New type of borophosphate anionic radical in the crystal structure of CsAl 2 BP 6 O 20

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CTURE OF INORGANIC COMPOUNDS

New Type of Borophosphate Anionic Radical in the Crystal Structure of CsAl2BP6O20 L. V. Shvanskayaa,b,*, O. V. Yakubovichb, and V. I. Belikb a

MISIS National University of Science and Technology, Leninskii pr. 4, Moscow, 119049 Russia b Faculty of Geology, Moscow State University, Moscow, 119992 Russia *e-mail: [email protected] Received December 4, 2015

Abstract—The crystal structure of a new borophosphate CsAl2BP6O20 obtained by spontaneous crystallization in a multicomponent Cs–Cu–B–P–O system is determined by X-ray diffraction (a = 11.815(2), b = 10.042(2), and c = 26.630(4) Å; space group Pbca, Z = 8, V = 3159.5(10) Å3; R1 = 0.043). A new type of borophosphate anionic 2D radical characterized by the lowest B : P = 1 : 6 ratio and containing P3O10 phosphate groups is found in the compound. A mixed-type anionic framework consisting of vertex-sharing BO4 and PO4 tetrahedra and AlO6 octahedra is distinguished in the structure. Large cesium atoms are located in the channels of the framework. Topological relationships are revealed between the structures of the CsAl3(P3O10)2 and CsAl2BP6O20 phases having different cationic compositions. These compounds can be considered quasi-polytypic phases. DOI: 10.1134/S1063774516050205

INTRODUCTION The class of borophosphates includes about 40 synthetic and two natural compounds and exhibits a wide variety of crystal structures. The systematics of borophosphates is based on two main criteria, namely, type of fundamental structural unit and boron-to-phosphorus ratio B : P [1–4]. Borophosphates with low B : P ratios and anionic radicals containing vertex-sharing PO4 tetrahedra are rather rare in occurrence. At first, the absence of P–O–P bonds in the known borophosphate radicals was explained by Pauling’s fourth rule. However, since 2010 a series of compounds with diphosphate groups included in borophosphate anions has been synthesized. All these compounds— Cs2Cr3(BP4O14)(P4O13), CsFe(BP3O11) [5], Li2Cs2B2P4O15, Li3M2BP4O14 (M = K, Rb) [6], and K7B2P5O19 [7]— belong to the class of anhydrous borophosphates. They were prepared by solid-phase synthesis or flux synthesis. Among the above borophosphates with the P–O–P bonds, the minimal ratio B : P = 1 : 4 is char− acteristic of the anionic radical of the [B(P2O7)2]5n n type found in the crystal structures of Cs2Cr3(BP4O14)(P4O13) [5] and isotypic phases Li3K2BP4O14 and Li3Rb2BP4O14 [6]. The same structural units form a one-dimensional fragment, namely, − the [B(P2O7)2]5n anionic chain, in the former case n and a two-dimensional construction, namely, the layer of the same composition, in the latter case.

Another example of the anionic borophosphate radical with high phosphorus content (B : P = 1 : 4) is provided by the isolated radical consisting of the central BO4 tetrahedron with all vertices shared with PO4 tetrahedra. Representatives of borophosphates whose crystal structures contain isolated [B(PO4)4] clusters are presented in [1]. In addition to the phases mentioned above, compounds with relativ