Dynamic and structural properties of orthorhombic rare-earth manganites under high pressure

We report a high-pressure study of orthorhombic rare-earth manganites $A{\mathrm{MnO}}_3$ using Raman scattering (for $A=Pr$, Nd, Sm, Eu, Tb, and Dy) and synchrotron x-ray diffraction (XRD), for $A$ = Pr, Sm, Eu, and Dy. In all cases, a phase transition was evidenced by the disappearance of the Raman signal at a critical pressure that depends on the $A$ cation. For the compounds with $A=Pr$, Sm, and Dy, XRD confirms the presence of a corresponding structural transition to a noncubic phase, so that the disappearance of the Raman spectrum can be interpreted as an insulator-to-metal transition. We analyze the compression mechanisms at work in the different manganites via the pressure dependence of the lattice parameters, the shear strain in the $ac$ plane, and the Raman bands associated with out-of-phase ${\mathrm{MnO}}_6$ rotations and in-plane O2 symmetric stretching modes. Our data show a crossover across the rare-earth series between two different kinds of behavior. For the smaller $A$ cations considered in this study (Dy and Tb), the compression is nearly isotropic in the $ac$ plane, with only small evolutions of the tilt angles and cooperative Jahn-Teller distortion. As the radius of the $A$ cation increases, the pressure-induced reduction of Jahn-Teller distortion becomes more pronounced and increasingly significant as a compression mechanism, while the pressure-induced tilting of octahedra chains becomes conversely less pronounced. We finally discuss our results in light of the notion of chemical pressure and show that the analogy with hydrostatic pressure works quite well for manganites with the smaller $A$ cations considered in this paper but can be misleading with large $A$ cations.

Figure 1

XRD patterns of (a) ${\mathrm{PrMnO}}_3$ and (b) ${\mathrm{SmMnO}}_3$, recorded at PETRA III (λ = 0.29118 Å), (c) ${\mathrm{EuMnO}}_3$, and (d) ${\mathrm{DyMnO}}_3$, recorded at ID27 (λ = 0.3738 Å) at selected pressures (expressed in GPa).

Figure 2

Pressure dependence of the pseudocubic lattice parameters and volume of ${\mathrm{PrMnO}}_3$, ${\mathrm{SmMnO}}_3$, ${\mathrm{EuMnO}}_3$, and ${\mathrm{DyMnO}}_3$: red circles $a_{pc}$, green diamonds $c_{pc}$, blue squares $b_{pc}$. The vertical dashed lines mark the transition pressure. The solid lines in the volume plots are the best fits to the Birch-Murnaghan equation of state [see Eq. (1)]. For ${\mathrm{SmMnO}}_3$, whose high-pressure structure could not be unambiguously determined, we do not show the lattice parameters in the high-pressure phase.

Figure 3

Unpolarized Raman spectra of the $A{\mathrm{MnO}}_3$ compounds at ambient conditions. The out-of-phase ${\mathrm{MnO}}_6$ rotation mode $[A_g$(4)] and the in-plane O2 symmetric stretching mode $[B_{2g}$(1)] are labeled in this figure. The sketches show the atomic motions involved in these modes.

Figure 4

Raman spectra of the $A{\mathrm{MnO}}_3$ compounds, recorded at room temperature and at fixed hydrostatic pressure (expressed in GPa). The bands marked with asterisks correspond to Raman modes arising from ${\mathrm{N}}_2$ and ${\mathrm{O}}_2$ solid phases, which were unintentionally introduced as a minor phase in a helium transmitting medium in the DAC preparation procedure. For the ${\mathrm{PrMnO}}_3$ and ${\mathrm{NdMnO}}_3$ cases, a background Raman spectrum measured next to the sample in the pressure transmitting medium is also shown for comparison.

Figure 5

Critical pressures as a function of ionic radius determined from XRD patterns (circles) and Raman spectra (squares) upon increasing pressure. The value of transition pressure for ${\mathrm{LaMnO}}_3$ and ${\mathrm{GdMnO}}_3$ were obtained from Refs. [5, 15], respectively.

Figure 6

(a) Mode Grüneisen parameter γ of the out-of-phase ${\mathrm{MnO}}_6$ rotations $A_g$(4) (circles) and of the in-plane O2 symmetric stretching $B_{2g}$(1) (squares) modes. The inset depicts the coefficient β, defined by Eq. (4). (b) Compressibility of the lattice parameters as a function of the $A$ cation size: ${\delta }_a$ circles, ${\delta }_b$ squares, and ${\delta }_c$ diamonds. Closed symbols are our data; open symbols for ${\mathrm{LaMnO}}_3$ and ${\mathrm{TbMnO}}_3$ were calculated from the data reported in Refs. [5, 12], respectively. The dashed lines are guides for the eyes.

Figure 7

Pressure dependence of the shear strain $e_4$ for $A{\mathrm{MnO}}_3$, with $A$ = La, Pr, Sm, Eu, Gd, Tb, and Dy. Data for ${\mathrm{LaMnO}}_3$ and ${\mathrm{GdMnO}}_3$ were taken from Refs. [5, 15], respectively.

Figure 8

Pseudocubic lattice parameters of some orthorhombic rare-earth manganites, as a function of $V_{pc}{}^{1/3}$, measured at room conditions and at different fixed hydrostatic pressures.