High pressure behavior of Ga-doped LaMnO3: a combined X-ray diffraction and optical spectroscopy study

The pressure effects on the Jahn-Teller (JT) distortion of three representative compounds belonging to the LaMn1-xGaxO3 (x = 0.2, 0.3, 0.4) family were widely investigated by means of X-ray diffraction and Raman spectroscopy. A compound with a fully coherent JT distorted structure (x = 0.2), one with regular octahedra (x = 0.6) and one in an intermediate configuration (x = 0.3) were selected. A pressure induced transition from the orthorhombic Pbnm phase towards structures with higher symmetry was observed in all the samples. Both Raman and X-ray data confirm that the most important structural effect of pressure is that of reducing the octahedral distortion. The appearance of a feature in the lattice parameter behavior connected to a structural instability was also detected, pointing out the key role of the JT distortion in stabilizing the manganite structures. On the other hand, the complete suppression of the JT distortion in the high-pressure phases cannot be claimed. The Raman spectra collected on the more distorted compounds (x = 0.2, 0.3) reveal clearly the coexistence of domains of distorted and more regular octahedra in a certain pressure range. The first sketch of the pressure vs. Ga-content phase diagram was drawn.