing the Nb concentration, as the M sharplyincreases below 10~K even at 2.5\% substitution. Furthermore, ac susceptibilitydata show the spin-glass behavior in $x=$ 0.1 sample. We find that the densityof states near the Fermi level decreases and the activation energy increases,which results in the decreasing conductivity with higher Nb concentration. Asignificant shift in the peak position of A$_{2g}$ phonon mode has beenobserved using Raman spectroscopy, which indicates the change in the couplingdue to the structural distortion with Nb substitution. The core-levelphotoemission study confirms that the Nb is present in 5+ valence state. Ourstudy reveals that the nonmagnetic Nb$^{5+}$ ($d^0$) substitution convertsCo$^{3+}$ ions to Co$^{2+}$ and stabilize both in the high-spin state. Ourresults suggest that structural and spin-state transitions as well as thedifference in the ionic radii between Nb$^{5+}$ and Co$^{3+}$ are playing animportant role in tuning the physical properties.

Anomalous magnetic and spin glass behavior in Nb substituted LaCo$_{1-x}$Nb$_x$O$_3$