Edinburgh Research Explorer High pressure synthesis of polar and non-polar cation-ordered polymorphs of Mn2ScSbO6

Two new cation-ordered polymorphs of Mn 2 ScSbO 6 have been synthesised at high-pressure. At 5.5 GPa and 1523 K Mn 2 ScSbO 6 crystallizes in the Ni 3 TeO 6 -type structure with polar R 3 space group and cell parameters a = 5.3419 (5) Å and c = 14.0603 (2) Å. Below T C = 42.0 K it exhibits ferrimagnetic order with a net magnetization of 0.6 μ B arising from unusual site-selective Mn/Sc disorder and is thus a potential multiferroic material. A double perovskite phase obtained at 12 GPa and 1473 K crystallizes in the non-polar P 2 1 / n monoclinic space group with cell parameters a = 5.2909 (3) Å, b = 5.4698 (3) Å, c = 7.7349 (5) Å and β = 90.165 (6) º . Magnetization and neutron diffraction experiments reveal antiferromagnetic order below T N = 22.3 K with the spins lying in the ac plane.


I. INTRODUCTION:
Material properties are dependent upon atomic arrangement and the degree of order can be tailored by controlling the pressure-temperature conditions and bulk composition. At a given pressure P and temperature T, there are different possible atomic arrangements that correspond to local minima of free energy, the lowest-energy conformation being the thermodynamically-stable phase. However, high pressure and high temperature synthesis conditions (HPHT) may favour higher-energy minima, metastable phases, making them kinetically stable at ambient conditions and therefore recoverable.
Amongst transition metal oxides, HPHT helps to stabilize unusual oxidation states and environments that result in useful properties, e.g. the room temperature ferromagnetic metal CrO2. 1 Moreover, HPHT promotes interesting structural mechanisms. The ilmenite (IL) FeTiO3, for example, crystallizes in the space group R-3 with Fe and Ti stacked into alternate layers along the c-axis. 2 It transforms into an unquenchable distorted perovskite (Pv) structure (space group Pbnm) at 16 GPa and converts back into a polar LiNbO3-type (LN, space group R3c) with Fe and Ti cations being ordered in the same layers. 3,4 Moreover, ABO3 oxides with Mn 2+ on the A site are of fundamental interest due to the electronic and magnetic phenomena that emerge from the coupling of spin, charge and orbital degrees of freedom. HPHT conditions are often needed to stabilize MnBO3 materials, e.g. perovskite-type MnVO3 with an incommensurate magnetic structure and metallic conductivity, 5 and LiNbO3-type MnTiO3-II with a weak ferromagnetism through anisotropic exchange interactions. 6 The use of HPHT on ordered quaternary systems (AA'B2O6 or A2BB'O6) is a relatively less explored area with only a few reports. 7,8 Amongst these, Mn2FeB'O6 (B'= Ta and Nb) with LNtype structure 9 or (B' = Mo, W) with the Ni3TeO6-type (NTO) 10,11 order show polar and magnetic properties, while Mn2BSbO6(B = Fe and Cr) show polymorphism between the ILand double perovskite-types (DPv) depending on whether they are synthesized at moderate (3)(4)(5) or higher pressures . .12,13 In this work, we present the order-disorder effects in the high pressure polymorphs of the Mn2ScSbO6 oxide. Mn and Sc cations are randomly distributed in a corundum-related type structure when this material is synthesized at ambient conditions and it shows no long-range magnetic ordering. 14 The double perovskite (DPv) can be achieved with pressures higher than 10 GPa, but below 5.5 GPa, the polar NTOstructure is obtained. NTO_Mn2ScSbO6 shows an unusual ferrimagnetism due to partial substitution of non-magnetic Sc 3+ at just one of the two Mn 2+ sites, whereas DPv_Mn2ScSbO6 is antiferromagnetic. Combined X-ray and powder neutron diffraction refinements and electron micro-diffraction experiments confirm the non-centrosymmetry of the NTO_Mn2ScSbO6 polymorph allowing a structural polarization that is predicted to be 28.3 µC/cm 2 at room temperature. These results demonstrate that cation ordering can be induced by high pressure synthesis providing access to new metastable

II. EXPERIMENTAL SECTION:
Mn2ScSbO6 was previously synthesized at ambient pressure by Kosse et al 15,16 and subsequently studied by Ivanov et al. 14 NPD studies showed a random distribution of Mn and Sc crystallizing in a corundum-related structure (R-3) with no long-range magnetic ordering. We prepared a sample at 1373 K and SF1 shows the Rietveld fit to the XRD pattern that agrees with a random cation distribution.
Both double perovskite and Ni3TeO6-type polymorphs were synthesized under high pressure and high temperature conditions. The precursor powder, prepared from grinding stoichiometric amounts of Mn2O3, Sc2O3 and Sb2O3 oxides, was treated at 12 GPa and 1473 K during 20 minutes in a Walkertype multianvil apparatus for the preparation of the perovskite phase. The application of 5.5 GPa and 1523 K during 35 minutes in a belt-type press led to the formation of the Ni3TeO6 polymorph. In both cases, the sample was quenched by rapid cooling and the pressure was progressively released down to ambient conditions. The crystal structures were first characterized by Rietveld refinement of powder X-ray diffraction collected on a Phillips X'Pert Pro Alfa 1 diffractometer using Cu Kα1 radiation, equipped with a Ge (111) monochromator and working in Bragg-Brentano geometry. The diffractograms were collected between 10 and 120 degrees with a step size of 0.017 º.
For transmission electron microscopy (TEM) studies, samples were ground in n-butyl alcohol and ultrasonically dispersed. A few drops of the resulting suspension were deposited on a carbon-coated grid. The study of reciprocal space by selected area electron diffraction (SAED) and microdiffraction was carried out in a JEOL JEM2100 microscope operating at 200 kV with a double tilt (± 42 °) goniometer. High resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) experiments were performed with a JEOL JEM 3000F microscope operating at 300 kV (double tilt (± 25 °) point resolution 0.17 nm), fitted with an energy-dispersive X-ray spectroscopy (XEDS) microanalysis system (OXFORD INCA) and ENFINA spectrometer with an energy resolution of 1.3 eV.
Simulations of the HRTEM images were performed with the software MacTempas X 17 , using the refined structures from neutron diffraction data. The oxidation state of Mn in both materials was determined by EELS with the relation between the white-line intensity ratio (L3/L2) and the oxidation state. 18 Neutron powder diffraction (NPD) patterns were collected at 300 K on the high resolution D2B diffractometer (Institut Laue-Langevin, Grenoble) between 0 and 160 º with a stepwidth of 0.05 º, using a neutron wavelength λ = 1.594 Å and a standard He cryostat. The NTO polymorph was also cooled down to 4 K, where a long scan was measured in the same conditions. The evolution of the magnetic structures was studied using sequential patterns collected each degree from 3 K up to 55 K on the high intensity D1B instrument. Each pattern was measured in the angular range 0 º ≤ 2θ ≤ 130 º with a step-width of 0.1 º using λ = 2.520 Å. Long scans were also taken at 3 K.
The nuclear structures were refined from room temperature D2B data, using Rietveld analysis through the FullProf software package 19 and considering a Thomson-Cox-Hastings function to optimize the shape of the peaks. The magnetic symmetry analysis of the low temperature D1B data was performed by means of the program BasIreps 20 .
Magnetic susceptibility measurements were performed on a Quantum Design XL-MPMS SQUID magnetometer under DC Zero-Field-Cooling (ZFC) and Field-Cooling (FC) conditions in the temperature range 2 K < T < 300 K under a magnetic field of 3 kOe. Magnetization dependence on the magnetic field strength was studied at 5, 20, 40 and 90 K for the perovskite polymorph up to 5 T and at 2, 20 and 40 K for the NTO-type oxide up to 7 T.

III. RESULTS AND DISCUSSION: A) NTO_ Mn2ScSbO6
-Structural characterization: The lower pressure polymorph of Mn2ScSbO6 can be obtained at 5.5 GPa and 1523 K. Figure 1 (bottom) shows the XRD diffraction pattern of this material. It can be indexed in an hexagonal cell with a = 5.34186 (5) Å and c = 14.0603 (2) Å cell parameters. This cell is indicative of a corundum-type related structure, as observed in other Mn2BB'O6 oxides. [9][10][11][12][13] The presence of the (003) and (101) reflections (inset Figure 1 bottom), demonstrates the absence of a c-glide plane and therefore excludes the corundum-(R-3c) and LN-type (R3c) structures, suggesting an ilmenite-type ordering (R-3). Rietveld refinement was performed with the IL-Mn2FeSbO6 structure as starting model. The refinement converged to Rwp = 11.3 % and Rp = 6.24 % agreement factors.
However, NPD experiments on the same compound revealed no intensity for the (003) and (101) reflections ( Figure  1-top) and the IL-type model obtained from XRD refinement showed a poor fit to the neutron data. The data can be fitted assuming a LN-type ordering (R3c) with Rwp = 4.40 % and Rp= 3.41 % agreement factors.
Figures. 2d and 2e show the microdiffraction of [2-1-10] zone axis. The no periodicity difference between the zero and first order Laue zones (ZOLZ and FOLZ) indicates the absence of a c-glide plane, in agreement with XRD. Moreover, the analysis of the whole pattern (WP) of the [2-1-10] and [01-10] zone axes (Fig. 2f) unequivocally distinguishes between R-3 (two-fold axis) and R3 (no symmetry). 21 A model with R3 symmetry was then tested. In order to break the inversion centre of the IL-type model, we assume a complete ordering within the (00z)-layers between Mn1/Sc This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3

Please do not adjust margins
Please do not adjust margins and Mn2/Sb; this cation arrangement is adopted in the Ni3TeO6-type structure with the polar R3 space group. NTOtype structure has also been observed in the related compounds Mn2FeMoO6 and Ni2ScSbO6. 10,22 The refinement converged smoothly; Figure 1 shows the Rietveld refinements of the NPD (top) and XRD (bottom) data to the NTO-Mn2ScSbO6 model and Table I summarizes the structural details and agreement factors resulting from this combined refinement.
The final model clarifies the apparent contradiction between XRD and NPD results. The cation arrangement of the R3 NTO-type structure is illustrated in Figure 3 (left). (00z) layers alternate between Mn1/Sc and Mn2/Sb composition. For XRD, Sc (Z = 21) and Mn (Z =25) look similar when compared with the consecutive layer of Mn and Sb (Z = 50). Therefore, IL-type ordering is apparent and (003) and (101) reflections are observable. On the other hand, the difference in neutron scattering lengths for Mn, Sc and Sb (-3.73, 12.29 and 5.57 fm respectively) breaks the apparent inversion centre, as (012) planes are alternatively constituted by Mn or by Sc/Sb, which makes the scattering distribution resemble that of a LN-type. For clarity, both IL-and LN-type orders are highlighted in Figure 3 (left) with black and red dashed lines respectively. In conclusion, the combination of SAED, NPD and XRD techniques unequivocally leads to the true NTO-type structure of this compound.   The displacement of the different cations from their ideal sites in reference to the oxygen octahedra are labeled in Figure  3(left). Mn cations show larger displacements in comparison with the d 0 Sc or d 10 Sb, and in accordance with the greater distortion found for their polyhedra (see Table I).
The theoretical value of polarization along the z axis has been calculated from P = Σjqjuj z , where qj stands for the charge over a generic j ion and uj z for its displacement along the z axis, resulting in Pexpected = 28.3 µC/cm 2 . This estimate neglects the effects of Mn/Sc disorder. Attempts were performed to measure the experimental polarization, using a modified Sawyer-Tower circuit. 23 Due to the low resistivity, sample polarization is masked by conducting effects and typical inconclusive P vs E ellipse-shaped cycles were obtained.   The magnetic susceptibility of NTO_Mn2ScSbO6, represented in Figure 4a with its inverse, follows a Curie-Weiss behaviour above 75 K with a calculated effective moment of μNTO = 6.1 μB per Mn 2+ and θNTO = -138 K. A ferro-or ferrimagnetic transition is observed at TC = 42 K. There is no significant divergence between ZFC and FC branches, and field dependent magnetization measurements (inset in Figure 4a and SF3 top) show that NTO_Mn2ScSbO6 is a very soft ferrimagnet with a saturated magnetization of 0.6 µB / f.u. at 2 K.
From  The 4 K NPD data collected for NTO_Mn2ScSbO6 at D2B instrument is shown in Figure 4b. The magnetic structure of this polymorph (Figure 3 right) was determined from the fit to these data, where 27 º -30 º and 46 º -48 º angular ranges are excluded due to the presence of peaks coming from the sample holder. Additional magnetic neutron diffraction peaks are observed below TC = 42 K, the most intense being (003) and (101), which can be indexed with the propagation vector k = [0 0 0]. Fits to the magnetic peaks confirm a FM coupling of Mn1 spins within 00z layers, where spins are confined to the ab plane, but antiferromagnetically coupled to the Mn2 cations of the adjacent layers. The refined moment per Mn 2+ cation is 4.62 (3) μB. The spin order and Mn site occupancies observed by NPD reveal the origin of the ferromagnetic behavior of NTO_Mn2ScSbO6. Antiparallel Mn1 and Mn2 spins are symmetry-inequivalent so a small uncompensated ferromagnetic moment is expected, but this is greatly enhanced by site-selective disorder as Mn1 sites are fully occupied by Mn 2+ spins, but 12.3 % of Mn2 sites are substituted by non-magnetic Sc 3+ . The predicted uncompensated moment (12.3 % *4.62 μB= 0.57 μB) is in excellent agreement with the observed magnetization of 0.6 μB. Hence, the ferromagnetic component arises from an unusual site-selective substitution of non-magnetic Sc 3+ at just one of the two Mn 2+ sites. A similar ferrimagnetism due to selective disorder was recently observed in DPv-type La3Ni2SbO9. 24 Magnetic diffraction from the ferromagnetic component in NTO_Mn2ScSbO6 is expected to be very weak and is not observed in the difference pattern between NPD data sets collected at 50 K and 3 K on the D1B instrument (SF4 lower panel).
The evolution of the magnetic moment below the Curie temperature (inset on Fig. 4b), was fitted to a critical law in the (TC / 2) < T < TC temperature range between 21 K and 41 K. It led to the parameters TC = 42.0 K and β = 0.37, consistent with 3D Heisenberg behaviour for which β = 0.36 is predicted.
Overall, NTO_Mn2ScSbO6 is notable as a potential multiferroic material, with ferroelectricity arising from the acentric cation order in the NTO-type arrangement, and ferrimagnetism from the uncompensated antiparallel order of Mn 2+ spins enhanced by site-selective Mn/Sc disorder. This material seems to be a type I multiferroic as the two orders are decoupled. The magnetoelectric coupling between the electrical and magnetic polarizations may be small as they are orthogonal, being respectively parallel and perpendicular to the c-axis, although this could lead to an unusual switching mechanism as predicted for LiNbO3-type MnTiO3-II. 6

B) DPv_ Mn2ScSbO6 -Structural characterization:
As in other related Mn2BSbO6 compounds (B = Fe, V, Cr, Ga, Al) 25 , high pressure synthesis was needed to stabilize the small Mn 2+ cation in the highly coordinated A site of the perovskite structure. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Figure 5a (top) shows the Rietveld refinement of NPD data of DPv_Mn2ScSbO6 collected at room temperature. It was fitted using the Mn2BSbO6 (B = Fe, Cr) 12, 13 structure as starting model and crystallizes in the P21/n monoclinic space group with lattice parameters a = 5.2909 (3) Å, b = 5.4698 (3) Å, c = 7.7349 (5) Å and β = 90.165 (6) º. The resulting crystallographic parameters, interatomic distances and bond angles are summarized on Table II. Anti-site disorder was found between Sc and Mn, resulting in 9.1 % of the A-site occupied by Sc 3+ . The secondary phase included in the refinement of DPv_Mn2ScSbO6 is the NTO polymorph formed at lower pressures during the synthesis reaction. A 20.1% of this phase has been found to crystallize within this high pressure compound. The presence of such a high proportion of NTO_Mn2ScSbO6 is due to the high stability of NTO-type structure at high temperatures.The effect of this secondary phase in the magnetic behaviour of DPv_Mn2ScSbO6 is discussed below. Figure 5b depicts the nuclear (and magnetic) structure of DPv_Mn2ScSbO6; Sb and Sc are six-fold coordinated and ordered in rock-salt configuration. Mn cations occupy the highly distorted cuboctahedral voids. The tilt angle, calculated as Φ = (180 ° -θ)/2 from <B-O-B'> bond angles (see Table II The ZFC and FC (black and red circles, respectively) molar magnetic susceptibilities for DPv_Mn2ScSbO6 polymorph and the reciprocal susceptibility (open circles) as a function of temperature under an applied field of 3 kOe is depicted in Figure 5c. It follows Curie-Weiss behaviour with an additional 0.0579 emu -1 molOe temperature independent paramagnetic (TIP) contribution above 100 K. The Weiss temperature is θDPv = -94 K and the effective paramagnetic moment μDPv = 5.5 μB per Mn 2+ , is slightly lower than the expected value for S = 5/2 Mn 2+ (5.92μB). The magnetic susceptibility increases abruptly at ~ 42 K due to the presence of the NTO polymorph as a secondary phase. This obscures the antiferromagnetic transition at 22.3 K, determined below for this polymorph from NPD. The absence of magnetic cations neither in B nor in B' -sites yields the only possible magnetic interactions in this compound to be those operating between nearest neighboring Mn 2+ cations. The long Mn -Mn distance and the high coordination of A -site, favors the development of cooperative superexchange interactions through oxygen anions. The nature and strength of these interactions is defined by the Mn -O -Mn angles, which take the values of 104 (1) º, 113 (1) º and 123 (1) º. According to Goodenough-Kanamori rules 27 these angles rule antiferromagnetic interactions among d 5 cations, as observed in magnetization curves (see SF3 bottom). From the Rietveld refinement, it has been found that 9.1 % of A-sites are occupied by diamagnetic Sc 3+ cations, resulting in a magnetic dilution effect on the overall Mn 2+ sublattice. Please do not adjust margins The magnetic structure of DPv_Mn2ScSbO6 was determined by NPD using data from the D1B instrument. Additional magnetic neutron diffraction peaks are observed below 22 K. Figure 5a (bottom) shows the fit to the 3 K data. The magnetic reflections can be indexed with the k = [0 0 0] propagation vector in the P21/n space group. Irreducible representations and their basis vectors were obtained by using the program BASIREPS. Figure 5b shows the resulting magnetic structure. It can be described as an antiparallel orientation of Mn 2+ spins located in the ac-plane, with a total moment of 4.54 (1) μB at 3 K. The deviation of the spins with respect to the c axis has been calculated to be γ = 20.34 º (1), resulting in 1.58 (1) μB and 4.26 (1) μB components along the a-and c-axis respectively. The inset in Figure 5a shows the thermal evolution of the ordered magnetic moment, fitted to the critical law. The fitting results in TN = 22.3 K and β = 0.37, where β agrees again with 3D Heisenberg antiferromagnetic behaviour. Table II. Crystallographic parameters, selected interatomic distances (Å) and angles (°) for DPv_Mn2ScSbO6 from the room temperature NPD Rietveld refinement in the P21/n space group. a a Fitting residuals: Rp = 2.00 %, Rwp = 2.51 %, RB = 6.04 % and RF = 4.36 %,. b Vi = ƩjSiji= exp(r0 -rij/0.37). Values calculated using rij = 1.79 Å for Mn 2+ , 1.849 Å for Sc 3+ and 1.942 Å for Sb 5+ . Polyhedral distortions calculated from ∆ = 1/n * ∑[(di-dav)/dav] 2

Conclusions
Our results demonstrate that two new phases of Mn2ScSbO6 are accessible by high pressure -high temperature synthesis and their structural and microstructural details and magnetic properties have been investigated. Although cations are completely randomly located within the room pressure polymorph, frustrating any long range magnetic order, the synthesis of this oxide under high pressure conditions induces different cationic arrangements leading to interesting properties. The NTO-type moderate pressure modification crystallizes in the rhombohedral R3 polar space group, which allows a predicted room temperature polarization of 28.3 µC/cm 2 . It has ferrimagnetic order below 42 K, with the spins lying in the ab plane. A ferromagnetic component of 0.6 µB has been determined to arise from an unusual site-selective Mn/Sc disorder. NTO_Mn2ScSbO6 is thus a potential multiferroic material. The high pressure phase has a double perovskite structure crystallizing in the P21/n space group, which exhibits an antiferromagnetic order below 22.3 K. Its magnetic structure has antiparallel Mn 2+ spins in the ac plane.