Structural distortions in rare-earth transition-metal oxide perovskites under high pressure

Owing to their structural complexity and wide range of possible chemical combinations, perovskite oxides exhibit many technically important physical properties. Pressure is a thermodynamic parameter which is useful for tuning physical properties; however, the response of the complex crystal structure to high pressure has not been thoroughly studied and rationalized. This study focuses on in situ high-pressure x-ray diffraction of the orthorhombic perovskite oxides $A^{3+}{B}^{3+}{\mathrm{O}}_3$, commonly found for the rare-earth transition-metal oxides of the $RM{\mathrm{O}}_3$ formula. Each of the four families of $RM{\mathrm{O}}_3$ ($M=\mathrm{Ti}$, Cr, Mn, Fe) perovskites in this study all crystallize in the same orthorhombic perovskite structure with the Pbnm space group. The lanthanide contraction in these materials leads to varying degrees of orthorhombic distortions that are primarily associated with octahedral site rotations. The pressure-induced change of the lattice parameters demonstrates an evolution from a suppression to an enlargement of the orthorhombic distortion for substitution of the rare-earth element from $R=\mathrm{La}$ to Lu in $RM{\mathrm{O}}_3$ perovskites. This unusual crossover of the lattice parameters’ dependence on pressure contradict the results from first-principles calculation but can be rationalized by the intrinsic distortion of the perovskite structure.

Figure 1

The lattice parameters and the orthorhombic distortion parameter S as a function of ionic radii (IR) of rare earths (bottom $x$ axis) or tolerance factor $t$ (top $x$ axis). Solid symbols represent results from neutron diffraction, hollow symbols represent results from x-ray diffraction. The dashed lines are the SPuDs simulation results. The lower right inset shows the structural model of octahedra in a-b plane; the lower middle inset shows an octahedron projected through the apical oxygen.

Figure 2

Upper panels: lattice parameters as a function of pressure for two titanates, which are typical for the $RM{\mathrm{O}}_3$ with larger IR ($\mathrm{NdTi}{\mathrm{O}}_3$) and the smaller IR ($\mathrm{YbTi}{\mathrm{O}}_3$). Lower panels: S factor as a function of pressure for the two families, $R\mathrm{Cr}{\mathrm{O}}_3$ and $R\mathrm{Fe}{\mathrm{O}}_3$. S factor as a function of pressure for the $R\mathrm{Ti}{\mathrm{O}}_3$ and $R\mathrm{Mn}{\mathrm{O}}_3$ families are provided in Figs. S2 and S6 [35].

Figure 3

The reduced S factor as a function of pressure for all four families of $RM{\mathrm{O}}_3$ ($M=\mathrm{Ti}$, Cr, Mn, Fe).

Figure 4

Polar plot of the octahedral-site distortion in $R\mathrm{Mn}{\mathrm{O}}_3$ and $R\mathrm{Fe}{\mathrm{O}}_3$ orthorhombic perovskites. In the plot, an octahedron with two long bonds and four equally short bonds is located at 0° (with the long bonds along $z$ axis), 120° (long bonds on $x$ axis), and 240° (long bonds on $y$ axis). An octahedron with two long bonds, two medium bonds, and two short bonds is represented by a spot at an angle between these three designated angles. Two neighboring M ions in the $ab$ plane in the orthorhombic perovskite structure have their long bond pointing to the $x$ axis and $y$ axis of the primary cell, alternatively. Since the difference between the medium bond and the short bond is small, the octahedral-site distortions are described by two groups of points, one group near 120° and another near 240°. Mapping out the octahedral-site distortion for all members in the JT-inactive $R\mathrm{Fe}{\mathrm{O}}_3$ indicates a continuous evolution with a maximum distortion at $R=\mathrm{Gd}$. Similar plots can be obtained for other JT-inactive $RM{\mathrm{O}}_3$ perovskites, meaning that it is an intrinsic local distortion for the orthorhombic perovskite oxides. In the JT active $R\mathrm{Mn}{\mathrm{O}}_3$ perovskites, the bond-length splitting of an octahedron increases dramatically. The structural distortions for all members are concentrated in the small areas near 120° and 240°; a magnified plot (inset) reveals an evolution of ${\rho }_0$ as a function of IR in the family.

Figure 5

The bulk modulus vs inverse cell volume for three families of $RM{\mathrm{O}}_3$ perovskites.