The eﬀect of manganese oxidation state on antiferromagnetic order in SrMn 1 (cid:2) x Sb x O 3 (0 o x o 0.5) perovskite solid solutions †

The mixed-valence manganese (Mn 3+ /Mn 4+ ) solid solution, SrMn 1 (cid:2) x Sb x O 3 , was prepared for the first time. Two ranges of solid solutions were found: (1) SrMn 1 (cid:2) x Sb x O 3 (0.025 r x r 0.09) with monoclinically distorted 6H-SrMnO 3 polytype (sp. gr. C /2 c ) and (2) SrMn 1 (cid:2) x Sb x O 3 (0.17 r x r 0.50) with a tetragonal unit cell (sp. gr. I 4/ mcm ). Crystal structure refinement using X-ray and neutron powder diﬀraction data showed that the structure of the monoclinic solid solution consists of corner-sharing octahedra around sites occupied by manganese and antimony ions and face-sharing octahedra around sites occupied by manganese ions only, while the tetragonal solid solution has a random distribution of B-site cations. The presence of long-range antiferromagnetic order with a Ne´el temperature of about 148 K for SrMn 0.80 Sb 0.20 O 3 and about 280 K for SrMn 0.925 Sb 0.075 O 3 was found from the results of DC and AC susceptibility and neutron diﬀraction experiments at 5 K and 80 K.


Introduction
Manganites with perovskite-like structures attract considerable attention due to their unique electronic, magnetoelectrical and magnetic properties. Spontaneous electric polarization and magnetic ordering in the Sr 1Àx Ba x MnO 3 solid solution at x Z 0.45, 1 ferromagnetic coupling and metallic conductivity in La 1Àx Ca x Mn 4Àx Mn 4+x O 3 , 2 strong magnetoelectric effects near room temperature in single-crystal BaMnO 2.99 and its derivative BaMn 0.97 M 0.03 O 3 (M = Li or K), 3 soft ferromagnetic behaviour in La 0.75 K 0.25 AMnTiO 6 (A = Sr or Ba) at low-temperature, 4 collinear-magnetism-driven ferroelectricity in the Ising chains in Ca 3 CoMnO 6 5,6 are just a few examples. Such variety of properties results from the interplay between charge, exchange and phonon interactions, 7 which can be adjusted by substituting either A-site cations or Mn with other cations (usually cations of different sizes/charges). The latter influences the ratio of Mn cations in different oxidation states in the final compound and can also lead to the formation of different structural modifications within the perovskite type structure. In this work we studied structural and magnetic properties of SrMn 4+ O 3 -Sr 2 Mn 3+ SbO 6 solid solutions. In order to explain why we chose to study this system, let's first look at the structure and properties of the two parent compounds SrMnO 3 and Sr 2 MnSbO 6 . They differ considerably in chemical composition, oxidation state of manganese, physical properties and their crystal structure.
The hexagonal 6H structure of SrMnO 3 can be stabilized at normal pressures by partial replacement of Sr or Mn by other elements. During the synthesis of the 15-layered rhombohedral (15R) perovskite, SrMn 0.9 Fe 0.1 MnO 3Àx , 12,13 a 6H-layered perovskite of the same composition was found to be an intermediate phase. The latter phase was obtained in pure form at 1350 1C. In the Sr 1Àx La x MnO 3 solid solutions, a 6H-layered perovskite was also found to form at x = 0.1. 14 The crystal structure and magnetic properties of the double perovskite Sr 2 MnSbO 6 have been extensively studied. [15][16][17][18][19] Depending on the synthesis method, different crystal structure modifications of Sr 2 MnSbO 6 can be formed. Samples prepared at low temperatures have a random distribution of Mn 3+ and Sb 5+ cations at the octahedral positions, resulting in the space group I4/mcm. 15 In ref. 17 and 18, the structure of Sr 2 MnSbO 6 was described in the space group I4/m with an ordered distribution of cations at the octahedral positions. The presence of the Jahn-Teller cation Mn 3+ distorts the coordination environment and contributes considerably to octahedral tilting. Accordingly, 19 this structure of Sr 2 MnSbO 6 is stable in the temperature range of 2-750 K. Above 750 K, a cubic modification with space group Fm% 3m is formed.
The magnetic properties of SrMn 4+ O 3 and Sr 2 Mn 3+ SbO 6 differ considerably because the latter has half of the octahedral positions occupied by diamagnetic Sb 5+ ions. Cubic SrMnO 3 is antiferromagnetic with the Néel temperature T N = 240 K. 8 4H-SrMnO 3 also possesses antiferromagnetic properties with T N = 280 K, 20 and short-range antiferromagnetic interactions are retained in the paramagnetic region. The magnetic properties of 6H-SrMnO 3 are expected to be defined by Mn-Mn interand intradimer interactions, and the temperature dependence of the magnetic susceptibility by the dimeric behaviour. However, in ref. 8 it was established that the temperature dependence of the magnetic susceptibility w(T) for 6H-SrMnO 3 is typical of a three-dimensional antiferromagnetic compound with a maximum at T N = 235 K. The magnetic susceptibility of 6H-SrMnO 3 above T N follows the Curie-Weiss law with a Curie-Weiss constant y of À743(3) K and an effective magnetic moment m eff of 4.099(6) m B (the theoretical value m theor for the Mn 4+ cation is 3.873 m B ). The ratio y/T N = 3.2 is indicative of the presence of frustration. The magnetization vs. magnetic field curves are linear up to 10 kOe, and there are upturn deviations from the linear behaviour for higher magnetic fields.
The magnetic susceptibility of Sr 2 MnSbO 6 exhibits a divergence between field cooling (FC) and zero field cooling (ZFC) curves at 25 K 18 and nonequilibrium dynamics and memory at low temperatures. 19 No long-range magnetic ordering was found at 2 K. It is believed that Sr 2 MnSbO 6 reaches an unconventional spin-glass state at low temperatures. The magnetization vs. magnetic field dependence at 5 K displays a Brillouinlike curvature with a hysteresis loop. The Curie-Weiss law w = C/(T À y) is fulfilled only in the temperature range of 300-400 K with a positive Weiss constant. The experimental value of m eff (5.68 m B ) is significantly higher than the calculated value m cal = 4.90 m B for Mn 3+ . These results show that the magnetic behaviour of Sr 2 MnSbO 6 is determined by weak cluster-type ferromagnetic interactions. 18 The reasons for the lack of magnetic ordering in Sr 2 MnSbO 6 are not yet completely clear.
However, magnetic ordering of B-site cations in Sr 2 MnSbO 6based compounds is possible to realise through heterovalent substitution of Sr 2+ by La 3+ . 21 The then obtained SrLaMnSbO 6 has a monoclinically distorted double-perovskite structure with ordered Mn 2+ and Sb 5+ cations and shows long-range antiferromagnetic (AFM) ordering below T N = 10 K and significant ferromagnetic correlation below 90 K. At the same time, it has been shown that the magnetic properties of the rhombohedral perovskite La 0.67 Ba 0.33 Mn 1Àx Sb x O 3 (x = 0.01, 0.03 and 0.07) can be modified by varying the Sb doping. 22 A small increase of the Sb content changes the Mn 3+ /Mn 4+ ratio and thus influences the double exchange interactions and leads to a decrease in the Curie temperature (T C ) from 326 to 296 K. 22 The above data motivated us to examine systems based on SrMn 4+ O 3 and Sr 2 Mn 3+ SbO 6 manganites to identify the structural and magnetic properties of compounds and/or solid solutions with mixed manganese oxidation states Mn 3+ /Mn 4+ . The wide variety of properties of manganites is determined not only by the differences in chemical composition, but also by the differences in their structure, in particular the disordered or ordered arrangement of B-site cations (Mn and Sb) at octahedral positions. The aim of this study was to find out how the structure of substituted manganites SrMn 1Àx Sb x O 3 with a mixed oxidation state of manganese, Mn 4+ /Mn 3+ , affects their magnetic characteristics. The purity of the synthesized product was checked using X-ray powder diffraction (XRPD). XRPD patterns were collected at room temperature on a XRD Shimadzu 7000S diffractometer using Cu Ka radiation. The possible impurity phases were checked by comparing their XRPD patterns with those in the PDF2 database (ICDD, USA, Release 2016). For the crystal structure refinements, XRPD patterns were collected on a STADI-P (Stoe) diffractometer in transmission geometry with a linear mini-PSD detector, using Cu Ka 1 radiation in the 2y range 51 to 1201 with a step of 0.021. Polycrystalline silicon (a = 5.43075(5) Å) was used as an external standard.

Experimental
The microstructure and chemical composition of obtained samples were analysed using a JEOL JSM-5900 LV microscope equipped with a JEOL energy-dispersive X-ray detector (EDX).
Neutron powder diffraction patterns (NPD) were collected at the ILL facility at Grenoble (France) using the D2B high-resolution two-axis diffractometer (l = 1.594 Å) equipped with a cryofurnace, which enables to perform measurements at temperatures in the range of 2-500 K. For all measurements, the samples were placed in a vanadium cylinder with a diameter of 8 mm.
The crystal structure refinement was carried out with the GSAS 23,24 program suite using the NPD data for low and high temperatures and a combination of XRPD and NPD data for room temperature. The peak profiles were fitted with a pseudo-Voigt function, I(2y) = x Â L(2y) + (1 À x) Â G(2y) (where L and G are the Lorentzian and Gaussian part, respectively). The angular dependence of the peak width was described by the relation (FWHM) 2 = Utg 2 y + Vtgy + W, where FWHM is the full line width at half maximum. The background level was described by a combination of thirty-sixth-order Chebyshev polynomials. The absorption correction function for a flat plate sample in transmission geometry has been applied. Since neutron scattering lengths for Mn and Sb are À3.73 and 5.57 fm, respectively, the resulting scattering length of the mixed sites with Mn/Sb ratios equal to 0.667/0.333 would be À3.73 Â 2/3 + 5.57 Â 1/3 = À0.63 fm, as close to zero as the value of À0.3824 fm tabulated for vanadium which is considered as transparent for low energy neutrons and used as a material for the capillary for neutron diffraction data collection. In this regard, we have fixed the Mn/Sb ratios to 0.667/0.333 and set the values of the thermal motions to 0.025 for this site for all full-profile refinements of the SrMn 0.665 Sb 0.335 O 3 structure based on neutron data only, namely for the temperatures of 5 K and 500 K. In the analysis of the low-temperature NPD data, a two-phase refinement for every temperature was performed. In each case, the crystal structure was refined taking as starting parameters those obtained at room temperature. The magnetic structure was refined as an independent phase with the P1 space group for which only magnetic atoms were defined. The scale, atomic coordinates and displacement parameters were constrained for both the nuclear and magnetic structures. The bond valence sums (BVS) were calculated using the VaList program by Wills 25 according to equations given by Brown and Altermatt 26 and the bond-valence parameters, r 0 and b, tabulated by Brese and O'Keeffe. 27 A scanning transmission electron microscopy (STEM) study was performed using a FEI (S)TEM Titan 80-300 microscope, operated at 300 kV. For the STEM study an ethanol suspension of the sample was prepared and kept in an ultrasonic bath for 5 minutes; then a drop of this suspension was put onto a holey carbon film supported on a Cu grid. High-temperature transmission electron microscopy investigations were performed using a double-tilt heating holder (Gatan model 652) on a JEOL 2100 transmission electron microscope. The selectedarea electron diffraction (SAED) patterns were recorded on a Gatan Orius 200D camera.
Magnetic measurements were performed on a SQUID magnetometer MPMS-XL-5 produced by QUANTUM DESIGN. 30 The temperature was varied from 5 K to 400 K. The controlled magnetic field strength H was set to 0.5 kOe. Measurements were carried out when a specimen was cooled in zero and controlled (ZFC and FC) magnetic field. A specimen was first cooled in the zero field to 5 K, after which the magnetization (M) on heating up to 400 K (ZFC mode) was measured, followed by measurements when cooling from 400 K to 5 K (FC mode). From these measurements we found the static magnetic susceptibility w = M/H, whereas the AC susceptibility measurement technique was used to determine the real w 0 and imaginary w 00 components of the dynamic susceptibility for an amplitude value of the variable magnetic field of 4 Oe and a frequency of 80 Hz.

Results and discussion
In order to obtain comprehensive information about the SrMnO 3 -Sr 2 MnSbO Table S1 (ESI †), all the lattice parameters and the unit cell volumes increase within the monoclinically distorted 6H solid solution as a result of Sb-for-Mn substitution, which is due to a reduction of the average oxidation state of manganese and a larger size of both Mn 3+ and Sb 5+ cations as compared with Mn 4+ . All diffraction patterns of SrMn 0.925 Sb 0.075 O 3 collected in the temperature range of 1.5-500 K have been indexed in a monoclinic unit cell with the space group C2/c (#15) ( Table 1). The obtained lattice parameters increase with temperature, whereas the monoclinic angle decreases (Fig. 2a), which means that the degree of monoclinic distortion decreases when heated. The crystal structure of SrMn 0.925 Sb 0.075 O 3 (Table 1) has been shown to possess a monoclinically distorted 6H-perovskite structure (Fig. 3) consisting of corner-sharing octahedra around sites 4a randomly occupied by manganese and antimony ions in the ratio 0.775/ 0.225 and face-sharing octahedra around sites 8f occupied by manganese ions (Fig. 3a and c).
Strontium ions in sites 4e and 8f, respectively, center two polyhedra 12-fold coordinated by oxygen ions. Linear and volume thermal expansion coefficients (Table 1) are expressed by (1) respectively, where L and V are unit cell parameters and volume, respectively, and dL/dT and dV/dT are their rates of change with temperature. Note that neither lattice parameters nor volume vary linearly. Therefore, the calculated values are just the average between two temperatures and are not true values at a given temperature. However, they allow us to estimate the order of magnitude and the trend in change of thermal expansion.
1.71 (8) 1.67 (7) 1.26 (5) 2.65(10) Mn/Sb(1)-4a (0, 0, 0) U i /U e Â 100 1.20 (6)  The magnitude of the thermal expansion is small, comparable with those of iron and quartz, and anisotropy is clearly seen for the b axis at around room temperature, which is most probably a result of the rotation of polyhedra into configuration corresponding to the ideal hexagonal lattice. The oxidation state of manganese for this composition is +3.92. The calculated bond valence sum (BVS) values (Table S2, ESI †) were in good agreement with expected oxidation states, with the exception of the BVSs of Mn(1) and Sb(1), which were found to be 3.722 and 6.202, respectively, not really reflecting the expected +4 and +5 that could be explained by mixed occupation of this site.
Analysis of the selected-area electron diffraction (SAED) patterns confirmed that SrMn 0.925 Sb 0.075 O 3 crystalizes in a monoclinic unit cell with the space group C2/c at room temperature.
Both HAADF-STEM images taken along the 100 direction and SAED patterns confirm the absence of planar defects or local cation ordering in the structure ( Fig. 4 and Fig. S1, ESI †).
Tetragonal solid solution. Both XRPD and NPD patterns of SrMn 1Àx Sb x O 3 with x = 0.20, 0.335 and 0.415 collected in the temperature range of 2-500 K have been indexed with a tetragonal body-centered unit cell (Table S3, ESI †). The lattice parameter a increases with temperature, whereas c decreases (Fig. 2b), which means that the degree of tetragonal distortion is minimized when heated, and which results in a polymorphic transition at 500 K to the cubic perovskite structure type for SrMn 0.80 Sb 0.20 O 3 . This transition was confirmed at the submicrometer scale during in situ heating inside the transmission electron microscope. The h0l reflections with h + l = 2n disappear at around 500 K, which can be explained by movement of the oxygen atoms from the general (x, x + 1/2, 0) position in the tetragonal unit cell to the special (1/2, 0, 0) site in the cubic unit cell (Fig. 6).
Linear and volume thermal expansion coefficients (Table S3,    described as a double perovskite with lattice parameters related to the cubic perovskite as follows: a t B a c O2 and c t B 2a c and results from tilting of the octahedra (I4/mcm -a 0 a 0 c À ) (Fig. 5a).
Since there is a large amount of discussion on the ordering of manganese and antimony among octahedral positions [15][16][17][18][19] we tested all the proposed models and found that our synthetic route produces materials with random distribution of cations, having the highly symmetrical space group I4/mcm. The hightemperature modification of SrMn 0.80 Sb 0.20 O 3 observed at 500 K is also described with the highest symmetry as a cubic perovskite (Fig. 5c) with the space group Pm% 3m (Pm% 3m -a 0 a 0 a 0 ) and unit cell parameter a = 3.87998(2) Å, unlike the structure published for Sr 2 MnSbO 6 at 900 K in ref. 19.
One should note that all full-profile refinements for all temperatures give the same R-values and quality of fit for models with and without refinement of ordering of Mn and Sb and oxygen shifts. No displacements from special sites of oxygen atoms, no ordering of manganese and antimony atoms exceeding experimental uncertainties have been detected, and the corresponding standard deviations for such parameters were much higher than those for other similar variables. These observations, along with the instability of the refinement for space groups I4/m and Fm% 3m for the high-temperature modification usually arising from strong correlations indicate an excess in degrees of freedom and a wrong or too-low symmetry of the model. So, we performed final refinements (Table S2, (Table S2, ESI †), except for those of Mn(1) and Sb(1) because manganese is present in two oxidation sates, +3 and +4, in addition to pentavalent antimony.  Average interatomic distances (Table S4, ESI †) are in good agreement with the sums of the crystal radii according to Shannon. 28 Mean  (2) in SrMn 0.925 Sb 0.075 O 3 in face-sharing octahedra is short and are 2.491(1) Å, 2.510(9) Å, 2.508(3) Å and 2.508(9) Å at 1.5 K, 80 K, 298 K and 500 K, respectively, but agrees well with 2.511(2) Å reported for the undistorted 6H-SrMnO 3 polymorph. 8 Small substitution of larger antimony for manganese in corner-sharing octahedra sites stabilizes the monoclinically distorted high-pressure modification of SrMnO 3 . While further increase of antimony content results in a morphotropic transition, and the phase diagram contains two solid solubility regions based on monoclinic and tetragonal structures and a miscibility gap in the region from x = 0.09 to 0.17, where both modifications coexist without a change of unit cell parameters, only their mass fractions are changed.
Magnetic properties of monoclinic SrMn 1Àx Sb x O 3 (x = 0.05 and 0.075) solid solutions. The temperature dependence of the magnetic susceptibility (w) and the AC susceptibility (w 0 ) for 6H-SrMn 0.95 Sb 0.05 O 3 and SrMn 0.925 Sb 0.075 O 3 measured in the ZFC and FC modes in a magnetic field of 500 Oe are presented in Fig. 7.
At high temperatures, both solid solutions are paramagnetic, and the magnetic susceptibility in the temperature range of B250-400 K follows the Curie-Weiss law w = C/(T À Y) (Fig. 7b and d) with negative Y values (À500 K for x = 0.05 and À368 K for presence of very strong antiferromagnetic interactions and magnetic frustration appearing due to the structural peculiarities of these perovskites. The crystal structure of SrMn 1Àx Sb x O 3 with x = 0.05 and 0.075-a monoclinically distorted 6H-perovskite-can be considered as a two-dimensional triangular magnet, 29 in which the Mn sublattice is divided by double layers occupied by manganese and antimony ions. The presence of magnetic disorder, due to the implantation of diamagnetic cations Sb 5+ , promotes a decrease in T N compared with 6H-SrMnO 3 8 and an abrupt growth of susceptibility when the temperature lowers ( Fig. 7a and c). Similar to 4H-SrMnO 3 , 20 for the monoclinic solid solution SrMn 1Àx Sb x O 3 (x = 0.05 and 0.075), the short-range antiferromagnetic interactions persist in the paramagnetic region. The w(T) curves for the samples with x = 0.05 and 0.075 cooled in the ZFC mode and in measured field (FC mode) diverge at 190 K and B280 K, respectively. For SrMn 0.925 Sb 0.075 O 3 , a considerable increase in w measured in the FC mode is observed at 160 K (Fig. 7c). During further cooling of the sample, another abrupt increase in w is observed at 41 K and a maximum at 21 K appears on the ZFC curve. The effect on the AC susceptibility at 21 K (Fig. 7d) may be an indication that at low temperature not all of the manganese cations are involved in long-range antiferromagnetic order and can form clusters that form a spin-glass state. These data suggest that the temperature vs. susceptibility dependence is determined by dimeric behavior for the monoclinic solid solution SrMn 1Àx Sb x O 3 (x = 0.05 and 0.075); in contrast to 6H-SrMnO 3 . 8 The insets in Fig. 7c display the magnetization measurement results as a function of the magnetic field at 5 K. The measurements were performed during a reduction of the magnetic field from 50 kOe on samples cooled under ZFC conditions. The fact that the s vs. H curves are nonlinear and that the magnetization does not reach saturation at 50 kOe suggest the presence of frustrated spins in the magnetic system. These results, together with the w = f (T) data ( Fig. 7a-d), may be indicative of a rather complicated model of antiferromagnetic ordering of monoclinic SrMn 1Àx Sb x O 3 solid solutions below the temperatures at which the divergence between FC and ZFC magnetic susceptibility data is observed (190 K and about 280 K for x = 0.05 and 0.075, respectively). Distinct from 6H-SrMnO 3 produced at 6 GPa, 11 the w(T) curve which displays a sharp maximum near T N = 235 K, there is only a flat maximum near 190 K on the ZFC curve (Fig. 7a) and the w 0 = f (T) curve (Fig. 7b) of the solid solution with x = 0.05.
Magnetic ordering in SrMn 0.925 Sb 0.075 O 3 . The magnetic structure determination for SrMn 0.925 Sb 0.075 O 3 was performed from NPD data at 1.5 K and 80 K. An analysis of profile fits after refinement of the nuclear structure showed extra intensities for (11% 1) at d E 4.47 Å and (11% 2) at d E 3.88 Å (Fig. 8).
These reflections of magnetic origin can be indexed with a magnetic cell of the same dimensions and space group as the structural one, which means that we need the single propagation vector k = (0, 0, 0). The magnetic symmetry analysis was performed using the program BasIreps in the FullProf suite. 31 Full details on the determination of the magnetic structure are given in the ESI † (including Tables S5-S7)   The relations between the cells and the arrangement of magnetic moments in monoclinic and parent hexagonal lattices are shown in Fig. 3; the difference in the fit of the NPD pattern of SrMn 0.925 Sb 0.075 O 3 at 1.5 K without taking into account the magnetic ordering and with long-range magnetic ordering is presented in Fig. 8. In general, Fourier coefficients are (u, v and w) and (Àu, v and Àw) and both magnetic subsystems might have antiferromagnetic spin canting in the direction of the a axis and ferromagnetic in the direction of the b axis, which means that spins are slightly tilted rather than being parallel, and a nonzero net moment is possible. In first approximation, the magnetic structure is well described as A-type antiferromagnetic, where spins are parallel to the c axis. In-plane, the coupling is ferromagnetic, while the inter-plane coupling is antiferromagnetic (Fig. 3b). The refined magnetic moments for SrMn 0.925 Sb 0.075 O 3 at 1.5 K and 80 K are presented in Table S8 (ESI †). The magnitude of the saturated moments at 1.5 K, 1.85(9) m B for 4a and 1.13(6) m B for 8f sites, respectively, is much lower than the predicted value of 2.6 m B for Mn 4+ with a degree of covalency. 33 The combined populations on both sites of 92.5% of S = 3/2 Mn 4+ and 7.5% of S = 2 Mn 3+ ions mainly leads to antiferromagnetic interactions, although local orbital ordering of Mn 3+ could create some ferromagnetic couplings causing frustration. Similar long-range magnetic ordering has been reported for the 9R-type perovskite, BaRu 0.2 Mn 0.8 O 3 34 and hematite, a-Fe 2 O 3 , 35 below the Morin temperature. More recently, the same magnetic ordering has been published for undistorted 6H perovskite Ba 3 Fe 2 TeO 9 . 36 Magnetic properties of the tetragonal solid solution SrMn 1Àx Sb x O 3 (x = 0.20 and 0.335). The substitution of Mn for 33% Sb in SrMn 0.5 Sb 0.5 O 3 results in an increase in the average oxidation state of Mn from +3 to +3.5 in SrMn 0.665 Sb 0.335 O 3 , which leads to a variation of the Y constant in the Curie-Weiss law from a positive 18,19 to a negative value (À81 K) (Fig. 9a, data for w 0 ). The positive value of Y is retained only above the point of the structural transition to the cubic modification (350 K). At low temperatures (Fig. 9b), the form of the w = f (T) curve is similar to that found in ref. 18 for Sr 2 MnSbO 6 and is characterized by the divergence of ZFC and FC data at 28 K in the 5 kOe magnetic field and a maximum on the ZFC dependence at 10 K. The magnetization vs. magnetic field dependence at 2 K displays a Brillouin-like curve with a very small hysteresis (inset on Fig. 9b).
The increase in the Mn content and in the average oxidation state to +3.75 in tetragonal SrMn 0.80 Sb 0.20 O 3 qualitatively changes the temperature dependence of the magnetic susceptibility (Fig. 9c). The appearance of a maximum on the w = f (T) dependence at 248 K and a very narrow region of validity of the Curie-Weiss law on the 1/w = f (T) dependence (in the range of 250-350 K, Y = 1300 K, not shown) allow us to suppose a long-range magnetic ordering of this solid solution, with T N = 248 K. The presence of a maximum at this temperature is also confirmed by the results of AC susceptibility measurements (inset on Fig. 9c). A small anomaly in the ZFC-susceptibility at 43 K may be the evidence of the magnetic transition in Mn 3 O 4 , which was not found by X-ray diffraction. The magnetization-field dependence does not show significant hysteresis (not shown). The transformation of this solid solution into the cubic form is also seen on the w = f (T) curve at about 350 K.
These reflections can be indexed with a magnetic cell of the same dimensions as the structural one, but the crystallographic unit cell is I-centered, whereas the magnetic unit cell is primitive, which means that we need the single propagation vector k = (1, 0, 0). Full details on the determination of the magnetic structure is given in the ESI † (including Table S9).
The refinement showed that the G 7 model perfectly describes the extra peaks (Fig. 10c). The magnetic structure is described as C-type antiferromagnetic, where spins are parallel to the c axis with antiferromagnetic intraplanar and ferromagnetic interplanar coupling (Fig. 5b). The refined saturated magnetic moments at 5 K, 2.79(2) m B and 2.00(4) m B for SrMn 1Àx Sb x O 3 with x = 0.20 and 0.335, respectively (Table S10, ESI †), fit well to the predicted value for Mn 4+ , but are lower than those for Mn 3+ .

Conclusions
Solid solutions of the SrMn 1Àx Sb x O 3 system with a perovskite structure and mixed-manganese valence Mn 4+ /Mn 3+ were prepared for the first time by conventional solid-state synthesis. Two ranges of solid solutions were found: (1) SrMn 1Àx Sb x O 3 (0.025 r x r 0.09) with a monoclinically distorted 6H-SrMnO 3 polytype (sp. gr. C/2c) and (2) SrMn 1Àx Sb x O 3 (0.17 r x r 0.5) with a tetragonal unit cell (sp. gr. I4/mcm). The crystal structure refinement was carried out using the NPD data for low and high temperatures and a combination of XRPD and NPD data for room temperature. The crystal structure of the monoclinic solid solution consists of corner-sharing octahedra around sites 4a randomly occupied by manganese and antimony ions and facesharing octahedra around sites 8f occupied by manganese ions only. The tetragonal solid solution SrMn 1Àx Sb x O 3 has a random distribution of cations. The high-temperature modification observed at 500 K has been described with the highest symmetry: a cubic perovskite with a space group Pm% 3m and a unit cell parameter a = 3.87998(2) Å (x = 0.20).
Magnetic susceptibility measurements and low-temperature NPD data show that the solid solutions adopt antiferromagnetic ordered structures. The magnetic structure of the tetragonal solid solutions SrMn 1Àx Sb x O 3 (x = 0.20 and 0.335) is described as C-type AFM, where spins are parallel to the c axis with AFM intraplanar and FM interplanar coupling. The refined saturated magnetic moments at 5 K, 2.79(2) m B and 2.00(4) m B for SrMn 1Àx Sb x O 3 for x = 0.20 and 0.335, respectively, fit well to the predicted value for Mn 4+ , but are lower than those for Mn 3+ . In first approximation, the magnetic structure of the monoclinic solid solution is well described as an A-type AFM, where spins are parallel to the c axis. Inside the plane, coupling is ferromagnetic while inter-plane coupling is antiferromagnetic.
Our results clearly demonstrate that antiferromagnetic order can be induced in both perovskite structures of the SrMn 1Àx Sb x O 3 system through chemical substitution. This result contradicts earlier published data for the SrMn 0.5 Sb 0.5 O 3 tetragonal perovskite in which magnetic order was not found. 18,19 The magnetic behavior of SrMn 0.5 Sb 0.5 O 3 was described by a spin-glass state with a weak ferromagnetism at low temperatures and a positive Weiss constant in the paramagnetic area indicating the presence of ferromagnetic clusters; there was no convincing explanation for the lack of a magnetic ordered state in Sr 2 MnSbO 6 given so far. However, similar magnetic properties of Sr 2 MnTaO 6 at low temperature 37 were explained by competing double-exchange (ferromagnetism) and super-exchange (antiferromagnetism) interactions. This conclusion was based on the assumption that Mn in this compound has a mixed oxidation state between +3 and +4. At the same time, in isostructural manganites (monoclinic Sr 2 FeSbO 6 and Sr 2 FeTaO 6 ) but with sufficient structural disorder, a partial cation ordering in the structure leads to the coexistence of a magnetically-ordered spin state and a spin-glass state. 38 It is important that in our work, as a result of the replacement of 33% of Sb 5+ cations by manganese in SrMn 0.5 Sb 0.5 O 3 , the Weiss constant in SrMn 0.67 Sb 0.33 O 3 began to have a negative value thus pointing towards domination of AFM interactions and the formation of antiferromagnetic frustrated-spin clusters. Considering that according to our NPD data the oxygen content in the solid solutions corresponds precisely to the theoretical values, we suggest that the mean oxidation states of manganese in the solid solutions with x = 0.33 and 0.20 are +3.5 and +3.75, respectively. Consequently, the emergence of antiferromagnetic ordering in these solid solutions can be explained by the formation of mixed-valence manganese Mn 3+ /Mn 4+ states due to decrease of the diamagnetic Sb 5+ ion content in antiferromagnetic frustrated-spin clusters.
Further substitution of Mn cations by Sb 5+ is also a reason for the formation of the monoclinic structure with mixed-valence Mn 3+ /Mn 4+ states. Thus, a change in composition provokes a change in structure and magnetic properties, inducing antiferromagnetic ordering in the tetragonal (x = 0.20 and 0.33) and the monoclinic (at x = 0.025 and 0.075) solid solution based on SrMn 1Àx Sb x O 3 .
This conclusion can be further supported by the analysis of results of the other works already mentioned in the introduction, in which possibilities of modification of the crystal structures of manganites and their magnetic properties by chemical replacements are clearly shown.

Conflicts of interest
There are no conflicts to declare.