Edinburgh Research Explorer Double and double double perovskites in the RMnMnTaO6 series

A B S T R A C T A new family of perovskites RMnMnTaO 6 has been synthesised at high pressure for representative R ¼ La, Nd, Sm, Eu, Gd, Y and Lu that span the whole rare earth series. The structure type changes from tetragonal P 4 2 / n double double perovskite (DDPv) type with columnar R 3 þ /Mn 2 þ cation order (R ¼ La, Nd, Sm) to monoclinic P 2 1 / n double perovskite (DPv) (R 0.5 Mn 0.5 ) 2 MnTaO 6 (R ¼ Sm, Gd, Y, Lu) as R 3 þ cation size decreases. No antisite disorder or nonstoichiometry is observed. Spin ordering transitions are observed below 70 K and a neutron diffraction study of DDPv NdMnMnTaO 6 reveals ferrimagnetic orders at T C1 ¼ 65 K and T C2 ¼ 40 K with Mn-spin reorientation due to Nd moment order at T C2 leading to magnetoelastic coupling.

R 3þ cation size is well-known as a tuning effect on perovskite properties, and more subtle electronic effects on synthesis conditions have also been reported [10].Within the RMnMnSbO 6 series, R 3þ size was found to lead to a dramatic switch in cation ordering, from double double (DDPv) AA'BB'O 6 order for larger R ¼ La, Pr, Nd, Sm to a double perovskite (DPv) (A 0.5 A 0 0.5 ) 2 BB'O 6 structure where A/A 0 cations are statistically disordered for smaller R ¼ Eu, Gd [13].The two structure types are shown in Fig. 1.To discover whether the same structural change can be observed in other families, we have explored the new RMnMnTaO 6 series across the full R ¼ La-Lu, Y range, and detailed results are reported in this paper.Both DDPv and DPv polymorphs of the R ¼ Sm member (and CaMnMnTaO 6 ) were discovered as a function of synthesis temperature at pressure, as reported elsewhere [20].

Experimental details
RMnMnTaO 6 compositions for representative R ¼ La, Nd, Sm, Eu, Gd, Y and Lu were treated under high pressure-temperature conditions using a Walker-type multi anvil apparatus.Stoichiometric ratios of RTaO 4 and MnO were packed in a Pt capsule, compressed to 10 GPa pressure and heated at temperatures from 1200 to 1650 C for 30 min.Temperature was quenched followed by slow pressure release to ambient.RTaO 4 precursors were obtained by standard solid-state reaction; a pellet of preheated R 2 O 3 and Ta 2 O 5 was heated at 1623 K for 24 h with intermediate regrinding and repelleting.
The polycrystalline products were characterised by powder X-ray diffraction (XRD) using a Bruker D2 instrument employing Cu Kα radiation.Neutron powder diffraction (NPD) data were collected for the DDPv NdMnMnTaO 6 sample on beamline D20 at the ILL facility; with wavelength λ ¼ 1.54 Å at 300 K for refinement of the crystal structure; and with λ ¼ 2.41 Å at 1.7, 50 and 80 K for studying magnetic ordering.Short scans with λ ¼ 2.41 Å from 1.5 to 80 K in 1 K intervals were also recorded to follow the temperature dependence of multiple spin ordering and potential spin reorientation transitions.
Magnetisations were measured on a Quantum Design MPMS SQUID magnetometer.Magnetic susceptibilities were recorded using zero-field cooled (ZFC) and field cooled (FC) conditions for an external field of 0.1 T. Isothermal magnetisation measurements at several temperatures were undertaken using fields from À5 to 5 T.

Phase formation and XRD
Seven RMnMnTaO 6 compositions were investigated (R ¼ La, Nd, Sm, Eu, Gd, Y and Lu).Laboratory X-ray diffraction patterns (Fig. 2) showed that R ¼ La and Nd formed the DDPv double double perovskite phase at all temperatures, as a characteristic peak for columnar A-site R/Mn cation ordering was observed near 2θ ¼ 16 .Smaller R 3þ cations from Eu to Lu formed the double perovskite DPv phase without A-site R/Mn ordering.R ¼ Sm shows a crossover from DDPv to DPv structures with increasing synthesis temperature as reported elsewhere [20].Rietveld fits to the diffraction patterns (Fig. S1) revealed small amounts of RTaO 4 and Mn 3 O 4 secondary phases.The fits confirmed that the DDPv phases have the tetragonal P4 2 /n structure found previously for analogous materials, while the DPv's have the commonly found monoclinic P2 1 /n structure.Lattice parameters and cell volumes in Fig. 3 show systematic changes consistent with increasing R 3þ ionic radius.Detailed crystallographic information is summarised in Tables S1-S3.A phase diagram displaying synthesis results at 10 GPa is shown in Fig. 4.

Magnetisation
Magnetisation measurements were performed for each sample except R ¼ Lu as this contained a relatively large amount of secondary phases.Representative data for the R ¼ Nd DDPv and the R ¼ Gd DPv phases are shown in Fig. 5 and data for other R are in SI.Extracted magnetic parameters are shown in Table 1.High-temperature susceptibilities for all samples follow the Curie-Weiss equation except for R ¼ Eu where a temperature-dependent moment is evident and the data do not follow Curie-Weiss behaviour.Fitted effective paramagnetic moments in Table 1    are near to the theoretical composite values for spin-only S ¼ 5/2 Mn 2þ and R 3þ cations in their ground J-states, except for R ¼ Y which is attributed to the sample containing a significant amount of secondary phases.Negative Weiss temperatures evidence dominant antiferromagnetic interactions between spins.
All samples show a ferro-or ferri-magnetic ordering at a transition T C1 between 26 and 69 K, and some materials containing magnetic R 3þ cations show a second transition at T C2 ¼ 13-40 K, suggesting respective ordering of the Mn 2þ and R 3þ moments at the two transitions.
Divergence of ZFC and FC susceptibilities is observed at low temperatures and M-H loops show corresponding hysteresis loops below T C1 with small coercive fields (<0.11T) at 5 K. Saturated magnetisations at 5 K extrapolated to zero field are small (<0.2 μ B ) for R ¼ La, Sm and Y samples, suggesting that the likely spin order is ferrimagnetic with antiparallel coupling of inequivalent Mn 2þ moments at A and B sites.R ¼ Nd, Eu and Gd samples have larger saturated moments at 5 K, in particular a value of 6.2 μ B for R ¼ Gd, showing that Mn 2þ and R 3þ moments are involved in the ferrimagnetic order.

Table 1
Magnetic parameters for RMnMnTaO 6 samples with DDPv or DPv structures.Curie temperatures (T C ), theoretical (μ th ) and experimental (μ exp ) effective paramagnetic moments, and Weiss constants (θ) are from susceptibility data.Saturation (M S ) and remnant (M R ) magnetisations and coercive field (B C ) values are from magnetisationfield loops at 5 K, with values at other temperatures also shown for some samples.Curie-Weiss fitting was not performed for R ¼ Eu as the moment varies with temperature.Values for DDPv and DPv polymorphs of R ¼ Sm were reported in ref. 20

Neutron powder diffraction study of NdMnMnTaO 6
NdMnMnTaO 6 was chosen as a representative DDPv for NPD study to obtain accurate oxygen positions and to determine the low-temperature spin structures.Rietveld fits to high resolution data collected at 300 K showed that all crystallographic sites were fully occupied and no antisite cation disorder was observed.This reflects the large cation size difference between Nd 3þ (1.29 Å) and Mn 2þ (0.96 Å) at A-sites, as well as a large charge difference between Mn 2þ and Ta 5þ at B-sites.Full results are shown in SI.
On cooling below T C1 ¼ 65 K, magnetic intensities appear on nuclear peaks such as (110), ( 111), (200) (Fig. 6a) and are indexed by the [000] propagation vector.Magnetic intensities at 50 K data are fitted well by a simple ferrimagnetic model (R mag ¼ 2.43%) in which A-and B-site Mn 2þ spins are antiparallel and aligned along the c-direction (Fig. 7a), without Nd 3þ moments being ordered at this temperature.A and B site Mn spins were constrained to have the same magnitude in the refinements, although magnetisation data show that they differ slightly, by 0.24 μ B at 50 K (Table 1).Dominant antiferromagnetic coupling between ferromagnetic A and B sublattice spins is consistent with the negative Weiss temperature.Further cooling below T C2 ¼ 40 K leads to changes in magnetic intensity distribution, as shown in Fig. 6b and c where the magnetic contribution to (200) decreases while the contribution to (002) increases.This evidences a spin reorientation transition and the magnetic NPD data at 1.7 K are fitted well (R mag ¼ 1.96%) by the model shown in Fig. 7b.The Mn 2þ spins reorient from being parallel to the c-axis to being perpendicular on cooling through T C2 , with an ordered moment of 4.5 (1) μ B at 1.7 K close to the ideal spin-only value of 5 μ B .Nd 3þ moments are ordered ferromagnetically in the ab-plane below T C2 with a magnitude of 1.2(1) μ B at 1.7 K indicating a J eff ¼ ½ ground state due to crystal electric field (CEF) effects that lead to significant anisotropy.Single-ion anisotropy from the Nd 3þ moments likely drives the reorientation of the Mn 2þ   Fits to the variable temperature NPD data collected between 1.7 and 80 K for NdMnMnTaO 6 were used to extract the lattice parameters and volume shown in Fig. 8. Volume increases smoothly across this range but the lattice parameters show a clear anomaly at T C2 with negative expansion of a below 40 K.Comparison of the thermal evolution of the tetragonal strain e ¼ (c/a) À 1 to the ordered moment variations in Fig. 9 shows that a negative excess strain develops through magnetoelastic coupling below T C2 .Although lattice parameters and volume evolve smoothly through T C2 , the tetragonal strain shows a discontinuity indicating that the transition is first order.This is expected as the magnetic structures above and below T C2 have different irreducible symmetry representations (Γ 1 and Γ 3 respectively [14]) and so the transition between them cannot be continuous.
Magnetoelastic coupling between strain e and magnetisation M typically has a linear-quadratic dependence, proportional to eM 2 [21].This is verified by the fit of a critical function m ¼ m 0 [1 À (T/T C )] β where m is the ordered Nd 3þ moment to data below T C2 ¼ 40 K with exponent β ¼ 0.5, in keeping with mean field theory, as shown on Fig. 9, while excess tetragonal strain shows a linear (i.e.β ¼ 1) dependence over a wide temperature range below 37 K.The interval between 37 and T C2 ¼ 40 K may correspond to a two phase region at the first order transition although this is not resolved in the magnetic fits.A fit of the critical function to the Mn 2þ moment variation approaching the higher temperature magnetic transition gives T C1 ¼ 64.4(2)K with critical exponent β ¼ 0.36(1), in excellent agreement with the theoretical value of β ¼ 0.365 for a 3D-Heisenberg magnet.Hence the spin order at T C1 may be viewed as a purely magnetic transition while the combined Mn 2þ moment reorientation and Nd 3þ moment order at T C2 is a magnetostructural transition with significant coupling of spins to the lattice through single-ion anisotropy of Nd 3þ .

Conclusions
In conclusion, cation-ordered perovskites RMnMnTaO 6 are found to exist at 10 GPa pressure across the whole rare earth series.The structure type changes from tetragonal P4 2 /n double double perovskite (DDPv) type with columnar R 3þ /Mn 2þ cation order (R ¼ La, Nd, Sm) to monoclinic P2 1 /n double perovskite (DPv) (R 0.5 Mn 0.5 ) 2 MnTaO 6 (R ¼ Sm, Gd, Y, Lu) as R 3þ cation size decreases.A similar trend was found for in the RMnMnSbO 6 series.Both structure types are observed for R ¼ Sm but not for other R. Low temperature magnetic transitions are observed and a detailed neutron diffraction study of DDPv reveals ferrimagnetic orders at T C1 ¼ 65 K and T C2 ¼ 40 K with Mn-spin reorientation due to Nd moment order leading to magnetoelastic coupling below T C2 .Fig. 9. Thermal evolution of tetragonal strain and ordered Mn 2þ and Nd 3þ magnetic moments for DDPv NdMnMnTaO 6 from Rietveld fits to short NPD scans.A linear fit to the excess strain below 37 K and critical fits to the moments in the range T C /2 < T < T C are shown, as described in the text.

Fig. 1 .
Fig. 1.(a) Tetragonal P4 2 /n double double perovskite (DDPv), and (b) monoclinic P2 1 /n double perovskite (DPv) RMnMnTaO 6 crystal structures.A-site Mn/ R cations are disordered in the DPv structure but have a columnar ordering in the DDPv with the A-column Mn cations having alternating tetrahedral (Mn AT , dark grey sphere) and square-planar (Mn AS , light grey sphere) coordination environments as shown to the left.R 3þ cations are shown as large grey spheres.Mn 2þ and Ta 5þ are in octahedral coordination with rock-salt type ordering in both structures.

Fig. 2 .
Fig. 2. Powder X-ray diffraction (XRD) patterns of RMnMnTaO 6 samples synthesised at 1200 C under 10 GPa pressure.A-site ordering in the DDPv samples is evidenced by the arrowed (110) peak (R ¼ La, Nd, Sm) and is absent for DPv materials (R ¼ Eu, Gd, Y, Lu).RTaO 4 secondary phases are present in all samples, with increasing amounts for small R ¼ Y and Lu, and the most intense peak is marked by a star on the R ¼ Eu profile.Rietveld fits to these data are shown in the SI.

Fig. 3 .
Fig. 3. Refined lattice parameters and cell volume plotted against the R 3þ ionic radius for RMnMnTaO 6 samples, with R shown at the top.The broken vertical line shows the boundary between tetragonal P4 2 /n DDPv's and monoclinic P2 1 / n DPv's.a and b lattice parameters for the latter are scaled by √2 for comparison.Error bars are smaller than the data points.

Fig. 4 .
Fig. 4. Phase diagram for the RMnMnTaO 6 series as a function of synthesis temperature and R 3þ ionic radius with R shown at the top.Double perovskite (DPv) and double double perovskite (DDPv) domains are indicated, with a narrow intermediate region of DPv/DDPv coexistence for R ¼ Sm [20].

Fig. 5 .
Fig. 5. Magnetisation plots for representative RMnMnTaO 6 samples; (a) and (b), DDPv R ¼ Nd, and (c) and (d), DPv R ¼ Gd.(a) and (c) show ZFC and FC susceptibilities and inverse ZFC susceptibilities with a Curie-Weiss fit to high temperature points.(b) and (d) show magnetisation-field data collected at several temperatures.

Fig. 6 .
Fig. 6.Low angle neutron diffraction data (λ ¼ 2.41 Å) for NdMnMnTaO 6 .(a) Comparsion of long scans at several temperatures showing starred magnetic contributions appearing below the spin ordering transitions at T C1 ¼ 65 K and T C2 ¼ 40 K.(b) Short-scan thermodiffraction neutron intensities showing magnetic contributions at low temperatures, with the expansion in (c) revealing redistribution of magnetic intensity from the (200) to the (002) peak on cooling through T C2 that evidences spin reorientation.

Fig. 7 .
Fig. 7. Ferrimagnetically ordered structures of DDPv NdMnMnTaO 6 at (a) 50 K and (b) 2 K with the c-axis vertical.Long/short arrows represent Mn/Nd moments.Up Mn-A and down Mn-B spin sublattices are ordered parallel to the c-axis at 50 K, but reorient perpendicular to this direction with down Nd, up Mn-A, and down Mn-B spin directions at 2 K. Dark and light grey arrows represent moments on Mn AT and Mn AS sites, respectively.

Fig. 8 .
Fig. 8. Temperature evolution of refined lattice parameters and cell volume for tetragonal DDPv NdMnMnTaO 6 from Rietveld fits to short NPD scans.
and are shown here for comparison.
a Measured at 50 K.b Measured at 30 K.