Synthesis and magnetic properties of ALnO2 (A=Cu or Ag; Ln=rare earths) with the delafossite structure

doi:10.1016/j.jssc.2009.05.035

Journal of Solid State Chemistry

Volume 182, Issue 8, August 2009, Pages 2104-2110

https://doi.org/10.1016/j.jssc.2009.05.035 Get rights and content

Abstract

Synthesis, structures, and magnetic properties of ternary rare earth oxides ALnO₂ (A=Cu or Ag; Ln=rare earths) have been investigated. CuLnO₂ (Ln=La, Pr, Nd, Sm, Eu) were synthesized by the direct solid state reaction of Cu₂O and Ln₂O₃, and AgLnO₂ (Ln=Tm, Yb, Lu) were obtained by the cation-exchange reaction of NaLnO₂ and AgNO₃ in a KNO₃ flux. These compounds crystallized in the delafossite-type structure with the rhombohedral 3R type (space group: R-3m). Magnetic susceptibility measurements showed that these compounds are paramagnetic down to 1.8 K. Specific heat measurements down to 0.4 K indicated that CuNdO₂ ordered antiferromagnetically at 0.8 K.

Graphical abstract

Ternary rare earth oxides ALnO₂ (A=Cu or Ag; Ln=rare earths) crystallized in the delafossite-type structure with the rhombohedral 3R poly-type (space group: R-3m). Magnetic susceptibility measurements showed that these compounds are paramagnetic down to 1.8 K. Specific heat measurements down to 0.4 K indicated that CuNdO₂ ordered antiferromagnetically at 0.8 K.

Introduction

It is well known that oxides containing rare earth elements show a variety of magnetic properties due to the behavior of unpaired 4f electrons. When the rare earth ions are arrayed in a structurally characteristic manner, interesting magnetic behavior has been often found. Here, we focus our attention on compounds with the delafossite-type (CuFeO₂) structure.

The synthesis, crystal structure, and electric transport properties of delafossite-type compounds were reported by Shannon, Rogers and Prewitt [1], [2], [3] and reviewed by Cann et al. [4]. Compounds with this structure have been of immense interest due to the discovery of the p-type transparent conductivity for CuAlO₂ [5]. Recently, the optical and electrical properties of silver delafossites were reported [6].

Delafossite compounds belong to a family of ternary oxides with the general formula ABO₂. In this structure, the A cation is linearly coordinated to two oxygen ions and occupied by a noble metal cation in the +1 oxidation state. Typical A cations include Pd, Pt, Cu, or Ag. The B cation is located in distorted edge-shared BO₆ octahedra with a central metal cation having a +3 charge. The delafossite structure can be visualized as consisting of two alternating layers: a planar layer of A cations in a triangular pattern and a layer of edge-sharing BO₆ octahedra flattened with respect to the c axis. Depending on the stacking of the double layers (close-packed A cations and BO₆ octahedra), the delafossite structure can form as one of two polytypes, i.e., the hexagonal 2H type and the rhombohedral 3R type.

In this structure, the B cations adopt triangular geometric arrangements. If the magnetic ions are located in the B sites, and if there exists an antiferromagnetic interaction between the nearest-neighbor magnetic ions, they can show magnetic frustrations.

There are some reports on the delafossite-type compounds containing rare earths. Haas et al. reported the preparation of CuLnO₂ (Ln=La, Pr, Nd, Sm, and Eu) [7]. Oxygen excess in delafossite structures was reported in the CuLnO₂ family, where several researchers have demonstrated the possibility to insert oxygen atoms in the Cu layer, namely in the center of Cu triangles [8], [9], [10]. Very recently, the preparation of one silver delafossite-type compound AgYbO₂ was reported [11].

In this study, we tried to synthesize a series of ALnO₂ (A=Cu or Ag; Ln=rare earths) and determined their crystal structures through the Rietveld analysis for the powder X-ray diffraction data. In these compounds, paramagnetic Ln ions adopt the triangle-based array. Therefore, anomalous magnetic properties reflecting the geometric frustration may be observed. In order to elucidate basic magnetic properties of ALnO₂ compounds containing rare earths, their magnetic susceptibilities were measured in the temperature range between 1.8 and 400 K. In addition, specific heat measurements of CuNdO₂ were performed in the temperature range between 0.4 and 300 K.

Section snippets

Sample preparation

The CuLnO₂ (Ln=La, Pr, Nd, Sm, Eu) were prepared by heating 1:1 mixtures of Cu₂O and Ln₂O₃ in a flowing atmosphere of Ar gas at 1123 K for a day. For La₂O₃ and Nd₂O₃, they absorb moisture in air and easily form lanthanide hydroxides Ln(OH)₃. Therefore, these compounds were preheated at 1073 K for a day.

Silver delafossite compounds AgLnO₂ were prepared by the following cation-exchange reactions using NaLnO₂ as precursors;NaLnO₂+AgNO₃→AgLnO₂+NaNO₃.
The reactions were carried out in a flux of AgNO₃

Preparation and crystal structure

For A=Cu, we could successfully prepare a series of CuLnO₂ compounds with Ln=La, Pr, Nd, Sm, Eu. A representative powder X-ray diffraction profile is shown in Fig. 1 for CuPrO₂. The observed diffraction peaks were indexed on a rhombohedral cell with the space group R-3m. The X-ray diffraction data for all the compounds prepared in this study were analyzed by the Rietveld method on the basis of the same space group. The refined structural parameters for CuPrO₂ are listed in Table 1. The lattice

Acknowledgement

This work was supported by Grant-in-aid for Scientific Research, No. 20550052 from the Ministry of Education, Science, Sports, and Culture of Japan.

References (21)

M.A. Marquardt et al.
Thin Solid Films
(2006)
M. Trari et al.
J. Solid State Chem.
(1994)
Y. Hashimoto et al.
J. Solid State Chem.
(2003)
J. Li et al.
Solid State Sci.
(2004)
R.D. Shannon et al.
Inorg. Chem.
(1971)
C.T. Prewitt et al.
Inorg. Chem.
(1971)
D.B. Rogers et al.
Inorg. Chem.
(1971)
H. Kawazoe et al.
Nature
(1997)
W.C. Sheets et al.
Inorg. Chem.
(2008)
H. Haas et al.
Kristallografiya
(1969)

There are more references available in the full text version of this article.

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Synthesis and magnetic properties of ALnO2 (A=Cu or Ag; Ln=rare earths) with the delafossite structure

Abstract

Graphical abstract

Introduction

Section snippets

Sample preparation

Preparation and crystal structure

Acknowledgement

Thin Solid Films

J. Solid State Chem.

J. Solid State Chem.

Solid State Sci.

Inorg. Chem.

Inorg. Chem.

Inorg. Chem.

Nature

Inorg. Chem.

Kristallografiya

Synthesis and magnetic properties of ALnO₂ (A=Cu or Ag; Ln=rare earths) with the delafossite structure