Single-crystal growth of the high-pressure phase B 2 O 3 II
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TAL GROWTH
SingleCrystal Growth of the HighPressure Phase B2O3 II L. F. Kulikova, T. I. Dyuzheva, N. A. Nikolaev, and V. V. Brazhkin Vereshchagin Institute for HighPressure Physics, Russian Academy of Sciences, Troitsk, Moscow oblast, 142190 Russia email: [email protected] Received July 19, 2010
Abstract—The technique and results of the hydrothermal singlecrystal growth of the highpressure phase B2O3 II are described. Transparent colorless crystals 450 × 450 × 150 μm in size have been grown under hydro thermal conditions (pressure 5 GPa, temperature range 1425–1025 K, and cooling rate ~100 K/h). DOI: 10.1134/S106377451103014X
INTRODUCTION
(a) 10.0.01
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O3
O3 B1
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Boron oxide B2O3 is the second (after SiO2) most important oxide material for use in glassy systems. The B2O3 melt is very viscous and it is practically impossi ble to crystallize it by cooling without a seed or a cata lyst. Under high pressures, the B2O3 melting tempera ture increases while the melt viscosity decreases, which is why both the lowpressure crystalline phase B2O3 I (at P < 4 GPa) and highpressure phase B2O3 II (P > 4 GPa) can be grown under pressure. Under nor mal conditions, the B2O3 I phase is stable (P31, a = 4.336 Å, c = 8.340 Å, Z = 3, and ρ = 2.55 g/cm3 [1]; the space group was refined to P3121 based on the same diffraction data in [2]). Boron atoms in the B2O3 I structure are located in triangles of oxygen atoms forming infinite ribbons (Fig. 1a). The dense B2O3 II modification was obtained more than 40 years ago at Р = 3.5 GPa and Т = 800 K [3]. The structure of the highpressure B2O3 II phase (Ссm21, a = 4.613(1) Å, b = 7.803 (2) Å; c = 4.129 (1) Å; Z = 4, and ρcalcd = 3.11 g/cm3 [4]) can be considered a distorted wurtzite structure (Fig. 1b). Oxygen atoms form a distorted hexagonal close packing where boron occupies 2/3 tetrahedra. Such a large (~27%) increase in density can be explained by the change in the coordination number from 3 to 4. Although the B2O3 II phase has been known for a long time, there are no data on its physical properties. The purpose of this work was to grow B2O3 II single crystals to study their physical properties.
O1
O2
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O1 O3 O3 O2 B2 O1
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O3 O2
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O1 O3 O2 B2
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B1 O1
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O3 B1
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O3 O3
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O(1)
O(2) O(2)
O(2'')
EXPERIMENTAL
Fig. 1. Structures of (a) the initial modification B2O3 I [1, 2] and (b) the B2O3 II phase [4].
The equilibrium P–T phase diagram of B2O3 was investigated in [5]; we chose the experimental condi tions based on this diagram (Fig. 2). Hydrothermal synthesis was used for the singlecrystal growth of B2O3 II because the initial glassy B2O3 is very hygro
scopic and contains a certain amount of water, which cannot be taken into consideration. The samples were grown in a Konak highpressure chamber with automatic pressure control and the pro
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SINGLECRYSTAL GROWTH OF THE HIGHPRESSURE PHASE B2O3 II I/I0 1300 1200 1100 1000 900 800 700
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