X-ray diffraction study of phonon and magnon properties of Eu2Cu6P5 ferromagnet

doi:10.1016/j.jmmm.2020.167271

Journal of Magnetism and Magnetic Materials

Volume 514, 15 November 2020, 167271

https://doi.org/10.1016/j.jmmm.2020.167271 Get rights and content

Highlights

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Temperature changes of cell parameters Eu₂Cu₆P₅ revealed anisotropy at 5–300 K.
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Kinks on a(T) and c(T) curves at T_C ≈ 36 K are due to magnetic phase transition.
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The parameters of the Debye-Einstein model for cell volume V_u(T) were determined.
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Grüneisen parameter growth with temperature is due to impact of anharmonicity.

Abstract

Ternary europium copper phosphide Eu₂Cu₆P₅ was synthesized from the elements by the standard ampule method as a phase-pure product and studied by means of X-ray powder diffraction. Temperature dependencies of the tetragonal unit cell parameters and volume revealed pronounced anisotropy. The observed anomalies on the a(T) and c(T) curves at about T_C ≈ 36 K were related to magnetic phase transition in the phosphide under study. The linear spontaneous magnetostrictions in the basal plane and in the direction of the c axis have different signs that indicates a pronounced anisotropy of the thermal expansion at the temperatures of magnetic ordering. Temperature changes of the unit cell volume V_u(T) were analyzed within the Debye-Einstein approximation. The parameters of the model applied (Debye and Einstein characteristic temperatures, shares of the correspondence contributions) were determined. It was shown that the ratio of the shares of the Debye and Einstein contributions to the thermal characteristics are 0.85/0.15; consequently, in the region of moderately low temperatures, the thermal properties of Eu₂Cu₆P₅ are determined mainly by the Debye vibrations of the Cu/P framework. Experimental data on thermal expansion and calculated heat capacity were used to calculate the temperature-dependent Grüneisen parameter, γ(T), for the further analysis of the lattice dynamics of Eu₂Cu₆P₅. The revealed monotonic growth of γ(T), which is a measure of the anharmonicity of lattice vibrations, indicates an increasing influence of anharmonicity on the thermal properties of Eu₂Cu₆P₅. The anisotropy of the vibrations of europium atoms, as well as local regions of structural heterogeneity of the sample that leads to the appearance of local stresses in the crystal structure can be the reasons for the increasing influence of anharmonicity.

Introduction

Compounds of divalent europium frequently exhibit ordering of Eu²⁺ cations at low temperatures. Depending on the nature of interactions, ferromagnetic and antiferromagnetic ordering prevails; moreover, the detailed analysis of the magnetic systems shows that, as a rule, both ferromagnetic and antiferromagnetic correlations coexist in one compound, and the resulting property is governed by a competition of correlations [1]. In the majority of compounds, the Eu²⁺ cations have large coordination numbers and do not interact with each other directly; instead, the RKKY type of interaction through conduction charge carriers is a typical mechanism. Coordination number of Eu²⁺ in the majority of compounds varies from 16 to 24. The coordination numbers of 20 and 24 are typical for clathrates and clathrate-like compounds, which exhibit ferromagnetic ordering at temperatures from 3 to 36 K [2], [3], [4], [5], with a sole exception of AFM-ordered EuNi₂P₄ [6]. At lower coordination numbers, from 16 to 18, Eu²⁺ typically forms layered or pseudo-layered compounds, of which the overwhelming majority belongs to two the structure family of ThCr₂Si₂; most of them exhibit antiferromagnetic ordering of Eu²⁺ cations [7], [8], although rare examples of ferromagnetism are also documented in the literature [9], [10]. We note that low coordination numbers are very rare for Eu²⁺; members of the LaFeAsO structure family with C.N. of 8 can be given as an example; these phases also exhibit AFM ordering [11].

Eu₂Cu₆P₅ is a complex phosphide of europium and copper; it has a crystal structure derived from ThCr₂Si₂ by introduction of an additional Cu₂P layer between EuCu₂P₂ layers along the c axis of the tetragonal unit cell (Fig. 1). In contrast with EuCu_2−xP₂, Eu₂Cu₆P₅ is a perfectly ordered compound with no sign of vacancies or any other positional disorder in its crystal structure. Emphasizing the polyhedral arrangement, the crystal structure of Eu₂Cu₆P₅ can be represented as an alternation of two layers of 16-vertex Frank-Kasper EuCu₈P₈ polyhedra with a single layer made of P(1)Cu₈ cubes (Fig. 1, right panel) along the tetragonal c axis.

Eu₂Cu₆P₅ has a metallic type of conductivity. At temperatures below T_C = 34.6 K, it transforms to the ferromagnetic state, which is manifested by a break in the temperature dependence of the electrical resistivity ρ(T), and also corresponds to a maximum in the temperature dependence of the effective magnetic moment μ_eff(T). The divalent state of europium (4f⁷, ⁸S_7/2) was inferred as a result of the analysis of the μ_eff(T) and M(T) dependences [12].

Recently, we have shown that low-temperature properties of FM and AFM ordered Eu²⁺ complex pnictides, such as heat capacity and thermal expansion, exhibit anomalies, which reflect details of the crystal structure, in particular, coordination and dynamics of the Eu²⁺ cation [13], [14].

The aim of this study was to establish the laws of the effect of magnetic phase transformation in Eu₂Cu₆P₅as well as of behavior of the Eu²⁺ cations in the crystal structure on the dynamics of its crystal lattice in the temperature range of 5–300 K.

Section snippets

Experiment

A polycrystalline sample of Eu₂Cu₆P₅ was synthesized using a standard ampule technique according to the literature procedure [10]. In brief, a stoichiometric mixture of copper and red phosphorus powders and crushed pieces of europium was pressed at room temperature into a pellet, which was placed in a graphitized quartz ampule. After slow heating at the rate of 0.1 K min⁻¹, annealing was carried out at 600 °C for 12 h, followed by regrinding and annealing twice at 850 °C for 20 h with another

Results and discussion

Temperature dependences of the unit cell parameters a(T) and c(T) of Eu₂Cu₆P₅ show variable behavior depending on the temperature range (Fig. 3). At T > 100 K both parameters are monotonically increasing curves without visible anomalies. The approximation of these dependences in the Debye-Einstein (D-E) model [17] satisfactorily reproduces the experimental data in this temperature range. At lower temperatures, we revealed noticeable deviations of the experimental values of a(T) and c(T) from

Conclusion

An X-ray diffraction study of the thermal expansion of Eu₂Cu₆P₅ over a wide range of low temperatures, including helium temperatures, made it possible to establish a number of important patterns in the behavior of its phonon and magnon subsystems. The transition from the paramagnetic to the ferromagnetic state at temperatures below T_C = 36 K is accompanied by the phenomenon of spontaneous magnetostriction - a deviation of the experimental temperature dependences of the unit cell parameters and

CRediT authorship contribution statement

V.V. Novikov: Conceptualization, Project administration, Writing - original draft. N.V. Mitroshenkov: Investigation. A.V. Morozov: Software. I.V. Plokhikh: Investigation. A. Pfitzner: Investigation. A.V. Shevelkov: Conceptualization, Writing - review & editing.

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.

Acknowledgement

The study was supported by the Russian Science Foundation (project No. 16-12-00004 P).

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Cited by (1)

Structural diversity of the Zintl pnictides with rare-earth metals
2021, Handbook on the Physics and Chemistry of Rare Earths
Citation Excerpt :
The formation of late transition element defects becomes a necessary feature to satisfy the electron count [243–246], and the formulation reflecting partially occupied transition metal site helps to approach the charge balance: (R3 +)(M2 +)2 − x(Pn3 −)2(e−)x. An interesting example worth mentioning in this subsection is the Eu2Cu6P5 phase [247]. Compared to the defect-structure of EuCu2 − xP2, Eu2Cu6P5 is perfectly ordered.
Zintl phases have been purely a laboratory curiosity for decades, recently becoming a familiar subject in many research topics of practical importance, such as thermoelectrics for example. This distinctive group of compounds holds a special place among the broader intermetallic family because its members are commonly composed of the electropositive alkali or alkaline-earth metals and the much more electronegative post-transition metalloid elements. The structures of the Zintl phases are often very complex, but they can be conceptually understood by assuming an electron transfer from the atoms of the less electronegative to the atoms of the more electronegative elements in a way that the classic valence rules are satisfied. In this manner, the donated electrons are used by the electronegative constituents to form bonds, usually through relatively strong covalent interactions, whereas the cation-anion interactions ought to be considered to be more ionic than covalent in character. Because of the peculiar nature of the chemical bonding, Zintl phases are located at the border between the valence compounds, which are typical insulators, and the intermetallic compounds, which are typical metals, often exhibiting intermediate properties, that is, Zintl phases are by nature narrow bandgap semiconductors.
A wide variety of main-group based Zintl phases have already been synthesized and structurally characterized. Their structures are truly diverse and many are remarkable on their own. In the past two to three decades, the chemistry of the Zintl phases has been extended to include lanthanides and some of the transition metals. This chapter provides a comprehensive overview of the crystal chemistry and some properties of the Zintl pnictides containing rare-earth metals, recounting many structures of binary, ternary and multinary compounds.

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Research articlesX-ray diffraction study of phonon and magnon properties of Eu2Cu6P5 ferromagnet

Highlights

Abstract

Introduction

Section snippets

Experiment

Results and discussion

Conclusion

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgement

J. Magn. Magn. Mater.

J. Phys. Chem. Solids.

J. Alloys Comp.

J. Magn. Magn. Mater.

J. Alloys Compd.

J. Alloys Compd.

J. Magn. Magn. Mater.

J. Alloys Compd.

Solid State Commun.

Physics of Rare Earth Solids

Introducing a magnetic guest to a tetrel-free clathrate: synthesis, structure, and properties of EuxBa8– xCu16P30 (0 ≤ x ≤ 1.5)

Inorg. Chem.

Giant magnetocaloric effect in clathrates

J. Appl. Phys.

Relationship between composition and charge carrier concentration in Eu8Ga16−xGe30+x

Phys. Rev. B.

Two New Arsenides, Eu7Cu44As23 and Sr7Cu44As23, With a New Filled Variety of the BaHg 11 Structure

Inorg. Chem.

EuNi2P4, the first magnetic unconventional clathrate prepared via a mechanochemically assisted route

Inorg. Chem. Front.

Correlating Itinerant Magnetism in RCo2Pn2 Pnictides (R = La, Ce, Pr, Nd, Eu, Ca; Pn = P, As) to Their Crystal and Electronic Structures

Acc. Chem. Res.

Research articles
X-ray diffraction study of phonon and magnon properties of Eu₂Cu₆P₅ ferromagnet

Introducing a magnetic guest to a tetrel-free clathrate: synthesis, structure, and properties of Eu_xBa_{8– x}Cu₁₆P₃₀ (0 ≤ x ≤ 1.5)

Relationship between composition and charge carrier concentration in Eu₈Ga_16−xGe_30+x

EuNi₂P₄, the first magnetic unconventional clathrate prepared via a mechanochemically assisted route

Correlating Itinerant Magnetism in RCo₂Pn₂ Pnictides (R = La, Ce, Pr, Nd, Eu, Ca; Pn = P, As) to Their Crystal and Electronic Structures