Orthorhombic YBaCo 4 O 8.4 crystals as a result of saturation of hexagonal YBaCo 4 O 7 crystals with oxygen
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CTURE OF INORGANIC COMPOUNDS
Orthorhombic YBaCo4O8.4 Crystals As a Result of Saturation of Hexagonal YBaCo4O7 Crystals with Oxygen N. V. Podberezskayaa, N. B. Bolotinab, V. Yu. Komarova, c, M. Yu. Kamenevaa, L. P. Kozeevaa, A. N. Lavrova, and A. I. Smolentseva a
Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, pr. Akademika Lavrent’eva 3, 630090 Russia b Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia c Novosibirsk National Research State University, ul. Pirogova 2, Novosibirsk, 630090 Russia e-mail: [email protected] e-mail: [email protected] e-mail: [email protected] Received December 30, 2014
Abstract—Hexagonal YBaCo4O7 crystals (sp. gr. P63mc, ah = 6.3058(4) Å, ch = 10.2442(7) Å, Z = 2) are saturated with oxygen to the YBaCo4O8.4 composition and studied by X-ray diffraction (XRD) analysis. The saturation is completed by a structural first-order phase transition to orthorhombic crystals (sp. gr. Pbc21, ao = 31.8419(2) Å, bo = 10.9239(5) Å, co = 10.0960(5) Å, Z = 20). The connection of two lattices is expressed in terms of the action of matrix (500/120/001) on the hexagonal basis. Five structural fragments of the same type but with different degrees of order alternate along the long axis of the oxygen-saturated orthorhombic structure. The XRD data on single crystals differ from the results obtained by other researchers on ceramic samples; possible causes of these differences are discussed. DOI: 10.1134/S1063774515040161
INTRODUCTION Valldor [1] was the first to study the physical properties and determine the crystal structure of cobaltates belonging to the R‒Ba‒Co‒O system (R is a rare earth element or Y) with the element ratio Y : Ba : Co : O = 1 : 1 : 4 : (7 + x) [1–14]. RBaCo4O7 + x (R114) compounds continue to attract much attention due to the unusualness and great variety of their physical properties (ferro- and antiferromagnetism, catalytic activity, etc.). The unique ability of these materials to exchange oxygen is of special interest. In particular, YBaCo4O7 + x compounds can absorb and reversibly return up to 1.4 oxygen atom per formula unit in the temperature range 300‒400°C [5–7]. The reversibility of this process was shown in [5]. It was established that the saturation has a character of the first-order phase transition [7] from the initial hexagonal phase to the oxygen-enriched orthorhombic phase. The initial YBaCo4O7 compound belongs to the structure type of mineral swedenborgite NaSbBe4O7 [15–17] (hexagonal system, sp. gr. P63mc, a = 5.4317(2) Å, c = 8.8571(4) Å, V = 226.31(2) Å3, Z = 2), the base of which is a threedimensional framework composed of Be tetrahedra with shared vertices. The building unit of the framework is a group of four tetrahedra with a shared vertex Be4O13, which is translational in the (001) plane and
passes to the z + 1/2 level under the action of the 63 axis. The layers of oxygen atoms in the bases of three tetrahedra, which are oriented perpendicula
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