ABO₃ oxides have proven to accommodate a wide variety of chemical compositions, to crystallise with several structures in competition and to develop diverse physical properties. Hence, they are intensively studied in the search for new functional materials. Among them, the use of high-pressure and high-temperature synthesis techniques allows the stabilisation of the small Mn²⁺ cation in the larger A site. Some of the most exciting A-site manganites are spintronic (e.g. perovskite MnVO₃-II) or multiferroic (e.g. LiNbO₃-type MnTiO₃-II) [1,2]. Mixing different cations into the A and/or B sites induces cation order and further magnetic complexity. Recent studies on high pressure Mn₂BB’O₆ compounds have evidenced the accessibility to new structural derivatives, such as the double double perovskite structures (DDPv) or triple perovskites (TPv) with 1:2 order of the B-site cations [3,4]. The possibility to tune both structure and properties as a function of the chemical composition has also been observed, for instance in the Mn_3-xCo_xTeO₆ double perovskite – Ni₃TeO₆-type solid solutions [5].

Here we present a revision on the strongly frustrated magnetic structures of A-site manganites with ordered corundum or perovskite derivative structures (Fig.1). Among the corundum derivatives, magnetic frustration arises as a consequence of the stacking of honeycomb and/or triangular magnetic sublattices. In the case of the perovskite superstructures, it is usually the competition between several magnetic interactions and the combination of dⁿ with d⁰ /d¹⁰ cations what induces large frustration indexes. As a result of such frustration both types of polymorphs develop complex magnetic structures, including incommensurate helices, temperature dependent propagation vectors, elliptical and sinusoidal modulation of the magnetic moments, lock-in spin transitions and split of the main magnetic phase into coexisting ground states.

Figure 1. Representative examples of cation order and magnetic frustration in corundum (a) and perovskite (b) derivatives in high pressure A-site manganites. a) Stacked honeycomb/ triangular sublattices (top left), temperature dependence of the propagation vector in Co₃TeO₆ (right) with split into circular and elliptical helices (bottom left). b) DDPv and 1:2 TPv structures of MnRMnSbO₆ and Mn₃MnNb₂O₉ respectively with several magnetic interactions in competition. Complex thermodiffraction of Mn₃MnNb₂O₉ developing a SDW modulated structure and lock-in transition at low temperatures.

[1] Markkula, M., Arevalo-Lopez, A. M, Kusmartseva, A., Rodgers, J. A. Ritter, C., Wu, H. & Attfield J.P. (2011) Phys. Rev. B. 84, 094450.

[2] Arevalo-Lopez, A. M. & Attfield, J. P. (2013) Phys. Rev. B. 88, 104416.

[3] Solana-Madruga, E., Arévalo-López, A. M., Dos Santos‐García, A. J., Urones‐Garrote, E., Ávila‐Brande, D., Sáez‐Puche, R. & Attfield, J. P. (2016) Angew. Chem. Int. Ed. 55, 9340.

[4] Solana-Madruga, E., Aguilar-Maldonado, C., Ritter, C., Mentré, O., Attfield, J. P. & Arevalo-Lopez, A. M. (2021) Angew. Chem. Int. Ed. Under revision.

[5] Solana-Madruga, E., Aguilar-Maldonado, C., Ritter, C., Huvé, M., Mentré, O., Attfield, J. P. & Arevalo-Lopez, A. M. (2021) Chem. Commun. 57, 2511-2514.

External stimuli such as temperature, electric and magnetic field, light or guest molecules can be used to control the magnetic properties of molecule-based solids. This in turn can lead to interesting magnetic switching behavior.[1, 2] Molecular magnets are also very susceptible to mechanical stress and yet external pressure is rarely used in this field to study magneto-structural correlations. This is most probably caused by a common belief that molecular crystals are mechanically fragile. In fact, they show very good stability under high quasi-hydrostatic conditions even up to 3 GPa, which is sometimes accompanied by astonishing changes/transformations.

Herein a combined structural, magnetic and spectroscopic study of a family of octacyanoniobate(IV)-based molecular magnets {[M^II(pyrazole)₄]₂[Nb^IV(CN)₈]×4H₂O}_n [3] MNb (M = Mn, Fe, Co or Ni) will be presented and discussed. The four compounds are isostructural and exhibit a three-dimensional (3-D) cyanide-bridged framework with a diamond-like topology (Fig. 1). The 3d transition metal ions M define their optical and magnetic properties under ambient pressure: MnNb (yellow) and FeNb (dark violet) are ferrimagnetic with the critical temperatures of 25 and 9 K, while CoNb (dark yellow) and NiNb (greenish) are both ferromagnetic with Curie temperatures of 6 and 13 K, respectively. The MNb family shows also stunning differences in their pressure responses depending on the metal ion M. The MnNb exhibits one of the highest shifts of the magnetic ordering temperature from 24 K to 37 K in response to pressure,[4] FeNb is a pressure-induced spin-crossover photomagnet based on the LIESST effect (LIESST = light induced excited spin state trapping),[4] while the long range magnetic ordering in CoNb switches from ferromagnetic to ferrimagnetic character under pressure. Finally, NiNb shows significant lowering of the Curie temperature under pressure – completely opposite to MnNb.[5] The thorough high pressure magnetic studies of MNb are correlated with the high pressure single-crystal X-ray diffraction structural analysis, enabling a full understanding of the observed pressure-induced changes [4, 5].

Perovskites ABO3 are of great interest due to their large variety of electronic and magnetic properties. Their compositions can be modified to induce different cation orderings giving double perovskites AA’B2O6 or A2BB’O6, and even more complex double double perovskites (AA’BB’O6) [1]. Recently, by using high-pressure and high-temperature (HPHT) techniques, we reported a new type of double double perovskite derivatives (DDPv) where columnar ordering at A-site and rock-salt ordering at B site are combined [2]. These crystallise with space group P42/n and two families have been established; those with R (= rare earth) cations at A sites in RMnMnSbO6 [2]; and those with Ca e.g. CaMnMReO6 (M = Mn, Fe) [3].

We have successfully synthesised a new R-based series of HPHT perovskites, RMnMnTaO6. Large R cations (R = La-Sm) result in a DDPv structure with space group P4₂/n; whereas a disordered A-site DPv structure has been observed for the smaller R =Eu-Y, with space group P2₁/n. By increasing the temperature, a structural transition from DDPv to DPv was observed for the very first time (Fig.1), confirming the structural phase boundary for the RMnMnTaO₆.

Magnetic measurements show a ferrimagnetic ordering for the DDPv and a ferromagnetic ordering for the DPv. Two magnetic transitions with spin reorientation has been found for the DDPv Nd-compound. All information above indicates a very rich structural and magnetic behaviour for the RMnMnTaO₆ family.

[1] King, G., Woodward, P. M. (2010). J. Mater. Chem. 20, 5785.

[2] Solana‐Madruga, E., Arévalo‐López, Á. M., Dos Santos‐García, A. J., Urones‐Garrote, E., Ávila‐Brande, D., Sáez‐Puche, R. & Attfield, J. P. (2016). Angew. Chem. Int. Ed. 55, 9340.

[3] McNally, G. M., Arévalo-López, Á. M., Kearins, P., Orlandi, F., Manuel, P., & Attfield, J. P. (2017). Chem. Mater. 29, 8870.

The compositional flexibility of double perovskites A₂BB’O₆ gives this family of materials a huge range of properties,[1] and interest in accommodating 5d cations such as Os⁶⁺ on a B sites stems from the stronger spin-obit coupling for these heavier cations (compared with 3d transition metal cations). This gives interesting electronic and magnetic properties of these phases and the A₂NiOsO₆ (A = Ca, Sr, Ba) series spans insulating to metallic phases, with ferromagnetic to antiferromagnetic order,[2-4] and with half-metallicity proposed for Sr₂NiOsO₆.[5] These properties are very sensitive to Ni – O – Os bond lengths and angles and therefore to A²⁺ cation size.[2]

The lower symmetry environments favoured by 6s² “inert pair” cations such as Pb²⁺ give structures and properties that can be quite different from those observed for the group 2 A cation analogues.[6, 7] However, high pressure synthetic routes are often required to access these lead analogues.[1]

This presentation describes work on the structural characterisation and properties of Pb₂NiOsO₆ synthesised at high pressure (6 GPa, 1575 K).[8] The rocksalt ordering of NiO₆ and OsO₆ octahedra combined with octahedral tilts gives a crystal structure of P2₁/n symmetry (similar to Ca₂NiOsO₆). Strong coupling between Ni²⁺ and Os⁶⁺ moments gives long-range magnetic order below 58 K, with the collinear magnetic structure described by magnetic propagation vector k = (½ 0 ½) (similar to Pb₂CoOsO₆[6]). This magnetic order, imposed on the (B-site ordered) crystal structure, breaks inversion symmetry.[8]

[1] Vasala, S.; Karppinen, M., (2015), Progress in Solid State Chemistry 43, 1-36.

[2] Morrow, R.; Samanta, K.; Saha Dasgupta, T.; Xiong, J.; Freeland, J. W.; Haskel, D.; Woodward, P. M., (2016), Chemistry of Materials 28, 3666-3675.

[3] Macquart, R.; Kim, S.-J.; Gemmill, W. R.; Stalick, J. K.; Lee, Y.; Vogt, T.; zur Loye, H.-C., (2005), Inorganic Chemistry 44, 9676-9683.

[4] Feng, H. L.; Calder, S.; Ghimire, M. P.; Yuan, Y.-H.; Shirako, Y.; Tsujimoto, Y.; Matsushita, Y.; Hu, Z.; Kuo, C.-Y.; Tjeng, L. H.; Pi, T.-W.; Soo, Y.-L.; He, J.; Tanaka, M.; Katsuya, Y.; Richter, M.; Yamaura, K., (2016), Physical Review B 94, 235158.

[5] Ghimire, M. P.; Hu, X., (2016), Materials Research Express 3, 106107.

[6] Princep, A. J.; Feng, H. L.; Guo, Y. F.; Lang, F.; Weng, H. M.; Manuel, P.; Khalyavin, D.; Senyshyn, A.; Rahn, M. C.; Yuan, Y. H.; Matsushita, Y.; Blundell, S. J.; Yamaura, K.; Boothroyd, A. T., (2020), Physical Review B 102, 104410.

[7] Jacobsen, H.; Feng, H. L.; Princep, A. J.; Rahn, M. C.; Guo, Y.; Chen, J.; Matsushita, Y.; Tsujimoto, Y.; Nagao, M.; Khalyavin, D.; Manuel, P.; Murray, C. A.; Donnerer, C.; Vale, J. G.; Sala, M. M.; Yamaura, K.; Boothroyd, A. T., (2020), Physical Review B 102, 214409.

[8] Feng, H. L.; Kang, C.-J.; Manuel, P.; Orlandi, F.; Su, Y.; Chen, J.; Tsujimoto, Y.; Hadermann, J.; Kotliar, G.; Yamaura, K.; McCabe, E. E.; Greenblatt, M., (2021), Chemistry of Materials 33, 4188-4195.

Due to their specific peculiarities neutrons are a very useful probe for structural studies on various hot topics related to physics, chemistry and mineralogy. The neutron single crystal diffractometer HEiDi at the Heinz Maier-Leibnitz Zentrum (MLZ) offers high flux, high resolution and large q range, low absorption and high sensitivity for light elements. These properties apply in a similar way to its polarized sister diffractometer POLI, which is optimized for detailed studies on magnetic structures.

In 2016 a project was launched in order to allow studies on tiny samples < 1 mm³ and to develop new pressure cells for HEiDi which can be combined with its existing low temperature equipment in order to study structural properties down to temperatures below 10 K, e.g. MnFe₄Si₃ and related magnetocaloric compounds [1]. This work was supported by the Bundesministerium für Bildung und Forschung (BMBF) (grand no. 05K16PA3). As part of this project various neutron-optical components (Cu220-monochromator, solid state collimators, neutron guides) were developed and optimized in order to generate a sufficiently high flux density at the sample location at the wavelength λ = 0.87 Å. Very tiny single crystal samples (down to < 0.1 mm³) were successfully studied using various new diamond anvil cells (DAC) - developed by A. Grzechnik - up to several GPa, either with a panoramic pressure cell in combination with low temperatures [2] or in a transmission pressure cell, which allows simultaneous studies of the same sample using neutron, synchrotron as well as laboratory x-ray sources [3].

Recently, a follow up project has been launched (BMBF No. 05K19PA2) to focus on further improvements of the high pressure capabilities on HEiDi and POLI and the development of optimized pressure cells for further instruments at the MLZ (POLI, DNS and MIRA), namely advanced clamp cells (see corresponding contribution by A. Eich).

[1] A. Grzechnik et al.; Single-Crystal Neutron Diffraction in Diamond Anvil Cells with Hot Neutrons; J. Appl. Cryst. 51, 351-356 (2018).

[2] A. Eich et al.; Magnetocaloric Mn₅Si₃ and MnFe₄Si₃ at variable pressure and temperature; Mater. Res. Express 6, 096118 (2019).

[3] A. Grzechnik et al.; Combined X-ray and neutron single-crystal diffraction in diamond anvil cells; J. Appl. Cryst. 53(1), 1 - 6 (2020).

Frustrated magnets with spiral magnetic phases are currently being intensively studied owing to their ability for inducing ferroelectricity. This could potentially be exploited in spintronics and low power memories devices.[1-2] However, the low magnetic order temperatures (typically < 100 K) in most of frustrated magnets greatly restrict their fields of application. One of the most notable exceptions are Cu/Fe-based layered perovskites, featuring magnetic spiral phases whose ordering temperatures can be continuously tuned far beyond RT. [3-5]. However, the influence of magnetic field on the magnetic structures especially spiral phases, imperative for further cross-control of the magnetic and ferroelectric orders, is barely known.

Here, we report a comprehensive description of the evolution of magnetic order in the layered perovskite YBaCuFeO₅ under the application of magnetic fields up to 9.0 T and at temperatures between 1.5 K and 300 K. Using bulk magnetization measurements and neutron powder diffraction we reveal the existence of a new incommensurate magnetic phase with a weak ferromagnetic component stable at low magnetic fields. Moreover, we observe a field-induced spin reorientation in the collinear phase. The resulting H-T phase diagram of YBaCuFeO₅ will be discussed, with emphasis in the magnetic phases with the largest potential to display strong magnetoelectric effects. [6]

[1] Eerenstein, W., Mathur, N.D. & Scott, J.F. (2006). Nature. 442, 759. [2] Kimura, T., Goto, T., Shintani, H., Ishizaka, K., Arima, T.H. & Tokura, Y. (2003). Nature 426, 55. [3] Morin, M., Scaramucci, A., Bartkowiak, M., Pomjakushina, E., Deng, G., Sheptyakov, D., Keller, L., Rodriguez-Carvajal, J., Spaldin, N.A., Kenzelmann, M., Conder, K. & Medarde, M. (2015). Phys. Rev. B 91, 064408. [4] Morin, M., Canévet, E., Raynaud, A., Bartkowiak, M., Sheptyakov, D., Ban, V., Kenzelmann, M., Pomjakushina, E., Conder, K. & Medarde, M. (2016). Nat. Commun. 7, 1. [5] Shang, T., Canévet, E., Morin, M., Sheptyakov, D., Fernández-Díaz, M. T., Pomjakushina, E. & Medarde, M. (2006). Sci. Adv. 4, eaau6386. [6] Lyu, J. et al. in preparation.