# From ferrimagnetism to spin-glass behavior in alkali- and rare-earth metal crichtonites

Ruiqi Chen, José Luis Rosas-Huerta, Clemens Ritter, Claire Minaud, Oleg Siidra, Marie Colmont, Ángel M Arévalo-López

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Ruiqi Chen, José Luis Rosas-Huerta, Clemens Ritter, Claire Minaud, Oleg Siidra, et al. From ferri-magnetism to spin-glass behavior in alkali- and rare-earth metal crichtonites. Journal of Alloys and Compounds, 2025, 1038, pp.182889. 10.1016/j.jallcom.2025.182889. hal-05379793

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Submitted on 24 Nov 2025

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From Ferrimagnetism to spin-glass behavior in alkali- and rare-earth metal crichtonites.

Ruiqi Chen,<sup>a,b</sup> José Luis Rosas-Huerta,<sup>*</sup><sup>a,c,d</sup> Clemens Ritter,<sup>c</sup> Claire Minaud,<sup>a</sup> Oleg Siidra,<sup>b</sup> Marie Colmont,<sup>a</sup> Ángel M. Arévalo-López<sup>*</sup><sup>a</sup>

<sup>a</sup> Unité de Catalyse et Chimie du Solide (UCCS) - UMR CNRS 8181, Université de Lille - Centrale Lille, Université Artois, ENSCL, Lille, F-59000, France.

<sup>b</sup> Department of Crystallography, Institute of Earth Sciences, St. Petersburg State University, University emb. 7/9, 199034, St. Petersburg, Russia.

<sup>c</sup> Facultad de Ciencias, Universidad Nacional Autónoma de México, AP 70-399, CDMX, 04510, Mexico.

<sup>d</sup> Escuela Superior de Ingeniería Mecánica y Eléctrica-Culhuacan, Instituto Politécnico Nacional. Av. Santa Ana 1000, Ciudad de México, 04440, México

<sup>e</sup> Institut Laue-Langevin, BP 156, 38042 Grenoble Cedex, France.

* corresponding author: joseluis_rosas@comunidad.unam.mx; angel.arevalo-lopez@univ-lille.fr

In this work, nine new synthetic alkali- and rare-earth metal crichtonite-type compounds with $AB_3C_{18}O_{38}$ general formula are presented. The effect on the magnetic properties as consequence of the substitution of divalent and trivalent cations in the *A* site, as well as the introduction of iron at the *B* and *C* sites is studied. Ferrimagnetic behavior was identified in the crichtonites $CaMn_3Ti_{18}O_{38}$, $BaMn_3Ti_{18}O_{38}$ and $CaFe_3Ti_{18}O_{38}$, which changes to a spin-glass type for the $BaMn_3Ti_{14}Fe_4O_{38}$ and *A*-trivalent $REMn_3Ti_{18}O_{38}$ (RE = La, Ce and Nd), $LaFe_3Ti_{18}O_{38}$ and $LaMn_3Ti_{13}Fe_5O_{38}$. A comprehensive comparison of the AC and DC magnetic measurements for whole series along with the FiM structure obtained by neutron powder diffraction is discussed. These results will expand the comprehension on this almost unexplored magnetic family of compounds.

Complex oxides can crystallize in a wide variety of structures such as spinel, pyrochlores, perovskites, crichtonites, etc. These contain different atomic species that allow a wide variety of potential applications such as photovoltaics, nuclear waste containers, spintronic, magnetoresistive, superconductors, multiferroic materials, etc. [1-3] Substitution of the different constituents in these materials is the common strategy to fine tune their properties. For instance, in the double perovskites $A_2FeMoO_{6-δ}$ with $A = Ca, Sr$ and $Ba$, the mixed valences $Fe^{2+/3+}$-$Mo^{6+/5+}$ changes to mostly $Fe^{2+}$-$Mo^{6+}$ as the *A* cation size increases, thus reducing the number of Mo itinerant electrons. [4] This size change also promotes the increase of symmetry from monoclinic to tetragonal and finally cubic for $Ca, Sr$ and $Ba$, respectively. [5] Furthermore, the substitution of rare-earth (RE) elements in $RE_2NiIrO_6$ (RE = La, Pr and Nd) results in the enhancement of the Néel temperature ($T_N$) as the ionic radius decreases and the number of free electrons increases, $T_N = 75$ K, 103 K and 121 K for La, Pr and Nd, respectively. [6]

In this article we focus on the chrichtonite-family and following the strategy sketched above, we study substitutions of alkali- and rare-earth metals in their composition and the influence on their magnetic properties. Crichtonites can be represented with the $AB_3C_{18}O_{38}$ general formula. They are a large family of minerals named accordingly to the majority element in the *A* site which can be monovalent, divalent or trivalent elements, like mathiasite (K), davidite (U), loveringite (Ca), lindsleyite (Ba), crichtonite (Sr) and rare earth elements. They
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have mainly been studied from natural samples, but some have been synthesized via high temperature and pressure, hydrothermal or solid-state techniques. [7] However, until now the main focus of these studies has been on the determination of their composition and structure. [8-14] The synthetic SrFe<sub>3</sub>V<sub>18</sub>O<sub>38</sub> [15] and BaMn<sub>3</sub>Ti<sub>18</sub>O<sub>38</sub> [9] were reported containing a V<sup>3+</sup>/V<sup>4+</sup> and Ti<sup>3+</sup>/Ti<sup>4+</sup> mixed valence state, which inspired our previous study of SrMn<sub>3</sub>Ti<sub>18</sub>O<sub>38</sub> (SrMTO) and SrMn<sub>3</sub>Ti<sub>14</sub>Fe<sub>4</sub>O<sub>38</sub> (SrMTFO) compounds. [16] In this work, we extend our studies on this relatively unexplored crichtonite family and present nine compounds along with their magnetic properties, crystal and magnetic structures and the rationale of the different substitution effects. The studied compositions are AMn<sub>3</sub>Ti<sub>18</sub>O<sub>38</sub> with A = Ca, Ba, La, Ce and Nd (AMTO), CaFe<sub>3</sub>Ti<sub>18</sub>O<sub>38</sub> (CaFTO), BaMn<sub>3</sub>Ti<sub>14</sub>Fe<sub>4</sub>O<sub>38</sub> (BaMTFO), LaFe<sub>3</sub>Ti<sub>18</sub>O<sub>38</sub> (LaFTO) and LaMn<sub>3</sub>Ti<sub>13</sub>Fe<sub>5</sub>O<sub>38</sub> (LaMTFO).

Crichtonites compounds were synthesized through solid-state reaction under vacuum. Chemical reagents (MnO, Ti, TiO₂, MnO₂, Fe, Fe₂O₃, La₂O₃, CeO₂, Nd₂O₃, CaCO₃ and BaCO₃) with a purity higher than 99.9% were used in our syntheses. They were mixed in respective stoichiometric ratios to obtain the desired compounds. For the A-site divalent compositions (Ca, Ba), the precursors (Ca,Ba)Ti₁₄O₂₉, (Ca,Ba)Ti₁₆.₅O₃₄ and CaTi₁₁O₂₃ were first prepared from reactants stoichiometric mixtures; calcium and barium carbonates and anatase, which were weighed, mixed thoroughly and heated at 900 °C for 12h in air to decompose the carbonate. As a second step, the reagents, along with decarbonated precursors when needed, were weighed stoichiometrically, mixed thoroughly and pressed into pellets weighing 80 mg. The pellets were placed in vacuum sealed quartz tubes. The synthesis starts with heating to 650 °C at a rate of 5°/min and held for 12 h, the temperature was then increased to 1100 °C at the same rate with a dwell time of 48 h. Finally, it was cooled to room temperature by turning off the furnace.

The purity of synthesized compounds was initially examined by X-ray diffraction (XRD) at room temperature in a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.54 Å), see Figure S1 and S2 in the Supplementary Information (SI). The crystal and magnetic structures were studied by means of neutron powder diffraction (NPD) carried out at the Institut Laue-Langevin in Grenoble, France (ILL) in the D20 beamline with a λ = 1.54 Å wavelength for high resolution and λ = 2.42 Å for magnetic structure resolution. ISODISTORT was used to determine the possible magnetic structures and space groups. [17] Rietveld refinement analyses and visualizations of the magnetic structure were made through the Fullprof suite and Fullprof Studio software. [18, 19] DC and AC magnetic susceptibility and heat capacity measurements were carried out in a Dynacool Physical Property Measurement System (PPMS) from Quantum Design. DC magnetic susceptibility (χ) was measured from 2 to 300 K under zero-field cooling (ZFC) and field cooling (FC) conditions, under 0.1 T external magnetic field. Magnetization (M) as a function of applied magnetic field (H) was measured from -9 to 9 T at different temperatures. AC-magnetic susceptibility was measured at different frequencies with an oscillating ±16 Oe magnetic field. Heat capacity measurements were made by the relaxation method.

Besides the AB₃C₁₈O₃₈ crichtonite general formula, the crystal structure can be better described by the formula
xiiA1VI B1IV B2VI C16VI C26VI C36O38 where A, B and C sites stand for six different metal positions where the
oxygen coordination is indicated by the left roman numeral exponent. The dodecahedron A1-site is the largest
and it is normally occupied by a divalent or trivalent cation, while B and C sites are occupied by transition
metals. [20, 21] The structure is built by a rhombohedral sequence of three layers (TOT*)₃ in the R-3 space
group. The T layer is composed by C1 and C2 octahedra sharing edges creating hexagonal rings linked by the
B2 tetrahedra and A1, layers T* and T are related via an inversion center as shown in Figure 1. O layer is
composed by twelve C3 octahedra creating a larger hexagonal arrangement with the B1 octahedron at the center.
The connection between T and O layers is only through vertices while inside T and T* layers is through C edges.
Main crystallographic data obtained by Rietveld refinements analyses are shown in Table 1, S1, S2 and S3 for
the nine different compounds studied in this work. High resolution NPD representative Rietveld refinement fits
are shown in Figure 1 for BaMTO, CaMTO and CaFTO, remaining fits are given in Figure S1 and S2.
<figure><img src="image_2.png" /><figcaption>**Figure 1.** a) Crystal structure of crichtonite compounds along (110) plane, b) (001) plane at z = 0.9 and c) at z = 0.2. Brown, green, light blue and red balls represent the (Ca,Ba,RE), (Mn,Fe), (Ti,Fe) and O atoms, respectively. NPD Rietveld refinements (λ = 1.54 Å) at room temperature for d) BaMTO, e) CaMTO and f) CaFTO compounds are shown; red empty circles, black and blue lines and vertical bars are the observed data, fitted, difference and Bragg reflections, respectively. Lower tick marks refer to secondary phases detailed in Table S1.</figcaption></figure>

Rietveld refinements indicate that all the synthesized crihtonites crystalize in the rhombohedral R-3 space group, some secondary phases were also identified as detailed in SI with TiO₂ being the most common one. When comparing the cell parameters for the A-site divalent CaMTO and BaMTO, an unexpected larger *a* parameter is observed for the former. This is due to 0.15 Mn by Ca partial substitution in the *B*1 site and Ca ionic radii in octahedral coordination being larger than Mn (1.00 vs. 0.83 Å for Ca and Mn respectively). This mixed site occupation has also been reported in Ca₁.₅Mn₁.₈Ti₁₈.₇O₃₈ or Ca₂Zn₄Ti₁₆O₃₈ crichtonites. [22, 23] On the other hand, when Mn²⁺ is replaced by Fe²⁺ (CaMTO vs. CaFTO), the expected decrease in *c* lattice parameter (IVMn²⁺ = 0.66 Å, VIIMn²⁺ = 0.83 Å vs. IVFe²⁺ = 0.63 Å, VIFe²⁺ = 0.78 Å) is again not followed. In this case, two possibilities arise, a 65% Fe²⁺/35% Ti³⁺ *B*1 mixed occupancy or Ca²⁺ fully occupying the *B*1 site instead of Fe thus generating a larger cell volume than CaMTO. Intriguingly, the same happens when comparing LaMTO and LaFTO, after careful refinement it was also found that 0.44 of La³⁺ doping in the *B*1 site. This has also been observed in the crichtonite La₂Ti₁₀.₂₇Ga₉.₆₃O₃₈ with *B*1 site fully occupied by lanthanum. [24] The effect of Ca replacing Fe on the *B*1 site is clearly reflected in the octahedral bond distances of this site which increase from 2.222(5) Å and 2.2077(1) Å for CaMTO and LaMTO to 2.2793(3) Å and 2.267(1) Å for CaFTO and LaFTO with 100/0 % and 44/56 % Ca/Fe and La/Fe substitution respectively. Ti sites were analyzed through bond valence sum (BVS) calculations, presented in Table S1 and S2. These indicate a predominant Ti⁴⁺ at C1 and C2 site, while C3 is mixed Ti³⁺/⁴⁺ valent in CaMTO, CaFTO, BaMTO and NdMTO crichtonites. On the other hand, a preferential Ti³⁺ at C3 site is observed in the remaining compounds: BaMTFO, LaMTO, LaFTO, LaMTFO and CeMTO. This is in agreement with previous reports on BaMTO [9], SrMTO and SrMTFO [9].

In contrast, the cell parameters evolution follows the expected contraction trend when different rare-earths are substituted into AMTO (A = La, Ce and Nd), see Table 1. Finally, in order to study the influence of free *d* electrons increasing in the *C*3 site, BaMn₃Ti₁₄Fe₄O₃₈ (BaMTFO) and LaMn₃Ti₁₃Fe₅O₃₈ (LaMTFO) were prepared. These should have 4Fe³⁺ and 5Fe³⁺ instead of their respective Ti³⁺ counterpart in BaMTO and

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LaMTO. The cell parameters show a decrease due to ionic radii size effects (0.67 and 0.645 Å for Ti³⁺ and Fe³⁺ respectively). A similar behavior was observed for SrMTO and SrMTFO. [16] All these data indicate the introduction of iron into B and C sites in A-site divalent and trivalent crichtonites and the effect on their magnetic properties is presented below.

<table><caption>**Table 1.** Cell parameters and BVS for A1 and C3 sites for the different crichtonite compounds. Full crystallographic data is given in supplementary Tables. The data were obtained from Rietveld fitting made to the high-resolution NPD (λ = 1.54 Å) or XRD (λ = 1.54 Å, labelled with an asterisk) at 293 K.</caption><thead><tr><th rowspan="2">Compound</th><th colspan="3">Lattice parameters</th><th rowspan="2">BVS A1</th><th rowspan="2">BVS C3</th></tr><tr><th>a (Å)</th><th>c (Å)</th><th>Volume (Å³)</th></tr></thead><tbody><tr><td>CaMn₃(Ti⁴⁺₁₄Ti³⁺₄)O₃₈<br>CaMTO</td><td>10.4041(2)</td><td>20.8157(6)</td><td>1951.32(8)</td><td>1.36(1)</td><td>Ti3 3.61(4)</td></tr><tr><td>CaFe₃(Ti⁴⁺₁₄Ti³⁺₄)O₃₈<br>CaFTO</td><td>10.3852(1)</td><td>20.9221(4)</td><td>1954.17(5)</td><td>1.34(6)</td><td>Ti3/Fe3<sup>a</sup><br>3.7(1)/3.2(1)</td></tr><tr><td>BaMn₃(Ti⁴⁺₁₄Ti³⁺₄)O₃₈<br>BaMTO</td><td>10.4029(1)</td><td>20.8181(4)</td><td>1951.10(5)</td><td>2.7(1)</td><td>Ti1 3.55(3)</td></tr><tr><td>*BaMn₃(Ti⁴⁺₁₄Fe³⁺₄)O₃₈<br>BaMTFO</td><td>10.4240(5)</td><td>20.8400(1)</td><td>1961.09(2)</td><td>1.70(2)</td><td>3.35(6)/3.07(6)</td></tr><tr><td>*LaMn₃(Ti⁴⁺₁₃Ti³⁺₅)O₃₈<br>LaMTO</td><td>10.4174(1)</td><td>20.8206(1)</td><td>1956.78(3)</td><td>2.05(2)</td><td>Ti3 3.07(4)</td></tr><tr><td>*LaFe₃(Ti⁴⁺₁₃Ti³⁺₅)O₃₈<br>LaFTO</td><td>10.4100(3)</td><td>21.0887(8)</td><td>1979.16(9)</td><td>2.23(2)</td><td>Ti3 3.05(5)</td></tr><tr><td>*LaMn₃(Ti⁴⁺₁₃Fe³⁺₅)O₃₈<br>LaMTFO</td><td>10.4089(1)</td><td>20.8052(2)</td><td>1952.18(2)</td><td>2.61(4)</td><td>Fe1 3.13(1)</td></tr><tr><td>*CeMn₃(Ti⁴⁺₁₃Ti³⁺₅)O₃₈<br>CeMTO</td><td>10.4165(1)</td><td>20.8099(1)</td><td>1955.42(2)</td><td>2.28(2)</td><td>Ti3 3.25(6)</td></tr><tr><td>NdMn₃(Ti⁴⁺₁₃Ti³⁺₅)O₃₈<br>NdMTO</td><td>10.3891(2)</td><td>20.7918(6)</td><td>1943.49(8)</td><td>2.15(1)</td><td>Ti3 3.60(1)</td></tr></tbody></table>

<sup>a</sup> Refined site occupancy factors for Ti3/Fe3 were 0.83(1)/0.17(1)%. More details about the refined parameters are shown in Table S1.

DC Magnetic susceptibility measurements for all the compounds are shown in Figure 2a-c. Their Curie-Weiss behavior at high-temperatures are summarized in Table 2. For CaFTO and LaFTO a Pauli paramagnetic contribution precluded a proper fit, this may be originated from an unknown minority phase, since the expected character for the electrons in these compounds is to be localized. BaMTO, CaMTO and CaFTO show a sharp increase of the magnetic susceptibility below 30 K indicating magnetic transitions at T<sub>N</sub> = 14.8(2) K, 14.4(1) K and 6.7(3) K respectively obtained from dχ/dT (Figure 2a). This increase is less pronounced for RE crichtonites [LaMTO (4.4(1) K), LaFTO (3.5(1) K), CeMTO (3.6(1) K) and NdMTO (4.0(1) K)] and even softer in the crichtonites with iron in the C3 sites [BaMTFO (10.5(1) K) and LaMTFO (12.2(1) K)], see Figure 2b and c. NPD confirmed the ferrimagnetic (FiM) nature of BaMTO, CaMTO and CaFTO transitions as described later.

The calculated effective magnetic moments (μ<sub>eff</sub>) are slightly reduced from their expected values (μ<sub>theo</sub>); this could be due to the small amount of secondary phases and thus an error in the considered mass even after corrected or to a reduced fitted range. The field-dependent magnetizations [M(H)] at 2 K for all the compounds are shown in Figure 2d-f. The M(H) behavior follows the results obtained by the χ(T) measurements: the
crichtonites BaMTO, CaMTO and CaFTO along with LaMTO, CeMTO and NdMTO exhibit a more intense response to changes with external field whereas M(H) for BaMTFO, LaFTO and LaMTFO are less sensitive.

BaMTO, CaMTO and CaFTO show an abrupt increase in their magnetic moments at 2 K, reaching 8.9 µB, 7.7 µB and 5.9 µB/f.u., for 0.4 T, 0.3 T and 0.6 T applied fields respectively. Their magnetic moments continue to increase in a slower rate and reach a saturation magnetization (M_s) of 10.5 (BaMTO), 9.4 (CaMTO) and 9.6 µB/f.u. (CaFTO) at 9 T. For REMTO (RE = La, Ce and Nd) the initial response is softer and at 9 T they reach 10.1 µB/f.u., 11.9 µB/f.u. and 12.1 µB/f.u. respectively. The remaining BaMTFO, LaMTFO and LFTO crichtonites present a smoother increase with the field, only reaching 5.9 µB/f.u. (BaMTFO), 6.1 µB/f.u. (LaMTFO), 5.8 µB/f.u. (LaFTO), at 9 T. No appreciable hysteresis was found in any of the compounds.

Heat capacity measurements for BaMTO, CaMTO and CaFTO crichtonites are shown in Figure 2g and SI. Sharp transitions are observed at ~14 K for BaMTO and CaMTO in agreement with T_N observed from χ(T). After subtraction of the lattice contribution (see Figure S4), the magnetic entropy (S_m) for BaMTO and CaMTO are ~44 and ~40 J mol⁻¹ K⁻¹ respectively. These values are 65 and 60 % of the theoretical expected value for S_m = R(3xln(2S + 1) + 4xln(2S + 1)) = 67.74 J mol⁻¹ K⁻¹ for 3 x Mn²⁺ (S = 5/2) and 4 x Ti³⁺ (S = 1/2), where R = 8.314 J mol⁻¹ K⁻¹ is the ideal gas constant. On the other hand, CaFTO shows no sharp magnetic transition and S_m = 43.2 J mol⁻¹ K⁻¹ which is 86.7% of the expected S_m = 49.81 J mol⁻¹ K⁻¹ for 2 x Fe²⁺ (S = 5/2) and 4 x Ti³⁺ (S = 1/2). A broad maximum appears in the heat capacity response at low temperatures for CaMTO and BaMTO, as the applied field increases, this moves to higher temperature and diffuses. It is associated to a Schottky anomaly as previously identified in SrMTO. [16] Its contribution follows the relation: C_Sch = NR(Δ/k_B T)² e^(Δ/k_B T) / (1 + e^(Δ/k_B T))², where Δ is the Schottky gap and k_B is the Boltzmann constant. The BaMTO and CaMTO respective Δ/k_B values are 22.9 K and 29.8 K. Both increased linearly with the magnetic field, see SI, typical of orphan spins or magnetic impurities. [25]

The introduction of RE elements in A site could also generate new magnetic interactions in the T and T* layers in these compounds, these would interfere with the long-range magnetic order and thus result in a lower T_N or spin-glass phase, as similarly reported in pyrochlore compounds. [26-27] The signatures of non-long-range magnetic order for the other six compounds were explored through AC magnetic susceptibility measurements, see Figure 2h-i, Figure S3 and Table S4. All these show a frequency (ω) dependence indicating a spin-glass behavior. The freezing temperature (T_f) is properly modeled with the Vogel-Fulcher equation ω = ω₀exp(-E_a/κ_B(T_f - T₀)), where t₀ = 1/ω₀, E_a and T₀ are the intrinsic relaxation time, the activation energy and Vogel-Fulcher temperature, respectively. Considering the Vogel-Fulcher fits made within the two limits 10⁻¹³ s < t₀ < 10⁻⁹ s which are in the interval for spin-glass behaviors [26, 27], T_f values do not change more than 0.34 K and activation energies are of the order of 10⁻⁵ eV when the frequency change four orders of magnitude. The relative shift per frequency decade (δ) was calculated by the relation δ_{T_f} = (ΔT_f / T_fΔlog(ω)). [28, 29] The obtained δ values of 0.022 (LaFTO), 0.010 (BaMTFO), and 0.007 (LaMTO, LaMTFO, CeMTO and NdMTO), indicate a canonical spin-glass for LaFTO, while a FM cluster-glass state for the others with one order of magnitude slower.
<figure><img src="image_3.png" /><figcaption>Figure 2. Magnetic measurements for the crichtonite compounds. a-c) $\chi(T)$ and $\chi^{-1}(T)$, d-f) $M(H)$ at 2 K, g) $C_p$ for BaMTO and h-i) AC measurements for LaMTO and LaFTO.</figcaption></figure>

Neutron powder diffraction for BaMTO, CaMTO, CaFTO and NdMTO above and below their respective magnetic transitions revealed the appearance of new diffraction maxima for the first three compounds, see Figure 3 and SI. However, the absence of any extra intensity or new maxima for NdMTO confirmed its spin glass behavior as discussed before. The extra reflections for BaMTO, CaMTO and CaFTO at low temperature establish the long-range magnetic ordering, these were indexed with the same $k_0 = [0 0 0]$ magnetic propagation vector. Rietveld refinements revealed that the three compounds also follow the same $m\Gamma 1^+$ irreducible representation resulting in the R-3 (#148.17) magnetic space group. The magnetic structure consists of a ferrimagnetic (FiM) arrangement between parallel $Mn^{2+}$ or $Fe^{2+}$ (B1 and B2 sites) aligned along the z axis, coupled antiferromagnetically with $Ti^{3+}$ atoms (C3 site). For BaMTO and CaMTO, the refinement of the magnetic moments in B1 and B2 sites were constrained together since they were converging to the same values. On the contrary, for CaFTO, B1 site is occupied by Ca *i.e.* without a magnetic contribution. The refined magnetic moments at 1.6 K for CaMTO and BaMTO are 3.90(1) $\mu_B$ and 4.36(1) $\mu_B$ for Mn at B1 and B2 sites and 0.17(1) $\mu_B$ and 0.24(1) $\mu_B$ for Ti at C3 site, respectively. For CaFTO, the refined magnetic moments are 0.95(3) $\mu_B$ for Fe at B2 site only and 0.22(2) $\mu_B$ for Ti at C3 site. Attempted refinements assuming no magnetic moment in the C3 site were performed. However, they result in lower quality fits *e.g.* for BaMTO, $\chi^2 = 1.52$ vs. 1.85 % and $R_{mag} = 7.67$ vs. 8.98 % for considering *vs.* ignoring Ti3 contribution. This C3 site contribution was also observed in the previously reported SrMTO. [16] Moreover, the $M_S$ from 9 T for BaMTO ($M_S$ = 10.5 $\mu_B$) and CaMTO ($M_S$ = 9.4 $\mu_B$) obtained from DC magnetization measurements also support these FiM arrangements
since they are close to the ideal value of 11 µB by considering 15 µB and -4 µB coming from the 3xMn2⁺ (S = 5/2) and 4xTi3⁺ (S = 1/2), see Table 2. The difference of M<sub>S</sub> values at 9T between BaMTO and CaMTO comes from the amount of secondary phases found of 6% and 12.4%, respectively, and by the 27% occupancy of Ca at Mn1 site (see Table S1). Also, the different A ionic radius induce a bond distance change modifying the orbital overlapping which affects the magnetic interactions. Conversely, for CaFTO, the ideal magnetic moment is calculated to be 4 µB considering 8 µB and -4 µB coming from 2xFe2⁺ (S = 2) and 4xTi3⁺ (S = 1/2), respectively. Nevertheless, one needs to also consider 17% of iron doping in the C3 site, reducing even further the expected moment. Still, the measured M<sub>S</sub> for CaFTO (9.6 µB) is considerable higher than the ideally expected value in the FiM configuration, this could be due to spin-orbit interactions in Fe2⁺.[30] The ferromagnetic alignment of the B sites is via B1 - O - C3 - O - B2 super-super-exchange interaction with the help of the Ti4⁺ 3d<sup>6</sup> linking octahedra, this is schematically shown in Figure 3c. Ferromagnetic super-super exchange has been observed in CrI<sub>3</sub> layers via Cr - I - Te - Fe [31] electron hopping or in a similar tetrahedra - octahedra - tetrahedra motif in Ba<sub>5</sub>Co<sub>5</sub>FO<sub>13</sub>. [32, 33] The thermal evolution of the Mn and Ti magnetic moments in BaMTO are shown in SI. Mn moment is properly modeled by the critical law µ(T) = µ<sub>0</sub>[1 - (T/T<sub>N</sub>)]<sup>β</sup> with µ<sub>0</sub> = 4.4(1) µ<sub>B</sub>, T<sub>N</sub> = 14.7(1) K and β = 0.36(3), related to a 3D Heisenberg magnet. Fully unsaturated magnetic Mn moments have also been reported in the double perovskite Mn<sub>2</sub>FeReO<sub>6</sub> (3.5 µ<sub>B</sub>), in the LiNbO<sub>3</sub>-type MnTiO<sub>3</sub>-II (3.9 µ<sub>B</sub>) [34, 35] and in the crichtonite SrMTO (4.4 µ<sub>B</sub>). [16]

<table><caption>**Table 2.** Magnetic properties of the crichtonite compounds. Curie-Weiss temperature (θ<sub>CW</sub>), Curie constant (C), effective magnetic moment (μ<sub>eff</sub>), theoretical magnetic moment (μ<sub>teo</sub>), T<sub>Néel</sub>/T<sub>C</sub>, magnetic moment (MM) observed at 9T and ideal M<sub>S</sub> expected considering FiM alignment between the ions in B and C3 sites. CaFTO and LaFTO show a Pauli paramagnetic contribution that precluded a proper fit.</caption><thead><tr><th>Compound</th><th>θ<sub>CW</sub> (K)</th><th>C (mol<br>emu<sup>-1</sup> K<sup>-1</sup>)</th><th>μ<sub>eff</sub><br>(μ<sub>B</sub>/f.u.)</th><th>μ<sub>theo</sub><br>(μ<sub>B</sub>/f.u.)</th><th>T<sub>N</sub>/T<sub>f</sub><br>(K)</th><th>MM at 9T<br>(μ<sub>B</sub>/f.u.)</th><th>M<sub>S</sub><br>(μ<sub>B</sub>/f.u.)</th></tr></thead><tbody><tr><td>CaMn<sub>3</sub>Ti<sub>18</sub>O<sub>38</sub></td><td>-7.6(2)</td><td>11.8(1)</td><td>9.7(2)</td><td>10.82<sup>1</sup></td><td>14.4</td><td>9.36</td><td>11.0</td></tr><tr><td>Ca<sub>2</sub>Fe<sub>2</sub>Ti<sub>18</sub>O<sub>38</sub></td><td>-</td><td>-</td><td>-</td><td>7.74<sup>2</sup></td><td>6.7</td><td>9.64</td><td>4.0</td></tr><tr><td>SrMn<sub>3</sub>Ti<sub>18</sub>O<sub>38</sub>*</td><td>15.0(1)</td><td>11.5(1)</td><td>9.6(1)</td><td>10.82<sup>1</sup></td><td>15.0</td><td>10.70</td><td>11.0</td></tr><tr><td>SrMn<sub>3</sub>(Ti<sub>14</sub>Fe<sub>4</sub>)O<sub>38</sub>*</td><td>-82.6(4)</td><td>21.4(1)</td><td>13.1(1)</td><td>15.65<sup>3</sup></td><td>8.0</td><td>5.80</td><td>5.0</td></tr><tr><td>BaMn<sub>3</sub>Ti<sub>18</sub>O<sub>38</sub></td><td>3.3(3)</td><td>11.2(1)</td><td>9.5(3)</td><td>10.82<sup>1</sup></td><td>14.8</td><td>10.45</td><td>11.0</td></tr><tr><td>BaMn<sub>3</sub>(Ti<sub>14</sub>Fe<sub>4</sub>)O<sub>38</sub></td><td>-11.1(1)</td><td>16.0(6)</td><td>11.0(1)</td><td>15.65<sup>3</sup></td><td>10.5</td><td>5.93</td><td>5.0</td></tr><tr><td>LaMn<sub>3</sub>Ti<sub>18</sub>O<sub>38</sub></td><td>-9.4(1)</td><td>11.9(1)</td><td>9.8(1)</td><td>10.96<sup>4</sup></td><td>4.4</td><td>10.10</td><td>11.0</td></tr><tr><td>LaMn<sub>3</sub>(Ti<sub>13</sub>Fe<sub>5</sub>)O<sub>38</sub></td><td>-111.4(2)</td><td>24.8(1)</td><td>14.1(1)</td><td>16.74<sup>5</sup></td><td>12.2</td><td>6.06</td><td>10.0</td></tr><tr><td>LaFe<sub>3</sub>Ti<sub>18</sub>O<sub>38</sub></td><td>-</td><td>-</td><td>-</td><td>9.32<sup>6</sup></td><td>3.5</td><td>5.84</td><td>8.0</td></tr><tr><td>CeMn<sub>3</sub>Ti<sub>18</sub>O<sub>38</sub></td><td>-30.7(2)</td><td>13.6(1)</td><td>10.4(1)</td><td>11.24<sup>7</sup></td><td>3.6</td><td>11.88</td><td>11.0<sup>**</sup></td></tr><tr><td>NdMn<sub>3</sub>Ti<sub>18</sub>O<sub>38</sub></td><td>-4.2(1)</td><td>12.2(1)</td><td>9.9(1)</td><td>11.67<sup>8</sup></td><td>4.0</td><td>12.12</td><td>11.0<sup>**</sup></td></tr></tbody></table>

<footnote n="1">¹ [3x Mn2⁺ (S = 5/2) + 4xTi3⁺ (S = 1/2)]. ² [2xFe2⁺ (S = 2) + 4xTi3⁺]. ³ [3xMn2⁺ + 4xFe3⁺ (S = 5/2)]. ⁴ [3xMn3⁺ + 5xTi3⁺]. ⁵ [3xMn2⁺ + 5xFe3⁺]. ⁶ [3xFe2⁺ + 5xTi3⁺]. ⁷ [1xCe3⁺ (S = ½) + 3xMn2⁺ + 5xTi3⁺]. ⁸ [1xNd3⁺ (S = 3/2) + 3xMn2⁺ + 5xTi3⁺]. * from reference [16]. ** Without moment from RE.</footnote>
<figure><img src="image_4.png" /><figcaption>Figure 3. a) NPD difference at low temperature for BaMTO. b) Magnetic structure with Mn/Ti1 moments in purple/blue. Spheres in cyan/gray represent Ba/Ti. c) Schematic of the Mn(↓e<sub>g</sub>)-O-O-Mn(↓t<sub>2g</sub>) ferromagnetic super-super-exchange via Ti<sup>4+</sup>.</figcaption></figure>

<table><caption>Table 3. NPD data.</caption><thead><tr><th>Sample name</th><th></th><th>CaMTO</th><th>SrMTO*</th><th>BaMTO</th><th>CaFTO</th></tr></thead><tbody><tr><td>A-site cation</td><td></td><td>Ca<sup>2+</sup></td><td>Sr<sup>2+</sup></td><td>Ba<sup>2+</sup></td><td>Ca<sup>2+</sup></td></tr><tr><td>Ionic radii</td><td>(Å)</td><td>1.34</td><td>1.44</td><td>1.61</td><td>1.34</td></tr><tr><td rowspan="2">Unit cell parameters</td><td>a (Å)</td><td>10.4041(2)</td><td>10.4282(1)</td><td>10.4324(1)</td><td>10.3852(1)</td></tr><tr><td>c (Å)</td><td>20.8157(6)</td><td>20.8410(3)</td><td>20.8765(1)</td><td>20.9221(4)</td></tr><tr><td>Sample purity</td><td>(%)</td><td>90.5(5)</td><td>97.8(7)</td><td>93.9(5)</td><td>55.0(2)</td></tr><tr><td>Transition temperature</td><td>(K)</td><td>14.4</td><td>14.4</td><td>14.8</td><td></td></tr><tr><td>Magnetization at 9T</td><td>(μ<sub>B</sub>/f.u.)</td><td>9.36</td><td>10.71</td><td>10.45</td><td>9.64</td></tr><tr><td>Magnetic moments</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mn1,2</td><td>(μ<sub>B</sub>)</td><td>3.90(1)</td><td>4.11(2)</td><td>4.36(1)</td><td>Fe2 - 0.95(3)</td></tr><tr><td>Ti3</td><td>(μ<sub>B</sub>)</td><td>0.17(1)</td><td>0.30(2)</td><td>0.24(1)</td><td>Ti3/Fe3 - 0.22(2)</td></tr></tbody></table>

<footnote>*Data taken from reference [16].</footnote>
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Summarizing, we have successfully synthesized nine new members of the AB₃C₁₈O₃₈ crichtonite family. They all crystallize in their typical rhombohedral structure with space group R-3. Stoichiometric compositions were obtained for most of the materials except for CaMTO, CaFTO and LaFTO, in which Ca doped 15 % and 100 % and La 44 % on the B1 site, respectively. Long-range magnetic ordering at low temperatures was only found for the divalent crichtonites BaMTO, CaMTO and CaFTO with T<sub>N</sub> = 14.8, 14.4 and 6.7 K respectively. The change from a ferrimagnetic spin arrangement to spin-glass behavior in the studied crichtonites was determined to be due to: 1) cation disorder between Ti and Fe in the C3 site, as observed from BaMTO (FiM) to BaMTFO (spin-glass) and 2) introduction of competing interactions via RE substitution into the A site. This work sheds light over the potential of divalent crichtonites over those containing rare earths elements, thus marking the line to follow in future research.

Acknowledgments

We thank the ILL for beamtime provided at D20 (DOI:https://doi.org/10.5291/ILL-DATA.5-31-2921). J. L. R. H. thanks the SECTEI/098/2022-Mexico and SECIHTI for the post-doctoral fellowships. R. C. thanks Region Hauts-de-France for a PhD scholarship. The Chevreul Institute (FR 2638), Region Hauts-de-France, and FEDER are acknowledged for funding the X-ray diffractometers and the PPMS magnetometer.

Supplementary materials

Supplementary material associated with this article can be found in the online version.

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