Phonon Raman scattering of $R$CrO${}_3$ perovskites ($R=\mathrm{Y}$, La, Pr, Sm, Gd, Dy, Ho, Yb, Lu)

We report a systematic investigation of orthorhombic perovskite-type $R$CrO${}_3$ powder samples by Raman scattering for nine different rare earth $R^{3+}$ cations ($R$ $=$ Y, La, Pr, Sm, Gd, Dy, Ho, Yb, and Lu). The room-temperature Raman spectra and the associated phonon mode assignment provide reference data for structural investigation of the whole series of $R$CrO${}_3$ orthochromites and phonon ab-initio calculations. The assignment of the chromite spectra and comparison with Raman data on other orthorhombic perovskites allows correlating the phonon modes with the structural distortions in the $R$CrO${}_3$ series. In particular, two $A_g$ modes are identified as octahedra rotation soft modes, as their positions scale linearly with the octahedra tilt angle of the CrO${}_6$ octahedra.

Figure 1

Variation of the pseudocubic cell parameters and the tolerance factor for $R$CrO${}_3$ as a function of the $R^{3+}$${}_{\mathrm{VIII}}$ ionic radius. The lattice parameters for NdCrO${}_3$ are taken from Ref. 38, for all other chromites from Ref. 10.

Figure 2

Raman spectra at 300 K of $R$CrO${}_3$ powders ($R$ $=$ La, Pr, Sm, Gd, Dy, Ho,Yb, and Lu) collected with a 632.8-nm He-Ne laser line. I, II, III, and IV denote specific spectral regions discussed in the text. The red/dark gray arrows illustrate for a specific mode, how the phonons can be followed through the series of $R$CrO${}_3$.

Figure 3

Comparison of Raman spectra for of HoCrO${}_3$ (bottom, black), YCrO${}_3$ (middle, red/dark gray), and DyCrO${}_3$ (top, black) illustrating a significant shift of the low wave number modes for YCrO${}_3$. The assignment of $A_g$, $B_{1g}$, $B_{2g}$, and $B_{3g}$ modes is based on Ref. 24 for YCrO${}_3$. The inset shows the mass-dependent evolution of the two low wave number bands.

Figure 4

Detailed view of the mid wave number spectral region III. (a) Raman modes and (b) evolution of the band positions with $r_R$ for $R$CrO${}_3$.

Figure 5

Detailed view and assignment of the Raman modes in the low wave number and mid wave number regions for LaCrO${}_3$, PrCrO${}_3$, and SmCrO${}_3$. The band at 160 cm${}^{-1}$, marked with an asterisk is a spurious line, likely related to an impurity phase. The assignment of LaCrO${}_3$, specifically the position and assignment of the 104-cm${}^{-1}$ $A_g$ band marked by a red/dark gray line, is from Ref. 22.

Figure 6

Raman phonon wave numbers of $R$CrO${}_3$ as a function of the rare earth $R^{3+}$ ionic radius. All lines are guides to the eye only. The dashed lines for the low wave number modes indicate schematically the region of mode mixing, discussed in the text.

Figure 7

Phonon wave numbers as a function of octahedra tilt angles for $A_g$ (3) [red/dark gray squares] and $A_g$ (5) [black circles] modes of those $R$CrO${}_3$ for which the tilt angle can be calculated from available atomic positions. Raman band positions for NdCrO${}_3$ (Ref. 38) and ErCrO${}_3$ (Ref. 37) are from the literature.