(Ca0.5Mn0.5)2MnTeO6 – An Anomalously Stable
High-Pressure Double Perovskite

Azizah Almadhi, Kunlang Ji, Sean D. Injac, Clemens Ritter, J. Paul Attfield

► To cite this version:

Azizah Almadhi, Kunlang Ji, Sean D. Injac, Clemens Ritter, J. Paul Attfield. (Ca0.5Mn0.5)2MnTeO6 – An Anomalously Stable High-Pressure Double Perovskite. Chemistry - An Asian Journal, 2024, 19 (14), pp.e202400280. 10.1002/asia.202400280. hal-05330159

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(Ca₀.₅Mn₀.₅)₂MnTeO₆ – An Anomalously Stable
High-Pressure Double Perovskite

Azizah Almadhi,[a] Kunlang Ji,[a] Sean D. Injac,[a] Clemens Ritter,[b] and J. Paul Attfield*[a]

Dedicated to Prof. C.N.R. Rao on the occasion of his 90th Birthday

High pressure high temperature treatments of the composition CaMnMnTeO₆ are found to yield only an A₂BB'O₆-type double perovskite (Ca₀.₅Mn₀.₅)₂MnTeO₆, rather than a AA'BB'O₆ double double perovskite with A- and B- site cation order as found in analogs CaMnMnReO₆ and CaMnMnWO₆ with similar cation sizes. Double perovskite (Ca₀.₅Mn₀.₅)₂MnTeO₆ adopts a mono-

clinic structure in space group P2₁/n with a framework of highly tilted MnO₆ and TeO₆ octahedra enclosing disordered Ca²⁺ and Mn²⁺ cations. Magnetic measurements show that (Ca₀.₅Mn₀.₅)₂MnTeO₆ is a highly frustrated spin glass with a freezing transition at 5 K, and no long-range spin order is apparent by neutron diffraction at 1.6 K.

Introduction

Transition metal oxides, and notably those with perovskite and
related structures, have many outstanding physical and chem-
ical properties leading to a variety of applications.[1,2] Cation
ordering at A or B sites of the ABO₃ perovskite structure can
produce many superstructure types such as AA'B₂O₆ and A₂BB'O₆
double perovskites (DPVs).[3] 1:1 cation ordering at both sites to
give an AA'BB'O₆ double double perovskite (DDPv) is rarer but
an extensive family of DDPvs has recently been discovered
based on the high-pressure stabilization of Mn²⁺ at perovskite
A-sites.[4]

B-site manganites are well-known and much studied for magnetoresistance, charge-ordering, and other properties,[5] but high-pressure high-temperature synthesis allows Mn²⁺ to occupy A-sites while other magnetic or non-magnetic cations occupy B-sites to produce A-site manganites with multiple magnetic sublattices.[4] Simple MnBO₃ perovskites are rare and MnVO₃, which has incommensurate low-temperature spin order,[6] is the only recovered Mn²⁺B⁴⁺O₃ example while ζ-Mn₂O₃ (a high pressure distorted-perovskite polymorph) has a more complex charge distribution,[7] and MnTiO₃ and MnSnO₃ are observed at high pressure but are not recoverable.[8] However many A-site Mn DPvs and DDPvs have been recovered from high pressure high temperature (HPHT) conditions in recent years.[4] Many of these are Mn₂BB'O₆ double perovskites

with rocksalt B/B' cation ordering which is a very common
double perovskite type.[9] They mostly have antiferromagnetic
ground states at low temperatures, e.g. Mn₂ScSbO₆[10]
Mn₂CrSbO₆[11] Mn₂MnTeO₆[12] Mn₂CoTeO₆[13] Mn₂FeSbO₆[14] and
Mn₂NiReO₆[15] but cluster spin glass behaviour was reported in
Mn₂LiReO₆.[16] AMnB₂O₆ DPvs with columnar A²⁺/Mn²⁺ order
driven by cation size difference are also known, and CaMnTi₂O₆
is notable for ferroelectric properties below a Curie temperature
of 630 K.[17] Combining the two ordering motifs, i.e. columnar
A²⁺/Mn²⁺ and rocksalt B/B' orders, leads to a large family of
AMnBB'O₆ double double perovskites with a tetragonal (space
group P4₂/n) structure.[4,18]

The AMnBB'O₆ DDPvs are notable for a wide range of magnetic properties.[4] CaMnFeReO₆ and Cu-substituted derivatives Ca(Mn₀.₅Cu₀.₅)FeReO₆ and CaCuFeReO₆ are ferrimagnets with Curie temperatures over 500 K and varying magnetoresistance properties.[19,20] RMnMnSbO₆ and RMnMnTaO₆ families (R=La-Sm) are also ferrimagnets and additional spin reorientation transitions are observed for magnetic R³⁺ cations.[18,21] This DDPv family and other cation-ordered A-site manganites such as Mn₃MnNb₂O₉[22] with 1:2 B-site ordering have recently been reviewed.[4] Some attempts to prepare HPHT AMnBB'O₆ compositions without a high charge difference between B and B' cations have led to simple disordered perovskites, such as 'CaMnMnTaO₆' and'DyMnMnTiO₆' which respectively gave GeFeO₃-type Pnma (Ca₀.₅Mn₀.₅)(Mn₀.₅Ta₀.₅)O₃ and (Dy₀.₅Mn₀.₅)(Mn₀.₅Ti₀.₅)O₃ perovskites without A- or B-site cation orders.[23,24]

A further interesting aspect is that some apparent AMnBB'O₆ double double perovskite compositions are found to adopt double perovskite structures, written as (A₀.₅Mn₀.₅)₂BB'O₆ to show that the A and Mn²⁺ cations are disordered over perovskite A-sites while B/B' retain rocksalt type long-range order. This was first discovered in the RMnMnSbO₆ series where materials with R in the range La-Sm were found to form the DDPv structure with columnar R/Mn A-site order, whereas for Eu and later R the (A₀.₅Mn₀.₅)₂BB'O₆ DPv with disordered A-site R/Mn was formed.[18] A similar observation was made in the RMnMnTaO₆ series, but it was found that both DDPv and DPv

<figure><img src="image_3.png" /></figure>

<footnote>[a] A. Almadhi, Dr. K. Ji, Dr. S. D. Injac, Prof. J. P. Attfield
Centre for Science at Extreme Conditions (CSEC) and School of Chemistry,
University of Edinburgh,
Peter Guthrie Tait Road, EH9 3FD Edinburgh, UK
E-mail: j.p.attfield@ed.ac.uk</footnote>

<footnote>[b] Dr. C. Ritter
Institute Laue-Langevin,
38042 Grenoble, France</footnote>

<footnote>© 2024 The Authors. Chemistry - An Asian Journal published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
forms of SmMnMnTaO₆ could be recovered.[21,25] The same discovery was made for the CaMnMnWO₆ composition where a fully-ordered DDPv form was recovered from synthesis at 1000°C and 10 GPa, while the A-site disordered DPv polymorph (Ca₀.₅Mn₀.₅)₂MnWO₆ resulted from treatment at 1600°C and 10 GPa.[25] The change from Ca/Mn order to disorder led to an interesting switch of magnetic properties as DDPv-CaMnMnWO₆ is ferrimagnetic below T_c=45 K whereas DPv-(Ca₀.₅Mn₀.₅)₂MnWO₆ is highly frustrated with a spin glass freezing temperature T_g=8 K. The possibility of generating both double double and double perovskite polymorphs of the same composition is exciting as it provides a method for directly observing how cation order or disorder affects physical properties of these complex materials. We have therefore explored the Te-analog of the latter material, CaMnMnTeO₆ to discover whether DDPv and/or DPv forms are accessible under similar synthesis conditions, as reported here.

## Results

X-ray powder diffraction (XPD) patterns from initial HPHT products synthesized at 10 GPa and temperatures 700–1250°C (Samples 1–7 in Figure 1) showed that a double perovskite (Ca₀.₅Mn₀.₅)₂MnTeO₆ was present in all samples. A DDPv form of CaMnMnTeO₆ would be favoured at higher pressures and/or lower temperatures than the DPv polymorph, however, reactions at 12 and 15 GPa with temperature held at 700°C (Samples 5 and 7 in Figure 1), and at 600 and 550°C for 10 GPa pressure (Samples 4 and 6), gave only the monoclinic DPv form with no XPD evidence for any tetragonal DDPv polymorph being formed. Sample 6, heated at 550°C, also showed unreacted and other secondary phases indicating that this is below the temperature for formation of a single-phase

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Powder X-ray patterns for CaMnMnTeO₆ samples recovered from HPHT conditions as shown. All patterns show a single phase of the monoclinic double perovskite (Ca₀.₅Mn₀.₅)₂MnTeO₆ except for Sample 6 which also contains unreacted or secondary phases.</figcaption></figure>

perovskite product. Hence, we conclude that only the DPv form of CaMnMnTeO₆ is formed in the explored reactions, whereas both DPv and DDPv polymorphs of the CaMnMnWO₆ analog were obtained previously under similar HPHT conditions.[25]

The new DPv-(Ca₀.₅Mn₀.₅)₂MnTeO₆ phase has a monoclinic structure in space group P2₁/n, as found for the analog DPv-(Ca₀.₅Mn₀.₅)₂MnWO₆ and many other A₂BB'O₆ double perovskites.[9] The structure was confirmed by a Rietveld fit to XPD data from Sample 7 (Figure 2a), giving 300 K cell parameters *a*=5.4071(4), *b*=5.5706(4), *c*=7.8110(5) Å, and β=90.258(9)°, but it was not possible to refine atomic parameters accurately. Structure refinement against 1.6 K neutron powder data (NPD) was used to provide a precise structural model (as no long-range magnetic ordering occurs as described later). Results are shown in Figures 2b and 3, and Tables 1 and 2. No deficiencies were observed at the three oxygen sites and their occupancies were fixed at 100% in the final refinement. Accurate refinement of cation site occupancies was made possible by the contrast in neutron scattering lengths (Ca/Mn/Te=4.70/-3.73/5.80 fm), although it is notable that the Ca₀.₅Mn₀.₅ site has a small average coherent scattering length (~1.0 fm) leading to less precise coordinates than for the O atoms in Table 1 and less precise Ca/Mn–O bond distances than for B-site Mn–O and Te–O in Table 2. The results in Table 1 show a slight Ca/Mn off-stoichiometry (55/45%) at the A-sites and a small (6%) Mn/Te inversion disorder at the B-sites, but otherwise confirms that the new (Ca₀.₅Mn₀.₅)₂MnTeO₆ double perovskite has been synthesised.

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Rietveld fits of the monoclinic double perovskite (Ca₀.₅Mn₀.₅)₂MnTeO₆ model in space group P2₁/n to (a) 300 K CuKα X-ray, and (b) 1.6 K neutron (λ=1.54 Å), powder diffraction profiles.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Monoclinic P2₁/n double perovskite (Ca₀.₅Mn₀.₅)₂MnTeO₆ crystal structure from the 1.6 K neutron refinement. A-site Ca/Mn cations (pink/light blue) are disordered while MnO₆ (blue) and TeO₆ (brown) octahedra result from rocksalt cation ordering over the B-sites.</figcaption></figure>

Results from magnetic measurements on DPv-(Ca₀.₅Mn₀.₅)₂MnTeO₆ are shown in Figure 4. The magnetic susceptibility in Figure 4a shows paramagnetic behavior at high temperatures, and this is confirmed by the linear magnetisation-field relationship at 300 K in Figure 4b. Inverse susceptibility varies linearly in the 100–300 K region showing that a Curie-Weiss law is followed. The fitted paramagnetic moment of μₐ₇eff = 8.35 μₐ₈ per (Ca₀.₅Mn₀.₅)₂MnTeO₆ formula unit is in excellent agreement with the spin-only value of 8.37 μₐ₈ for two S = 5/2 Mn²⁺ while the negative fitted Weiss temperature of θ = -128 K shows that the dominant spin-spin interactions are antiferromagnetic. However, inverse susceptibility deviates below the Curie-Weiss limit on cooling below 100 K, indicating that ferro- or ferri- magnetic spin clustering occurs, and a susceptibility maximum is observed at 5 K with divergence of zero field cooled (ZFC) and field cooled (FC) data at lower temperatures. This is similar to the behavior of DPv-(Ca₀.₅Mn₀.₅)₂MnWO₆, [25] which showed a spin glass transition at T₉ = 8 K, and the non-linear magnetization-field variation for DPv-(Ca₀.₅Mn₀.₅)₂MnTeO₆ at 2 K is also typical of a spin glass.

Comparison of neutron powder diffraction (NPD) data for DPv-(Ca₀.₅Mn₀.₅)₂MnTeO₆ collected at several temperatures in Figure 5 demonstrates that no magnetic diffraction peaks appear on cooling below the spin freezing transition at 5 K,

<table><caption>**Table 2.** Metal-oxygen distances and selected bond angles for (Ca₀.₅Mn₀.₅)₂MnTeO₆ double perovskite at 1.6 K from the NPD refinement</caption><thead><tr><th>Bond</th><th>Length (Å)</th><th>Bond</th><th>Length (Å)</th></tr></thead><tbody><tr><td>Ca/Mn–O1</td><td>2.49(2)</td><td>Mn–O1</td><td>2.138(4)</td></tr><tr><td>Ca/Mn–O1</td><td>2.33(2)</td><td>Mn–O2</td><td>2.212(4)</td></tr><tr><td>Ca/Mn–O1</td><td>2.57(2)</td><td>Mn–O3</td><td>2.173(5)</td></tr><tr><td>Ca/Mn–O2</td><td>2.24(2)</td><td>&lt;Mn–O&gt;</td><td>2.174(4)</td></tr><tr><td>Ca/Mn–O2</td><td>2.47(2)</td><td>Te–O1</td><td>1.893(4)</td></tr><tr><td>Ca/Mn–O2</td><td>2.92(2)</td><td>Te–O2</td><td>1.892(4)</td></tr><tr><td>Ca/Mn–O3</td><td>2.07(2)</td><td>Te–O3</td><td>1.928(5)</td></tr><tr><td>Ca/Mn–O3</td><td>2.46(2)</td><td>&lt;Te–O&gt;</td><td>1.904(4)</td></tr><tr><td>&lt;Ca/Mn–O&gt;</td><td>2.44(2)</td><td></td><td></td></tr></tbody></table><table><thead><tr><th>Bond</th><th>Angle (°)</th><th>Bond</th><th>Angle (°)</th></tr></thead><tbody><tr><td>O1–Mn–O2</td><td>89.8(2)/90.2</td><td>O1–Te–O2</td><td>88.1(3)/91.9</td></tr><tr><td>O1–Mn–O3</td><td>84.8(3)/95.2</td><td>O1–Te–O3</td><td>89.2(3)/90.8</td></tr><tr><td>O2–Mn–O3</td><td>84.9(3)/95.1</td><td>O2–Te–O3</td><td>89.9(3)/90.1</td></tr><tr><td>Mn–O1–Te</td><td>146.83(15)</td><td>Mn–O2–Te</td><td>140.53(14)</td></tr><tr><td>Mn–O3–Te</td><td>142.50(18)</td><td></td><td></td></tr></tbody></table>

although a small impurity magnetic peak appears between 300 and 45 K. This may be from an Mn-rich secondary phase in view of the observed Mn-deficiency of the DPv phase. Absence of intrinsic magnetic diffraction peaks is also confirmed by the Rietveld fit to the 1.6 K data described above and displayed in Figure 2b which shows that the DPv nuclear model accounts for the observed intensities. Growth of a broad, diffuse background scattering feature at 2θ ≈ 15–30° on cooling from 45 to 1.6 K is apparent and this is likely due to short-range spin-spin correlations. Neutron scattering thus confirms that DPv-(Ca₀.₅Mn₀.₅)₂MnTeO₆ has a spin glassy ground state below T₉ = 5 K, consistent with the above magnetic measurements and the previously-reported behavior of the DPv-(Ca₀.₅Mn₀.₅)₂MnWO₆ analog. [25]

## Discussion

The above results show that a new double perovskite (Ca₀.₅Mn₀.₅)₂MnTeO₆ is obtained by HPHT treatment of the composition CaMnMnTeO₆ over the investigated ranges of

<table><caption>**Table 1.** Refined fractional coordinates, isotropic thermal parameters and site occupancies for (Ca₀.₅Mn₀.₅)₂MnTeO₆ double perovskite at 1.6 K from the NPD fit in monoclinic space group P2₁/n. Lattice parameters; *a* = 5.3833(2), *b* = 5.5444(2), *c* = 7.7681(3) Å, and β = 90.309(4)°.</caption><thead><tr><th>Atom</th><th>x</th><th>y</th><th>z</th><th>B<sub>iso</sub>/Å<sup>2</sup></th><th>Occupancy</th></tr></thead><tbody><tr><td>Ca/Mn</td><td>0.036(3)</td><td>0.106(3)</td><td>0.733(3)</td><td>0.0005(6)</td><td>0.543(5)/0.457</td></tr><tr><td>Mn(Te)</td><td><sup>1</sup>/<sub>2</sub></td><td>0</td><td>0</td><td>0.0005</td><td>0.945(6)/0.055</td></tr><tr><td>Te(Mn)</td><td>0</td><td><sup>1</sup>/<sub>2</sub></td><td>0</td><td>0.0005</td><td>0.945/0.055</td></tr><tr><td>O1</td><td>0.2912(7)</td><td>0.3213(7)</td><td>0.9521(5)</td><td>0.0130(5)</td><td>1.0</td></tr><tr><td>O2</td><td>0.3281(7)</td><td>0.2804(6)</td><td>0.5705(6)</td><td>0.0130</td><td>1.0</td></tr><tr><td>O3</td><td>0.8905(7)</td><td>0.4478(7)</td><td>0.7663(6)</td><td>0.0130</td><td>1.0</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Magnetization data for DPv-(Ca<sub>0.5</sub>Mn<sub>0.5</sub>)<sub>2</sub>MnTeO<sub>6</sub>. (a) Susceptibility (showing ZFC and FC data) and inverse ZFC susceptibilities in a field of 1000 Oe between 2 and 300 K, with a Curie-Weiss fit to the inverse data shown. (b) Magnetization-field loops at 2 and 300 K.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 5.** Low angle neutron powder diffraction data at several temperatures show that no magnetic diffraction peaks from DPv-(Ca<sub>0.5</sub>Mn<sub>0.5</sub>)<sub>2</sub>MnTeO<sub>6</sub> appear below the spin freezing transition at 5 K, although a diffuse feature is apparent at 2θ ≈ 15–30° in the 1.6 K profile and a small impurity magnetic peak appears at 2θ = 25° between 300 and 45 K.</figcaption></figure>

temperature (550–1250 °C) and pressure (10–15 GPa), with no double double perovskite (DDPv) phase observed. This is surprising as previous investigations of CaMnMnB'O₆ analogs with B' = Re and W at similar HPHT conditions reported a DDPv phase for both metals as well as a DPv phase for B' = W at higher temperatures.[19,25] Observed HPHT phases for all reported Ca²⁺Mn²⁺B²⁺B'⁶⁺O₆ compositions are compared in Table 3.

Structural crossovers in perovskite families are primarily related to the tolerance factor, t = <A–O>/√2. <B–O> where <A,B–O> are average A,B-cation to oxygen distances. The DDPv to DPv change in the RMnMnB'O₆ (B' = Sb and Ta) DDPv series is driven by decreasing R³⁺ size and t across the rare earth cations. Size differences between A and A' or between B and B' cations might also be important factors for the cation-ordered AA'BB'O₆ DDPvs. Comparison of the average <r(B)> and the difference Δr(B) between B and B' cation radii[28] in the Ca²⁺Mn²⁺B²⁺B'⁶⁺O₆ compositions in Table 3 shows that the B=Mn, B'=Re, Te, W materials have the largest <r(B)> (and hence smallest t) values, and so might be expected to change from DDPv to DPv structures. However, as the ⁶B'⁶⁺ cation radii are in the order Re (0.55) <Te (0.56) <W (0.60 Å),[28] it is unclear why CaMnMnTeO₆ gives only a DPv phase while B'=Re and W analogs both form DDPvs (plus a DPv for B'=W) at the same pressures and temperatures.

A possible explanation for the anomalous stability of the double perovskite (Ca<sub>0.5</sub>Mn<sub>0.5</sub>)<sub>2</sub>MnTeO<sub>6</sub> in comparison to B'=Re and W analogs is that significant formation of Te⁴⁺ occurs under the HPHT synthesis conditions. This could result from oxygen deficiency (x), i. e. CaMnMn<sup>2+</sup>B<sup>(6-2x)</sup>+O<sub>6-x</sub>, or from charge transfer (q) between B-site Mn<sup>2+</sup> and B'⁶⁺ cations which tends to equalize the sizes of the MnO₆ and B'O₆ octahedra thereby reducing lattice strain, i. e. CaMnMn<sup>(2+q)</sup>+B<sup>(6-q)</sup>+O₆. Reduction of Te⁶⁺ gives Te⁴⁺ which is anomalously large compared to the corresponding transition metal cations due to the 5s² lone electron pair of Te⁴⁺; their ⁶B'⁴⁺ cation radii are in the order Re (0.63) <W (0.66) <Te (0.97 Å).[28] Hence formation of significant amounts of Te⁴⁺ during HPHT synthesis would increase the average <r(B)>, decreasing tolerance factor t and thus stabilizing the DPv over the DDPv structure. In situ HPHT diffraction experiments will be needed to test this hypothesis. Exploration of CaMnMTeO₆ compositions with smaller M²⁺ cations than Mn, e. g. M=Co, Ni, will also be worthwhile as this should decrease <r(B)> relative to M=Mn which will tune the structure back through the DPv to DDPv structural transition enabling both polymorphs to be recovered from the given HPHT conditions.

X-ray and neutron powder diffraction at ambient pressure confirm that DPv-(Ca<sub>0.5</sub>Mn<sub>0.5</sub>)<sub>2</sub>MnTeO<sub>6</sub> adopts a monoclinic P2<sub>1</sub>/n structure with similar lattice parameters to those of the DPv-(Ca<sub>0.5</sub>Mn<sub>0.5</sub>)<sub>2</sub>MnWO<sub>6</sub> analog.[25] Detailed NPD structural analysis shows that almost full Mn/Te ordering occurs over the B-sites with 6% antisite disorder, whereas only 0.8% antisite disorder was reported in DPv-(Ca<sub>0.5</sub>Mn<sub>0.5</sub>)<sub>2</sub>MnWO<sub>6</sub>.[25] This is also consistent with the above hypothesis, as the presence of some large Te⁴⁺ during HPHT synthesis decreases the B/B' cation size difference leading to more antisite disorder. The MnO₆ octahe-
<table><caption>**Table 3.** Comparison of the 6-coordinate ionic radii $^6r$ for *B* and *B'* cations,²⁸ and their average $<^6r(B)>$ and difference $\Delta r(B) = ^6r(B^+) - ^6r(B^+)$, for reported $Ca^{2+}$, $Mn^{2+}$, $B^{2+}$, $B^{6+}$, $O_6$ compositions, showing whether double perovskite (DPv), double double perovskite (DDPv) or both phases were recovered from HPHT synthesis (typically at ~1000 °C and 10 GPa).</caption><thead><tr><th><i>B</i><sup>2+</sup></th><th><sup>6</sup>r(<i>B</i><sup>2+</sup>)/Å</th><th><i>B</i>'<sup>6+</sup></th><th><sup>6</sup>r(<i>B</i><sup>6+</sup>)/Å</th><th>&lt;<sup>6</sup>r(<i>B</i>)&gt;/Å</th><th>Δ<sup>6</sup>r(<i>B</i>)/Å</th><th>DPv/DDPv</th><th>Ref.</th></tr></thead><tbody><tr><td>Mn</td><td>0.83</td><td>W</td><td>0.60</td><td>0.715</td><td>0.23</td><td>DPv/DDPv</td><td>[25]</td></tr><tr><td>Co</td><td>0.745</td><td>W</td><td>0.60</td><td>0.673</td><td>0.145</td><td>DDPv</td><td>[29]</td></tr><tr><td>Mn</td><td>0.83</td><td>Te</td><td>0.56</td><td>0.695</td><td>0.27</td><td>DPv</td><td>this work</td></tr><tr><td>Mn</td><td>0.83</td><td>Re</td><td>0.55</td><td>0.69</td><td>0.28</td><td>DDPv</td><td>[19]</td></tr><tr><td>Co</td><td>0.745</td><td>Re</td><td>0.55</td><td>0.648</td><td>0.195</td><td>DDPv</td><td>[30]</td></tr><tr><td>Ni</td><td>0.69</td><td>Re</td><td>0.55</td><td>0.62</td><td>0.14</td><td>DDPv</td><td>[30]</td></tr></tbody></table>

dra are somewhat distorted with Mn–O distances in the range 2.14–2.21 Å and cis–O–Mn–O angles deviating by up to 5.2° from 90°, whereas the TeO₆ are more regular with Te–O distances of 1.89–1.93 Å and cis–O–Te–O angles deviating by only up to 1.9° from 90°. This reflects the differing stiffness of Mn²⁺–O and Te⁶⁺–O bonds. The observed average $<\text{Mn–O}> = 2.17$ and $<\text{Te–O}> = 1.90$ Å distances are both 0.05 Å shorter than estimates of 2.22 and 1.95 Å based on ionic radii for Mn²⁺ and Te⁶⁺, [28] reflecting the metastable nature of this recovered high pressure phase and ruling out the presence of significant Te⁴⁺ at ambient pressure. The presence of small and differently-sized Ca²⁺ and Mn²⁺ cations at the A-sites leads to substantial octahedral tilts, calculated as $(180° – \text{Mn–O–Te angle})/2$, in the range 16.6–19.7°. The resulting Ca/Mn coordination is very irregular and although the coordination number in P2₁/n DPvs is conventionally reported as 8, DPv-(Ca₀.₅Mn₀.₅)₂MnTeO₆ has 7 shorter Ca/Mn–O distances in the range 2.07–2.57 Å and a long distance of 2.92 Å, so the A-site is better described as being 7- or 7+1-coordinate. The average $<\text{Ca/Mn–O}>$ distance of 2.44 Å is in perfect agreement with the estimate of 2.44 Å from ionic radii, [28] allowing for slightly off-stoichiometric Ca/Mn occupancies shown in Table 1.

DPv-(Ca₀.₅Mn₀.₅)₂MnTeO₆ is notable as a highly frustrated magnet with a frustration factor $f = -\theta/T_g = 26$, even higher than the value of $f = 18$ reported for the (Ca₀.₅Mn₀.₅)₂MnWO₆ analog. [25] These DPvs are unusual in having two interpenetrating types of frustrated spin lattice within the same structure – a geometrically frustrated spin lattice, from the *B*-site Mn²⁺ spins as their network of Mn²⁺–Mn²⁺ superexchange interactions consists of edge-sharing tetrahedra, and a disorder-frustrated sublattice of A-site Mn²⁺ spins randomly diluted by non-magnetic Ca²⁺. Mn–O–*B*′–O–Mn superexchange within the *B*-cation network is expected to be weaker for *B*′ = Te than for *B*′ = W as the latter has vacant d-orbitals available while Te does not, and this is borne out by their respective Weiss temperatures of $\theta = -28$ and $-145$ K in the DPv-(Ca₀.₅Mn₀.₅)₂MnB'O₆ phases.

## Experimental Section

The composition CaMnMnTeO₆ was explored under high pressure-high temperature (HPHT) conditions using a Walker-type multianvil

apparatus. [26] The reaction precursors were CaMnO₃ (prepared by heating a pellet of CaCO₃ and MnO₂ at 1200 °C), MnO, and TeO₂. The precursors were mixed in stoichiometric ratios, packed in a platinum capsule, and treated at different HPHT conditions in the ranges 550–1200 °C and 10–15 GPa for 30 minutes before quenching to room temperature and slowly depressurising to ambient. X-ray powder diffraction (XPD) patterns were collected on a Bruker D2 diffractometer at room temperature in the angular range (5° ≤ 2θ ≤ 70°) using CuKα radiation. Variable temperature magnetization data under a field of 1000 Oe and magnetization-field loops were measured on a Quantum Design PPMS-Dynacool instrument. Neutron powder diffraction (NPD) data were collected at 1.6, 45, and 300 K on the D20 beamline at the ILL facility at a wavelength of 1.54 Å. XPD and NPD data were fitted using FullProf software. [27]

## Conclusions

High temperature and pressure treatments at 550–1250 °C and 10–15 GPa of the composition CaMnMnTeO₆ are found to form only an A₂BB'O₆-type double perovskite (Ca₀.₅Mn₀.₅)₂MnTeO₆, rather than a AA'BB'O₆ double double perovskite with A- and *B*-site cation order that was found in analogs CaMnMnReO₆ and CaMnMnWO₆ with similar cation sizes. This anomalous structural behavior is attributed to formation of some Te⁴⁺ under synthesis conditions which increases the perovskite tolerance factor favoring the double perovskite phase. Exploration of CaMnMnTeO₆ compositions with smaller M=Co, Ni cations will thus be worthwhile as this should tune the structure back through the double to double double perovskite crossover enabling both polymorphs to be recovered. The double perovskite (Ca₀.₅Mn₀.₅)₂MnTeO₆ adopts a monoclinic P2₁/n structure with a framework of highly tilted MnO₆ and TeO₆ octahedra enclosing disordered Ca²⁺ and Mn²⁺ cations. Magnetic measurements show that (Ca₀.₅Mn₀.₅)₂MnTeO₆ is a highly frustrated spin glass with a freezing transition at 5 K, and no long-range spin order is apparent by neutron diffraction at 1.6 K.

## Acknowledgements

We acknowledge EPSRC and STFC for support and the provision of beam time at the ILL neutron facility.
**Conflict of Interests**

The authors declare no conflict of interest.

**Data Availability Statement**

The data that support the findings of this study are openly available in ILL at https://doi.org/10.5291/ILL-DATA.5-31-2948, reference number 5312948.

**Keywords:** Double Perovskite · Double double Perovskite · Frustrated Spin glass

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Manuscript received: March 12, 2024

Revised manuscript received: May 7, 2024

Accepted manuscript online: May 10, 2024

Version of record online: June 11, 2024