Relationship between polymorphic structures and
magnetic properties of La2–xA’ Ni7 compounds
(A’ = Sm, Gd)

Valérie Paul-Boncour, Véronique Charbonnier, Nicolas Madern, Lotfi Bessais,
Judith Monnier, Junxian Zhang

► To cite this version:

Valérie Paul-Boncour, Véronique Charbonnier, Nicolas Madern, Lotfi Bessais, Judith Monnier, et al. Relationship between polymorphic structures and magnetic properties of La2–xA’ Ni7 compounds (A’ = Sm, Gd). Journal of Magnetism and Magnetic Materials, 2025, 618, pp.172859. 10.1016/j.jmmm.2025.172859. hal-04969969

HAL Id: hal-04969969

https://hal.science/hal-04969969v1

Submitted on 27 Feb 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Relationship between polymorphic structures and magnetic properties of La₂ₓA'ₓNi₇ compounds (A' = Sm, Gd)

Valérie Paul-Boncour¹*, Véronique Charbonnier¹,², Nicolas Madern¹,³, Lotfi Bessais¹, Judith Monnier¹, Junxian Zhang¹

¹Univ. Paris-Est Créteil, CNRS, ICMPE, UMR7182, F-94320 Thiais, France

²Energy Process Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.

³ Mines Paris, PSL University, MAT - Centre des Matériaux, CNRS UMR 7633, BP 87, 91003, Evry, France

## Abstract

La₂Ni₇ compounds (A = rare earth) are of interest for their fundamental structural and magnetic properties such as weak itinerant magnets when A = Y and La. In this study, the crystal structure and magnetic properties of La₂ₓA'ₓNi₇ compounds with magnetic rare earth elements (A' = Sm, Gd) have been investigated combining X-ray powder diffraction and magnetic measurements. These intergrowth compounds crystallize in a mixture of 2H hexagonal (Ce₂Ni₇-type) and 3R rhombohedral (Gd₂Co₇-type) polymorphic structures which are related to the stacking of [AB₅] and [A₂B₄] subunits along the c-axis. The average cell volume decreases linearly versus A' content, whereas the c/a ratio reaches a minimum at x = 1, due to geometric constraints upon A' for La substitution between the two different subunits. The magnetic properties strongly depend on the structure type and the A' content. Hexagonal La₂Ni₇ is a weak antiferromagnet (wAFM) at low field and temperature and undergoes metamagnetic transitions towards weak ferromagnetic state (wFM) under applied field. Under an applied field of 0.1 T, La₂xA'ₓNi₇ intermetallic compounds display two different transition temperatures T₁ and T₂ that both increase with x. T₁ is associated with a wFM-wAFM transition in the 2H phase for A' = Sm, whereas T₂ is related to the Curie temperature of both 2H and 3R phases. A metamagnetic behaviour is observed between T₁ and T₂ with transition field µ₀Hₜrans between 2 and 3.5 T for compounds with A' = Sm. The La₂ₓSmₓNi₇ compounds (x > 0) behave as hard magnets with a large coercive field µ₀Hc at low temperature (µ₀Hc > 9 T at 5 K for x = 2), whereas the La₂ₓGdₓNi₇ compounds (x > 0) are soft ferrimagnets with a linear increase of the saturation magnetization versus Gd content.

**Keywords:** Intermetallic compounds, Intergrowth compounds, polymorphic structures, metamagnetism, ferrimagnetism

https://doi.org/10.1016/j.jmmm.2025.172859

<footnote>*Corresponding author: valerie.paul-boncour@cnrs.fr, Phone number: +33 1 49 78 12 07</footnote>
# 1-Introduction

$AB_n$ intermetallic compounds ($A$ = rare earth and $B$ = transition metal, $n=1-13$) exhibit a wide range of magnetic properties that have been the subject of extensive research, both at the fundamental level and with regard to their potential applications, including permanent magnets, magnetocaloric properties and magnetostrictive materials [1]. Furthermore, numerous studies have been conducted on these compounds with regard to their hydrogenation and electrochemical properties, particularly in relation to their potential use as negative electrodes in Ni-MH batteries [2].

The metamagnetic properties of La₂Ni₇ have been the subject of interest for many years [3-13]. Due to its weak itinerant antiferromagnetic properties, it can be considered as a fragile magnet, i.e. a compound where the magnetism (ordering temperature and magnetic moment) can be suppressed by perturbations such as doping, applied pressure or applied field [14]. Such systems are good candidates to find new superconducting materials near quantum critical point. In addition, recent studies of both La₂Ni₇ and Y₂Ni₇ intermetallic compounds have established a correlation between their crystal structure and their magnetic properties [10, 11]. These two compounds crystallize in two different polymorphic intergrowth structures: La₂Ni₇ is hexagonal (Ce₂Ni₇-type, *P*6₃/*mmc* space group), whereas Y₂Ni₇ is rhombohedral (Gd₂Co₇-type, *R*-3*m* space group). These structures result from the stacking of one [A₂Ni₄] and two [ANi₅] subunits along the *c*-axis repeated twice for the hexagonal *2H* phase, and three times for the rhombohedral *3R* phase.

La₂Ni₇ with *2H* structure was first considered as a weak antiferromagnet (wAFM) at low field with Néel temperature, *T*<sub>N</sub> = 55(2) K [4, 8, 10, 12, 13]. Y₂Ni₇ with *3R* structure is a weak ferromagnet (wFM) with Curie temperature, *T*<sub>C</sub> = 53(2) K [15-19]. Their magnetic properties are related to a narrow band at the Fermi level, which is close to the onset of ferromagnetism according to the Stoner criterium [10, 19]. The wAFM order observed in La₂Ni₇ has been related to its hexagonal structure, the symmetry of which allows an inversion of the direction of the Ni moments around the Ni atoms located in the [La₂Ni₄] subunit. This has been proposed first by ab-initio DFT calculations [10] and then confirmed by neutron diffraction on a single crystal [13]. This last study revealed the existence of three magnetic structures at low field: one commensurate and two incommensurate wAFM structures. The temperature at which the transition occurred was measured at zero field (*T*₁ = 61.0 ±0.2K, *T*₂ = 56.5 ± 0.2K, and *T*₃ = 42.2 ± 0.2K) and found to vary with the applied field [12, 13, 20]. La₂Ni₇ undergoes a metamagnetic transition towards a ferromagnetic state with a transition field μ₀*H*<sub>trans</sub> of 4.6 T at 5 K [9, 11].

The experimental study of the La₂-xYxNi₇ pseudobinary compounds has shown that for $x \le 1$ the compounds crystallize in the hexagonal structure with Y atoms substituted to La only in the [A₂Ni₄] subunit, whereas for $1 < x < 2$ a mixture of *2H* and *3R* phases is observed when Y is substituted on both [A₂Ni₄] and [ANi₅] subunits [11]. The pseudobinary La₂-xYxNi₇ ($x \le 1$) compounds, with only hexagonal structure, display a wAFM ground state with a Néel
temperature increasing with the Y content. Furthermore, a metamagnetic transition from wAFM
towards a wFM state is observed at low temperature, with the transition field increasing with
increasing Y content [11]. This indicates that the wAFM state becomes more stable than the
wFM state upon Y substitution in the 2H structure. For x > 1, the samples contain a mixture of
2H and 3R phases and are wFM with the same Tc = 55 K independently of Y content and the
phase composition [11].

Several studies have been published for binary A₂Ni₇ compounds with magnetic A element [3, 5, 21]. Most of them relate to their structural and hydrogenation properties [22-27]. It has been observed that the relative ratio of hexagonal (2H) versus rhombohedral (3R) structure depends on the size of the A elements [28]. La₂Ni₇ and Ce₂Ni₇, which have the largest A atom radius, can be easily synthesized to crystallize only in the hexagonal structure [4, 29]. A₂Ni₇ compounds with the smallest A radii (A = Y, Ho, Dy, Er) are obtained with only the rhombohedral (3R) structure [28]. The A₂Ni₇ compounds with intermediate A radii (A = Pr to Tb) generally show a mixture of both 2H and 3R phases, the ratio of which depends on the radius of the A atom [22, 25, 27, 28]. However, only a few studies have been devoted to the magnetic properties of A₂Ni₇ compounds. Their Curie temperature Tc increases from 50 K for Ce₂Ni₇ [5] to a maximum of 116 K for Gd₂Ni₇ [21] and then decreases progressively down to 67 K for Er₂Ni₇ [30, 31]. Their Tc are higher than those of ANi₃, ANi₂ and ANi₅ compounds, but lower than those of A₂Ni₁₇ compounds [30]. Zhou et al. [32] investigated the field dependence of the magnetization at 4.2 K and up to 35 T of A'₂ₓYₓNi₇ compounds (A' = Gd, Tb, Ho) with x ≥ 1.8. They have used these results to determine the magnetic coupling between the rare earth and Ni JₐNiᵢ with a mean field model and found them twice larger than for A₂Ni₁₇ compounds. Nevertheless, these studies have not been reproduced and no recent work has been published on the magnetic properties of the A₂Ni₇ compounds (A = magnetic rare earth).

To gain a better understanding of the magnetic properties of A₂Ni₇ compounds containing a
magnetic rare earth, the study of pseudobinary La₂ₓA'ₓNi₇ compounds was undertaken. Non-
magnetic La was substituted by either a light (A' = Sm) or a heavy (A' = Gd) magnetic rare
earth, which influence the sign of the magnetic interaction with neighbouring transition metals.
Sm and Gd single ions are also characterized by different free ion moments (μSm = 0.84 μB and
μGd = 7.94 μB) and anisotropy (elongated ellipsoid for Sm and spherical shape for Gd). Several
La₂ₓA'ₓNi₇ compounds were synthesized for 0 ≤ x ≤ 2 and characterized by X-ray diffraction
and magnetic measurements. The replacement of La by Sm or Gd magnetic rare earths is
expected to induce ferromagnetic or ferrimagnetic order due to their localized 4f moments
compared to the weak itinerant magnetism of Ni atoms. In addition, the potential correlation
between the polymorphic structure of these compounds and their magnetic properties, as
observed in the La and Y systems, prompted this investigation. The results will be compared
and discussed in the context of previous literature on the subject.

**2-Experimental methods**

Polycrystalline samples with composition La₂ₓA'ₓNi₇ (A' = Sm, Gd, 0 ≤ x ≤ 2) were prepared
by induction melting of the pure elements La (99.9 %), Gd and Sm (Alfa Aesar 99.9%), Ni
(Praxair 99.95%) under a purified argon atmosphere. The La₂ₓGdₓNi₇ alloys were sealed under argon in silica tubes and were annealed for 7 days at 1273 K [27, 33]. As for the La₂ₓSmₓNi₇ alloys, an excess of Sm was necessary. These ingots were crushed into powder, pressed into pellet, wrapped in Ta foils, sealed under argon in a stainless steel crucible and annealed 3 days at 1223 K [27].

The samples were characterized by X-ray diffraction (XRD) using a D8 diffractometer from Bruker with Cu Kα radiation. The XRD patterns were refined by the Rietveld method using the FullProf code [34]. The XRD patterns have already been published in [11, 33]. The analysis of their chemical composition by electron probe micro-analysis was already reported in [27, 33].

Magnetic measurements were performed using a Physical Properties Measurement System (Quantum Design PPMS-9) operating from 2 to 300 K, with a maximum applied field of 9 T. The measurements were done with few mg of sample (bulk piece or powders fixed with a resin) which were placed in gelatin capsules.

# 3. Results and discussion

## 3.1. Structural properties

In Figure SI1 (Supplemental Information), the percentage of 2H and 3R phases of binary A₂Ni₇ compounds annealed at 1273 K has been plotted versus the A atom radius (see ref [35] and references herein). The data clearly demonstrate the significant influence of geometric constraints induced by the lanthanide contraction on the stability of the polymorphic structure [22, 25, 27, 28]. The following presents the investigation of two solid solutions between La₂Ni₇ and Sm₂Ni₇ and Gd₂Ni₇ binary compounds, which will be denoted La₂xA'_xNi₇, where A' represents the specific rare earth element substituted for La.

All the synthesized La₂xA'_xNi₇ compounds crystallize in hexagonal (Ce₂Ni₇-type, P6₃/mmc) and/or rhombohedral (Gd₂Co₇-type, R-3m) structures as evidenced by their XRD patterns published in our previous work [33]. In these structures, the A atoms occupy two different Wyckoff sites belonging either to [A₂Ni₄] or to [ANi₅] subunits, while the Ni atoms are in 5 different Wyckoff sites (one in the [A₂Ni₄] subunit, two in the [ANi₅] subunit, and two at the interfaces between the two subunits) as detailed in ref. [11]. The results presented here are for A' = Sm or Gd, but the results previously obtained for A' = Y in ref. [11] have been added to the figures 1 and 2 to highlight the influence of the A' atom size on the cell parameter variation. The cell parameters of the La₂xA'_xNi₇ compounds (A' = Y, Sm and Gd) in both 2H and 3R structures are shown in Figure 1 and the weight percentage of the hexagonal phase in Figure 2. The c parameter of the 3R phase is multiplied by 2/3 to compare them with the hexagonal phase, while the cell volume is expressed in formula units. In all cases, the a, c and V parameters decrease with the A' content due to their smaller radii compared to that of La. No significant difference in relative a and c cell parameters is observed between 2H and 3R phases. The influence of the size of the A' element is clearly observed in the variation of the relative cell
parameters (La > Sm > Gd > Y) whatever the polymorph structure. The relative cell volumes all decrease linearly with the rate of substitution following the Vegard law expected for a solid solution. The dV/dx slopes are -4.27(9), -3.83(7) and -3.13(4) Å³/f.u./x for A' = Y, Gd and Sm, respectively.

In all systems and regardless of the structure, *c/a* shows a minimum for *x* = 1 that can be explained by the preferential substitution of La by a smaller A' element in the [A₂Ni₄] subunit up to *x* = 1. Then, once all the La atoms of the [A₂Ni₄] have been substituted by A' atoms, the substitution of A' for La in the [ANi₅] subunit starts for *x* > 1 [11]. The cell contraction is more pronounced along the *c*-axis to accommodate the strains induced by the substitution solely in the [A₂Ni₄] subunit (for *x* ≤ 1). Then, for *x* > 1, the contraction occurs in the (*a*, *b*) plane when La is replaced by A' element in both types of subunits. As the A' radius increases (*r*<sub>Y</sub> < *r*<sub>Gd</sub> < *r*<sub>Sm</sub>) the *c/a* ratio variation becomes smaller. This shows that the A' substitution affects the anisotropic lattice variation and that the strains are greater when the La and A' atom radii differ more. The samples crystallize in a single hexagonal phase up to *x* = 1 for A' = Y and Gd (Fig. 2) whereas all compounds with A' = Sm contain a mixture of 2H and 3R phases. For A' = Y and Gd and *x* > 1, a mixture of 2H and 3R phases is observed with a progressive reduction of the 2H fraction (Fig. 2). As mentioned above, only Y₂Ni₇ was obtained in the pure rhombohedral structure.

<figure><img src="image_2.png" /></figure>

Fig. 1. Cell parameters of La₂-xA'xNi₇ compounds: a) *a* parameter, b) *c* parameter, c) *c/a* ratio and d) unit cell volume *V*. To facilitate the comparison between the two polymorphs the *c* cell parameters have been multiplied by 2/3 for the rhombohedral phase.
<figure><img src="image_3.png" /><figcaption>Fig. 2. Weight percent of hexagonal phase (2H) determined by XRD pattern Rietveld refinement for La₂-xA'xNi7 compounds with A' = Sm, Gd or Y. The results for A' = Y are from ref. [11] and those of A' = Sm and Gd from [33].</figcaption></figure>

**3.2. Magnetic properties**

The magnetic curves M(T) of La₂-xSmxNi7 and La₂-xGdxNi7 compounds measured under an applied field of 0.1 T are compared in Fig. 3a and b respectively. Several M(T) curves (for x > 0) indicate the existence of two distinct ordering transition temperatures, hereafter denoted T₁ and T₂, with T₁ < T₂. The evolution of the transition temperatures (Fig. 4a and b) demonstrates that both T₁ and T₂ increase with the A' content. The Néel temperature T_N of La₂Ni₇ is between T₁ and T₂ for xSm ≤ 1 and for xGd ≤ 2. The observed increase in temperature versus A' content provides evidence that these variables are related to the influence of magnetic rare earths on magnetization. In order to gain insight into the origin of these two transition temperatures, the magnetic behaviour of the Sm₂Ni₇ and Gd₂Ni₇ binary compounds will be first investigated. This will be followed by an examination of the solid solutions.
<figure><img src="image_4.png" /><figcaption>Fig. 3. $M(T)$ curves of a) La$_{2-x}$Sm$_x$Ni$_7$ and b) La$_{2-x}$Gd$_x$Ni$_7$ compounds measured under an applied field of 0.1 T in field-cool (FC) mode. The $T_1$ and $T_2$ temperatures are indicated by arrows for the Sm$_2$Ni$_7$ and Gd$_2$Ni$_7$ binary compounds.</figcaption></figure>

The two different ordering transition temperatures $T_1$ and $T_2$ are reported in Table 1 for the A'$_2$Ni$_7$ binary compounds. Both Sm$_2$Ni$_7$ and Gd$_2$Ni$_7$ compounds are composed of a mixture of 2H and 3R phases, which can exhibit distinct magnetic behaviours. It is noteworthy that the relative intensity of magnetization before and after $T_1$ differs markedly between the two compounds. It seems reasonable to posit that this discrepancy is attributable to the relative weight fraction (wt%) of the 2H and 3R phases. The determination of the magnetic phase weight fractions and the magnetic ordering temperatures $T_1$ and $T_2$ are detailed in supplementary materials (Figs. SI2 and SI3 for Sm$_2$Ni$_7$ and Gd$_2$Ni$_7$ respectively). The contribution of the magnetization in the paramagnetic range is first extrapolated linearly to low temperature and then subtracted from that of the magnetically ordered phase. Then two values $\Delta M_1$ and $\Delta M_2$ are obtained from the jump of the magnetization at 5 K and in the middle of the plateau between $T_1$ and $T_2$ respectively. The relative weight fraction $W_{Mag}$ of the magnetic phase between $T_1$ and $T_2$, is then calculated according to:

$$W_{Mag} = 100 \times \frac{\Delta M_2}{\Delta M_1} \qquad (1)$$

It can be assumed that at 5 K, the magnetization of the 2H and 3R phases as well as their magnetic anisotropy are very close. Consequently, $W_{Mag}$ is compared to the weight fractions of 2H and 3R phases ($W_{XRD}$) obtained from the refinement of the XRD patterns (Table 1). The close values obtained for $W_{Mag}$ and $W_{XRD}$ of the 3R phase suggest that the magnetization
between $T_1$ and $T_2$ is predominantly associated with the contribution of the rhombohedral phase, whereas that of the hexagonal phase becomes significantly less pronounced. A similar quantitative analysis of the $M(T)$ curves at 0.1 T was performed for all other samples and compared to $W_{XRD}$ of the 3R phase (Figs. 4c and 4d). A relatively good agreement is observed between these two values for all compounds (except for $x = 1.4$ and $A' = Gd$).

<figure><img src="image_5.png" /><figcaption>Fig. 4. Magnetic ordering temperatures of La$_{2-x}$A'$_x$Ni$_7$ for (a) A' = Sm and (b) A' = Gd, $W_{XRD}$ of 3R phase and $W_{Mag}$ from magnetization measurements for (c) A' = Sm and (d) A' = Gd.</figcaption></figure>

To further understand the origin of both transitions at $T_1$ or $T_2$, the magnetization curves versus applied field have been measured for all compounds. The compounds with $A' = Sm$ will be initially presented, commencing with Sm$_2$Ni$_7$.

<table><caption>**Table 1:** Magnetic transition temperatures ($T_1$ and $T_2$), relative magnetization ($\Delta M_1$ and $\Delta M_2$), magnetic weight percentage between $T_1$ and $T_2$, and weight percentage of 2H and 3R phases measured by XRD in Sm$_2$Ni$_7$ and Gd$_2$Ni$_7$.</caption><thead><tr><th>Phase</th><th><i>T</i><sub>1</sub><br>(K)</th><th><i>T</i><sub>2</sub><br>(K)</th><th>Δ<i>M</i><sub>1</sub><br>(emu/g)</th><th>Δ<i>M</i><sub>2</sub><br>(emu/g)</th><th><i>W</i><sub>Mag</sub><br>(wt %)</th><th>2<i>H</i> <i>W</i><sub>XRD</sub><br>(wt %)</th><th>3<i>R</i> <i>W</i><sub>XRD</sub><br>(wt %)</th></tr></thead><tbody><tr><td>Sm<sub>2</sub>Ni<sub>7</sub></td><td>75(1)</td><td>114(1)</td><td>6.8(1)</td><td>1.3(1)</td><td>19(1)</td><td>79(1)</td><td>21(1)</td></tr><tr><td>Gd<sub>2</sub>Ni<sub>7</sub></td><td>44(1)</td><td>117(1)</td><td>23.4(1)</td><td>16.5(1)</td><td>70(1)</td><td>37(1)</td><td>63(1)</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>Fig. 5. a) Comparison of the *M*(*μ*₀*H*) magnetization curves of Sm₂Ni₇ at different temperatures, b) *M*(*T*) at 0.1 and 3 T (FC), *M*<sub>R</sub> and *M*<sub>S</sub> obtained from the *M*(*μ*₀*H*) curves.</figcaption></figure>

Fig. 5a presents the *M*(*μ*₀*H*) curves of Sm₂Ni₇ at selected temperatures. Hysteresis loops were measured for temperatures between 5 and 100 K, starting from 9 T and decreasing the applied field down to -9 T, then increasing again to 9 T. At 5 K, only positive values of the magnetization could be measured with a magnetization of 0.033 μ<sub>B</sub>/f.u. at -9 T. This means that the coercive field μ<sub>0</sub>H<sub>C</sub> is smaller than the maximum available magnetic field (9T). The extrapolation of *M*(*μ*₀*H*) to 0, yields to an estimated coercive field μ<sub>0</sub>H<sub>C</sub> = 9.2(1) T. Upon heating, the coercive field μ<sub>0</sub>H<sub>C</sub> and the remanent magnetization *M*<sub>R</sub> both decrease (Fig. 6a and b). The magnetization curve at 100 K (i.e. at T<sub>1</sub> < *T* < T<sub>2</sub>) displays a metamagnetic behaviour with a transition field μ<sub>0</sub>H<sub>Trans</sub> of 2 T and a hysteresis loop closing at the same field (Fig. 5a). The S shape of the Arrott plot (*M*<sup>2</sup> = f(μ<sub>0</sub>*H*/M) at 100 K demonstrates the first order character of this transition. The *M*(*μ*₀*H*) curves at 150 K and above show a linear behaviour, which is characteristic of a paramagnetic state.

The saturation magnetization (M<sub>S</sub>) extrapolated from magnetization at high field reaches 0.9 μ<sub>B</sub>/f.u. at 5 K. Assuming a collinear ferromagnetic orientation of Sm and Ni moments and a total Ni moment of 0.6 μ<sub>B</sub>/f.u. as in La<sub>2</sub>Ni<sub>7</sub>, the calculated Sm moment (0.15 μ<sub>B</sub>/Sm) is significantly smaller than that of the free ion value of 0.84 μ<sub>B</sub>/Sm. In light of the aforementioned evidence, the diminished M<sub>S</sub> value can be ascribed to the presence of a canted magnetic structure, comprising different orientations of the Sm moments in the [A<sub>2</sub>B<sub>4</sub>] and [AB<sub>5</sub>] subunits. This observation corroborates the findings previously documented in the case of rhombohedral Er<sub>2</sub>Ni<sub>7</sub> by Burzo et al. [31] by neutron diffraction, which have revealed that the two Er sites
have different easy magnetization axes, and form a canted ferrimagnetic magnetic structure with a ferromagnetic component along the c-axis. It is established that Sm induces a magnetic anisotropy, which becomes particularly pronounced in uniaxial structures.

The existence of the large hysteresis loop at low temperature can be attributed to the energy required to align the Sm moments parallel to the magnetic field. The large coercive field (> 9T) estimated at 5 K can be enhanced by microstructural effects related to the method of preparation, specifically the compression of the powder into pellets prior to annealing treatment used for the samples containing Sm. Adapted heat treatments are currently used to tune the microstructural properties and increase the coercive field of permanent magnets such as SmCo₅ [36].

The metamagnetic behaviour observed at 100 K with a transition field µ₀H<sub>Trans</sub> = 2 T can be attributed to a transition from a smaller to a stronger magnetic structure. The *M*(*T*) curve measured 3 T (Fig. 5b) demonstrates that the abrupt transition observed at *T*₁ for an applied field of 0.1 T has ceased to exist, and instead, a gradual decline in the magnetization is evident up to *T*₂. The values of the remanent *M*<sub>R</sub> and saturation magnetization *M*<sub>S</sub> have been also reported in Fig. 5b and are found located in the curves at 0.1 and 3 T respectively.

Considering the preceding correlation between W<sub>mag</sub> and W<sub>XRD</sub> (3R), it can be postulated that the abrupt transition at *T*₁ and for 0.1 T is associated with a transition in the magnetic state of the 2H phase, whereby it shifts from a high to a low magnetic state. Meanwhile, the 3R phase maintains a high magnetic state and contributes mainly to the magnetization in this temperature range. The low magnetic state of the 2H phase above *T*₁, can be either a paramagnetic state or to a wAFM state. If it is the former, then *T*₁ could be identified as the Curie temperature of the hexagonal phase. Alternatively, if it is the latter, then it may be related to a spin reorientation.

In order to elucidate the genesis of this transition and of the metamagnetic phenomenon, it is instructive to draw a parallel with the different magnetic behaviours previously observed in 2H La₂Ni₇ and 3R Y₂Ni₇ [10, 11]. Notwithstanding the discrepancy in the magnetic ground state wAFM for 2H La₂Ni₇ and wFM for 3R Y₂Ni₇, their magnetic ordering temperatures are remarkably similar, with T_N = 50 K and T_C = 53 K respectively [11]. These transition temperatures were related to the total energy difference between the spin-polarized (magnetic state) and non-spin-polarized (paramagnetic state) for both compounds [10]. Given that the interatomic distances and coordination number remain similar between each Sm and Ni atom in both 2H and 3R Sm₂Ni₇ structures, it is not possible to justify a difference of 39 K in their Curie temperature. Consequently, it seems more probable that there is a spin reorientation of the 2H phase between two magnetic states. Note that a spin reorientation has been previously reported for Dy₂Ni₇ and Ho₂Ni₇ [37] at a temperature lower of 36 and 40 K than T_C respectively.

In addition, a correlation was established between the crystal structure symmetry and the magnetic phase diagram of 2H La₂Ni₇ and 3R Y₂Ni₇ [10]. At 5 K, La₂Ni₇ undergoes a metamagnetic transition from a wAFM state towards a wFM state with a transition field of 4.5 T. This has been interpreted, through band structure calculations, by the weak energy difference
between the wAFM and wFM states. The wAFM structure, is described by an inversion of the
direction of the Ni moments oriented along the c-axis below and above a Ni site located in the
[A₂B₄] subunit which has a magnetic moment equal to zero. As a sufficient magnetic field is
applied, this Ni site recovers a small magnetic moment and the wFM structure with all Ni
moment parallel to the c-axis becomes more stable. On the other hand, in the rhombohedral
structure, which results from the stacking of three times 1 [A₂B₄] and 2 [AB₅] subunits, such
[A₂B₄] Ni site acting as an inversion center is not available. Therefore, 3R Y₂Ni₇ has a wFM
ground state and no metamagnetic transition is observed. In view of these previous results, it
can be assumed that the metamagnetic behaviour observed at 100 K is related to a transition
from a wAFM towards a wFM state of the Ni sublattice in the 2H Sm₂Ni₇ phase.

In the case of Sm₂Ni₇, it is essential to consider the molecular field induced by the Sm moments, which has the potential to act as an external field and stabilize a wFM state for the Ni sublattice of the 2H phase below T₁. Upon heating, the Sm moment decreases, which results in a reduction of this molecular field. At T₁, this molecular field becomes insufficient to stabilize the wFM structure of the Ni magnetic sublattice belonging to the 2H phase. Consequently, at T₁ and in the presence of a low magnetic field, a transition from the wFM to a wAFM state may occur. The application of a transition field of 3 T is sufficient to restore a wFM Ni magnetic sublattice of the 2H phase, as the transition at T₁ is no longer observed at such a field (Fig. 5b). The Ni sublattice of the rhombohedral phase remains in a wFM state up to T₂, as observed in the case of Y₂Ni₇ [10].

The observation of paramagnetic behaviour above $T_2$ indicates that this transition temperature can be attributed to a Curie temperature. As only one transition temperature is observed in $M(\mu_0H)$ at 3 T, it can be assumed that the 2H and 3R phases, with both a wFM Ni sublattice, have the same Curie temperature. It is also possible for the $T_N$ value of the wAFM state of the 2H phase at 0.1 T to coincide with the $T_C$ of the 3R phase; however, this will be obscured due to the lesser contribution of the former.

The inverse of the magnetic susceptibility in the paramagnetic range ($T > T_2$) can be fitted by a Curie-Weiss law with an effective moment $\mu_{eff} = 1.98(2) \mu_B/f.u.$ and a positive paramagnetic Curie temperature $\theta_p = 103(2) \text{ K, which is slightly lower than } T_2 (114 \text{ K}).$

In the following, the results of the magnetization curves of the La₂ₓSmₓNi₇ pseudobinary compounds are presented, considering the results obtained for Sm₂Ni₇ and their interpretation. The $M(\mu_0H)$ curves of La₂ₓSmₓNi₇ compounds are reported in supplementary materials (Figs. SI5, SI6 and SI7 for $x=0.5, 1$ and $1.5$ respectively).

All the $M(\mu_0H)$ curves of La₂ₓSmₓNi₇ ($x \ge 0.5$) compounds display a hysteresis loop at low temperature. The thermal variation of the coercive field $\mu_0H_C$ and remanent magnetization $M_R$ of the La₂ₓSmₓNi₇ compounds ($0.5 \le x \le 2$) are compared in Fig. 6a and b. The absolute values of $M_R$ are reported for positive (+) and negative magnetizations (-). A discrepancy is evident for $x=1, 1.5$ and $2$ below 35, 50 and 70 K, respectively. This difference is because the maximum available field of 9 T is insufficient to fully saturate the magnetization as the field strength
decreases, resulting in incomplete saturation. The values of $\mu_0H_C$ and $M_R$ decrease as a function of temperature for all compounds, reaching zero above $T_2$. Furthermore, these values decrease in conjunction with the Sm content, supporting the hypothesis that the hysteresis loop depends on the anisotropic behaviour of Sm.

The saturation magnetization $M_S$ at 5 K, slightly decreases between $x = 0$ and $0.5$ and, then increases versus $x$ (Fig. 6c). However, all these values are smaller than calculated for a collinear ferromagnetic structure with the free ion value of Sm (0.84 $\mu_B$/Sm) and the total Ni moment measured for La$_2$Ni$_7$ (7.$\mu_{Ni}$ = 0.6 $\mu_B$) (dashed red curve in Fig. 6c). These lower values can be explained by a canting between the two Sm moments belonging to [A$_2$B$_4$] and [AB$_5$] subunits as explained for Sm$_2$Ni$_7$.

The $M(T)$ curves of La$_{1.5}$Sm$_{0.5}$Ni$_7$ at different fields from 1 to 9 T show different magnetic transitions versus applied field (supplementary materials, Fig. SI4). Some features correspond to a step decrease of the magnetization ($T_1, T_2$), whereas the presence bumps with maxima at $T_N$ suggest the existence of AFM states as for La$_2$Ni$_7$. This reveals a quite complex magnetic phase diagram as observed for La$_2$Ni$_7$ with probably commensurate and incommensurate AFM and/or canted magnetic structures [12, 13]. The $M(\mu_0H_C)$ curves present a metamagnetic behaviour from 5 to 60 K (supplementary materials, Fig. SI5). The transition field $\mu_0H_{Trans}$ of La$_{1.5}$Sm$_{0.5}$Ni$_7$ increases to a maximum value of 3.5 T between 40 and 50 K and that of LaSmNi$_7$ reach a maximum of 3.3 T between 50 and 60 K. Concerning La$_{0.5}$Sm$_{1.5}$Ni$_7$, the transition field of 3 T is only observed in the curve at 70 K.

These metamagnetic transitions are mainly observed between $T_1$ and $T_2$. The maximum transition field $\mu_0H_{Trans}$ decreases as a function of Sm content. This reveals the influence of the molecular field induced by Sm on the wAFM-wFM field induced transition of the Ni sublattice. The higher the Sm content, the lower the transition field required to induce this transition. The observed increase of $T_1$ versus Sm content lends support this interpretation. As the Sm content increases, the magnetic ordering temperature of Sm rises, and the molecular field contribution also increases in proportion. For $x \ge 1$, the weight percentage of 3R phase derived from $W_{XRD}$ and $W_{Mag}$ are in very good agreement confirming that the 3R phase keeps a wFM sublattice up the Curie temperature $T_2$ (Fig. 4a). In addition, the $dM/d\mu_0H$ slopes obtained from the linear part of the $M(\mu_0H)$ curves at high field present maxima at temperatures intermediate between $T_1$ and $T_2$ for all La$_{2-x}$Sm$_x$Ni$_7$ compounds (Fig. 6d). This agrees with a spin reorientation occurring for the 2H phase in this temperature range, a larger applied field being necessary to reach a full saturation.

As a paramagnetic behaviour of the $M(\mu_0H)$ curves is observed above $T_2$, this transition temperature can be associated with a Curie temperature for all pseudobinary compounds and both 2H and 3R phases. The increase of $T_2$ indicates stronger magnetic interactions induced by Sm.
<figure><img src="image_7.png" /><figcaption>Fig. 6. Evolution of (a) the coercive field $\mu_0H_C$, (b) the remanent magnetization $M_R$ with (+) = positive magnetization, (-) = negative magnetization (c) the saturation magnetization $M_S$ at 5 K and the calculated total moment assuming a collinear ferromagnetic structure and (d) the slope $dM/d\mu_0H$ at high field of La$_{2-x}$Sm$_x$Ni$_7$ compounds.</figcaption></figure>

Following an examination of the magnetic properties of compounds containing substitutions of samarium, we will proceed to analyse those of the gadolinium-based compounds. The transition temperature $T_1$ of La$_{2-x}$Gd$_x$Ni$_7$ compounds remains almost constant, whereas $T_2$ increases with the Gd concentration (Fig. 4b). The 3R weight percentage of La$_{2-x}$Gd$_x$Ni$_7$ compounds starts to increase for $x > 1$ (Fig. 4d). However, for $x = 1.4$, as it was not possible to distinguish clearly the two different transition temperatures, this is why $W_{Mag}$ is plotted equal to 100 %.

The $M(\mu_0H)$ curves of Gd$_2$Ni$_7$ at different temperatures are presented in Fig. 7a and the hysteresis loop at 4 K in Fig. 7b. At 4 K, no coercive field is observed, and the saturation is reached above 1 T. The magnetization decreases upon heating and the $dM/d\mu_0H$ slope becomes larger. A paramagnetic behaviour is observed above 150 K, i.e. above the Curie temperature ($T_2$ = 117 K). The weak ferromagnetic contribution observed in some of the $M(\mu_0H)$ curves above 150 K is most probably related to Ni particles segregated at the surface due to the partial oxidation of the rare earth when the samples were crushed into powder.
The saturation magnetization at 4 K, $M_S = 13(1) \mu_B/f.u.$, is in good agreement with that of Zhou et al. [32], who found $M_S = 12.8 \mu_B/f.u.$ from $M(\mu_0H)$ curves measured at 4.2 K and for field up to 38 T. $Gd_2Ni_7$ presents also two transition temperatures, $T_1$ and $T_2$, at 0.1 T (Fig. 7c) and the relative magnetic fraction $W_{Mag}$ is also close to $W_{XRD}$ of the 3R phase as already reported. However, the $M(\mu_0H)$ curves of $Gd_2Ni_7$ do not display a metamagnetic behaviour between $T_1$ and $T_2$. A sharp decrease of the initial slope $dM/d\mu_0H$ from 32(2) to 28(2) $\mu_B/f.u/T$ is observed below and above $T_1$ respectively (inset of Fig. 7c). A second sharp decrease of $dM/d\mu_0H$ is observed at $T_2$.

The evolution of the $M(T)$ curves for applied fields between 0.05 and 0.7 T is displayed in Fig. 7c. The transition observed at $T_1$ for $\mu_0H = 0.05$ to 0.3 T is progressively smeared out at larger field. To explain the decrease of the $dM/d\mu_0H$ slope above $T_1$ it is possible to assume a change of the magnetic structure of the hexagonal fraction above the transition. Nevertheless, with only this small variation of the $dM/d\mu_0H$ slope it remains difficult to identify more clearly the change of magnetic order for the 2H phase. Note that a small bump is observed at 60 K for $\mu_0H = 0.5$ and 0.7 T upon heating, but not upon cooling. As the $M(\mu_0H)$ curves measured at 50 and 60 K are crossing at 0.6 T (Fig. 7a), it can indicate another type of magnetic transition.

The magnetization curves $M(\mu_0H)$ of the $La_{2-x}Gd_xNi_7$ compounds are reported in Figures SI8, Si9 and SI10 (supplementary materials) for $x = 0.5, 1$ and $1.4$ respectively. They are characteristic of soft ferrimagnetic materials below $T_2$ and paramagnetic above this temperature, confirming that $T_2$ corresponds to a Curie temperature for all compounds.

The evolution of $M_S$ at 5 K in $La_{2-x}Gd_xNi_7$ compounds versus the rate of Gd (Fig. 8) exhibits a linear increase between $x = 0.5$ and 2. A linear fit to the data is given by:

$$M_S = x \cdot \mu_{Gd} - 7 \mu_{Ni} \qquad (2)$$

With $\mu_{Gd} = 6.73(5) \mu_B/Gd$ and $\mu_{Ni} = 0.057(1) \mu_B/Ni$.

Assuming a collinear ferrimagnetic structure, the measured Gd moment is close to that of the free ion moment ($\mu_{Gd} = 7 \mu_B/Gd$), whereas the mean Ni moment is slightly smaller than measured for ferromagnetic 3R-Y$_2$Ni$_7$ with $\mu_{Ni} = 0.08(1) \mu_B/Ni$ [10, 11]. This behaviour can be compared with the results of Zhou et al. [32], who investigated the pseudobinary Y$_2$Gd$_xNi_7$ compounds for low Gd content ($x \le 0.2$). In both our work and that of Zhou et al., the Gd is diluted in a non-magnetic rare earth (i.e. La or Y). The exchange-coupling constants $J_{GdNi}/k$ obtained from the magnetization curves at 4.2 K by Zhou et al. [32] was changing from -15.1 K for $x = 0.08$ to -10.8 K for $x = 0.15$ and then to -15.4 K for $x = 0.2$. The shape of the $M(\mu_0H)$ curves for low Gd content ($x = 0.10$ and 0.15) suggested that a field induced ferri-ferromagnetic transition can occur with parallel orientation of Gd and Ni moments at high field. This is no longer the case for $x \ge 0.2$ even for a field up to 38 T [32].
<figure><img src="image_8.png" /><figcaption>Fig. 7. Comparison of a) the $M(\mu_0H)$ curves at different temperatures, b) $M(\mu_0H)$ at 4.2 K with a zoom of the hysteresis loop in inset, c) $M(\mu_0H)$ curves at low field and initial $dM/dH$ slope in inset d) $M(T)$ curves for several applied fields between 0.05 and 0.7 T of Gd$_2$Ni$_7$.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8. Evolution of the saturation magnetization $M_s$ versus Gd content at 5 K in La$_{2-x}$Gd$_x$Ni$_7$ compounds ($x$= 0 to 2). The dashed red line corresponds to a calculated value of $M_s$ using equation 1. The measurements for Y$_{2-x}$Gd$_x$Ni$_7$ from Zhou *et al.* [32] were performed at 4.2 K.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 9. Evolution of a) the Curie constant and b) the paramagnetic temperature versus Gd content in La₂ₓGdₓNi₇ compounds (x = 0 to 2), c) the effective moment, d) the RWR factor. The data for La₂Ni₇ are from ref. [12].</figcaption></figure>

Analysis of the magnetization curves in the paramagnetic region (150-300 K) shows a weak spontaneous magnetization which can be related to the presence of segregated Ni particles on the surface of the grains. This can occur when the samples are crushed to powder. The slopes of these curves were used to obtain the paramagnetic susceptibility χ (T) of each compound and to calculate the paramagnetic Curie temperature θp and the Curie constant C. These values are plotted in Fig. 9 as a function of Gd content and display a linear variation. The values added for La₂Ni₇ are from ref. [12], they were obtained from the magnetic measurements of a single crystal. Both θp and C (x > 0) can be fitted with a linear equation, the values of which are plotted in fig. 9. Extrapolation of the calculated curves to x = 0 gives larger values than reported for La₂Ni₇. This may be due to the influence of the Gd sublattice on the Ni sublattice. It can be observed that θp is significantly lower than the Curie temperature T₂ (30 to 50 K lower) and that extrapolation to x = 0 gives a negative value. This reflects the competition between ferromagnetic and antiferromagnetic interactions between the Gd and Ni magnetic sublattices. The effective moments are presented in Fig. 9c. For Gd₂Ni₇ μₐff=10.5(1) μB is smaller than the saturation magnetization (Mₛ=13.1 μB/f.u.at 5 K). The Rhodes–Wohlfarth ratio (RWR = Pc/Mₛ) with μₐff² = pc(pc+2) which characterize the itinerant character of a ferromagnet is plotted in Fig. 9d. The value of RWR= 0.73 for Gd₂Ni₇ is lower than 1, indicating the localized character
of the magnetic transition. This value is comparable with the values obtained for Dy₂Ni₇ and Ho₂Ni₇ (0.76 and 0.88 respectively) [37] and smaller than for La₂Ni₇ (3.2) [11]. As the Gd content decreases the RWR factor progressively increases, reaching 1.7 for *x* = 0.5 (Fig. 9d). This evolution reveals a change from itinerant towards localized character upon Gd for La substitution.

The present study has revealed that the magnetic properties of the La₂-xSmxNi₇ and La₂-xGdxNi₇ pseudobinary compounds are influenced by the difference in crystal symmetry between the two *2H* and *3R* structures. This finding is consistent with observations made on La₂-xYxNi₇ compounds [10, 11]. The contribution of the magnetic rare earth A' substituted for La should be considered to explain the evolution of the magnetic properties. The analysis of the magnetization curves has shown that it is possible to discriminate the behaviour of the hexagonal and rhombohedral magnetic phases by the existence of a low and sharp magnetic transition at *T*₁ for a field around 0.1 T. Below *T*₁, the *2H* and *3R* phases both order in a canted magnetic structure for A' = Sm or a collinear ferrimagnetic structure for A' = Gd as expected for a heavy rare earth. The influence of the nature rare earth element is also visible at low temperature: for Sm substituted compounds the large hysteresis curves are related to the influence of Sm anisotropy in an uniaxial structure, whereas for Gd, which presents a spherical electronic cloud (corresponding to α<sub>J</sub> = 0, α<sub>J</sub> being the second order Stevens coefficient [38]) no hysteresis is observed.

On heating, the rhombohedral phase retains the same magnetic state with a progressive decrease in the *A*' and Ni moments up to the Curie temperature *T*₂. For Sm compounds, the hexagonal phase undergoes a magnetic transition at *T*₁ from a wFM to a wAFM Ni magnetic sublattice due to the decrease of the molecular field induced by the Sm moments. Between *T*₁ and *T*₂ the metamagnetic behaviour look like that of hexagonal La₂Ni₇. In Gd compounds, the transition at *T*₁ corresponds to a small decrease of the initial slope dM/dμ₀H of the magnetization curves (Fig. 7c).

The variation of the transition temperatures *T*₁ and *T*₂ versus the *A'* concentration presents for both systems some similarities. The transition temperature *T*₁ of La₁.₅*A*'₀.₅Ni₇ is smaller than *T*ₙ = 55 K in La₂Ni₇ (with *T*₁ = 20.4 K for *A'* = Sm and 30.5 K for *A'* = Gd). As the Sm content increases, *T*₁ increases noticeably until it reaches 75 K for the binary Sm₂Ni₇. In the case of Gd, *T*₁ also increase with the Gd content, but more moderately, saturating at around 44 K for Gd₂Ni₇. The Curie temperature, *T*₂, increases as a function of *A*' content for both systems reaching 114 and 117 K for Sm₂Ni₇ and Gd₂Ni₇ respectively. The difference of Curie temperature between the two compounds remains small.

## Conclusions

La₂xA'Ni₇ (*A'* = Sm, Gd) pseudobinary compounds crystallize in a mixture of hexagonal (Ce₂Ni₇-type, *P*6₃/*mmc*) and rhombohedral (Gd₂Co₇-type, *R*-3*m*) phases, both constituted by the stacking of two [*AB*₅] and one [*A*₂*B*₄] subunits along the *c*-axis repeated two and three time respectively. Their cell volumes decrease linearly upon *A'* for La substitution due to the smaller
atomic radius of A'. On the other hand, the decrease in c/a ratio exhibits a minimum for x = 1. At low field, the magnetization versus temperature curves of La₂-xSmxNi7 and La₂-xGdxNi7 show two transition temperatures T₁ and T₂. T₁ has been attributed to a magnetic transition of the hexagonal phase from a high to a lower magnetic state through a spin reorientation of the Ni sublattice, whereas T₂ corresponds to the Curie temperature. For Sm compounds, a metamagnetic behaviour is observed between T₁ and T₂ comparable to that observed for hexagonal La₂Ni7, which can be therefore attributed to a wAFM-wFM transition. The isotherm magnetization curves below T₁ reveal that La₂-xSmxNi7 compounds display remarkable hard magnetic behaviour with a coercive field larger than 9 T for Sm₂Ni7 at 5 K. A canted magnetic structure with different orientations of the Sm moments is assumed to explain the saturation magnetization lower than expected for a collinear ferromagnetic structure with all moments parallel to the c-axis. La₂-xGdxNi7 compounds behave as soft ferrimagnets and the saturation magnetization at 5 K is well fitted assuming a ferrimagnetic structure with antiparallel Gd and Ni magnetic moments. The transition at T₁ is also attributed to the hexagonal phase and is characterized by a discontinuous reduction of the initial slope of the M(μ₀H) curves. Further first principles calculations will be useful to propose stable magnetic ground states and explain the evolution of the magnetic properties.

**Declaration of competing interest**

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

**Acknowledgements**

The project was funded by CNRS. This work has been supported by the French ANR (Agence Nationale de la Recherche) program PROGELEC under the contract MALHYCE ANR-2011-PRGE-006 01. The authors are also thankful to Michel Latroche which was responsible of the MALHYCE project.

**Declaration of generative AI and AI-assisted technologies in the writing process**

During the preparation of this work the authors used [DEEPL / WRITE] to improve the readability of the work. After using this tool/service, the authors reviewed and edited the content as needed and take(s) full responsibility for the content of the publication.

**Data availability**

Data will be made available on request.
References

[1] J.J.M. Franse, R.J. Radwanski, Magnetic properties of binary rare-earth 3d transition metal intermetallic compounds, in: K.H.J. Buschow (Ed.) Handbook of Magnetic Materials, Elsevier Science Publishers, 1993, pp. 307-486. https://doi-org inc bib cnrs fr/10 1016/S1567 2719(05)80046-0

[2] P.H.L. Notten, M. Latroche, Secondary Batteries: nickel batteries metal hydride alloys in: J. Garche (Ed.), Encyclopedia of Electrochemical Power Sources, Elsevier, North Holland, 2009, pp. 502-521. https://doi.org/10.1016/B978-044452745-5.00164-7

[3] K.H.J. Buschow, Magnetic properties of La₂Ni₇ and its hydride, Journal of Magnetism and Magnetic Materials 40 (1983) 224-226. https://doi.org/10.1016/0304-8853(83)90032-X

[4] F.T. Parker, H. Oesterreicher, Magnetic properties of La₂Ni₇, Journal of the Less Common Metals 90 (1983) 127-136. https://doi.org/10.1016/0022-5088(83)90124-8

[5] Y. Tazuke, R. Nakabayashi, S. Murayama, T. Sakakibara, T. Goto, magnetism of R₂Ni₇ and RNi₃ (R = Y, La, Ce), Physica B-Condensed Matter 186-88 (1993) 596-598. https://doi.org/inc bib cnrs fr/10.1016/0921-4526(93)90646-N

[6] Y. Tazuke, M. Abe, S. Funahashi, Magnetic properties of La-Ni system Physica B: Condensed Matter 237-238 (1997) 559-560. https://doi-org inc bib cnrs fr/10.1016/S0921-4526(97)00245-7

[7] M. Fukase, Y. Tazuke, H. Mitamura, T. Goto, T. Sato, Successive metamagnetic transitions in hexagonal La₂Ni₇, J. Phys. Soc. Jpn. 68 (1999) 1460-1461. https://doi.org/10.1143/JPSJ.68.1460

[8] M. Fukase, Y. Tazuke, H. Mitamura, T. Goto, T. Sato, Itinerant electron magnetism of hexagonal La₂Ni₇, Mater. Trans. JIM 41 (2000) 1046-1051. https://doi.org/10.2320/matertrans1989.41.1046

[9] Y. Tazuke, H. Suzuki, H. Tanikawa, Metamagnetic transitions in hexagonal La₂Ni₇, Physica B-Condensed Matter 346 (2004) 122-126. https://doi.org/inc bib cnrs fr/10.1016/j.physb.2004.01.033

[10] J.C. Crivello, V. Paul-Boncour, Relation between the weak itinerant magnetism in A₂Ni₇ compounds (A = Y, La) and their stacked crystal structures, Journal of Physics : Condensed Matter 32 (2020) 145802. https://iopscience.iop.org/article/10.1088/1361-648X/ab6045

[11] V. Paul-Boncour, J.-C. Crivello, N. Madern, J. Zhang, L.V.B. Diop, V. Charbonnier, J. Monnier, M. Latroche, Correlations between stacked structures and weak itinerant magnetic properties of La₂ₓYₓNi₇ compounds, Journal of Physics-Condensed Matter 32 (2020) 415804. https://iopscience.iop.org/article/10.1088/1361-648X/ab9d4c

[12] R.A. Ribeiro, S.L. Bud'ko, L. Xiang, D.H. Ryan, P.C. Canfield, Small-moment antiferromagnetic ordering in single-crystalline La₂Ni₇, Physical Review B 105 (2022) 014412. https://journals.aps.org/prb/abstract/10.1103/PhysRevB.105.014412

[13] J.M. Wilde, A. Sapkota, W. Tian, S.L. Bud'ko, R.A. Ribeiro, A. Kreyssig, P.C. Canfield, Weak itinerant magnetic phases of La₂Ni₇, Physical Review B 106 (2022) 075118. https://journals.aps.org/prb/abstract/10.1103/PhysRevB.106.075118

[14] P.C. Canfield, S.L. Bud'ko, Preserved entropy and fragile magnetism, Reports on Progress in Physics 79 (2016) 084506. https://iopscience.iop.org/article/10.1088/0034-4885/79/8/084506
[15] M. Shimizu, J. Inoue, Thermal spontaneous ferromagnetism in Y₂Ni₇ intermetallic compound, Journal of Physics F: Met. Phys. 17 (1987) 1221-1229.
https://iopscience.iop.org/article/10.1088/0034-4885/79/8/084506

[16] O.M. Tolkachev, Spin fluctuation mechanism for the temperature-induced magnetic moment (TIMM) in the electron liquid of Y₂Ni₇ alloy, Phys. Status Solidi B-Basic Res. 159 (1990) K115-K118. https://doi.org/10.1002/pssb.2221590259

[17] Y. Nishihara, S. Ogawa, Itinerant electron ferromagnetism in Y₂Ni₇ and Mossbauer-effect of Fe⁵⁷ doped in Y₂Ni₇, J. Phys. Soc. Jpn. 60 (1991) 300-303.
https://doi.org/10.1143/JPSJ.60.300

[18] R. Nakabayashi, Y. Tazuke, S. Murayama, Itinerant electron weak ferromagnetism in Y₂Ni₇ and YNi₃, J. Phys. Soc. Jpn. 61 (1992) 774-777.
https://ui.adsabs.harvard.edu/link_gateway/1992JPSJ...61..774N/doi:10.1143/JPSJ.61.774

[19] D.J. Singh, Electronic structure and weak itinerant magnetism in metallic Y₂Ni₇, Physical Review B 92 (2015) 174403. https://doi.org/10.1103/PhysRevB.92.174403

[20] Q.P. Ding, J. Babu, K. Rana, Y. Lee, S.L. Bud'ko, R.A. Ribeiro, P.C. Canfield, Y. Furukawa, Microscopic characterization of the magnetic properties of the itinerant antiferromagnet La₂Ni₇ by ¹³⁹LaNMR/NQR measurements, Physical Review B 108 (2023) 064413. https://link.aps.org/doi/10.1103/PhysRevB.108.064413

[21] S. Jaakkola, S. Parviainen, S. Penttila, Volume dependence of the Curie temperature of rare-earth-3d transition metal compounds, Journal of Physics F: Metal Physics 13 (1983) 491-502. http://dx.doi.org/10.1088/0305-4608/13/2/023

[22] H. Horikoshi, S. Takeda, Y. Komura, Polytypes and Intergrowth of the Intermetallic Compounds Sm₂Ni₇, Japanese Journal of Applied Physics 24 (1985) L210-L212.
http://dx.doi.org/10.1143/JJAP.24.L210

[23] V.A. Yartys, P. Vajeeston, A.B. Riabov, P. Ravindran, R.V. Denys, J.P. Maehlen, R.G. Delaplane, H. Fjellvag, Crystal chemistry and metal-hydrogen bonding in anisotropic and interstitial hydrides of intermetallics of rare earth (R) and transition metals (T), RT₃ and R₂T₇, Z. Kristall. 223 (2008) 674-689. https://doi-org inc bib cnrs fr/10.1524/zkri.2008.1030

[24] L. Lemort, M. Latroche, B. Knosp, P. Bernard, Elaboration and Characterization of New Pseudo-Binary Hydride-Forming Phases Pr₁.₅Mg₀.₅Ni₇ and Pr₃.₇₅Mg₁.₂₅Ni₁₉: A Comparison to the Binary Pr₂Ni₇ and Pr₅Ni₁₉ Ones, The Journal of Physical Chemistry C 115 (2011) 19437-19444. https://pubs.acs.org/doi/10.1021/jp2059134#Abstract

[25] K. Iwase, K. Mori, Crystal structure and hydrogen storage property of Nd₂Ni₇ superlattice alloy, International Journal of Hydrogen Energy 38 (2013) 5316-5321.
https://doi.org/10.1016/j.ijhydene.2013.02.063

[26] V. Levytskyy, V. Babizhetskyy, B. Kotur, V. Smetana, Diterbium hepta-nickel: a crystal structure redetermination, Acta Crystallographica E70 (2014) i42.
https://doi.org/10.1107/S1600536814015384

[27] V. Charbonnier, J. Monnier, J. Zhang, V. Paul-Boncour, S. Joiret, B. Puga, L. Goubault, P. Bernard, M. Latroche, Relationship between H₂ sorption properties and aqueous corrosion mechanisms in A₂Ni₇ hydride forming alloys (A = Y, Gd or Sm), Journal of Power Sources 326 (2016) 146-155. https://doi-org inc bib cnrs fr/10.1016/j.jpowsour.2016.06.126

[28] K.H.J. Buschow, A.S. Van Der Goot, The crystal structure of rare-earth nickel compounds of the type R₂Ni₇, Journal of the Less Common Metals 22 (1970) 419-428.

[29] A.V. Virkar, A. Raman, Crystal structures of AB₃ and A₂B₇ rare earth-nickel phases, Journal of the Less Common Metals 18 (1969) 59-66. https://doi.org/10.1016/0022-5088(69)90120-9
[30] H.R. Kirchmayr, C.A. Poldy, Magnetism in rare earth-3d intermetallics, Journal of Magnetism and Magnetic Materials 8 (1978) 1-42. https://doi.org/inc.bib.cnrs.fr/10.1016/0304-8853(78)90073-2

[31] J.K. Yakinthos, P.J. Rentzeperis, Explanation of the Magnetic Structure of Er₂Ni₇ with a Point-Charge Crystal Field Model, physica status solidi (b) 64 (1974) 643-647. https://doi.org/10.1002/pssb.2220640227

[32] G.F. Zhou, F.R. Deboer, K.H.J. Buschow, Magnetic coupling in rare-earth nickel compounds of the type R₂Ni₇, J. Alloy. Compd. 187 (1992) 299-303. https://doi.org/inc.bib.cnrs.fr/10.1016/0925-8388(92)90436-D

[33] J. Zhang, V. Charbonnier, N. Madern, J. Monnier, M. Latroche, Improvement of reversible H storage capacity by fine tuning of the composition in the pseudo-binary systems A₂-xLaxNi₇ (A = Gd, Sm, Y, Mg), J. Alloy. Compd. 852 (2021) 157008. https://doi.org/10.1016/j.jallcom.2020.157008

[34] J. Rodriguez-Carvajal, Recent Advances in Magnetic Structure Determination by Neutron Powder Diffraction, Physica B 192 (1993) 55-69. https://doi.org/inc.bib.cnrs.fr/10.1016/0921-4526(93)90108-I

[35] V. Charbonnier, Optimisation de la composition en terres rares pour des hydrures métalliques utilisés comme électrodes dans les accumulateurs Ni-MH, PhD Thesis, University of Paris Est Creteil, 2015

[36] R. Kütterer, H.R. Hilzinger, H. Kronmüller, The temperature dependence of the coercive field of Co₅Sm magnets, Journal of Magnetism and Magnetic Materials 4 (1977) 1-7. https://doi.org/10.1016/0304-8853(77)90004-X

[37] A. Bhattacharyya, S. Giri, S. Majumdar, Successive magnetic transitions and low temperature magnetocaloric effect in RE₂Ni₇ (RE=Dy, Ho), Journal of Magnetism and Magnetic Materials 323 (2011) 1484-1489. https://doi.org/10.1016/j.jmmm.2011.01.004

[38] R. Skomski, D.J. Sellmyer, Anisotropy of rare-earth magnets, Journal of Rare Earths 27 (2009) 675-679. https://doi.org/10.1016/S1002-0721(08)60314-2
Supplementary Material

Relationship between polymorphic structures and magnetic properties of La₂-xA'xNi₇ compounds (A' = Sm, Gd)

Valérie Paul-Boncour<fnref n="1" />*, Véronique Charbonnier<fnref n="1" />,<fnref n="2" />, Nicolas Madern<fnref n="1" />,<fnref n="3" />, Lotfi Bessais<fnref n="1" />, Judith Monnier<fnref n="1" />, Junxian Zhang<fnref n="1" />

<figure><img src="image_11.png" /></figure>

Fig. SI1. Weight percentage of hexagonal and rhombohedral phases of A₂Ni₇ compounds annealed at 1273 K versus atomic radius. The figure is from V. Charbonnier's PhD thesis [1]. The data are obtained from literature A = La, Ce, Er [2]; Y, Sm, Gd[3] ; Nd[4] ;Pr[5] ;Dy, Ho, [6]

<footnote n="1">¹Univ. Paris-Est Créteil, CNRS, ICMPE, UMR7182, F-94320 Thiais, France</footnote>

<footnote n="2">²Energy Process Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.</footnote>

<footnote n="3">³ Mines Paris, PSL University, MAT - Centre des Matériaux, CNRS UMR 7633, BP 87, 91003, Evry, France</footnote>

<footnote>*Corresponding author: valerie.paul-boncour@cnrs.fr, Phone number: +33 1 49 78 12 07</footnote>
<figure><img src="image_12.png" /><figcaption>Fig. SI2. Determination of the relative magnetic weight percentages and the transition temperatures $T_1$ and $T_2$, in the binary Sm$_2$Ni$_7$ compound.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. SI3. Determination of the relative magnetic weight percentages and the transition temperatures $T_1$ and $T_2$, in the binary Gd$_2$Ni$_7$ compound.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. SI4. (a) Evolution of the *M*(*T*) curves of La<sub>1.5</sub>Sm<sub>0.5</sub>Ni<sub>7</sub> at different applied fields, (b) magnetic transition temperatures as a function of the applied field for La<sub>1.5</sub>Sm<sub>0.5</sub>Ni<sub>7</sub></figcaption></figure>
<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /><figcaption>Fig. SI5. (a) Evolution of the $M(\mu_0H)$ curves of La$_{1.5}$Sm$_{0.5}$Ni$_7$ at different temperatures. (b) transition field versus temperature for La$_{1.5}$Sm$_{0.5}$Ni$_7$.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>Fig. SI6. Evolution of (a) the $M(\mu_0H)$ curves of LaSmNi$_7$ at different temperatures, (b) transition field versus temperature.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>Figure SI7: Evolution of the $M(\mu_0H)$ curves of La$_{0.5}$Sm$_{1.5}$Ni$_7$ at different temperatures.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>Fig. SI8. Evolution of the $M(\mu_0H)$ curves of La$_{1.5}$Gd$_{0.5}$Ni$_7$ at different temperatures.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>Fig. SI9. Evolution of the $M(\mu_0H)$ curves of LaGdNi$_7$ at different temperatures.</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>Fig. SI10. Evolution of the $M(\mu_0H)$ curves of La$_{0.6}$Gd$_{1.4}$Ni$_7$ at different temperatures.</figcaption></figure>
References

[1] V. Charbonnier, Optimisation de la composition en terres rares pour des hydrures métalliques utilisés comme électrodes dans les accumulateurs Ni-MH, PhD Thesis, University of Paris Est Creteil, 2015

[2] K.H.J. Buschow, A.S. Van Der Goot, The crystal structure of rare-earth nickel compounds of the type R₂Ni₇, J. Less-Common Met. 22 (1970) 419-428.

[3] V. Charbonnier, J. Monnier, J. Zhang, V. Paul-Boncour, S. Joiret, B. Puga, L. Goubault, P. Bernard, M. Latroche, Relationship between H₂ sorption properties and aqueous corrosion mechanisms in A₂Ni₇ hydride forming alloys (A = Y, Gd or Sm), J. Power source 326 (2016) 146-155. https://doi-org inc bib cnrs fr 10 1016 j jpowsour 2016 06 126

[4] K. Iwase, K. Mori, Crystal structure and hydrogen storage property of Nd₂Ni₇ superlattice alloy, Int. J. Hydr. Energ. 38 (2013) 5316-5321. https://doi.org/10.1016/j.ijhydene.2013.02.063

[5] L. Lemort, M. Latroche, B. Knosp, P. Bernard, Elaboration and Characterization of New Pseudo-Binary Hydride-Forming Phases Pr₁.₅Mg₀.₅Ni₇ and Pr₃.₇₅Mg₁.₂₅Ni₁₉: A Comparison to the Binary Pr₂Ni₇ and Pr₅Ni₁₉ Ones, J. Phys. Chem. C 115 (2011) 19437-19444. https://pubs.acs.org/doi/10.1021/jp2059134#Abstract

[6] A. Bhattacharyya, S. Giri, S. Majumdar, Successive magnetic transitions and low temperature magnetocaloric effect in RE₂Ni₇ (RE=Dy, Ho), J. Magn. Magn. Mat. 323 (2011) 1484-1489. https://doi-org inc bib cnrs fr 10 1016/j.jmmm 2011 01 004