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High - pressure synchrotron x-ray diffraction study of RMnO3 (R = Eu, Gd, Tb and Dy) upto 50 GPa

We have carried out synchrotron based high-pressure x-ray diffraction study of orthorhombic EuMnO3, GdMnO3, TbMnO3 and DyMnO3 up to 54.4, 41.6, 47.0 and 50.2 GPa, respectively. The diffraction peaks of all the four manganites shift monotonically to higher diffraction angles and the crystals retain the orthorhombic structure till the highest pressure. We have fitted the observed volume versus pressure data with the Birch-Murnaghan equation of state and determined the bulk modulus to be 185 ± 6 GPa, 190 ± 16 GPa, 188 ± 9 GPa and 192 ± 8 GPa for EuMnO3, GdMnO3, TbMnO3 and DyMnO3, respectively. The bulk modulus of EuMnO3 is comparable to other manganites, in contrast to theoretical predictions.


Introduction
Rare-earth manganites RMnO 3 (R = Eu, Gd, Tb, and Dy) have attracted a lot of interest due to their coupling between ferroelectric and anti-ferromagnetic properties [1]. RMnO 3 compounds which contain large R ions crystallize in the orthorhombic structure while those with smaller R ions crystallize in hexagonal structure. The boundary between these structures is present between Dy and Ho. At room temperature, R = Eu, Gd, Tb and Dy are paraelectric and paramagnetic with a distorted perovskite structure (due to tilting and deformation of MnO 6 octahedra) of orthorhombic symmetry with space group Pbnm [2 -4]. In comparison to many studies under temperature and magnetic field, only a few experimental studies have been reported under pressure in perovskite RMnO 3 , R = Tb and Dy [5,6] and R = La, Pr and Sm [7][8][9][10]. Theoretical calculations [11] have shown that the bulk moduli in RMnO 3 (R = Dy-La) decreases with increasing molar volume with an anomalous dip for EuMnO 3 . This motivated us to perform high pressure synchrotron based x-ray diffraction measurements on Eu, Gd, Tb and DyMnO 3 to determine the equation of state and look for possible phase transitions.

Experimental
High-pressure x-ray experiments were performed at room temperature at beamline X17C of the National Synchrotron Light Source at Brookhaven National Laboratory. Powder samples were loaded along with ruby chips, into a hole of ~100 µm diameter drilled in a pre-indented steel gasket. A Mao-Bell type diamond-anvil cell (DAC) was used with methanol:ethanol (4:1) mixture as pressuretransmitting medium. Pressure was monitored using the ruby fluorescence technique [12]. The  3 . The 2D images were transformed into plots of intensity versus 2θ by radial integration using the FIT2D software package. Since the ambient structure is known, the lattice parameters and unit cell volumes were determined at each pressure by profile refinement of all four samples using Jana2000 package [13] without doing structural refinement. The equation of state was determined from the observed volume versus pressure using Igor Pro software.

Results and discussion
Eu, Gd, Tb and DyMnO 3 crystallize in the orthorhombic structure (space group Pbnm -no. 62). With increasing pressure the diffraction peaks shifts monotonically to higher diffraction angle value till the highest pressures without any phase transition. The lattice parameters and unit cell volumes at each pressure were determined. The zero pressure bulk modulus is found to show a small increase with decreasing ionic radius (molar volume also decreases). Figure 1a shows x-ray diffraction patterns, after background subtraction at selected pressures of

GdMnO 3
The lattice parameters and the unit cell volume of GdMnO 3 at the lowest pressure were determined to be: a = 5.3202 Å, b = 5.8627 Å, c = 7.4291 Å and V = 231.7 Å 3 which are in good agreement with the reported values [2]. Figure 2a shows the variation of the unit cell volume with pressure. The solid line represents the Birch-Murnaghan EOS fit to the experimental data. This yields a bulk modulus of 190 ± 16 GPa and a first pressure derivative of 3.2 ± 1.6.

TbMnO 3
Profile refinement of the diffraction peaks collected at 0.2 GPa yields the lattice parameters: a = 5.2939 Å, b = 5.8407 Å, c = 7.3886 Å and V = 228.5 Å 3 which match with previously reported values [3]. Chen et al. have reported pressure-induced variations of the electronic state as well as structural distortion of Tb and DyMnO 3 but the equation state was not measured [5,6]. The pressure dependence of the unit cell volume for TbMnO 3 is shown in Figure 2b. The bulk modulus and its pressure derivative were determined by fitting the Birch-Murnaghan EOS to the data. The results are K o = 188 ± 9 GPa and K 0 ' = 8.9 ± 1.2.

DyMnO 3
The lattice parameters and unit cell volume (a = 5.2713 Å, b = 5.8335 Å, c = 7.3786 Å and V = 226.9 Å 3 ) were determined from the profile refinement of the diffraction pattern recorded at the lowest pressure (0.7 GPa). Figure 3 shows the pressure dependence of the unit cell volume. The Birch-Murnaghan EOS fit to the pressure versus volume data yields a bulk modulus of K o = 192 ± 8 GPa and its first pressure derivative K 0 ' = 1.6 ± 0.6.  and the structure deviates from orthorhombic to distorted orthorhombic. The unit cell volume decreases as the distortion increases correspondingly the bulk modulus increases. A noteworthy observation from figure 4 is that the experimental value of the bulk modulus for EuMnO 3 does not show an anomalous decrease as compared to the theoretical predictions [11].

Conclusion
In conclusion, we have performed a synchrotron based high-pressure study on rare-earth manganites, RMnO 3 (R = Eu, Gd, Tb and Dy) up to 54.4, 41.6, 47.0 and 50.2 GPa, respectively. Our studies do not reveal any phase transition, and structure remains orthorhombic till the highest pressures. The fit to the experimental data using the Birch-Murnaghan EOS yields the zero pressure bulk modulus to be 185, 190, 188 and 192 GPa in Eu, Gd, Tb and DyMnO 3 , respectively. These values are compared with the theoretical predictions [11].