2Material Science and Technology of Japan, 61（2024）90～93

Jun. 2024

Regular Paper
Optical Floating Zone Crystal Growth of Rare-Earth Tantalates
RE8Ta2O17 (RE = Gd, Y, and Lu)
Yueshen ZHOU1,2, Dongsheng YUAN1,* and Kiyoshi SHIMAMURA1,2,*
1National Institute for Materials Science, Tsukuba 305-0044, Japan.
2Graduate School of Advanced Science and Engineering, Waseda University, Shinjuku, Tokyo 169-8555,

Japan.
Received Jan. 23, 2024; accepted Apr. 23, 2024
Abstract
The bulk crystal growth of rare-earth tantalates RE8Ta2O17 (RE = Gd, Y, and Lu) was first achieved using the opticalfloating zone method. The crystals are colorlessness and exhibit a high transparency with an absorption edge at ~265nm. They are characterized in fluorite-type structure Fm-3m, and their compositions were found shifted from thenominal RE/Ta ratio of one, being smaller as the ionic radius of RE increases. Under light excitation of 230 nm, all theRE8Ta2O17 samples emit at ~280 nm originating from Ta-O charge transfer. However, undoped RE8Ta2O17 series onlyemit a weak luminescence due to the strong self-absorption of their UV emissions.
Keywords: Floating zone technique, Rare-Earth Tantalates, RE8Ta2O17, Crystal

1.

Introduction

Lu2O3-Ta2O5

compounds

throughsolid-state

reactions,Rare-earth tantalates exhibit promising potentials across various

constructing a phase diagram spanning 0-100 mol% Lu2O3 [6].

applications, including refractories, scintillators, laser hosts, andUnlike in the Gd-Ta and Y-Ta systems, the melt with a molar ratio

oxygen ion conductors [1-3]. They are characterized by highof 1:1 did not yield a single-phase LuTaO4 but with a coexistence

melting points due to the refractory nature of RE2O3 and Ta2O5,of Lu3TaO7. A single cubic phase Lu3TaO7 was also identified,particularly for compounds with large RE2O3 ratio, thus their

melting congruently within the range of 70-75 mol% Lu2O3. But

single-crystal growth is not easy by the conventional Czochralski

for the composition ~80% Lu2O3, the F phase became unstable atand Bridgeman methods. Consequently, most of the studies so far

high temperatures exceeding 1500℃ [6].
Considering the relatively high density of RE8Ta2O17 and theare focused on ceramics [4-9].
Among this family, Gd2O3, Y2O3, and Lu2O3 based binary

possible luminescence from Ta-O charge transfer centerstantalates attract the most attention as hosts for laser and(commonly observed in tantalates) [7, 11], its single crystal

luminescence. Yokogawa et al. studied the phase relations within

growth is highly demanded in order to evaluate the optical

the Gd2O3-Ta2O5 and Y2O3-Ta2O systems [4,5]. By solid-state

properties as intrinsic scintillators.

reactions and thermal analyses, they confirmed the congruently

Herein, we used the optical floating zone method to grow themelting compounds RETaO4 (RE = Gd, Y) at approximately 50%

cubic phase RE8Ta2O17. The crystals were characterized in terms

RE2O3. Furthermore, a fluorite-type cubic phase (F phase, spaceof crystal structure, chemical composition, optical transmittance,group Fm-3m) was found at around 80% RE2O3, being stableand photoluminescence properties.

above 1500℃ but decomposing into RE2O3 & RE3TaO7 during2.

cooling [4,5]. This F phase exhibited disordered cation and anion
sublattices [10]. In a similar vein, Xing et al. explored different

Experimental method
The growth of RE8Ta2O17 (RE = Gd, Y, and Lu) transparent

crystals was conducted in Ar atmosphere using an optical floating-*Corresponding author:YUAN.Dongsheng@nims.go.jp

zone furnace (FZ-T-12000-X-I-VPM-NS-PC, Crystal Systems

SHIMAMURA.Kiyoshi@nims.go.jp(90)