Structural induced Electronic Complexity controlled by low temperature Topotactic Reaction – SECTOR

During this project, we investigated in detail two oxygen intercalation reactions followed up in situ using synchrotron radiation on BM01@ESRF in a specially designed electrochemical cell. Such experiments are extremely challenging from the experimental setup as tiny crystals of < 50 µm size have to be oriented, shaped and glued, which requires sufficient know-how. We could realise another two very important experimental set-ups to follow structural changes by neutron and X-ray diffraction while modifying oxygen partial pressure in situ. The first device was realized in collaboration with the FRM2, where a large gas-proof quartz capillary was implemented in an existing mirror furnace on the single crystal diffractometer HEIDI, allowing to follow up structural changes in situ as a function of temperature and oxygen stoichiometry of any non-stoichiometric oxide, and this at temperatures above 900°C. This is not only a challenge in terms of in situ studies on single crystals but equally allows to make single crystal neutron measurements even above 900°C, which was a real problem in terms of availability at any EU neutron sources. The 2nd device concerns a high-pressure oxygen cell, suitable to explore by in situ single crystal X-ray diffraction the phase diagram of non-stoichiometric oxides. It allows to operate up to 800°C and 100 bars oxygen gas pressure, allowing to explore in situ the phase diagram of non-stoichiometric oxides using single crystals by diffraction methods. This allows to access to diffuse scattering and changes in the domain/twin structure, not available to access with powder methods.

The understanding of the extremely complex pattern of Pr2NiO4.25, obtained by single crystal synchrotron diffraction and especially on MX (biology) beamlines, as it turned out to be a 3D incommensurate phase with satellites up to 9th order and up to 8 incommensurate domains with different orientation. We are now thus able to identify unambiguously charge ordering present as checkerboard and spin charge ordering, which is an important breakthrough in this field. Also, the findings of an electronic phase separation in terms of charge ordering obtained from single crystal synchrotron diffraction shows the way to understand lattice dynamics and specifically magnetic excitations in terms of possible hour-glass type spectra related to the coherence length of charge order/disorder and respective domain size. We successfully studied the oxygen intercalation reaction on SrFeO2.5 as well as Pr2NiO4.25, yielding very important information in terms of diffuse scattering, twinning and time dependent intensities on specific twin individuals, not possible to obtain by classic powder diffraction. In addition, we have now a high oxygen gas pressure cell, suitable for single crystal X-ray diffraction, at our disposal. It allowed us to follow up pressure induced phase transitions on Pr2NiO4.25 at 500°C between 0 and 100 bars. An excellent opportunity to measure structural changes as a function of the oxygen stoichiometry by in situ using single crystal neutron diffraction has been realized in collaboration with the FRM2 staff. The modified mirror furnace allows in a gas-tight atmosphere to use any oxygen partial pressure for adjusting the required oxygen stoichiometry and to determine the respective structure with high resolution diffraction on HEIDI@FRM2.

An important work has been conducted on Pr2NiO4.25 and the isoelectronic phase in terms of hole concentration Pr1.5Sr0.5NiO4.0, in order to explore spin, charge and oxygen ordering as a function of hole-doping and applied external conditions as temperature and p(O2). We are extremely proud, having succeeded to solving the very puzzling and demanding diffraction pattern of Pr2NiO4.25, which in-fine turned out as a 3D incommensurate structure, using several synchrotron diffraction beamlines, including MX-type (BM01, ID29, ID28 side station, as well as ID23). While we could index satellite reflections up to the 9th order, the appearance of checkerboard-type charge ordering becomes clearly evident at already ambient. We consider today Pr2NiO4.25 to be one of the most complex transition metal oxides ever. Importantly we observed upon cooling the segregation into two distinct but well-defined charge ordered phases (checkerboard and stripe charge ordering), while the nuclear structure remains invariant. We could unambiguously attribute the reflections of charge ordering in a nightmare of reflections related to oxygen ordering. These studies have not only relevance to better understand large scale oxygen ordering, but also point towards a better understanding of the lattice dynamics, including magnetic excitations since nano-separation of charge ordered phase have been claimed to induce hour-glass type magnetic excitations, not only for La2-xSrxCoO4 (nature Commun. 5, 5731 (2014). In this regard, we intend to carry out inelastic measurements and check for possible changes in the magnetic excitations for ordered and quenched charge disorder. The synchrotron studies were completed with neutron diffraction studies on the low background diffractometer DMC@PSI, where in addition incommensurate spin ordering could be evidenced essentially for Pr2NiO4.25 and Pr1.5Sr0.5NiO4.0 opening the way for inelastic magnetic studies on EIGER@PSI and PUMA/PANDA@FRM2.

1. Solid-state reactivity explored in situ by synchrotron radiation on single crystals: from SrFeO2.5 to SrFeO3 via electrochemical oxygen intercalation, A. Maity, B. Penkala, R. Dutta, M. Ceretti, A. Lebranchu, D. Chernyshov, A. Perichon, A. Piovano, A. Bossak, M. Meven, W. Paulus, J Physics D: Applied Physics, Special issue: 100 years of crystallography: new dimensions offered by large scale facilities, J. Phys. D: Appl. Phys. 48 (2015), highlight 2015 IOP special issues doi:10.1088/0022-3727/48/50/504004 2. Evidence for monoclinic distortion in the ground state phase of underdoped La1.95Sr0.05CuO4: A single crystal neutron diffraction study”, Anar Singh, Jürg Schefer, Ravi Sura, Kazimierz Conder, Romain F. Sibille, Monica Ceretti, Matthias Frontzek, and Werner Paulus, Journal of Applied Physics 119, 123902 (2016); dx.doi.org/10.1063/1.4944797