Synthesis of lanthanum oxyhydroxide single crystals using an electrochemical method

doi:10.1016/j.jcrysgro.2007.03.025

Journal of Crystal Growth

Volume 304, Issue 2, 15 June 2007, Pages 448-451

https://doi.org/10.1016/j.jcrysgro.2007.03.025 Get rights and content

Abstract

Single crystals of lanthanum oxyhydroxide (LaOOH) were grown using an electrochemical method. Crystals having a maximum size of 7.0 mm in length, 1.5 mm in width, and 0.7 mm in thickness were grown. LaOOH crystallizes in a monoclinic structure of the space group P2₁/m with a lattice parameter of a=0.4417 nm, b=0.3926 nm, c=0.6573 nm, and β =112.32°. LaOOH crystals completely changed to lanthanum trihydroxide La(OH)₃ within 48 h in ambient atmosphere by water absorption. Maintenance at 390 °C by heat treatment was effective to maintain the structure of LaOOH for at least a week.

Introduction

Rare-earth compounds are of great research interest because of their various magnetic, electrical, optical, and catalytic properties. A great deal of study has been conducted on intermetallic compounds and oxides containing rare earth. Rare-earth oxyhydroxides LnOOH (Ln=La, Tb, Dy, Ho, Er, Tm, Yb, Lu) have a monoclinic structure of the space group P2₁/m and are all isostructual [1], [2], [3], [4], [5], [6], [7]. The thermal decomposition process of LnOOH has been investigated in detail [1], [8], [9], [10], [11], [12], but only a few magnetic and electrical properties of these compounds have been reported. TbOOH, DyOOH, and ErOOH are antiferromagnetic, with Néel temperatures of 10, 9, and 7.2 K, respectively, and HoOOH and YbOOH are not ordered magnetically at temperatures as low as 2.4 K [13], [14]. No magnetic ordered state could be found in TmOOH down to 0.04 K using gamma resonance spectroscopy [5]. The conductivity and the activation energies for conduction calculated from the Arrhenius plot of LaOOH were investigated [12]. However, these data were measured using polycrystalline specimens, and no measurement using single crystals has yet been performed. The lack of single-crystal data prevented the determination of the intrinsic properties of these compounds. Knowledge of the intrinsic properties of rare-earth oxyhydroxides is of importance in the application of lanthanides as ceramics and catalysts. Moreover, the subsequent dehydration process of rare-earth oxyhydroxides to rare-earth oxides by heat treatment has a potential for the formation of a nanostructure, which could be applied to optoelectronic devices and catalysis. Furthermore, the synthesis of crystals of rare-earth compounds from solution is useful for the development of a new route for the formation of nanoparticles. In this paper, we describe the results of the synthesis of LaOOH crystals of the order of millimeters using an electrochemical method.

Section snippets

Experimental procedure

Crystals of LaOOH were grown by using an electrochemical method [15]. A schematic of the apparatus for the synthesis of crystals is shown in Fig. 1. A zirconium crucible was used as a bath for the electrolyte. The crucible was placed in a stainless-steel container, which was paved with alumina powder to homogenize the temperature in the crucible. The container was covered with a stainless-steel cap with 1 atm ambient atmosphere. The temperature of the crucible was controlled by an electronic

Results and discussion

An electrical voltage of 0.4 V was applied between the electrodes and a current of 0.4–0.7 mA flowed during the deposition. LaOOH crystallized on the surface of the cathode at temperatures between 380 and 400 °C. Fig. 2 shows photographs of as-grown crystals synthesized at 390 °C. Fig. 2(a) is a photograph of crystals on an electrode, and Fig. 2(b) is a photograph of crystals that were removed from the electrode. The crystals could be removed from the electrode easily without any mechanical strain.

Conclusion

Lanthanum oxyhydroxide (LaOOH) crystals were grown using an electrochemical method with NaOH as an electrolyte. Crystals having a maximum size of 7.0 mm in length, 1.5 mm in width, and 0.7 mm in thickness were synthesized. LaOOH crystallizes in a monoclinic structure of the space group P2₁/m with a lattice parameter of a=0.4417 nm, b=0.3926 nm, c=0.6573 nm, and β=112.32°. Crystals grew only on the surface of the cathode, and a possible reaction process for crystal growth was proposed. LaOOH crystals

Acknowledgments

The work performed at Kobe University was supported by The Asahi Glass Foundation. The work conducted at Aoyama Gakuin University was supported by The 21st Century COE Program and The Private School High-tech Research Center Program of the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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Synthesis of lanthanum oxyhydroxide single crystals using an electrochemical method

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Conclusion

Acknowledgments

J. Solid State Chem.

Solid State Commun.

J. Catal.

Solid State Ion.

J. Solid State Chem.

Mater. Res. Bull.

J. Less-Common Met.

J. Solid State Chem.

Thermochim. Acta

J. Alloys Compounds