A structural study of PrCrO3 under extreme conditions: a comparison with the effects of doping

Abstract

The nuclear and magnetic structure of [Formula: see text] has been investigated using neutron and X-ray powder diffraction as a function of pressure and temperature. The orthorhombic symmetry (space group [Formula: see text]) remains stable up to the highest temperature (1500 K) and pressure (approx. [Formula: see text]) considered. There is a crossover in the magnitude of the a- and b-lattice parameters at approximately 1135 K, caused by competing effects of octahedral tilting and distortion. The material is antiferromagnetic ([Formula: see text] K) with [Formula: see text] symmetry, with a maximum moment of [Formula: see text] on the [Formula: see text] sites aligned along the direction of the [Formula: see text]-axis. The application of pressure shows an abnormal softening in the unit-cell volume, which is suggestive of a continuous approach to a second-order phase transition. Raman spectroscopy measurements at ambient temperature were collected as a function of pressure up to approximately [Formula: see text] GPa, with discontinuous mode behaviour further suggesting the existence of a transition above 7 GPa. The measured structural changes in [Formula: see text] are compared extensively in the wider context of other lanthanide orthochromites, and the comparative effects of A- and B-site substitution on the polyhedral tilts and distortion are discussed. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 1)'.

Keywords: distortion; lanthanide-perovskites; orthochromites; polyhedral-tilting; structure.

Figure 1.

(a) Structure of ${\mathrm{PrCrO}}_3$ under ambient conditions. The ${\mathrm{CrO}}_6$ octahedral units are shown in blue with oxygen atoms (red) at the shared corners. The ${\mathrm{PrO}}_n$ polyhedral units are shown in yellow. The isolated ${\mathrm{PrO}}_n$ units are shown below the main structure, for $n=8$ (lower-left) and $n=12$ (lower-right). The more obvious additional oxygen atoms for $n=12$ are indicated by an asterisk and the remaining additional oxygen atoms are behind other more visible oxygen atoms. (b) Ambient pressure/temperature neutron diffraction pattern and Rietveld refinement of Pbnm structure of ${\mathrm{PrCrO}}_3$. Data collected from Polaris $2\theta ={90}^{\circ }$ detector banks. The collected data are shown as open black circles, the Rietveld fit as the red trace and the residual of the fit to the data as the blue trace. The vertical tick marks show the expected reflection positions for the orthorhombic unit cell described in the main text. (Online version in colour.)

Figure 2.

(a) Temperature dependence of the lattice parameters of ${\mathrm{PrCrO}}_3$ reduced to their pseudo-cubic form (see main text). (b) Calculated tetragonal ($e_{tx}$) and shear ($e_4$) strain of the system as a function of temperature (the arrows indicate which dataset is plotted on each axis). In both figures, the data below 280 K (blue region) were collected from neutron powder diffraction (open symbols), and those above (red region) were collected using X-ray diffraction (solid symbols). (Online version in colour.)

Figure 3.

Calculated anti-phase, $a^{-}$ (a), and in-phase, $c^{+}$ (b), tilt angles plotted as a function of $A$-site ionic radius for a number of lanthanide orthochromites found in the literature [,–50], and from the present study. The ionic radii are compared assuming 3+ oxidation, and eightfold coordination [52]. The lines in both (a) and (b) are linear best fits to the neutron data at ambient temperature. Also shown are the calculated distortion mode amplitudes for X5+, oxygen octahedra (c) $A$-site cation (d). The outlier in (c) is for the Gd compound, so it is assumed that absorption corrections have led to an inaccurate structure determination from the neutron data. In all figures, the crosses are calculated from X-ray diffraction data (room temperature), while the squares (low temperature 2–20 K) and circles (room temperature) are calculated from neutron diffraction data. (Online version in colour.)

Figure 4.

Neutron diffraction pattern and Rietveld refined fit of ${\mathrm{PrCrO}}_3$ at 5.6 GPa. The experimental data are shown as open circles, the Rietveld fit as the red line and the blue line is the residual of the fit to the data. The tick marks show the expected positions of reflections from orthorhombic ${\mathrm{PrCrO}}_3$ (red), Pb pressure marker (blue), ${\mathrm{Al}}_2{\mathrm{O}}_3$ (green) and ${\mathrm{ZrO}}_2$ (purple), from top to bottom respectively. The ${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{ZrO}}_2$ contribution are from the anvil material used in the high-pressure assembly. (Online version in colour.)

Figure 5.

Behaviour of unit-cell parameters of ${\mathrm{PrCrO}}_3$ with pressure at 290 K. (a) Variation in pseudo-cubic lattice parameter where $a_{\text{pc}}=a_o/\sqrt{2}$ (open squares), $b_{\text{pc}}=b_o/\sqrt{2}$ (open circles) and $c_{\text{pc}}=c_o/2$ (open triangles). The transparent hexagon symbols are the pseudo-cubic lattice parameter ($a_{\text{pc}}$) determined from the orthorhombic unit cell volume ($V_o$) as $a_{\text{pc}}={(V_o/4)}^{1/3}$. (b) Change in unit-cell volume (squares) with pressure. The third-order Birch–Murnaghan EoS fit to the data is shown as solid red line and determined values given in main text. Error bars are shown but smaller than symbols. (c) Determined tetragonal strain ($e_{\text{tz}}$) and shear strain ($e_4$) with pressure (see text for details). (Online version in colour.)

Figure 6.

(a) Variation in the ${\mathrm{CrO}}_6$ (rectangle) and ${\mathrm{PrO}}_8$ (circle) polyhedral volumes with pressure in ${\mathrm{PrCrO}}_3$. (b) Variation in the average M–O bond length ($⟨\mathrm{M}\text{−}\mathrm{O}⟩$) within the ${\mathrm{CrO}}_6$ (rectangle) and ${\mathrm{PrO}}_8$ (circle) polyhedra. The values in both (a,b) are normalized against ambient pressure values.

Figure 7.

Raman spectra of ${\mathrm{PrCrO}}_3$ at ambient conditions, with mode assignments (top). Behaviour of Raman modes of ${\mathrm{PrCrO}}_3$ with increasing pressure (bottom). Note that there are breaks in the $y$-axes of all of the plots.